deliverable 3.4: results on control strategies of wpps ... · project report. deliverable 3.4:...

PROMOTioN – Progress on Meshed HVDC Offshore Transmission Networks Mail [email protected] Web www.promotion-offshore.net This result is part of a project that has received funding form the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691714. Publicity reflects the author’s view and the EU is not liable of any use made of the information in this report.

Deliverable 3.4: Results on control strategies of WPPs connected to DR-HVDC

DOCUMENT INFO SHEET

Document Name: Deliverable 3.4: Results on control strategies of WPPS connected to DR-HVDC

Responsible partner: USTRAT (University of Strathclyde)

Work Package: WP 3

Work Package leader: DTU

Task: 3.2 General control algorithms (M09-M24)

Task lead: DTU

DISTRIBUTION LIST

PROMOTioN partners, European Commission

APPROVALS

Name Company

Validated by: Kees Koreman TenneT

Willem Leterme KU LEUVEN

Task leader: Ömer Göksu DTU

WP Leader: Ömer Göksu DTU

WP Number WP Title Person months Start month End month

WP3 Wind Turbine – Converter Interaction 269 3 42

Deliverable

Number Deliverable Title Type

Dissemination

level Due Date

D3.4 Results on control strategies of WPPS connected to DR-HVDC Report Public 24

LIST OF CONTRIBUTORS

Work Package 3 and deliverable 3.4 involve a large number of partners and contributors. The names of the

partners, who contributed to the present deliverable, are presented in the following table.

PARTNER NAME

DTU Ömer Göksu, Nicolaos A. Cutululis, Oscar Saborío-Romano

UPV Ramon Blasco-Gimenez, Soledad Bernal-Perez, Salvador Añó-Villalba, Jaime Martinez-Turégano, Manuel Hernandez-Mejias

USTRAT Lie Xu, Rui Li, Grain Adams

ADWEN Carmelo Perez, Ilir Purellku

DONG Energy Lorenzo Zeni, Łukasz H. Kocewiak

Energinet.dk Walid Ziad El-Khatib

MVOW Tusitha Abeyasekera

RWTH Christina Brantl, Philipp Ruffing

Iberdrola Iñigo Azpiri

ABB Kanstantsin Fadzeyeu, Adil Abdalrahman

Statoil Wei He

CONTENT

Document info sheet ............................................................................................................................................................. ii

Distribution list ...................................................................................................................................................................... ii

Approvals ............................................................................................................................................................................. ii

List of Contributors .............................................................................................................................................................. iii

List of Definitions / abbreviations ........................................................................................................................................ 3

1. Introduction .................................................................................................................................................................... 4

2. Simulation results during normal operation of DR-HVDC system ........................................................................... 5

2.1. HVDC link and offshore AC grid start-up operation ................................................................................................. 8

2.1.1 Offshore AC grid energisation and string connection ...................................................................................... 9

2.1.2 WPP production ramped up to full available power ....................................................................................... 14

2.2. HVDC link and offshore AC grid disconnection operation ..................................................................................... 17

2.2.1 Stop power transmission ............................................................................................................................... 18

2.2.2 Complete de-energisation of the offshore AC grid ......................................................................................... 22

2.3. Intentional islanding ............................................................................................................................................... 26

2.3.1 Intentional islanding ....................................................................................................................................... 26

2.3.2 Re-synchronisation to external AC from ISL configuration ............................................................................ 29

2.4. Dynamic voltage control ........................................................................................................................................ 34

2.4.1 ISL ................................................................................................................................................................. 34

2.4.2 SAC ............................................................................................................................................................... 36

2.5. Wind farm power control ........................................................................................................................................ 40

2.6. Response to changes in reactive power sharing command .................................................................................. 43

2.6.1 Reactive power sharing command with DR configuration ............................................................................. 43

2.6.2 Reactive power sharing command in ISL configuration ................................................................................. 45

2.7. Response to active power reference commands when connected to external AC ............................................... 47

2.8. Disconnection / reconnection of a string / OWF .................................................................................................... 52

2.8.1 Disconnection of a string / OWF .................................................................................................................... 52

2.8.2 Reconnection of a string / OWF ..................................................................................................................... 54

2.9. Disconnection / reconnection of filters ................................................................................................................... 57

2.9.1 Disconnection of filters ................................................................................................................................... 58

2.9.2 Reconnection of filters ................................................................................................................................... 60

3. Fault ride-through and protection of DR-HVDC system .......................................................................................... 63

3.1. Distributed control strategy of WT FECs ............................................................................................................... 64

3.1.1 Inner current control ....................................................................................................................................... 64

3.1.2 Voltage control ............................................................................................................................................... 64

3.1.3 Active power control ...................................................................................................................................... 64

3.1.4 Reactive power sharing control ..................................................................................................................... 65

3.1.5 Distributed PLL-based frequency control ....................................................................................................... 65

3.1.6 Control strategy of WT converters connected with DR-HVDC and umbilical AC cable ................................. 66

3.2. Unintended transmission capability limitation ........................................................................................................ 67

3.2.1 Onshore grid faults ........................................................................................................................................ 68

3.2.2 DC cable faults .............................................................................................................................................. 74

3.2.3 Internal DRU faults ........................................................................................................................................ 80

3.3. Umbilical AC cable faults ....................................................................................................................................... 83

3.4. Offshore AC faults ................................................................................................................................................. 88

3.4.1 Symmetrical offshore AC faults ..................................................................................................................... 88

3.4.2 Asymmetrical offshore AC faults .................................................................................................................... 94

4. Summary .................................................................................................................................................................... 100

5. Bibliography ............................................................................................................................................................... 102

PROJECT REPORT. Deliverable 3.4: Results on control strategies of WPPs connected to DR-HVDC

3

LIST OF DEFINITIONS / ABBREVIATIONS

Term Meaning

WPP Wind Power Plant

WT Wind Turbine

FEC Front-End Converter

OWF Offshore Wind Farm VSC Voltage Source Converter

DR Diode Rectifier DRU Diode Rectifier Unit

Radial grid Grid that does not contain a loop Point-to-Point (Inter) connection between two points

OTS Offshore Transmission System WFG Wind Farm Group

TSO Transport (onshore) System Operator

MOG Meshed Offshore Grid Multi Terminal More than two stations

SAC Synchronised AC DRSAC Diode Rectifier and Synchronised AC

ISL Island Operation Cluster

See Figure 2-3 D3.1

Sub-cluster

Cluster Controller (CLC)

Master Controller (MC)

Offshore HVDC Conv. (OFC)

Onshore HVDC Conv. (ONC)


4

1. INTRODUCTION

This report is part of Task 3.2 General Control Algorithms, which includes the simulation results of the DR-

HVDC system in normal operation and during faults. Figure 1-1 shows the considered general OWFs connected

with point-to-point DR-HVDC link.

Section 2 presents the results of the DR-HVDC system during normal operation, which covers energization,

connection and disconnection of the offshore AC grid, operating point dynamic changing in different

configurations as specified in D3.2. The ancillary services provided by the DR-HVDC system is addressed in

D3.5.

Section 3 focuses on the fault ride-through and protection of the DR-HVDC system and various fault cases are

analysed, i.e., onshore grid fault, DC cable fault, internal DRU fault, umbilical AC cable fault, and offshore fault.

The performances of the DR-HVDC during the fault and after fault isolation are addressed.

Figure 1-1 Base line scenario with three DRU platforms (from D3.2)


5

2. SIMULATION RESULTS DURING NORMAL OPERATION OF DR-HVDC SYSTEM

This section includes the results corresponding to the normal operation of the complete DR-HVDC connected

wind power plant. The considered simulation cases are those defined in deliverable D3.2. The distance between

DRU platforms is 25km. The aggregation level used (as defined in D3.2 [1]) is:

Table 2-1: Aggregation level of the tested system.

Aggregation/Detail Level of simulation detail

OWF models Level 7

On-shore grid Level 3

On-shore MMC converter Level 3

Wind Turbine model Level 4

Figure 2-1 and Figure 2-2 show a simplified diagram of the system with the name and location of the different

variables considered. The subscripts i,j correspond to (i=wind farm number, j=wind turbine number within the

windfarm).

WT transformer

VT-i,j

PWT-i,j

QWT-i,jVTa-i,j

BKWTb-i,jVF-i,j

IF-i,j

PF-i,j

QF-i,jBKWTa-i,j BKOWF i,j

VF-i

WT Front-end

VW-i,j

IW-i,j

Figure 2-1: Aggregated string.

WPP1 400MW

BKOWF 1,1

BKOWF 1,2

BKOWF 1,3

BKOWF 1,4

BKOWF 1,5

BKOWF 1,6

VF-1

IF-1

PF-1

QF-1BKAC filter 1

AC filters

BKDRU1

66kV/43kV/43kV

ΔY

YY

Δ

ΔY

YY

Δ

On-shoreVariable

Transformer

BKUmb.2 BKUmb.3BKUmb.1

Vumb_on-shore

Iumb_on-shore

Pumb_on-shore

Qumb_on-shore

Vumb

Iumb

Pumb

Qumb

64MW

64MW

64MW

64MW

72MW

72MW

OWF3OWF2

IR-1

PR-1

QR-1

DRU2

DRU3

VDC

On-shore

Figure 2-2: DR-HVDC system.


6

The wind turbine control in normal operation for each individual wind turbine is shown in Figure 2-3.

Inner- PR Current loop Outer-PR Voltage loop

pol

rec

abc

PW

M

abc

ΔY

Y

YY

Y

Δ

Δ

abc

Vdc

VW-i,j

IW-i,jVT-i,j

PWT-i,j

QWT-i,j

BKWTb-i,j

BKWTa-i,j BKOWF i,j

VF-i

I w i, j I w i , j

V T i, j V T i, j

I T i, j I T i , j

IT-i,j

VT-i,j

IT-i,j

V w,0*

V synchro

synchro

V T i, j*

t i , j*

1s

VT i, j*

V T i, j

PR

VT i, j*

V T i, j

I T i, j

Iw i, j*

Iw i , j*

I T i , j

I w i, j

I w i , j

PR

V T i, j

V w i, j

V T i, j

V w i , jPRPR

PI

V Tad i , j

V T d i, j

PI0

V T q i , j

+

f

Synchronisation Controller

PQCalculation+

LPF

PWT i, j*

QWT i , j*

P

PD+

++

Droop-Controller

ACfilters

BKDRU

DRU X 3

VTa-i,j

Figure 2-3: WTG grid side converter control.

The proposed control consists of an inner proportional-resonant (PR) current control loop, an outer PR voltage

control loop, outer P/w, Q/V droops. Figure 2-3 also includes the off-shore grid synchronisation block. The

operation of the aforementioned blocks will be explained in detail as follows.

The proposed distributed control technique is based on a cascaded control approach, with an inner PR current

control loop in the stationary α-β frame. The outer PR control loop is responsible for the control of the VT voltage

magnitude and angle control. The bandwidth of the control loops is 200 Hz and 40 Hz, respectively. The internal

PR current loop allows for control of positive and negative sequence WTG currents and voltages and allow for

both current and voltage limitation.

Active and reactive power sharing between wind turbines is carried out by means of P/w and Q/V droops:

* * *d

dm P P m P P

dt (2-1)

* *0w wV V n Q Q (2-2)

where m and md are the proportional and derivate coefficients of active power P, and n is the proportional coefficient of the reactive power Q [2].


7

Total delivered power is regulated by means of secondary voltage control by the offshore wind farm controller.

This approach requires slow communications, albeit alternative control strategies which do not require

communication during normal operation are available in the literature as control strategy proposed in [3-6]. This

control strategy has been chosen as its principles of operation are relatively easy to understand.

The control system also includes synchronisation of each wind turbine with the 66kV offshore AC-grid.

Synchronisation is achieved by means of controlling the voltage magnitude and angle of voltage Vt to follow

those of Vta. The PI controllers in green allow the WT synchronisation by expressing Vt in a synchronous

rotating frame aligned with Vta voltage vector. When Vtq=0 and Vtd=Vtad for longer than 50 ms, the breaker

BKWT-i,j of the WTG is closed automatically.

As Figure 2-4 shows, a wind farm controller (magenta blocks) is implemented in order to regulate the total

power flowing to the HVDC-link through the DRUs. Other functions of the centralised control include sequencing

of the different operations, umbilical cable active power control, offshore AC grid voltage control,

synchronisation with the umbilical cable, and synchronisation of a string or OWF. This controller has a sampling

period of 100 ms. DRU active power control will be operational when the system is in DR or DRSAC

configurations. In SAC and ISL configuration, the centralised controller regulates the voltage of offshore AC grid.

Synchronisation of the offshore AC-grid with the umbilical cable is carried out by the wind farm controller, as

Figure 2-4 shows. The wind farm controller sets the corresponding offshore AC-grid voltage VFi to have the

same magnitude and phase as the offshore side umbilical cable voltage (Vumb in Figure 2-2). Synchronisation is

performed in a similar way as individual wind turbines, albeit with a larger time step due to the need for

communications of remote measurements and control actions. Once the voltage and phase angles of the

aforementioned voltages are within a certain range during 500 ms, the synchronising relay closes breaker

BKOWF i,1. The active power control through the umbilical cable is also included within the wind farm controller.

The parameters of the umbilical active power PI controller depend on the number of active wind turbines

NWTconn.

Figure 2-4: OWF control for wind farm synchronisation with umbilical cable.

The wind farm controller also includes the corresponding state machines to perform the required operations. All

test cases covered below have been simulated using a single PSCAD model, which takes references and

breaker positions from external files.


8

All results are shown in [pu] or Hz. Table 2-2 shows the base values of the different magnitudes.

Table 2-2: Base value of presented signals in simulation results of normal operation.

Magnitude Base value

Pumb*; Pumb; Pumb_onshore 40 MW

Qumb; Qumb_onshore 40 MVAr

Vumb;Vumb_onshore; VF*; VF; VFi 66 kV

Pwt1_1; Pwt1_2; Pwt2_1; Pwt2_2; Pwt3_1; Pwt3_2

72 MW

Pwt1_3; Pwt1_4; Pwt1_5; Pwt1_6; Pwt2_3; Pwt2_4; Pwt2_5; Pwt2_6; Pwt3_3; Pwt3_4; Pwt3_5; Pwt3_6;

64 MW

Qwt1_1; Qwt1_2; Qwt2_1; Qwt2_2; Qwt3_1; Qwt3_2

72 MVAr

Qwt1_3; Qwt1_4; Qwt1_5; Qwt1_6; Qwt2_3; Qwt2_4; Qwt2_5; Qwt2_6; Qwt3_3; Qwt3_4; Qwt3_5; Qwt3_6;

64 MVAr

PF*; PF; PFi 400 MW

QFi 400 MVAr

IFi 3.45 kA

2.1. HVDC LINK AND OFFSHORE AC GRID START-UP OPERATION

Figure 2-5 shows the considered energisation procedure, as described in deliverable D3.2 [1]. This procedure

includes the energisation of all the offshore system elements, as well as, all steps needed for the wind farm to

start normal production.

Simulation results cover 36 s of operation, starting with a totally de-energised offshore AC-grid and finishing with

the offshore wind farms through the DRU with the umbilical cable disconnected. For ease of presentation,

simulation results of the start-up procedure are shown in two parts. The first one consists of the energisation of

the offshore AC grid and the connection of all WT to the offshore AC grid (0-22 s). The second part shows the

procedure of production ramped up to full available power and the disconnection of umbilical cable (22-36 s).


9

Figure 2-5: Energization procedure.

The energization procedure includes voltage control/Static Var Compensator (SVC) mode of operation to

minimise reactive power through the umbilical cable, and HVDC standby mode when DRUs are connected but

not yet transmitting power.

2.1.1 OFFSHORE AC GRID ENERGISATION AND STRING CONNECTION

This section deals with the energisation of the offshore AC system and its control without transmission of active

power. The sequence of events is listed in Table 2-3.

Table 2-3: Events of offshore AC grid energisation and string connection.

Time (s) Events

0.1 1- Offshore AC grid is energised by closing BKumb1

0.3

2- WTi,1 synchronisation (energisation of string by closing BKOWF i,1 and then energisation). Synchronisation of 3 strings is done sequentially with a 1 second delay between connections. At this stage the WTGs can take over the P and Q requirements of the offshore AC grid.

3.5 3- Enable of umbilical active power control (P* = 0 MW).

5.0 4- Enable of offshore AC voltage control.

6.5 5- Synchronisation of the remaining strings (5 strings per OWF)

with a 1 second delay between connections.


10

For all the simulations, it has been considered that the tap changer of the umbilical cable is set at its lowest

position (0.85pu). In this way, it is possible to energise the offshore wind farm cables without overvoltage and

without additional offshore AC grid shunt compensators.


11

Main : Graphs

sec 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

(a)

Pumb Pumb_onshore

-1.75 -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50

(b)

Qumb Qumb_onshore

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(c)

Vumb Vumb_onshore

-0.020

0.000

0.020

0.040

0.060

0.080

0.100

(d)

Pwt1_i Pwt2_i Pwt3_i

-0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050 0.100

(e)

Qwt1_i Qwt2_i Qwt3_i

Figure 2-6: Simulation results during offshore AC grid start-up (string connection): (a) active power through umbilical cable

measured at umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at offshore and on-

shore ends; (c) offshore and on-shore AC voltages; (d) WTG active power PWT-i,j; (e) WTG reactive power QWT-i,j.


12

OWF_Control,Main : Graphs

sec 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0

-0.0200

-0.0150

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF* PF

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(b)

VF* VF

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

(c)

Pumb* Pumb

49.60 49.70 49.80 49.90 50.00 50.10 50.20 50.30 50.40

(d)

wf

Figure 2-7: Simulation results during offshore AC grid start-up (string connection): (a) total OWF active power; (b) offshore

voltage VF1; (c) active power through the umbilical cable; (d) frequency of the offshore AC grid.

Figure 2-6 shows the response of the umbilical cable and the power of each string during the events listed in

Table 2-3. At t=0.1 s, the offshore AC grid is energised as BKumb1 is closed. The reactive power produced by the

offshore cables is quite large and causes the reactive power delivered through the umbilical to increase beyond

1pu for a very short amount of time. It is possible to reduce the umbilical cable reactive power requirements by

energizing the offshore AC-grid in sections. In our case, we consider a short temporary overload of the umbilical

cable as acceptable.

After the cable energisation, Figure 2-6 shows the connection of each individual string. Initially, one string per

OWF (aggregated WTi,1) is connected at 1 second intervals. The sequence is as follows:


13

Each aggregated string generates about 0.025 pu active power after it connects to offshore AC grid. Now the

number of connected wind turbines can provide the required active and reactive power to continue energizing

the complete offshore wind farm.

Main : Graphs

sec 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0

-0.0200

-0.0150

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF1 PF2 PF3

-0.040

-0.030

-0.020

-0.010

0.000

0.010

0.020

(b)

QF1 QF2 QF3

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(c)

VF1 VF2 VF3

0.000

0.020

0.040

0.060

0.080

0.100

0.120

(d)

IF1 IF2 IF3

Figure 2-8: (d) Simulation results during offshore AC grid start-up (string connection): (a) active power delivered by each

OWF-i; (b) reactive power generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through each

OWF-i.

At t=5 s, the umbilical active power controller is enabled. We can see the effect in Figure 2-6 and Figure 2-7, as

the active power delivered by the WTGs increases and the active power through the umbilical cable goes to

zero. At the same time, the offshore AC voltage control is enabled. The connected WTGs start operating in

STATCOM mode. Voltage references are set so reactive power from the umbilical is zero.


14

From t=6.5 to t=22, the remaining strings are connected (at 1 second intervals). Connection procedure for each

string is the same as described before. However, in this case, each WTG contributed to active and reactive

power generation from its connection. Therefore, the active and reactive power contribution of each individual

wind turbine decreases with increasing the number of connected WTGs.

Figure 2-8 shows the active power, reactive power, voltage and current of the DRU platforms (see Figure 2-2)

during offshore AC grid start-up operation.

2.1.2 WPP PRODUCTION RAMPED UP TO FULL AVAILABLE POWER

Once all WTGs are connected and supplying all required active and reactive power for the offshore wind farm,

the active power reference to be delivered through the HVDC link is ramped up to 1 pu. At this stage the

umbilical cable is connected and normal DRU only (DR) operation is reached at the end. It is important to

highlight that, when both DRU and umbilical cable are connected, it is possible to transmit active power through

both umbilical and HVDC cable.

Table 2-4 shows the sequence of events leading to full power production through the DRU link.

Table 2-4: Events of WPP production ramped to full available power and umbilical cable disconnection.

Time (s) Events

22.0 6- Close DRU breakers BKDRU i (at 0.5 seconds intervals). DRU transformers get

energised

24.0 7- Enable active power control through DRUs.

24.0 8- Active power reference of OWF-i is increased to 0.2 pu [6].

26.5 9- DRU filters are connected by closing breakers BKAC filter i (at 1 second intervals).

31.0 10- Active power reference of OWF-i is increased from 0.2 pu to 1 pu.

34.5 11- Umbilical cable is disconnected (breaker BKumb1 opened).

As can be seen from Figure 2-9, Figure 2-10 and Figure 2-11, the energisation of DRU AC transformers (at 22

seconds) has a little effect in the frequency and the power through the umbilical cable.

There is a small coupling between the active power through the DRUs and the reactive power through umbilical

cable (at 24 seconds and 31 seconds). This effect is produced because the changes on the offshore AC grid

voltage required for DRU active power control cause changes of reactive power through the umbilical AC cable.

Figure 2-9 shows reactive power through umbilical cable, which depends on the voltage difference between on-

shore and offshore grid.


15

Main : Graphs

sec 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0

-0.150 -0.100 -0.050 0.000 0.050 0.100 0.150 0.200 0.250

(a)

Pumb Pumb_onshore

-1.20 -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40

(b)

Qumb Qumb_onshore

0.700 0.750 0.800 0.850 0.900 0.950 1.000 1.050 1.100

(c)

Vumb Vumb_onshore

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(d)


-0.40

-0.30

-0.20

-0.10

0.00

0.10

(e)


Figure 2-9: Simulation results during WPP production ramped to full available power: (a) active power through umbilical cable

measured at umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at offshore and on-

shore ends; (c) offshore and on-shore AC voltages; (d) WTG active power PWT-i,j; (e) WTG reactive power QWT-i,j.


16

It has been assumed that each DRU platform has three filter banks. Each bank is connected sequentially, in

order to reduce the voltage peaks during connection and disconnection (see point 2.9.2 for more information)

and the interaction with the umbilical cable. Clearly, the connection of DRU filters (at 26.5 seconds) produces an

increment of voltage, which acts as a disturbance to the active power and frequency controllers (see Figure 2-

10).


sec 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(a)

PF* PF

0.880 0.900 0.920 0.940 0.960 0.980 1.000 1.020

(b)

VF* VF

-0.150

-0.100

-0.050

0.000

0.050

0.100

0.150

(c)

Pumb* Pumb

49.800

49.850

49.900

49.950

50.000

50.050

50.100

(d)

wf

Figure 2-10: Simulation results during WPP production ramped to full available power: (a) total OWF active power; (b)

offshore voltage VF1; (c) active power through the umbilical cable; (d) frequency of the offshore AC grid.

Finally, the disconnection of the umbilical cables is carried out by first setting its active power to zero. It would

be possible to disconnect the umbilical cable at zero reactive power by acting on the transformer tap-changer.


17

Main : Graphs

sec 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(a)

PF1 PF2 PF3

-0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050 0.100

(b)

QF1 QF2 QF3

0.700 0.750 0.800 0.850 0.900 0.950 1.000 1.050

(c)

VF1 VF2 VF3

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(d)

IF1 IF2 IF3

Figure 2-11: (c) Simulation results during WPP production ramped to full available power: (a) active power delivered by each


OWF-i.

2.2. HVDC LINK AND OFFSHORE AC GRID DISCONNECTION OPERATION

This section covers programmed HVDC link transmission disconnection as a response to a disconnection

command. The considered sequence for ceasing active power transmission through the DRU and re-connecting

the umbilical cable consists of reducing the active power sent through the DRUs, then disconnecting the filters,

disconnecting the DRUs and then re-synchronising to the umbilical AC cable. The aforementioned sequence


18

(DR ISL SAC) is slightly different from that described in deliverable 3.2, however, other sequences are

possible.

Section 2.2.1 includes the sequence to stop power production and resume SAC operation and section 2.2.2

includes the total de-energisation of the OWF.

2.2.1 STOP POWER TRANSMISSION

The sequence for the change of mode of operation from DR ISL SAC is shown in Table 2-5.

Table 2-5: Events of offshore AC grid disconnection operation (DR ISL SAC)

Time (s) Events

4.5 1- Active power ramped down from 1 pu to 0.2 pu.

6.5 2- Disconnection of DRU filters (sequentially each 1 second opening BKAC filter i).

9.5 3- Active power reference down from 0.2 pu to 0pu.

10.5 4- Enable of offshore Voltage control, and disable DRU active power control.

11.5 5- Disconnection of DRUs (open breakers BKDRU i). Now the OWF is operating in

islanded mode (ISL)

12.0 6- Synchronisation with umbilical cable. Now the OWF is operating in synchronous

AC mode (SAC)

Figure 2-12 shows how the total active power is ramped down from 1pu to 0.2 pu. Also, it shows the voltage

response of the offshore AC grid, the active power through the umbilical cable and the frequency. Figure 2-12

(d) shows that the frequency variation is less than 0.2 Hz during the power transient. This variation is due to the

droop controller of each WT. In addition, the DRU filters disconnection also produces short frequency

deviations. Note frequency deviations occur during very short periods of time and the frequency 10 second

average is well within the specifications considered in D3.1. When a DRU filter is disconnected, the reactive

power absorbed by the WTGs decreases accordingly (see Figure 2-13).

At t=10 s, the DRUs stop transmitting power. At that time, voltage control by the OWF is enabled. Finally, at

t=12 s synchronisation with the umbilical cable is carried out. Therefore, the offshore AC-grid frequency and

voltage commands are used to set the offshore voltage to be the same phase and magnitude as the umbilical

cable voltage. Synchronisation is detected at t=14, and the umbilical cable breaker is closed. Figure 2-14 shows

the behaviour of the reactive power through umbilical cable during the aforementioned transient.


19


sec 4.0 6.0 8.0 10.0 12.0 14.0 16.0

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(a)

PF* PF

0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 1.000 1.025

(b)

VF* VF

-0.100

-0.050

0.000

0.050

0.100

(c)

Pumb* Pumb

49.60 49.70 49.80 49.90 50.00 50.10 50.20 50.30 50.40

(d)

wf

Figure 2-12: Simulation results during offshore AC grid disconnection operation (part 1): (a) total OWF active power; (b)



20

Main : Graphs

sec 4.0 6.0 8.0 10.0 12.0 14.0 16.0

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(a)

PF1 PF2 PF3

-0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(b)

QF1 QF2 QF3

0.800

0.850

0.900

0.950

1.000

1.050

(c)

VF1 VF2 VF3

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(d)

IF1 IF2 IF3

Figure 2-13: Simulation results during offshore AC grid disconnection operation (part 1): (a) active power delivered by each


OWF-i.


21

Main : Graphs

sec 4.0 6.0 8.0 10.0 12.0 14.0 16.0

-0.100

-0.050

0.000

0.050

0.100

(a)

Pumb Pumb_onshore

-0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

(b)

Qumb Qumb_onshore

0.800

0.820

0.840

0.860

0.880

0.900

0.920

(c)

Vumb Vumb_onshore

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(d)


-0.40

-0.30

-0.20

-0.10

0.00

0.10

(e)


Figure 2-14: Simulation results during offshore AC grid disconnection operation (part 1): (a) active power through umbilical

cable measured at umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at offshore and

on-shore ends; (c) offshore and on-shore AC voltages; (d) WTG active power PWT-i,j; (e) WTG reactive power QWT-i,j.


22

2.2.2 COMPLETE DE-ENERGISATION OF THE OFFSHORE AC GRID

The steps to carry out the total de-energisation of the offshore AC grid are shown in Table 2-6.

Table 2-6: Events of offshore AC grid disconnection operation (part 2).

Time (s) Events

17.0 7- WTi,j disconnection (and disconnection of string cables). At 1 second

intervals, until only one string per OWF remains connected.

35.0 8- Disable of central P and V controller

36.0 9- Disconnection of remaining WTGs

39.0 10- Disconnection of offshore AC grid (open breaker BKumb1).

The WTs disconnection procedure is carried out by disconnecting one WT from each OWF sequentially until

only one string per OWF remains connected (WT1,1, WT2,1 and WT3,1). At this stage, the P and V central control

is disabled. Then, WT3,1 is disconnected, later WT2,1, and finally WT1,1. Finally, the umbilical offshore side

breaker is opened.

Figure 2-15 shows how WTG disconnection and string de-energisation affect total power production, grid

frequency, and umbilical cable active and reactive power. Note frequency transients are larger as the number of

WTGs connected is reduced.

When the central P and V control is enabled, the total reactive power remains approximately constant (see

Figure 2-16 and Figure 2-17). However, when it is disabled, the offshore grid voltage begins to increase (see

Figure 2-16 and Figure 2-17) due to the capacitive behaviour of the cables. In addition, the reactive power is

higher than 1.0 pu when the centralised controller is disabled (see Figure 2-17), due to the capacitive nature of

the offshore grid. As with the connection transient, it is assumed that short overloads are permissible, otherwise

de-energisation should be carried out section-by-section, with central voltage control enabled.

Finally, note that the offshore grid voltage goes to 0 pu when the umbilical cable breaker BKumb1 is opened (see

Figure 2-15 (b).


23


sec 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0

-0.0160 -0.0140 -0.0120 -0.0100 -0.0080 -0.0060 -0.0040 -0.0020 0.0000 0.0020

(a)

PF* PF

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(b)

VF* VF

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

(c)

Pumb* Pumb

49.00 49.25 49.50 49.75 50.00 50.25 50.50 50.75 51.00

(d)

wf

Figure 2-15: Simulation results during offshore AC grid disconnection operation (part 2): (a) total OWF active power; (b)



24

Main : Graphs

sec 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0

-0.0200

-0.0150

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF1 PF2 PF3

-0.060 -0.050 -0.040 -0.030 -0.020 -0.010 0.000 0.010 0.020

(b)

QF1 QF2 QF3

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(c)

VF1 VF2 VF3

0.000

0.020

0.040

0.060

0.080

0.100

(d)

IF1 IF2 IF3

Figure 2-16: (c) Simulation results during offshore AC grid disconnectionoperation (part 2): (a) active power delivered by each


OWF-i.


25

Main : Graphs

sec 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

(a)

Pumb Pumb_onshore

-1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25

(b)

Qumb Qumb_onshore

0.750

0.800

0.850

0.900

0.950

1.000

1.050

(c)

Vumb Vumb_onshore

-0.025

0.000

0.025

0.050

0.075

0.100

(d)


-0.400 -0.350 -0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(e)


Figure 2-17: Simulation results during offshore AC grid disconnectionoperation (part 2): (a) active power through umbilical

cable measured at umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at offshore and

on-shore ends; (c) offshore and on-shore AC voltages; (d) WTG active power PWT-i,j; (e) WTG reactive power QWT-i,j.


26

2.3. INTENTIONAL ISLANDING

Intentional islanding, as defined in D3.2 is composed of two test cases [1]. The first one consists of the transition

from DR to ISL configuration (DR ISL). Second test case deals of the transition from ISL to SAC (ISL

SAC).

Operation of the proposed control strategy during both test cases has been shown in section 2.2.1 (DR ISL

SAC). Here the detail graphs are shown for completeness.

2.3.1 INTENTIONAL ISLANDING

Table 2-7 shows the sequence of events when changing from DR to ISL mode of operation:

Table 2-7: Events of intentional islanding.

Time (s) Events

4.5 1- Active power ramp down from 1pu to 0.2pu.

6.5 2- Disconnection of DRU filters (sequentially each 1 second opening BKAC filter i).

9.5 3- Active power down from 0.2pu to 0pu.

10.5 4- Enable offshore Voltage control, and disable active power control.

Intentional islanding test case starts when the system is completely energised and transmitting active power via

the HVDC Link (DR). From this state, the first step is to reduce the active power transmission through DRUs

from 1.0 pu to 0.2 pu. These results are shown in Figure 2-18, Figure 2-19 and Figure 2-20. The next step is the

DRU filters disconnection sequentially every 1 second (beginning at t=6.5). Each DRU filter is composed of

three C-type filter banks that are disconnected sequentially 80 ms apart from each other (note the three peaks

of frequency in Figure 2-18, or how reactive power is reduced in Figure 2-19 and Figure 2-20). At t=9.5, active

power transmission through the DRU is decreased to 0. Finally, voltage control is enabled and hence the

offshore grid AC-voltage goes to 0.9 pu.


27


sec 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(a)

PF* PF

0.800

0.850

0.900

0.950

1.000

1.050

(b)

VF* VF

-0.0100

-0.0050

0.0000

0.0050

0.0100

(c)

Pumb* Pumb

49.60

49.80

50.00

50.20

50.40

(d)

wf

Figure 2-18: Simulation results during transition from DR to ISL: (a) total OWF active power; (b) offshore voltage VF1; (c) active

power through the umbilical cable; (d) frequency of the offshore AC grid.


28

Main : Graphs

sec 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(a)

PF1 PF2 PF3

-0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(b)

QF1 QF2 QF3

0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 1.000 1.025

(c)

VF1 VF2 VF3

0.00

0.20

0.40

0.60

0.80

1.00

(d)

IF1 IF2 IF3

Figure 2-19: Simulation results during transition from DR to ISL: (a) active power delivered by each OWF-i; (b) reactive power

generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through each OWF-i.


29

Main : Graphs

sec 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(a)


-0.40

-0.30

-0.20

-0.10

0.00

0.10

(b)


Figure 2-20: Simulation results during transition from DR to ISL: (a) WTG active power PWT-i,j; (b) WTG reactive power QWT-i,j.

2.3.2 RE-SYNCHRONISATION TO EXTERNAL AC FROM ISL CONFIGURATION

In this case, the offshore AC grid is re-synchronised with the umbilical cable. To be able to synchronise with any

AC grid, the voltage and frequency of the offshore AC grid need to be controlled. Table 2-8 shows the sequence

of events to carry out in this procedure.

Table 2-8: Events of re-synchronisation.

Time (s) Events

2.5 1- Enable order of synchronisation.

4.8 2- Return to voltage control

Figure 2-21, Figure 2-22, Figure 2-23 and Figure 2-24 show how the synchronisation process is carried out.

Synchronisation control is enabled at t=2.5 following a synchronisation command. The synchronisation control

will change both magnitude and phase of the offshore grid to match that of the offshore end of the umbilical

cable (see Figure 2-21).

Note re-connection voltage depends on the umbilical transformer tap-changer setting. The voltage drop shown

can be minimised if the tap changer is operated so the umbilical cable offshore end voltage is close to the

existing offshore grid farm voltage.


30

OWF_Control : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0.800

0.820

0.840

0.860

0.880

0.900

(a)

Vumb VF

-0.70 -0.60 -0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10

(b)

VF_q

Figure 2-21: Simulation results during re-synchronisation with external AC grid (ISL SAC): (a) direct offshore voltage Vumb

and direct offshore voltage VF1; (b) quadrature offshore voltage VF1.

The breaker BKumb1 automatically closes when the grids are synchronised for longer than 500ms. After the

connection, the umbilical active power control is enabled. As shown in Figure 2-23 and Figure 2-24, the

complete procedure is carried out smoothly.

After connection, the central control restores the offshore AC grid voltage to its initial value (see Figure 2-21).


31

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0

-0.0100 -0.0075 -0.0050 -0.0025 0.0000 0.0025 0.0050 0.0075 0.0100

(a)

PF1 PF2 PF3

-0.0350 -0.0325 -0.0300 -0.0275 -0.0250 -0.0225 -0.0200 -0.0175

(b)

QF1 QF2 QF3

0.800

0.820

0.840

0.860

0.880

0.900

0.920

(c)

VF1 VF2 VF3

0.0200 0.0225 0.0250 0.0275 0.0300 0.0325 0.0350 0.0375 0.0400

(d)

IF1 IF2 IF3

Figure 2-22: Simulation results during re-synchronisation with external AC grid (ISL SAC): (a) active power delivered by

each OWF-i; (b) reactive power generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through

each OWF-i.


32


sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0

-0.0100 -0.0075 -0.0050 -0.0025 0.0000 0.0025 0.0050 0.0075 0.0100

(a)

PF* PF

0.800

0.820

0.840

0.860

0.880

0.900

0.920

(b)

VF* VF

-0.030

-0.020

-0.010

0.000

0.010

0.020

0.030

(c)

Pumb* Pumb

49.950

50.000

50.050

50.100

50.150

50.200

50.250

(d)

wf

Figure 2-23: Simulation results during re-synchronisation with external AC grid (ISL SAC): (a) total OWF active power; (b)



33

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0

-0.080 -0.060 -0.040 -0.020 0.000 0.020 0.040 0.060 0.080 0.100

(a)

Pumb Pumb_onshore

-0.60

-0.40

-0.20

0.00

0.20

0.40

(b)

Qumb Qumb_onshore

0.780 0.800 0.820 0.840 0.860 0.880 0.900 0.920

(c)

Vumb Vumb_onshore

0.0140 0.0150 0.0160 0.0170 0.0180 0.0190 0.0200 0.0210 0.0220

(d)


-0.150

-0.140

-0.130

-0.120

-0.110

-0.100

-0.090

(e)


Figure 2-24: Simulation results during re-synchronisation with external AC grid (ISL SAC): (a) active power through

umbilical cable measured at umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at

offshore and on-shore ends; (c) offshore and on-shore AC voltages; (d) WTG active power PWT-i,j; (e) WTG reactive power

QWT-i,j.


34

2.4. DYNAMIC VOLTAGE CONTROL

This tested case is to validate the AC offshore dynamic voltage control during SAC and ISL configuration.

2.4.1 ISL

The voltage control in ISL configuration is tested in order to validate the proposed controller. Additionally, it is

important to ascertain how the dynamic voltage control can affect the frequency of the offshore AC-grid. Table

2-9 shows the sequence of events for this tested case.

Table 2-9: Events of dynamic voltage control in ISL.

Time (s) Events

3.0 1- Decrease of the offshore AC grid voltage reference.

5.0 2- Increase oh the offshore AC grid voltage reference.

Figure 2-25 shows how the offshore grid voltage follows its reference in less than 1 second, considering a

central secondary voltage controller with a communication delay of 100 ms.

Figure 2-26 shows how offshore grid reactive power decreases when grid-voltage decreases.


35


sec 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF* PF

0.780 0.800 0.820 0.840 0.860 0.880 0.900 0.920

(b)

VF* VF

-0.0100

-0.0050

0.0000

0.0050

0.0100

(c)

Pumb* Pumb

49.970

49.980

49.990

50.000

50.010

50.020

50.030

(d)

wf

Figure 2-25: Simulation results during dynamic voltage control in ISL: (a) total OWF active power; (b) offshore voltage VF1; (c)

active power through the umbilical cable; (d) frequency of the offshore AC grid.


36

Main : Graphs

sec 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF1 PF2 PF3

-0.0340

-0.0320

-0.0300

-0.0280

-0.0260

(b)

QF1 QF2 QF3

0.780 0.800 0.820 0.840 0.860 0.880 0.900 0.920

(c)

VF1 VF2 VF3

0.0330

0.0340

0.0350

0.0360

0.0370

0.0380

0.0390

(d)

IF1 IF2 IF3

Figure 2-26: Simulation results during dynamic voltage control in ISL: (a) active power delivered by each OWF-i; (b) reactive

power generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through each OWF-i.

2.4.2 SAC

The voltage control in SAC configuration is more complex than in ISL configuration because umbilical cable is

connected and the active power controller through umbilical cable is enabled. The results show a certain degree

of coupling between active and reactive power in SAC configuration.

Table 2-10 shows the sequence of events for this test case.


37

Table 2-10: Events of dynamic voltage control in ISL.

Time (s) Events

3.0 1- Decrease of the offshore AC grid voltage reference.

5.0 2- Increase of the voltage reference of offshore AC grid.

As shown in Figure 2-28 there is a small coupling between AC grid reactive and active power through umbilical

cable. Offshore grid voltage changes produce some small variations on the offshore frequency (less than 0.04

Hz) (see Figure 2-27).


sec 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF* PF

0.780 0.800 0.820 0.840 0.860 0.880 0.900

(b)

VF* VF

-0.080 -0.060 -0.040 -0.020 0.000 0.020 0.040 0.060 0.080

(c)

Pumb* Pumb

49.960

49.980

50.000

50.020

50.040

(d)

wf

Figure 2-27: Simulation results during dynamic voltage control in SAC: (a) total OWF active power; (b) offshore voltage VF1;

(c) active power through the umbilical cable; (d) frequency of the offshore AC grid.


38

Main : Graphs

sec 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

-0.080 -0.060 -0.040 -0.020 0.000 0.020 0.040 0.060 0.080

(a)

Pumb Pumb_onshore

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40

(b)

Qumb Qumb_onshore

0.780 0.800 0.820 0.840 0.860 0.880 0.900

(c)

Vumb Vumb_onshore

0.0100 0.0120 0.0140 0.0160 0.0180 0.0200 0.0220

(d)


-0.1250 -0.1225 -0.1200 -0.1175 -0.1150 -0.1125 -0.1100 -0.1075 -0.1050

(e)


Figure 2-28: Simulation results during dynamic voltage control in SAC: (a) active power through umbilical cable measured at

umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at offshore and on-shore ends; (c)

offshore and on-shore AC voltages of umbilical AC cable; (d) WTG active power PWT-i,j; (e) WTG reactive power QWT-i,j.


39

Main : Graphs

sec 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

-0.0100 -0.0075 -0.0050 -0.0025 0.0000 0.0025 0.0050 0.0075 0.0100

(a)

PF1 PF2 PF3

-0.0350 -0.0325 -0.0300 -0.0275 -0.0250 -0.0225 -0.0200 -0.0175 -0.0150

(b)

QF1 QF2 QF3

0.780 0.800 0.820 0.840 0.860 0.880 0.900 0.920

(c)

VF1 VF2 VF3

0.000

0.010

0.020

0.030

0.040

0.050

0.060

(d)

IF1 IF2 IF3

Figure 2-29: Simulation results during dynamic voltage control in SAC: (a) active power delivered by each OWF-i; (b) reactive


From Figure 2-29 we can notice that, current and reactive power of OWF1 is different from that of OWF2 and

OWF3. The reason for this difference is that the umbilical cable is connected to DRU platform OWF1. Active

power PF2, reactive power QF2, voltage VF2 and current IF2 lay under active power PF3, reactive power QF3,

voltage VF3 and current IF3.


40

2.5. WIND FARM POWER CONTROL

In this section, power delivered by each individual wind farm is regulated by OWF controller while operating in

DR mode. Table 2-11 shows the sequence of events corresponding to this tested case.

Table 2-11: Events of OWF power control.

Time (s) Events

5.0 1- Active power reference of OWF-1 decreased from 1 pu to 0.5 pu



The generated power of each OWF is shown in Figure 2-30. There is a small coupling between the active power

generation of the different wind farms due to the large active power transients, which depends on the tuning of

the WTG droop coefficients.

Figure 2-31 shows that total power remains controlled. Moreover, Figure 2-31 shows how power reference

changes in each OWF affect the offshore grid frequency. This figure clearly shows how the power transmitted

through DRUs depends strongly on the offshore AC-grid voltage level.


41

Main : Graphs

sec 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

(a)

PF1 PF2 PF3

-0.250

-0.200

-0.150

-0.100

-0.050

0.000

0.050

(b)

QF1 QF2 QF3

0.960

0.970

0.980

0.990

1.000

1.010

(c)

VF1 VF2 VF3

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

(d)

IF1 IF2 IF3

Figure 2-30: Simulation results during power control: (a) active power delivered by each OWF-i; (b) reactive power generated

by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through each OWF-i.


42


sec 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10

(a)

PF* PF

0.880 0.900 0.920 0.940 0.960 0.980 1.000 1.020

(b)

VF* VF

-0.0100 -0.0075 -0.0050 -0.0025 0.0000 0.0025 0.0050 0.0075 0.0100

(c)

Pumb* Pumb

49.900 49.925 49.950 49.975 50.000 50.025 50.050 50.075 50.100

(d)

wf

Figure 2-31: Simulation results during power control: (a) total OWF active power; (b) offshore voltage VF1; (c) active power

through the umbilical cable; (d) frequency of the offshore AC grid.


43

2.6. RESPONSE TO CHANGES IN REACTIVE POWER SHARING COMMAND

During normal operation, all OWFs shall support the reactive power required to maintain the voltage within the

normal operation range for transmission. Additionally, in some cases it can be desired that some WTs/OWFs

generate more reactive power than others. This section includes the response to different reactive power

sharing commands when the system is in DR and ISL configurations.

2.6.1 REACTIVE POWER SHARING COMMAND WITH DR CONFIGURATION

This tested case consists on changing the reactive power production of the each OWF sequentially and

verifying the system behaviour when the reactive power sharing is carried out. Table 2-12 shows the sequence

of events for the DR configuration reactive power sharing command.

Table 2-12: Events of reactive power sharing in DR.

Time (s) Events

2.5 1- Increment reactive power reference of OWF-1.



Figure 2-32 shows the good behaviour of the offshore grid when there are changes in reactive power command.

Note that, total reactive power remains practically constant. Therefore, when the reactive power reference of

any OWF increases the other OWFs decrease its reactive power generation.

In addition, changes of reactive power sharing only marginally affect the active power delivered through the

DRU, and hardly affect the overall offshore AC-grid voltage level (system is working in DR mode).

Figure 2-33 also shows the frequency variations during changes in reactive power sharing command.


44

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540

(a)

PF1 PF2 PF3

-0.400 -0.350 -0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000

(b)

QF1 QF2 QF3

0.9500

0.9550

0.9600

0.9650

0.9700

0.9750

0.9800

(c)

VF1 VF2 VF3

0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675

(d)

IF1 IF2 IF3

Figure 2-32: Simulation results during reactive power sharing in DR: a) active power delivered by each OWF-i; (b) reactive



45

OWF Control,Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540

(a)

PF* PF

0.950 0.960 0.970 0.980 0.990 1.000 1.010 1.020

(b)

VF* VF

-0.0100 -0.0075 -0.0050 -0.0025 0.0000 0.0025 0.0050 0.0075 0.0100

(c)

Pumb* Pumb

49.900

49.950

50.000

50.050

50.100

(d)

wf

Figure 2-33: Simulation results during reactive power sharing in DR: (a) total OWF active power; (b) offshore voltage VF1; (c)

active power through the umbilical cable; (d) frequency of the offshore AC grid.

2.6.2 REACTIVE POWER SHARING COMMAND IN ISL CONFIGURATION

This tested case consists on changing the reactive power production of the each OWF sequentially and

verifying the system behaviour when the reactive power sharing is carried out. Table 2-13 shows the sequence

of events for the ISL configuration reactive power sharing command.


46

Table 2-13: Events of reactive power sharing in ISL.

Time (s) Events




Figure 2-34 shows the simulation results for changes in reactive power sharing when the system is operated in

ISL mode. As in the previous test, the total reactive power remains constant. Therefore, when the reactive

power reference of any OWF increase the remaining OWFs decrease its reactive power generation.

In addition, we can see the changes of reactive power sharing command only affect marginally the generated

active power and offshore AC-grid voltage.


47

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

-0.0100

-0.0050

0.0000

0.0050

0.0100

(a)

PF1 PF2 PF3

-0.070 -0.060 -0.050 -0.040 -0.030 -0.020 -0.010 0.000

(b)

QF1 QF2 QF3

0.8800 0.8850 0.8900 0.8950 0.9000 0.9050 0.9100 0.9150 0.9200

(c)

VF1 VF2 VF3

0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080

(d)

IF1 IF2 IF3

Figure 2-34: Simulation results during reactive power sharing in ISL: (a) active power delivered by each OWF-i; (b) reactive


2.7. RESPONSE TO ACTIVE POWER REFERENCE COMMANDS WHEN CONNECTED TO EXTERNAL AC

When the OWFs are connected to an external AC grid (SAC mode), they should be able to transmit active

power to the external AC grid through the umbilical cable, up to the power rating of this cable (around 40MW as

Table 2-2 shows). This power represents a 3.3 % of the HVDC link.


48

Power transmission through the umbilical cable is carried out by the central controller by means of a PI

controller acting on the overall OWF frequency (i.e. overall OWF voltage angle). Table 2-14 shows the

sequence of events for this test case.

Table 2-14: Events of active power control through umbilical cable in SAC.

Time (s) Events

2.5 1- Active power reference through umbilical cable increases from 0 pu to 1 pu

5.0 2- Active power reference through umbilical cable decreases from 1 pu to 0 pu

Figure 2-35 and Figure 2-36 show the behaviour of the system when active power is transmitted through the

umbilical cable. Note there is a small coupling between the voltage and frequency (see Figure 2-35).


49


sec 2.0 3.0 4.0 5.0 6.0 7.0

-0.0050 0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350 0.0400

(a)

PF* PF

0.8900

0.8950

0.9000

0.9050

0.9100

(b)

VF* VF

-1.20 -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20

(c)

Pumb* Pumb

49.850

49.900

49.950

50.000

50.050

50.100

50.150

(d)

wf

Figure 2-35: Simulation results during active power reference commands through umbilical cable: (a) total OWF active power;

(b) offshore voltage VF1; (c) active power through the umbilical cable; (d) frequency of the offshore AC grid.


50

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0

-1.20 -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20

(a)

Pumb Pumb_onshore

-0.40

-0.20

0.00

0.20

0.40

0.60

(b)

Qumb Qumb_onshore

0.820

0.840

0.860

0.880

0.900

(c)

Vumb Vumb_onshore

Figure 2-36: Simulation results during active power reference commands through umbilical cable: (a) active power through

umbilical cable measured at umbilical offshore and on-shore ends; (b) reactive power through umbilical cable measured at

offshore and on-shore ends; (c) offshore and on-shore AC voltages of the umbilical AC cable.


51

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0

0.000

0.010

0.020

0.030

0.040

(a)

PF1 PF2 PF3

-0.0400

-0.0350

-0.0300

-0.0250

-0.0200

-0.0150

-0.0100

(b)

QF1 QF2 QF3

0.8900 0.8925 0.8950 0.8975 0.9000 0.9025 0.9050 0.9075 0.9100

(c)

VF1 VF2 VF3

0.010

0.020

0.030

0.040

0.050

0.060

0.070

(d)

IF1 IF2 IF3

Figure 2-37: Simulation results during active power reference commands through umbilical cable: (a) active power delivered

by each OWF-i; (b) reactive power generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current

through each OWF-i.


52

2.8. DISCONNECTION / RECONNECTION OF A STRING / OWF

2.8.1 DISCONNECTION OF A STRING / OWF

Results regarding the controlled disconnection of an OWF are presented in this section. Table 2-15 shows the

sequence of events corresponding to this tested case.

Table 2-15: Events of to disconnect a OWF.

Time (s) Events

2.0 1- Start disconnection (OWF active power ramped down to 0 pu).

3.6 2- Reactive power reference is set to 0 pu

7.2 3- Disconnection completed

Figure 2-38 shows the controlled disconnection of an OWF. At t=2.0 s., the active power reference of OWF-1 is

ramped down from 0.5 pu to zero in 0.5 seconds. Then the reactive power contribution of OWF-1 is also set to

zero. At t=7.2 s OWF-1 is not delivering either active or reactive power and hence the breaker is opened to

complete OWF-1 disconnection, which is now operated in islanded mode.

At the end of the transient, the total active power delivered to the HVDC link is reduced due to the disconnection

of OWF-1 (see Figure 2-39 (a)). Furthermore, the disconnection of the OWF-1 has a little effect on the

frequency of the offshore AC-grid (see Figure 2-39 (d)).


53

Main : Graphs

sec 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

-0.30

0.70

(a)

PF1 PF2 PF3

-0.400 -0.350 -0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(b)

QF1 QF2 QF3

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(c)

VF1 VF2 VF3

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

(d)

IF1 IF2 IF3

Figure 2-38: Simulation results during disconnection of a OWF: (a) active power delivered by each OWF-i; (b) reactive power



54


sec 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0.250

0.300

0.350

0.400

0.450

0.500

0.550

(a)

PF* PF

0.9400

0.9450

0.9500

0.9550

0.9600

0.9650

0.9700

(b)

VF* VF

-0.0100 -0.0075 -0.0050 -0.0025 0.0000 0.0025 0.0050 0.0075 0.0100

(c)

Pumb* Pumb

49.900

49.950

50.000

50.050

50.100

(d)

wf

Figure 2-39: Simulation results during disconnection of a OWF: (a) total OWF active power; (b) offshore voltage VF1; (c) active


2.8.2 RECONNECTION OF A STRING / OWF

Table 2-16 shows the sequence of events for the reconnection of an OWF.

Table 2-16: Events of to reconnect a OWF.

Time (s) Events

2.0 1- Start synchronisation of OWF1 with to offshore AC grid.

3.9 2- Detection of synchronisation and connection of OWF1. (At 3.9 s the breaker

with pre-insertion resistance is closed and at 4.0s the pre-insertion resistance is by-passed).

6.0 3- Begin to generate active power.


55

In this scenario, two offshore wind farms, namely OWF-2 and OWF-3, are connected in DR mode and injecting

power through the DRU. OWF-1 is initially operating in islanding mode (ISL).

Figure 2-40 shows the reconnection of OWF-1. In order to synchronise OWF-1 with the rest of the system, a

synchronisation control loop, similar to that used for umbilical synchronisation is used. The synchronisation loop

brings the voltage magnitude VF1_d to be the same as the voltage magnitude of the ring-bus VF_d, and makes

sure that also the voltage angles are the same by making VF1_q=VF_q. At this stage, the breaker is closed and

now OWF-1 is synchronised with the rest of the system (Figure 2-42 clearly shows the breaker closing at t=3.9

s).

OWF_Control : Graphs

sec 1.50 2.00 2.50 3.00 3.50 4.00 4.50

35.50 35.75 36.00 36.25 36.50 36.75 37.00 37.25 37.50 37.75

(a)

VF_d VF1_d

-5.0 0.0 5.0

10.0 15.0 20.0 25.0 30.0 35.0

(b)

VF1_q

Figure 2-40: Simulation results during reconnection of a OWF: (a) direct offshore voltage VF1 and VF1,1; (b) quadrature offshore

voltage VF1,1.

Figure 2-41 shows the behaviour of OWFs after connection. From t=3.9s, OWF1 begins to contribute reactive

power to the offshore grid. At t=6 the active power reference of OWF-1 is ramped up.


56

Main : Graphs

sec 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60

(a)

PF1 PF2 PF3

-0.400 -0.350 -0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(b)

QF1 QF2 QF3

0.9350 0.9400 0.9450 0.9500 0.9550 0.9600 0.9650 0.9700

(c)

VF1 VF2 VF3

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

(d)

IF1 IF2 IF3

Figure 2-41: Simulation results during reconnection of a OWF: (a) active power delivered by each OWF-i; (b) reactive power



57


sec 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

0.300

0.350

0.400

0.450

0.500

0.550

(a)

PF* PF

0.9400 0.9450 0.9500 0.9550 0.9600 0.9650 0.9700 0.9750

(b)

VF* VF

-0.0100

-0.0050

0.0000

0.0050

0.0100

(c)

Pumb* Pumb

49.70

49.80

49.90

50.00

50.10

50.20

50.30

(d)

wf

Figure 2-42: Simulation results during reconnection of an OWF: (a) total OWF active power; (b) offshore voltage VF1; (c) active


2.9. DISCONNECTION / RECONNECTION OF FILTERS

When a transition between DR mode and islanded mode happens, OWF shall be able to connect/disconnect

DRU filters in order to avoid unnecessary reactive power compensation. For this tested case, it is considered

that the connection/disconnection of DRU filters occurs when the system is transmitting 0.2 pu of active power

through the DRUs.

Each DRU filter is composed of three smaller filter banks that are connected or disconnected sequentially each

80 ms, so the connection/disconnection effect on the offshore grid is reduced.


58

2.9.1 DISCONNECTION OF FILTERS

DRU filters disconnection causes reactive power changes, and hence voltage changes. Table 2-17 shows the

sequence of events for the disconnection of the DRU filters.

Table 2-17: Events of disconnection of filters in DR.

Time (s) Events

3.0 1- Disconnection of DRU filters by opening breakers BKAC filter i (sequentially

closed each 1 second).

Figure 2-43 shows the simulation results during DRU filter disconnections operation. As this figure shows, each

DRU filter is sequentially disconnected (at 1 second intervals). The transient behaviour is very good and all

variables remain within their operational limits during the transient.

As discussed before, the filter disconnection affects the reactive power injected by the wind farms (see Figure 2-

44 (b)). When a DRU filter is disconnected, the reactive power absorbed by each OWF is reduced, causing a

reduction on the current that each OWF is injecting (see Figure 2-44 (d)). Finally, the frequency response during

this case is shown in Figure 2-43 (d). The frequency deviation is less than 0.2 Hz, which represents a very good

frequency behaviour.


59


sec 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

0.1900 0.1925 0.1950 0.1975 0.2000 0.2025 0.2050 0.2075 0.2100

(a)

PF* PF

0.9353

0.9377

(b)

VF* VF

-0.100 -0.075 -0.050 -0.025 0.000 0.025 0.050 0.075 0.100

(c)

Pumb* Pumb

49.800 49.850 49.900 49.950 50.000 50.050 50.100 50.150 50.200

(d)

wf

Figure 2-43: Simulation results during disconnection of filters od DRUs (DR): (a) total OWF active power; (b) offshore voltage

VF1; (c) active power through the umbilical cable; (d) frequency of the offshore AC grid.


60

Main : Graphs

sec 2.0 3.0 4.0 5.0 6.0 7.0

0.1900 0.1925 0.1950 0.1975 0.2000 0.2025 0.2050 0.2075 0.2100

(a)

PF1 PF2 PF3

-0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(b)

QF1 QF2 QF3

0.9200

0.9250

0.9300

0.9350

0.9400

0.9450

0.9500

(c)

VF1 VF2 VF3

0.200

0.250

0.300

0.350

0.400

(d)

IF1 IF2 IF3

Figure 2-44: Simulation results during disconnection of filters od DRUs (DR): (a) active power delivered by each OWF-i; (b)

reactive power generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through each OWF-i.

2.9.2 RECONNECTION OF FILTERS

Table 2-18 shows the sequence of events for filter re-connection.

Table 2-18: Events of reconnection of filters in DR.

Time (s) Events

3.0 1- Connection of DRU filters by closing breakers BKAC filter i (sequentially

closed at 1 second intervals).


61

Figure 2-45 and Figure 2-46 show the offshore grid behaviour when the DRU filters are connected. Note that,

active power through DRUs increase a little bit when any DRU filter is connected as the filters are initially

charged. The voltage VF in Figure 2-45 changes depending on how many filters are connected. The differences

of reactive power in each DRU platform (if filters are connected in some DRU platforms, but not in others)

produce small differences in the AC voltage of each DRU platform.


sec 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

0.1900 0.1925 0.1950 0.1975 0.2000 0.2025 0.2050 0.2075 0.2100

(a)

PF* PF

0.9365

0.9370

0.9375

0.9380

0.9385

0.9390

0.9395

(b)

VF* VF

-0.0100

-0.0050

0.0000

0.0050

0.0100

(c)

Pumb* Pumb

49.800 49.850 49.900 49.950 50.000 50.050 50.100 50.150 50.200

(d)

wf

Figure 2-45: Simulation results during reconnection of filters od DRUs (DR): (a) total OWF active power; (b) offshore voltage

VF1; (c) active power through the umbilical cable; (d) frequency of the offshore AC grid.


62

Main : Graphs

sec 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

0.1900 0.1925 0.1950 0.1975 0.2000 0.2025 0.2050 0.2075 0.2100

(a)

PF1 PF2 PF3

-0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050

(b)

QF1 QF2 QF3

0.60

0.70

0.80

0.90

1.00

1.10

1.20

(c)

VF1 VF2 VF3

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

(d)

IF1 IF2 IF3

Figure 2-46: Simulation results during reconnection of filters od DRUs (DR): (a) active power delivered by each OWF-i; (b)

reactive power generated by each OWF-i, (c) voltage at terminals of each DRU platform; (d) current through each OWF-i.


63

3. FAULT RIDE-THROUGH AND PROTECTION OF DR-HVDC SYSTEM

The performances of DR-HVDC during various fault cases as illustrated in Figure 3-1 are presented in this

section, including:

onshore grid faults F1

DC cable faults F2

internal DRU faults F3

umbilical AC cable faults F4

offshore faults F5 and F6

The distributed control strategy of WT Front End Converters (FECs) are presented in Section 3.1 while tested

cases are detailed in other sections.

Bumb1 F4Bumb2 Bumb3

Umbilical cable

vumb1

iumb1 iumb2

Qumb1

Pumb1

String 11239

iS1

BS2~6

BC2

BC3

DRU 1BB13

BB31

BB21

BB12

BB23

BB32

F6Filter

Filter

Filter

G

DRU 2

DRU 3

onshoreoffshore

Cab5

Cab1 S1

S2

S3

iS2~6

iC2

iC3

BS1

iBS1

iBS2~6

UDCUDCrec

IDC

LDC

LDC

LDC

LDC

Bus-bar

Cluster 1

Cluster 2

Cluster 3

Cab3

String 2~6 Cab2

Cab4

Cab6

Cab7

F1

F2

F3

F5

BD1

BD2

BD3

Figure 3-1: Considered various fault cases.


64

3.1. DISTRIBUTED CONTROL STRATEGY OF WT FECS

When connected with DR-HVDC, the WT FECs have to work as grid-forming converters rather than grid-feeding

converters. In addition to balancing transmitted active power between WT and offshore network, FECs also

need to establish the offshore network frequency and voltage [4, 5]. The WT controls represented in this section

include the fault response functions as the faults are studied in this section. The fundamental operation (as

studied in the previous section), which has to be performed before and after fault, is being tested as well.

Figure 3-2 shows the overall structure of the system control for the WT FECs [3]. The detailed functions and

designs of the individual control block will be described in the following sections.

3.1.1 INNER CURRENT CONTROL

The inner current loop has been widely used for controlling VSC with the benefit of fast response and current

limiting during external AC faults. For the converter circuit shown in Figure 3-2, the WT FEC current loop

dynamics in the dq reference frame in which the q-axis component of the VSC filter bus voltage UF is

approximately 0, are expressed as

WdWd Cd Fd Wq

dIRI L U U LI

dt (3-1)

WqWq Cq Fq Wd

dIRI L U U LI

dt (3-2)

where ω is the angular frequency of the offshore network. With proportional-integral (PI) regulators, the current

control loop is illustrated in Figure 3-2.

3.1.2 VOLTAGE CONTROL

As shown in Figure 3-2, the voltage dynamics of the WT FECs in the dq reference frame are described as

FdWd sd Fq

dUC I I CU

dt (3-3)

FqWq sq Fd

dUC I I CU

dt (3-4)

By regulating the output current of the converter, the voltage at the VSC filter capacitor terminal UF can be

controlled to follow its reference. The control diagram of the voltage loop can be seen in Figure 3-2. The output

dq current limits are set according to the converter current rating.

3.1.3 ACTIVE POWER CONTROL

As the DC voltage of the DR-HVDC link is controlled at the rated value by the onshore MMC, the transmitted

active power is largely determined by the DC voltage produced by the diode rectifier (Udcr, Figure 3-2) [5]. The

DC voltage produced by the diode rectifier is given as

3

2(1.35 )dcr off dcU nU XI

(3-5)

where X is the reactance of the diode rectifier transformer and n is the transformer ratio. Thus, the active power

transmitted from the wind farm to the onshore HVDC is determined by the offshore AC grid voltage Uoff or VSC

filter capacitor terminal voltage UF. Therefore, a WT active power control loop can be implemented as shown in


65

Figure 3-2 whose output is the amplitude of the d-axis voltage reference, i.e. magnitude of the produced

offshore AC voltage. Uo is the AC voltage set point of the offshore WT converters.

refPFdU

FqCU

sdI

0U

1

s

0Q

FqU

FdCU

sqI

fk qk

0

FqU

FdU

FqU

cdU

cqU

dcI

dcrU

0

ref

wtP

wtQ

wtP wtQ

WdI

WqI

WdLI

FdrefU

FqrefU

WdrefI

WqrefI

WqLI

lilp

kk

s

pi

pp

kk

svi

vp

kk

s

vivp

kk

s

iiip

kk

s

iiip

kk

s

Figure 3-2: Control diagram of WT front-end converter.

3.1.4 REACTIVE POWER SHARING CONTROL

When a large number of WTs are connected to the diode rectifier, the reactive power needs to be shared

among the WTs to avoid over-currents and reactive current circulation. The adopted reactive power sharing

control [4, 6] is shown in Figure 3-2 where a reactive power frequency droop is used as

0 0ref q wtk Q Q (3-6)

If Qwt is represented as per unit value (positive Qwt defined as WTs providing capacitive reactive power to the

offshore AC network) where the power rating of the respective WT is used as the based power, same kq and Q0

can be used for all the WTs to achieve equal reactive power sharing (based on their respective power rating).

3.1.5 DISTRIBUTED PLL-BASED FREQUENCY CONTROL

The distributed PLL-based frequency control is illustrated in Figure 3-2. In addition to the frequency

controllability, such control ensures plug-and-play capability providing automatic synchronization of the offline

WTs and minimum impact during disconnection of some of the turbines with the offshore AC network.

In the existing voltage control for isolated converter based networks, the voltage amplitude of the network is

regulated by the d-axis voltage reference UFdref while the q-axis reference UFqref is normally set to zero. On the

other hand, the PLL takes UFq as the input and regulates the frequency output to ensure the q-axis voltage UFq

to zero, as shown in Figure 3-3. For example, if the measured UFq is slightly larger than zero for a short time, the

detected frequency of the voltage vector will increase, as shown in Figure 3-4 and (3-7):


66

0lp Fq li Fqk U k U dt (3-7)

Since the frequency and phase angle measured by the PLL will also drive (synchronize) the output of the

converter for the offshore AC network, the frequency of the offshore system will increase under such conditions.

This indicates that the q-axis voltage reference UFqref can be used to control the AC frequency and thus an

additional PLL-based frequency loop is used to generate the desired UFqref, as shown in Figure 3-2 and

expressed as

Fqref f refU k (3-8)

When ω < ωref (i.e. UFqref > 0), the PLL-based frequency control produces a positive UFqref feeding to the AC

voltage controller, as seen in Figure 3-2. The voltage and current loops ensure the converter produce the

required UFq according to its reference value produced by the frequency loop. Consequently, the frequency

measured by the PLL is increased (due to UFq >0) until becoming identical to the reference (ω=ωref). Similarly,

when ω > ωref (i.e. UFqref < 0), the frequency control produces a negative UFq so the frequency is reduced

accordingly. Such frequency control can be implemented at each FEC of the WTs and is able to operate

autonomously to contribute to the overall frequency regulation of the offshore AC network.

0

FqU 1

s

abc

dq

lilp

kk

s

Figure 3-3: Diagram of the PLL.

FU��

FdU

FqU

Figure 3-4: Voltage vector in dq reference frame.

3.1.6 CONTROL STRATEGY OF WT CONVERTERS CONNECTED WITH DR-HVDC AND UMBILICAL AC CABLE

After the start up of wind farm connected with DR-HVDC system, the umbilical AC cable is expected to be

disconnected and all the wind power will be transmitted by the DR-HVDC. In order to achieve smooth transition

from mixed operation (AC and DC parallel operation) to DC operation, it is significant to control of the active

power of umbilical cable near 0 before the disconnection. As the transmitted power of umbilical cable mainly

depends on the phase angle difference between offshore voltage and onshore voltage, the umbilical cable

power change can be achieved by the adjustment of the offshore voltage phase angle. Thus, a centralized


67

umbilical cable active power control is proposed to adjust the offshore frequency and phase angle, as shown in

Figure 3-5. For the WT FECs, the controller parameters are as follows:

Current loop: kip=1256, kii=98696

Voltage loop: kvp=2511, kvi=15775

PLL loop: klp=223.21, kli=7042.5

Frequency control: kf=0.1

Reactive power sharing control: kq=1

Active power control: kpp=0.2, kpi=2

Umbilical cable power control: kumb=0.05.

DC -

DC +

dqabc

-

+

dqabc

refPFdU

FqCU

sdI

0U

dqabc

Cable

Active power control

Voltage controlFrequency

control

Reactive power sharing controlCurrent control

PLL

+

+

+1

s

+-

++

-

+-

+

-

+

-+

0Q

FqU

FdCU

sqI+

-

+-

+

+

+-

++

fk qk

FqU+

+

FdU

FqU

cdU

cqU

+

UFUC Is dcI

dcrU

+

WT line-side converter

R L C

0

ref

wtP

wtQ

wtP wtQ

WdI

WqI

WdLI

FdrefU

FqrefU

WdrefI

WqrefI

WqLI

IW

UFIW

lilp

kk

s

Uoff

pi

pp

kk

svi

vp

kk

s

vivp

kk

s

iiip

kk

s

iiip

kk

s

-+

Umbilical AC cable active power control

+

+

0

0umbp

umbp0 p ste

Umbilical Cable

umbp umbq

G

umbk

Figure 3-5: Control diagram of WT FECs connected with DR-HVDC and umbilical AC cable.

3.2. UNINTENDED TRANSMISSION CAPABILITY LIMITATION

The performances of the DR-HVDC system during various fault cases are presented in this section, including

onshore grid faults, DC cable faults, internal DRU faults, umbilical AC cable faults, and offshore AC faults. The

parameters of the tested system are detailed in D3.2 [1].

The aggregation level used (as defined in D3.2 [1, 4, 5, 7]) is:


68

Table 3-1: Aggregation level of the tested system for fault test.

Aggregation/Detail Level of simulation detail

OWF models Level 6

On-shore grid Level 3

On-shore MMC converter Level 2

Wind Turbine model Level 4

The results presented in this section are all based on per unit values where the base units are defined as

follows:

Offshore rated AC voltage: 66 kV

Rated WT power: 8 MW, 328 MW, and 400 MW

Onshore MMC rated AC voltage / current / power: 400 kV / 1.732 kA / 1200 MVA

Rated DC voltage / current: 640 kV / 1.875 kA

Rated frequency: 50 Hz

3.2.1 ONSHORE GRID FAULTS

The onshore grid faults are tested first, considering both symmetrical and asymmetrical faults.

3.2.1.1 SYMMETRICAL ONSHORE GRID FAULTS

String 11239

iS1

BS2~6

BC2

BC3

DRU 1BB13

BB31

BB21

BB12

BB23

BB32

Filter

Filter

Filter

G

DRU 2

DRU 3

onshoreoffshore

Cab5

Cab1 S1

S2

S3

iS2~6

iC2

iC3

BS1

iBS1

iBS2~6

UDCUDCrec

IDC

LDC

LDC

LDC

LDC

Bus-bar

Cluster 1

Cluster 2

Cluster 3

Cab3

String 2~6 Cab2

Cab4

Cab6

Cab7

F1

BD1

BD2

BD3

Figure 3-6: Onshore grid faults.


69

Table 3-2: Timetable of the onshore grid fault test.

Time Events

0-0.5 s Normal operation

0.5 s Solid fault occurs at onshore transformer grid-side

0.64 s Fault is cleared

As shown in Figure 3-6, a symmetrical solid fault F1 is applied at the transformer grid-side at t=0.5 s and is

cleared at t=0.64 s, as listed in Table 3-2.

At 0.5 s, a solid three-phase onshore fault occurs at the transformer primary side and the onshore AC voltage

rapidly decreases to 0, as shown in Figure 3-7 (a). The onshore transmitted active power quickly reduces to 0

as shown in Figure 3-7 (e), while the WT and DRU station still try to transmit the generated active power, as

shown in Figure 3-8 (c). During the fault, the active current of the MMC is reduced using a voltage dependent

current order limit (VDCOL) while its reactive current is increased [8]. This unbalanced active power leads to the

increase of the HVDC-link voltage, as shown in Figure 3-7 (c), which increases from 1.0 pu to 1.38 pu in 0.08s.

The increase of the DC voltage reduces the power output from the WTs and transmitted to DC by the offshore

diode rectifier as can be seen in Figure 3-8 (c). When the DC voltage reaches 1.38 pu, the offshore AC voltage

is limited by the WT converters (set at 1.1 pu) as seen in Figure 3-8 (a) and thus no active power can be

generated and transmitted to the DC. The excess power in individual WT needs to be dealt with as part of WT

fault ride through strategy, e.g. using DC damping resistors in the WTs.


70

MmcAsym : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900 0.950

-1.20 -0.80

-0.40 0.00 0.40

0.80 1.20

(a)

Vabc_mmc

-1.50 -1.00

-0.50 0.00 0.50

1.00 1.50

(b)

Iabc_mmc

0.80

1.00

1.20

1.40

(c)

Vdc_mmc

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

(d)

Idc_mmc

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

(e)

P_mmc Q_mmc

Figure 3-7: Simulation results of onshore MMC during symmetrical onshore AC fault in pu terms: (a) three-phase AC

voltages, (b) three-phase AC currents, (c) DC voltage, (d) DC current, and (e) active and reactive powers.

After the fault initiation, the reactive power and q-axis current increases while the d-axis current decreases to

avoid overcurrent. In addition, the reactive powers are shared among WT converters and the offshore frequency

is largely controlled around 50 Hz, as displayed in Figure 3-8 (d) and (e).

At 0.64 s, the fault is cleared and the onshore AC voltage recovers, leading to the increase of onshore

transmitted active power. As seen from Figure 3-8 (c), the wind power generation is also quickly restored.


71

The HVDC link experiences overvoltage (1.38 pu) during the solid symmetrical onshore fault. DC chopper could

be used to reduce such DC cable overvoltage but this introduces additional cost and volume requirement.

WTC_pu,WTC_400MW_AV_pu,WTC_400MW_AV_pu_1,WTC_8MW_AV_pu : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900

-1.00

-0.50

0.00

0.50

1.00

(a)

Va_WT Vb_WT Vc_WT

-1.00

-0.50

0.00

0.50

1.00

(b)

Ia_WT Ib_WT Ic_WT

0.00

0.50

1.00

(c)

Pref_WT P_WT

-0.80

-0.60

-0.40 -0.20

0.00

0.20

0.40

(d)

Q1_WT Q2_WT Q3_WT Q4_WT

0.940

0.960

0.980

1.000 1.020

1.040

1.060

(e)

f_WT

Figure 3-8: Simulation results of WT converter during symmetrical onshore grid fault in pu terms: (a) three-phase AC

voltages, (b) three-phase currents, (c) active power, (d) reactive power, and (e) frequency.


72

3.2.1.2 ASYMMETRICAL ONSHORE GRID FAULTS

The performances of the DR-HVDC system during an asymmetrical onshore grid fault is tested in this section.

At t=0.5 s, a solid ground fault is applied at phase a of the onshore grid (F1, Figure 3-6) and is cleared at t=0.64

s, as listed in Table 3-2.

MmcAsym : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900

-1.50 -1.00

-0.50 0.00 0.50

1.00 1.50

(a)

Vabc_mmc

-2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00

(b)

Iabc_mmc

0.60

0.80

1.00

1.20

1.40

(c)

Vdc_mmc

0.60

0.80

1.00

1.20

1.40

(d)

Idc_mmc

-1.00 -0.50 0.00

0.50 1.00 1.50 2.00

(e)

P_mmc Q_mmc

Figure 3-9: Simulation results of onshore MMC during asymmetrical onshore AC fault in pu terms: (a) three-phase AC

voltages, (b) three-phase AC currents, (c) DC voltage, (d) DC current, and (e) active and reactive powers.

After the fault, the voltage of the faulty phase drops to zero, as shown in Figure 3-9 (a). With the negative-

sequence current reference set at zero, the three-phase currents of the MMC station are balanced and their

peaks during the fault transient are controlled to be lower than the set limit of 1.5 pu, as displayed in Figure 3-9


73

(b). As shown in Figure 3-9 (e), the active and reactive powers contain second-order harmonic oscillations (i.e.

100 Hz). The DR-HVDC voltage increases to 1.1 pu after fault and then restores to rated value, Figure 3-9 (c).

WTC_pu,WTC_400MW_AV_pu,WTC_400MW_AV_pu_1,WTC_8MW_AV_pu : Graphs

sec 0.40 0.50 0.60 0.70 0.80 0.90

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(a)

Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00 0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

0.980

0.990

1.000

1.010

1.020

(c)

Pref_WT P_WT

-0.50

-0.25

0.00

0.25

0.50

(d)


0.960

0.980

1.000

1.020

1.040

(e)

f_WT

Figure 3-10: Simulation results of WT converter during asymmetrical onshore AC fault in pu terms: (a) three-phase AC


Due to the increase of the DR-HVDC DC voltage after the fault initiation, the offshore voltage increases to 1.07

pu to remain the power transmission to the onshore grid and then gradually restores to rated value, as seen in

Figure 3-10 (a). The reactive powers are shared among the WT converters and the offshore frequency is

controlled around 50 Hz, as shown in Figure 3-10 (d) and (e).


74

During the tested asymmetrical onshore grid fault, the DR-HVDC link does not experience significant

overvoltage (1.07 pu) and the system can continue transmitting rated power. The power transmission capability

of the onshore MMC will be reduced in the event of a severe onshore gird voltage dip, e.g. during a phase-to-

phase fault. Once the generated power of WTs is greater the power transmission capability of the onshore

station, the DC voltage of the DR-HVDC link will automatically increase to limit the transmitted power, as

discussion in section 3.2.11. WT pitch control may be required to limit the generated power. After fault isolation,

the whole system automatically restores normal operation.

3.2.2 DC CABLE FAULTS

The system performances during DC cable fault are assessed in this section, considering both pole-to-pole and

pole-to-ground DC faults.

3.2.2.1 POLE-TO-POLE DC CABLE FAULTS

As illustrated in Figure 3-11, a permanent solid pole-to-pole fault is applied at the middle of the DC cable at

t=0.5 s. Under such a fault, it is impossible to transmit power to onshore grid through DR-HVDC link and thus

the WT converters are shut-down after 100 ms from fault initiation, as listed in Table 3-3, while the onshore

MMC station operates on STATCOM mode to support the onshore grid by providing reactive power.

Table 3-3: Timetable of the pole-to-pole DC cable fault test.

Time Events


0.5 s Pole-to-pole fault occurs at the middle of the DC cable

0.6 s WT converters are shut-down

The DRU DC voltage drops to zero after the DC fault, as shown in Figure 3-12 (a). WT FECs automatically

operate on current limiting mode to provide fault currents, which flow through the offshore grid and the DRU

station to feed the fault, as shown in Figure 3-12 (b) and Figure 3-13 (b). This contributes the establishment of

the offshore AC voltage to around 0.4 pu as shown in Figure 3-13 (a), even though the system suffers a solid

pole-to-pole DC fault. Oscillations are observed in the DC current of the DR-HVDC during fault transients due to

the passive R, L, and C components in the offshore AC and DC systems, as shown in Figure 3-12 (b). The

active power drops to zero while the reactive power increases to around 0.5 pu to try to restore the offshore grid

voltages, as seen in Figure 3-13 (c) and (d). As displayed in Figure 3-13 (e), the offshore frequency is largely

controlled at 50 Hz until the WT converters are blocked at t=0.6 s.

As shown in Figure 3-14 (a), (b), and (d), the DC voltage of the DR-HVDC link collapses after the pole-to-pole

DC fault and wind power transmission is interrupted. The onshore MMC controls its terminal DC voltage and DC


75

current around zero. The onshore FB-MMC operates on STATCOM mode and continues providing reactive

power to the grid, as shown in Figure 3-14 (d).

The WT converters provide fault currents during faults in the first 100 ms after the fault initiation, which enables

overcurrent protection and contributes the AC voltage control of the offshore network. The onshore FB-MMC

station operates on STATCOM mode during DC faults.

Figure 3-11: DC cable faults.

Main : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700

0.00 0.20

0.40

0.60 0.80 1.00

1.20

(a)

Vdc_DRU

0.00 0.50

1.00 1.50 2.00

2.50 3.00

(b)

Idc_DRU

Figure 3-12: Simulation results of DRU station during pole-to-pole DC cable fault in pu terms: (a) DC voltage and (b) DC

current.


76

WTC_pu : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50 (a

)Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00 0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

-0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(c)

Pref_WT P_WT

-0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60

(d)

Q1_WT

0.00

0.20

0.40

0.60 0.80

1.00

1.20

(e)

f_WT

Figure 3-13: Simulation results of WT converter during pole-to-pole DC cable fault in pu terms: (a) three-phase AC voltages,

(b) three-phase currents, (c) active power, (d) reactive power, and (e) frequency.


77

MMC_Statcom : Graphs

sec 0.450 0.475 0.500 0.525 0.550 0.575 0.600

0.00

0.30

0.60

0.90

1.20 (a

)Vdc_mmc

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

(b)

Idc_mmc

-1.40

-0.70

0.00

0.70

1.40

(c)

Ia_mmc Ib_mmc Ic_mmc

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

P_mmc Qmmc

Figure 3-14: Simulation results of onshore MMC during pole-to-pole DC cable fault in pu terms: (a) DC voltage, (b) DC

current, (c) three-phase currents, and (d) active and reactive powers.

3.2.2.2 POLE-TO-GROUND DC CABLE FAULTS

The asymmetrical DC fault ride-through operations of the OWF connecting with DR-HVDC link is investigated in

this section. As shown in Figure 3-11, a permanent solid pole-to-ground fault F2 is applied at the positive pole at

t=0.5 s, as listed in Table 3-4.

Table 3-4: Timetable of the pole-to-ground DC cable fault test.

Time Events


0.5 s Pole-to-ground fault occurs at the middle of the positive pole

0.5 s - Single pole operation of DR-HVDC link


78

After the fault initiation, the positive-pole DC voltage drops to zero. For the adopted symmetrical monopole

configuration, the onshore full-bridge submodule MMC station continues controlling the healthy negative-pole

DC voltage around the rated value during such an asymmetrical DC cable fault [9], as shown in Figure 3-15 (a).

The DR-HVDC link is thus operated at half of the rated DC voltage to avoid overvoltage of the healthy negative

pole and continuously transmit power. With reduced DC voltage, the DC current is increased to 1.25 pu and the

active power, thereby the onshore MMC AC current, is reduced to 0.625 pu, as shown in Figure 3-15 (b), (c)

and (d).

MMC_1 : Graphs

sec 0.30 0.40 0.50 0.60 0.70 0.80 0.90

-1.50 -1.00 -0.50

0.00 0.50 1.00 1.50

(a)

Vdcp_mmc Vdcn_mmc

-2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0

(b)

Idc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(c)


-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

P_mmc Q_mmc

Figure 3-15: Simulation results of onshore MMC during positive pole-to-ground DC cable fault in pu terms: (a) DC voltage, (b)

DC current, (c) three-phase AC currents, and (d) active and reactive powers.

Due to the reduced DR-HVDC link voltage, the output voltage of WT converters decreases to 0.65 pu and thus

the voltage control loop saturates and the converter outputs maximum current (1.25 pu), as shown in Figure

3-16 (a) and (b).

After the fault, the power generating capability of WTs reduces to 0.68 pu. The active power control loop thus

saturates, as displayed in Figure 3-16 (c). The reactive powers increase and are shared among the WT


79

converters, as shown in Figure 3-16 (d). The offshore frequency is largely controlled around the rated (50 Hz),

as shown in Figure 3-16 (e).

Assuming a permanent fault, the DR-HVDC link is operated with half of the rated DC voltage to continuously

transmit power through the healthy negative pole, where the WT FECs automatically operate on current limiting

mode.

WTC_pu,WTC_400MW_AV_pu,WTC_400MW_AV_pu_1,WTC_8MW_AV_pu,Main : Graphs

sec 0.30 0.40 0.50 0.60 0.70 0.80 0.90

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(a)

Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00 0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

0.20

0.40

0.60

0.80

1.00

1.20

(c)

Pref_WT P_WT

0.00

0.10

0.20

0.30

0.40

0.50

(d)


0.950

1.000

1.050

1.100

(e)

f_WT

Figure 3-16: Simulation results of WT converter during positive pole-to-ground DC cable fault in pu terms: (a) three-phase



80

3.2.3 INTERNAL DRU FAULTS

Before the fault, the system operated with 0.6 p.u active power and DRU 1 is suffered an internal fault F3 at

t=0.5 s as listed in Table 3-5, which leads to the short circuit of the DC terminal of DRU 1, as shown in Figure

3-17.

Table 3-5: Timetable of the internal DRU fault test.

Time Events


0.5 s DRU 1 is suffered an internal fault

0.64 s Breaker BD1 is opened

Figure 3-17: Internal DRU faults.

After the fault, the DC voltage of DRU 1 drops to zero while the DC voltages of the healthy DRUs 2 and 3

remain around the rated value, as shown in Figure 3-18 (a). The DC voltage of the DR-HVDC link is thus

reduced to two thirds of the rated DC voltage, as shown in Figure 3-20 (a). Such an internal DRU fault leads to

the short circuit of the AC side of the DRU 1 and large currents feed the fault from the WT converters through

circuit breaker BD1, as seen in Figure 3-18 (c). BD1 experiences overcurrent and thus is opened (assumed at

t=0.64 s for illustration) to isolate the fault.

However, due to the large fault current and leakage inductance of DRU transformer (0.18 pu), the offshore

voltages do not drop significantly during the fault, as seen in Figure 3-19 (a).


81

After the fault is isolated by BD1, the system is operated with reduced DRUs and the transmitted power

autonomously resumes the pre-fault conditions (0.6 pu), as shown in Figure 3-19 (c) and Figure 3-20 (d). ,

Figure 3-20 (c) displays that the onshore three-phase currents also restore to the pre-fault value. From the fault

initiation to the power transmission restoration, the offshore frequency is largely controlled around the rated (50

Hz), as shown in Figure 3-19 (e).

The WT converters automatically operate on current limiting during the fault and can provide fault currents,

which enables the fault detection of breaker BD1.

DR_1,DR,DR_2,Main : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

(a)

Vdc_DRU1 Vdc_DRU2 Vdc_DRU3

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

(b)

Idc_DRU

-5.0

-2.5

0.0

2.5

5.0

(c)

I_BC1

Figure 3-18: Simulation results of offshore DRU station during internal DRU fault in pu terms: (a) DRU DC terminal voltages,

(b) DC current, and (c) currents of breaker BD1.


82

WTC_8MW_AV_pu,WTC_400MW_AV_pu_1,WTC_pu,WTC_400MW_AV_pu : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(a)

Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00 0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

(c)

Pref_WT P_WT

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

(d)


0.90

1.00

1.10

1.20

(e)

f_WT

Figure 3-19: Simulation results of WT converter during internal DRU fault in pu terms: (a) three-phase voltages, (b) three-

phase currents, (c) active power, (d) reactive power, and (e) frequency.


83

MMC_1 : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900

0.40

0.60

0.80

1.00

1.20 (a

)Vdc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00

(b)

Idc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(c)


-0.20 0.00 0.20 0.40 0.60 0.80 1.00

(d)

P_mmc Q_mmc

Figure 3-20: Simulation results of onshore MMC during internal DRU fault in pu terms: (a) DC voltage, (b) DC current, (c) AC

currents, and (d) active and reactive powers.

3.3. UMBILICAL AC CABLE FAULTS

As shown in Figure 3-21, the offshore wind farm is connected with onshore grid by both DR-HVDC and umbilical

AC cable and the system operates on DRSAC mode. To test performances of the system during umbilical AC

cable fault, a symmetrical solid fault F4 is applied at umbilical cable at t=0.2 s as shown in Figure 3-21 and

listed in Table 3-6.

Table 3-6: Timetable of the umbilical AC cable fault test.

Time Events


0.2 s Umbilical AC cable fault occurs

0.34 s Breakers Bumb1 and Bumb2 are opened


84

Figure 3-21. Umbilical AC cable faults.

After the fault occurs at 0.2 s, the umbilical AC cable voltage drops to 0, as shown in Figure 3-22 (a). Both the

fault currents from the WT side (iumb1) and onshore grid side (iumb2) start to increase, as shown in Figure 3-22

(b), (c). The transmitted active power and reactive power from the WT decreases nearly to 0, as shown in

Figure 3-22 (d), while the frequency is still 50 Hz during the fault, as shown in Figure 3-22 (e).

After the fault, the fault currents provided by the WT converters and onshore grid feed to the fault through the

circuit breakers BUmb1 and BUmb2. By fault detection, breakers BUmb1 and BUmb2 are assumed to be opened at

t=0.34 s to isolate the fault. After 0.34 s, umbilical AC cable transmitted active power and reactive power

decrease to 0.


85

Main : Graphs

sec 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(a)

Va_umb Vb_umb Vc_umb

-20

-10

0

10

20

(b)

Ia_umb Ib_umb Ic_umb

-12.0 -8.0 -4.0 0.0 4.0 8.0

12.0

(c)

Ia_umb2 Ib_umb2 Ic_umb2

-0.020 0.000 0.020 0.040 0.060 0.080 0.100

(d)

P_umb Q_umb

0.80

1.00

1.20

1.40

1.60

(e)

f_umb

Figure 3-22. Simulation results of WT converter during umbilical AC cable fault in pu terms: (a) three-phase AC voltages, (b)

offshore three-phase currents, (c) onshore three-phase currents, (d) active power and reactive power, and (e) umbilical AC

cable frequency.

The umbilical AC fault also leads to the decrease of the WT AC voltage to around 0.1p.u, as shown in Figure

3-23 (a). Meanwhile, WTs lose the ability to transmit rated power, as the active power control and voltage

control saturates, as shown in Figure 3-23 (d). After the fault, WT converters increase the q-axis current to

provide fault currents whereas the d-axis current reduces to avoid overcurrent, as shown in Figure 3-23 (b).

During the fault, the offshore frequency is still 50HZ, as shown in Figure 3-23 (c).

When the umbilical AC cable fault is cleared at 0.34 s, the reactive current starts to decrease with the increase

of the active current. Then the active power and AC voltage control start to work again, as shown in Figure 3-23


86

(a) and (d). The wind power is transmitted to onshore by the DR-HVDC system without the connection of the

umbilical AC cable.

WTC_8MW_AV_pu,Main : Graphs

sec 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(a)

Va_WT Vb_WT Vc_WT

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(b)

Ia_WT Ib_WT Ic_WT

0.80

1.00

1.20

1.40

1.60

(c)

f_off

0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

Pref_WT P_WT

-0.80

-0.40

0.00

0.40

0.80

(e)

Qm2

Figure 3-23. Simulation results of WT converter during umbilical AC cable fault in pu terms: (a) three-phase AC voltages, (b)

three-phase currents, (c) offshore frequency, (d) active power, and (e) reactive power.

As the onshore grid is strong (the impedance of the umbilical AC cable and umbilical cable transformer is much

larger than that of the onshore grid), the umbilical AC fault F4 does not have a large impact on the onshore grid

voltage, as shown in Figure 3-24 (a). The DC voltage and reactive power is still under control, as shown in

Figure 3-24 (c) and (d). The onshore AC current experiences a slight increase during the umbilical AC cable

fault, as shown in Figure 3-24 (b). During the fault period, no wind power can be transmitted to the onshore

MMC.


87

When the umbilical AC fault is cleared, the offshore wind power is restored and transmitted through the DR-

HVDC, as can be seen from the Figure 3-24 (c).

The system is robust to the umbilical AC cable fault and then can automatically restore power transmission

through the DRU-HVDC, once the faulty umbilical AC cable is isolated.

MMC_FB : Graphs

sec 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(a)

Va_mmc Vb_mmc Vc_mmc

-0.100

-0.050

0.000

0.050

0.100

(b)


-0.100

-0.050

0.000

0.050

0.100

(c)

P_mmc Q_mmc

0.900

0.950

1.000

1.050

1.100

(d)

Udcm_mmc

-0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

(e)

Idc_mmc

Figure 3-24. Simulation results of onshore MMC during umbilical AC cable fault in pu terms: (a) three-phase AC voltage, (b)

three-phase AC currents, (c) active and reactive powers, (d) DC voltage, and (e) DC current.


88

3.4. OFFSHORE AC FAULTS

3.4.1 SYMMETRICAL OFFSHORE AC FAULTS

3.4.1.1 SYMMETRICAL OFFSHORE AC FAULT AT CLUSTER INTERCONNECTIN CABLE

To test performances of the DR-HVDC system during an offshore AC fault and after fault isolation, a

symmetrical solid fault F5 is applied at the cluster interconnection cable Cab5 as shown in Figure 3-25 at t=0.5 s

and is isolated by breakers BB13 and BB31 at t=0.64 s, as listed in Table 3-7.

Table 3-7: Timetable of the offshore AC fault at cluster interconnection cable.

Time Events

0-0.5 s Normal operation (DR mode)

0.5 s A solid fault is applied at the cluster interconnection cable Cab5

0.64 s Breakers BB13 and BB31 are opened

After the fault, the offshore AC voltages collapse and the converter quickly increases the q-axis current to

provide fault currents whereas the d-axis current reduces to avoid overcurrents, as shown in Figure 3-26 (a) and

(b). The fault currents provided by the WT converters (1.25 pu) feed to the fault through the circuit breakers BB13

and BB31. By differential fault detection, breakers BB13 and BB31 are assumed to be opened at t=0.64 s (for

illustration) to isolate the fault.

Figure 3-25: Offshore AC faults applied at cluster interconnection AC cable.


89

WTC_400MW_AV_pu,WTC_pu,WTC_400MW_AV_pu_1,Main : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50 (a

)Va_WT Vb_WT Vc_WT

-2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00

(b)

Ia_WT Ib_WT Ic_WT

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

(c)

Pref_WT P_WT

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20

(d)

Q1_WT Q2_WT Q3_WT

0.60

0.80

1.00

1.20

(e)

f_WT

-30 -20 -10

0 10 20 30 40

(f)

Ia_diff Ib_diff Ic_diff

Figure 3-26: Simulation results of WT converter during symmetrical offshore AC fault at cluster interconnection cable in pu

terms: (a) three-phase voltages, (b) three-phase currents, (c) active power, (d) reactive power, (e) frequency, and.(f) differential currents on the faulty cable.


90

With d-axis voltage reference set at 1.1 pu by the active power loop, the d-axis voltage control loop saturates

during the fault and gradually restores the offshore voltage after the fault is isolated by BB13 and BB31 at t=0.64 s,

Figure 3-26 (a). The power transmission and the DC current of the DR-HVDC link also gradually restore, as

displayed in Figure 3-26 (c) and Figure 3-27 (b).

During the entire process, the reactive powers are shared among the WT converters and the offshore frequency

is largely controlled around the rated (50 Hz), as shown in Figure 3-26 (d) and (e) respectively.

Figure 3-26 and Figure 3-27 demonstrate the WT converters automatically operate on current limiting during the

fault and can fast provide fault currents, which enables the fault detection. The system is robust to the cluster

interconnection cable fault and the can automatically restore power transmission.

MMC_FB : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900

0.80

0.90

1.00

1.10

1.20

(a)

Vdc_mmc

-0.80 -0.40 0.00 0.40 0.80 1.20 1.60

(b)

Idc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(c)


-0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

P_mmc Q_mmc

Figure 3-27: Simulation results of onshore MMC during symmetrical offshore AC fault at cluster interconnection cable in pu

terms: (a) DC voltage, (b) DC current, (c) AC currents, and (d) active and reactive powers.


91

3.4.1.2 SYMMETRICAL OFFSHORE AC FAULT AT STRING CABLE

To test performances of the DR-HVDC system during an offshore AC fault and after fault isolation, a

symmetrical solid fault F6 is applied at a sting cable as shown in Figure 3-28 at t=0.5 s and is isolated by

breaker BBS1 at t=0.64 s, as listed in Table 3-8.

Table 3-8: Timetable of the offshore AC fault at string cable.

Time Events


0.5 s A solid fault is applied at the string cable Cab1

0.64 s Breaker BS1 is opened

After the fault, the offshore AC voltages collapse and the converter quickly increases the q-axis current and the

reactive power to provide fault currents whereas the d-axis current reduces to avoid overcurrents, as shown in

Figure 3-29 (a), (d), and (b). The fault currents provided by the WT converters (1.25 pu) feed to the fault through

the circuit breakers BS1. By differential fault detection, breaker BS1 is assumed to be opened at t=0.64 s (for

illustration) to isolate the fault. The WT reactive power experiences slight oscillation during such a severe fault

and gradually stabilizes with the regulation of the controller, as observed in Figure 3-29 (d).

Figure 3-28: Offshore AC faults at string cable.


92

WTC_400MW_AV_pu,Main : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50 (a

)Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

(c)

Pref_WT P_WT

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

(d)

Q1_WT

0.60

0.70

0.80

0.90

1.00

1.10

1.20

(e)

f_WT

-30

-20

-10

0

10

20

30

(f)

I_BS1

Figure 3-29. Simulation results of WT converter during symmetrical offshore AC fault at string cables in pu terms: (a) three-

phase voltages, (b) three-phase currents, (c) active power, (d) reactive power, (e) frequency, and (f) currents flowing through

circuit breaker BS1.


93

With d-axis voltage reference set at 1.1 pu by the active power loop, the d-axis voltage control loop saturates

during the fault and gradually restores the offshore voltage after the fault is isolated by BS1 at t=0.64 s, as shown

in Figure 3-29 (a). The WT transmitted power and the DC current of the DR-HVDC link also gradually restore,

as displayed in Figure 3-29 (c) and Figure 3-30 (b).

During the entire process, the offshore frequency is largely controlled around the rated (50 Hz), as shown in

Figure 3-29 (e).

Figure 3-29 and Figure 3-30 demonstrate the WT converters automatically operate on current limiting during the

string fault and can quickly provide fault currents, which enables the fault detection. The system is robust to the

string cable fault and the can automatically restore power transmission.

MMC_FB : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900

0.90

1.00

1.10

1.20

(a)

Vdc_mmc

-0.80

0.00

0.80

1.60

(b)

Idc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(c)


-0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

P_mmc Q_mmc

Figure 3-30: Simulation results of onshore MMC during symmetrical offshore AC fault at string cables in pu terms: (a) DC

voltage, (b) DC current, (c) AC currents, and (d) active and reactive powers.


94

3.4.2 ASYMMETRICAL OFFSHORE AC FAULTS

3.4.2.1 ASYMMETRICAL OFFSHORE AC FAULT AT CLUSTER INTERCONNECTIN CABLE

To test performances of the DR-HVDC system in the event of an offshore asymmetrical AC fault, a solid ground

fault is applied at phase a of the cluster interconnection cable (F5, Figure 3-25) at t=0.5 s and is isolated by

breakers BB13 and BB31 at t=0.64 s, as listed in Table 3-7. To suppress the active power ripple, an additional

negative-sequence current controller is developed based on the generic control strategy as shown in Figure 3-2.

After the fault, the offshore grid side voltages of the faulty phase drop to zero while Figure 3-31 (a) shows the

WT converter side voltages, which exhibit different fault behaviour with grid side voltages. The peaks of the

three-phase currents of the WT converters are around 1.5 pu, as displayed in Figure 3-31 (b). As shown in

Figure 3-31 (c) and (d), the second-order oscillation of the active power is effectively suppressed by the

negative-sequence controller while the reactive power contains second-order harmonic. During the fault the

offshore frequency is controlled around 50 Hz, as seen in Figure 3-31 (e).

The DC terminal voltage of the onshore MMC station is regulated at the rated value while the DC current of the

DR-HVDC link exhibits second-order oscillations, as shown in Figure 3-32 (a) and (b). Thus, the imported DC

power of the onshore MMC station contains significant second-order component. This leads to relatively higher

submodule capacitor voltage ripple and slight disturbances are observed in the onshore grid currents as well as

active and reactive power, as displayed in Figure 3-32 (c) and (d) respectively.

During the tested asymmetrical offshore fault, the WT FECs do not experience significant overvoltage and

overcurrents (1.2 pu and 1.5 pu respectively). The whole system automatically restores normal operation after

fault isolation.


95

WTC_pu,Main : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50 (a

)Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

0.70 0.80 0.90 1.00 1.10 1.20 1.30

(c)

Pref_WT P_WT

-0.60

-0.30

0.00

0.30

0.60

0.90

1.20

(d)

Q1_WT

0.90

1.00

1.10

1.20

(e)

f_WT

-30 -20 -10

0 10 20 30 40

(f)

Ia_diff Ib_diff Ic_diff

Figure 3-31: Simulation results of WT converter during asymmetrical offshore AC fault at cluster interconnection cables in pu

terms: (a) three-phase voltages, (b) three-phase currents, (c) active power, (d) reactive power, (e) frequency, and (f) differential currents on the faulty cable.


96

MMC_FB_1 : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800

0.80

0.90

1.00

1.10

1.20 (a

)Vdc_mmc

-1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50

(b)

Idc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(c)


-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

P_mmc Q_mmc

Figure 3-32: Simulation results of onshore MMC during asymmetrical offshore AC fault at cluster interconnection cables in pu

terms: (a) DC voltage, (b) DC current, (c) three-phase AC currents, and (d) active and reactive powers.

3.4.2.2 ASYMMETRICAL OFFSHORE AC FAULT AT STRING CABLE

In addition to the aforementioned fault at the cluster interconnection cable, the asymmetrical offshore AC fault

applied at the string cable is considered in this subsection for sensitivity analysis. In this simulation scenario, a

solid ground fault is applied at phase a of the string cable (F6, Figure 3-28) at t=0.5 s and is isolated by breaker

BBS1 at t=0.64 s, as listed in Table 3-8.

After the fault, the offshore grid side voltages of the faulty phase drop to zero while Figure 3-33 (a) shows the

WT converter side voltages, which exhibit different fault behaviour with grid side voltages. The peaks of the

three-phase currents of the WT converters are around 1.5 pu, as displayed in Figure 3-33 (b). As shown in

Figure 3-33 (c) and (d), the second-order oscillation of the active power is effectively suppressed by the

negative-sequence current controller while the reactive power experiences second-order oscillations. During the

fault, the offshore frequency is controlled around 50 Hz, as seen in Figure 3-33 (e).


97

The DC terminal voltage of the onshore MMC station is regulated at the rated value while the DC current of the

DR-HVDC link exhibits second-order oscillations, as shown in Figure 3-34 (a) and (b). Thus, the imported DC

power of the onshore MMC station contains significant second-order component. This leads to relatively higher

submodule capacitor voltage ripple and slight disturbances are observed in the onshore grid currents and active

and reactive power, as displayed in Figure 3-34 (c) and (d) respectively.

During the tested asymmetrical offshore fault, the WT FECs do not experience significant overvoltage and

overcurrent. The whole system automatically restores normal operation after fault isolation.


98

WTC_pu,Main : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50 (a

)Va_WT Vb_WT Vc_WT

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(b)

Ia_WT Ib_WT Ic_WT

0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

(c)

Pref_WT P_WT

-0.50

0.00

0.50

1.00

1.50

(d)

Q1_WT

0.90

1.00

1.10

1.20

(e)

f

-30 -20 -10

0 10 20 30 40

(f)

I_BS1

Figure 3-33: Simulation results of WT converter during asymmetrical offshore AC fault at string cable in pu terms: (a) three-phase voltages, (b) three-phase currents, (c) active power, (d) reactive power, (e) frequency, and (f) currents flowing through

circuit breaker BS1.


99

MMC_FB_1 : Graphs

sec 0.400 0.450 0.500 0.550 0.600 0.650 0.700 0.750 0.800

0.80

0.90

1.00

1.10

1.20 (a

)Vdc_mmc

-1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50

(b)

Idc_mmc

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

(c)


-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20

(d)

P_mmc Q_mmc

Figure 3-34: Simulation results of onshore MMC during asymmetrical offshore AC fault at string cable in pu terms: (a) DC

voltage, (b) DC current, (c) AC currents, and (d) active and reactive powers.


100

4. SUMMARY

This document includes simulation results for normal and fault operation of WPPs connected to a point to point

DR-HVDC link.

For normal operation a simple controller has been used, considering inner PR current and voltage loops and

outer P/w Q/V loops with a secondary voltage control (P/V) for overall power production control. An OWF

controller is also included for the sequencing of operation change, umbilical cable power control and

synchronisation to external AC grid (either umbilical cable or an already operating section of the offshore AC-

grid).

The studied cases for normal operation include:

- HVDC link and offshore AC grid start-up operation

- HVDC link and offshore AC grid disconnection operation

- Intentional islanding and re-synchronisation to umbilical AC cable

- Dynamic voltage control during islanding operation (ISL) and during connection to synchronous AC system

(SAC)

- Wind farm power control

- Change in reactive sharing power commands during DR operation and during ISL operation

- Active power control through the umbilical for both SAC and DRSAC operation

- Disconnection of an OWF and re-synchronisation of an islanded OWF to an operational offshore AC grid.

- Disconnection and reconnection of DRU filters.

It is worth noting that both WTG and OWF controllers operated correctly during all considered scenarios, without

the need of re-tuning them for specific cases.

The fault ride-through operations of DR-HVDC connected WTs are presented in the report, considering:

- onshore grid faults

- DC cable faults

- internal DRU faults

- umbilical AC cable faults

- offshore symmetrical AC faults

- offshore asymmetrical AC faults.

The WT converters can automatically operate on current or voltage limiting mode during faults and do not suffer

overcurrent or overvoltage. During faults, the converters provide fast AC fault currents, which enables offshore

overcurrent and differential fault protection. The system is robust to the various faults and can automatically

restore power transmission after fault isolation.


101

This report primarily demonstrates the feasibility of the DR-HVDC technique for offshore wind energy

integration, considering both normal operation and during various faults.


102

5. BIBLIOGRAPHY

[1] "Deliverable 3.2 https://www.promotion-

offshore.net/fileadmin/PDFs/D3.2_Specifications_Control_strategies_and_simulation_test_case

s.pdf."

[2] J. M. Guerrero, J. C. Vasquez, J. Matas, L. G. d. Vicuna, and M. Castilla, "Hierarchical Control of

Droop-Controlled AC and DC Microgrids—A General Approach Toward Standardization," IEEE

Transactions on Industrial Electronics, vol. 58, pp. 158-172, 2011.

[3] L. Yu, R. Li, and L. Xu, "Distributed PLL-based Control of Offshore Wind Turbine Connected with

Diode-Rectifier based HVDC Systems," IEEE Transactions on Power Delivery, vol. PP, pp. 1-1, 2017.

[4] R. Blasco-Gimenez, S. A.-. Villalba, J. Rodríguez-D'Derlée, F. Morant, and S. Bernal-Perez,

"Distributed Voltage and Frequency Control of Offshore Wind Farms Connected With a Diode-Based

HVdc Link," IEEE Transactions on Power Electronics, vol. 25, pp. 3095-3105, 2010.

[5] R. Blasco-Gimenez, S. Anó-Villalba, J. Rodriguez-D'Derlée, S. Bernal-Perez, and F. Morant, "Diode-

Based HVdc Link for the Connection of Large Offshore Wind Farms," IEEE Transactions on Energy

Conversion, vol. 26, pp. 615-626, 2011.

[6] S. Seman, R. Zurowski, and C. Taratoris, "Interconnection of advanced Type 4 WTGs with Diode

Rectifier based HVDC solution and weak AC grids," in Proc. The 14th International Workshop on

Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for

Offshore Wind Farms, Energynautics GmbH, 2015, pp. 20-22.

[7] R. Li, L. Xu, and D. Guo, "Accelerated switching function model of hybrid MMCs for HVDC system

simulation," IET Power Electronics, 2017.

[8] L. Xu, L. Yao, and C. Sasse, "Grid Integration of Large DFIG-Based Wind Farms Using VSC

Transmission," IEEE Transactions on Power Systems, vol. 22, pp. 976-984, 2007.

[9] W. Xiang, W. Lin, L. Xu, and J. Wen, "Enhanced Independent Pole Control of Hybrid MMC-HVDC

System," IEEE Transactions on Power Delivery, vol. PP, pp. 1-1, 2017.

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