control and operation of multi termina vsc hvdc

Control and Operation of Multi-terminal VSC-HVDCVSC-HVDC

Temesgen HaileselassieKjetil Uhlenj

1

Conclusions / Hypothesisyp• A meshed HVDC grid has the potential to improve

it f l !security of supply!• A multi-terminal HVDC grid in the North Sea can

effectively integrate the synchronous interconnections (UK, UCTE and Nordic)

• Can be operated as ONE control area (if desirable)

R ( i d d ) b h d• Reserves (primary and secondary) can be shared without “technical constraints”

• Fast control and protection will enable network splitting to avoid risk of cascading outages and complete blackouts

• Fully integrate the power markets across the2

Fully integrate the power markets across the asynchronous areas.

O tliOutline• Introduction: Why HVDC – VSC HVDC

– Multi-terminal HVDC– Multi-terminal HVDC• Power system security and Control

objectives• Modelling and control designModelling and control design• Examples (illustrating security control

t )aspects)

3

HVDC Why?y1. To reduce total transmission loss for long distance

power linespower lines

Break even distance for onshore application is

4about 600km and less than 100km for subseatransmissions

Cont…2. AC transmission becomes weak (unstable) for very large

Cont…( ) y g

distances3. Subsea power transmission3. Subsea power transmission

- Large capacitive currents in AC cables severly limit the transmission capacity of long distance AC cables

- The most common application area of HVDC is for subsea power transmission

5

Limits of transmission capacity, Submarine HVAC cables (ref Nexans)

6

Why multiterminal VSC-HVDC (MTDC) in the N th S ?North Sea?

7

Grid integration

8

P t itPower system security

Security standards: Deterministic (N-1) or risk based

Ability to manage contingencies / outages:

Availability of reserves

Availability of transmission capacityAvailability of transmission capacity

Stability and control

9

Main challenges in operation andMain challenges in operation and control

• Primary control: Less primary reserves if new generation provide lessLess primary reserves if new generation provide less

frequency response• Secondary control:• Secondary control: More need for secondary reserves with more variable

generationgeneration• Tertiary control: Benefits with larger control areas and exchange of

reserves.

New possibilities with an offshore Multi-terminal HVDC grid!

10

HVDC grid!

Control objectives: DesiredControl objectives: Desired operational capabilitiesp p

• Balancing of offshore wind power variation• Resilience, e.g. to loss of a VSC-HVDC

terminal (≈load/generation loss) or line/cableterminal ( load/generation loss) or line/cable • Frequency response enhancement of AC grids• Market integrated operation• AC and DC fault handling capabilitiesAC and DC fault handling capabilities

C t ll h ld b d i d t tControllers should be designed to meet the requirements specified above.

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the requirements specified above.

Modelling:Active and Passive AC grid Connections

Passive AC grid connection:-AC voltage control at PCC

Time-average VSC model

AC voltage control at PCC-no synchronization-no control of currentno control of current

, ,2 2 2

1

a dc b dc c dca b c

m U m U m UV V V Active AC grid connection:-Grid synchronization

12o a a b b c cI m i m i m i -control of current flow

12

Control of VSC Connected to PassiveControl of VSC Connected to Passive AC Grid

AC voltage controlAC voltage control at PCC, |V|

13

Control of VSC Connected to ActiveControl of VSC Connected to Active AC grid g

Two options:

• |m|, δ control: uses phasor measurements

• decoupled axes (dq) control : usesinstantaneous measurementsinstantaneous measurements

14

|m|, δ control

• Suitable for modelling VSC control in phasor basedl t i i l ti t l ( DI SILENT)electric power simulation tools (eg: DIgSILENT).

• Voltage and current phasor measurements introduceadditional time constant into the controllers

0o

+-

|m|*additional time constant into the controllers

δ*

+ cos+

ma*ωt ×

PIδ*

P* +

120o

+-

cosωt m *×

|m|*PI

P-

PI+ |m|*

|V|* or |Q|*δ*

cos+

ωt mb×

-

|V| or |Q|

240o

+-

cosωt *

|m|*

15δ*

cos+

ωt mc×

Decoupled axes (dq) controlDecoupled axes (dq) control

• Involves abc/dq transf.• Fast control responses

Mostl sed in practice• Mostly used in practicefor VSC control

• Modelling is possibleg pwith electromagnetictransient softwares such

PSCADas PSCAD

16

Outer Controllers• Set the references to active & reactive (inner) current controllers• Active power and/or DC bus voltage control (there types shown below)• Reactive power and/or AC voltage control (not shown here)

17

Safe operating area of a VSCSafe operating area of a VSCController actions are limited within safe operating areaController actions are limited within safe operating area

Safe operating area of a VSC: (a) U vs I safe operating region (b) U vs P safe operating region

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(b) U vs P safe operating region

Constant power control within the safe operating region

P* = power referenceP* = power reference

19

Constant DC voltage control within the safe operating region

U* = DC voltage referenceU* = DC voltage reference

20

DC voltage droop control within the safe operating region

U* = DC voltage referenceU* = DC voltage reference

21

Two terminal VSC-HVDC ControlHVDC

S L

Rectifier(Source converter)

Inveter(Load converter)

transmission

PS PL

Control modes (3x3=9 combinations) RemarksRectifier Inverter

Fixed power Fixed power X (Not viable)Fixed power DC Droop √ (With risk of DC overvoltage)Fixed power DC Droop √ (With risk of DC overvoltage)Fixed power Fixed DC voltage √ (With risk of DC overvoltage)DC Droop Fixed power √ (Good P control by Inv )DC Droop Fixed power √ (Good, P control by Inv.)DC Droop DC Droop √ (Good, P control by Both)DC Droop Fixed DC voltage √ (Good P control by Rect )DC Droop Fixed DC voltage √ (Good, P control by Rect.)Fixed DC voltage Fixed power √ (Good, P control by Inv.)Fi ed DC oltage DC Droop √ (Good P control b In )

22

Fixed DC voltage DC Droop √ (Good, P control by Inv.)Fixed DC voltage Fixed DC voltage X (Not viable)

DC Voltage Control HVDC

S L

Rectifier(Source converter)

Inveter(Load converter)

Responses(Primary & secondary DC voltage control)

HVDC

transmission

( y y g )PS PL

UDCPL0

Operatingpoint PL1

UDC PL0

PL1

UDC PL0

C

PL1

PS

PS1

B

A

C

PS0

PS1

A

C

PS0

A

B

P P

PS0

399

400

401

ge

399

400

401

e

399

400

401 P

BS0

395

396

397

398

DC voltag

Rectifier (source)terminal DC bus voltage

395

396

397

398

DC voltage


395

396

397

398

DC voltage


10 20 30 40 50 60 70

950

1000

1050

)

950

1000

1050

W)

10 20 30 40 50 60 70395

950

1000

1050

10 20 30 40 50 60 70395

800

850

900

Power (M

W)

Power taken byinverter (load) terminal

800

850

900

Power (M

W


800

850

900

Power (M

W)


2310 20 30 40 50 60 70

750

Time (s)

10 20 30 40 50 60 70

750

Time (s)

10 20 30 40 50 60 70

750

Time (s)

(a) (b) (c)

Power flow control in DC grid :Power flow control in DC grid :achieved by DC voltage droop

No need for communication between terminals No need for communication between terminals Many converter terminals contribute to DC voltage regulation DC analogy to distributed frequency droop control in AC

24

DC analogy to distributed frequency droop control in AC systems

Similarities of AC grid frequency droop control d DC lt d t l f MTDCand DC voltage droop control of MTDC

Steady state characteristic of: (a)synchronous generator (b)VSC-HVDC terminal (The dots at the end of the characteristic lines

25

HVDC terminal (The dots at the end of the characteristic lines show tripping points.)

Test model: ”North Sea DC grid”Test model: North Sea DC gridOffshore load

(Oil/gas platform)

Prated=250 MWC t t P

All cable resistances: r=0.01 Ω/kmAll bl i 5 F/k

Prated=450 MW Prated=1000 MWDC d d

l45=250

5

Constant Power terminal

All cable capacitances:c=5 μF/kmBipolar DC transmission for all cases

Rated DC voltage =+/-200 kV

DC droop mode DC droop mode

l14=500 km

50 Km

1 4NORDEL GridScotland

l12=300 km l34=700 km

l23=600 km

UCTE Grid

UK National Grid

Prated=800 MWDC droop mode

2 3Prated=750 MWDC droop=∞ 6

l26=120 km

England

Prated=600 MWVariable (wind) power

26Offshore windfarm

S it l iSecurity analysisExamples illustrating security aspects p g y p

related to operation and control

• Managing normal wind variations• Outage of DC connections• Outage of DC connections• Outage of generation (wind farm tripping)

P i t l t id• Primary control response to ac grid contingency (exchange of primary reserves)

• Need for secondary control

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Variations of DCVariations of DC voltage with

fluctuating wind 460

480

500

W)

fluctuating wind power and DC droop control

380

400

420

440

Power (M

W Generated wind power from offshore WF

presponses 10 20 30 40 50 60 70 80 90 100 110

360

380

402

404

DC bus 1 (Scotland)

396

398

400

402

voltage (kV)

DC bus‐1 (Scotland)

DC bus‐2 (England)

DC bus‐3 (UCTE)

DC bus‐4 (NORDEL)

l45 =250 Km

10 20 30 40 50 60 70 80 90 100 110392

394

396DC

600

DC bus‐5 (Oil/gas platform)

DC bus‐6 (Offshore Windfarm)

m

400

500

600

(MW)

From Scotland (via Conv‐1)

To England (via Conv‐2)

To UCTE (via Conv‐3)

l26 =120 km

200

300Power To UCTE (via Conv‐3)

From NORDEL (via Conv‐4)

To oil/gas platform (via Conv‐5)

From offshore WF (via Conv‐6)

2810 20 30 40 50 60 70 80 90 100 110

Time (s)

Outage of DC line 1-2

Terminal-1 (Conv-1) ti t d 5

200

250

MW)

Power via Line 1‐2

Power via Line 1‐3continues to draw power from DC grid via lines 1-3 and 1-4 when 50

100

150

Line Pow

er (M



line 1-2 is disconnected. 85 90 95 100 1050

L

402


398

400

ltage (kV)



DC bus‐3 (UCTE)

DC bus 4 (NORDEL)

l45 =250 Km

394

396DC vol DC bus‐4 (NORDEL)



85 90 95 100 105

450

500

550

W)



l26=120 km

250

300

350

400

Power (M

W To UCTE (via Conv‐3)



29 85 90 95 100 105150

200

Time (s)


Outage of connection to windfarmOutage of connection to windfarm

500

550

From Scotland

400

450

500 From Scotland(via Conv‐1)

To England(via Conv 2)

300

350

00

W)

(via Conv‐2)

To UCTE(via Conv‐3)

l45 =250 Km 200

250

Power (M

W

From NORDEL(via Conv‐4)

100

150To oil/gasplatform(via Conv‐5)

l26 =120 km

0

50 From offshore WF(via Conv‐6)

Terminals 1, 2 and 4 compensate for lost power

83 84 85 86 87 88 89 90 91 92

Time (s)

30flow from offshore wind farm.

Frequency response enhancement of AC grids by VSC HVDCof AC grids by VSC-HVDC

Frequency droop control implemetation on VSC-HVDC with: ( ) t t t l (b) DC lt d t l(a) constant power control (b) DC voltage droop control

31

Frequency response enhancement example: two terminal VSC-HVDC

32

Effect of frequency droop control (in VSC-HVDC ) AC id fHVDCs) on AC grid frequency responses

33

Frequency response enhancement of AC grids by MTDCof AC grids by MTDC

1.001

u.)

UK national grid

0.998

0.999

1

frequency (p.u

UCTE grid

NORDEL grid

Oil/gas platform

l45 =250 Km

402100 110 120 130 140 150 160 170 180

0.997

0.998

Grid f

Oil/gas platform

Offshore WF

l26 =12

398

400

402

age (kV)



DC bus‐3 (UCTE)

120 km

392

394

396DC volta DC bus‐4 (NORDEL)



UCTE grid frequency500

600

W)



100 110 120 130 140 150 160 170 180392

UCTE grid frequency supported by NORDEL and UK (via two HVDC

i l d300

400

Power (M

W

To UCTE (via Conv‐3)



F ff h WF ( i C 6)

34stations: one at England and the other in Scotland) 100 110 120 130 140 150 160 170 180

200

Time (s)


Comparison of grid responses (UCTE grid): with and without frequency support from DC gridand without frequency support from DC grid

0.9995

1

0.999

)

0.998

0.9985

equency (p.u.

With frequency support by MTDCWithout frequency support by MTDC

0.997

0.9975

Grid fre

0.9965

0.997

100 110 120 130 140 150 160

0.996

Time (s)

35Frequency response improves with presence of frequency support from DC grid.

( )

Precise control of power flowSchedule/ dispatch Control

typeTerminal No

PDC(MW) UDC (kV)No. (MW)

1 600 - Droop

2 - 400 Droop

3 -750 - Fixed P

4 550 - Droop

5 -900 - Fixed P

Control references Control typeTerminal

NoPDC

(MW) UDC (kV)No. (MW) DC ( )

1 600 400 Droop

2 500 400 Droopp

3 -750 400 Fixed P

4 550 400 Droop

N t i !36

5 -900 400 Fixed P Not precise!

Cntd…Schedule/ dispatch Control

typeTerminal No.

PDC(MW) UDC (kV)( )

1 600 - Droop2 - 400 Droop3 750 Fi d P3 -750 - Fixed P4 550 - Droop5 -900 - Fixed P

DC Power flow analyisis

Control references Control tTermina PDC

flow analyisis

typeTerminal No.

PDC(MW) UDC (kV)

1 600.00 399.550 Droop

2 535.94 400.000 Droop

3 -750.00 396.613 Fixed P

4 550 00 396 928 D Precise!37

4 550.00 396.928 Droop

5 -900.00 385.247 Fixed P

Precise!(Desired power flow achieved)

F t k MTDCFuture works on MTDC• Protection schemes

F lt d t ti d l li ti• Fault detection and localization algorithms

• Communication based (?)Communication based (?)

38

Future worksFuture works…Impact of wide area MTDCImpact of wide area MTDC (Stability, operational, …)

39

Thank you!

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control and operation of multi termina vsc hvdc

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