lecture on basic concept operation and control of hvdc

Lecture on Basic Concept, Operation

and Control of HVDC September 2, 2008

09.00-16.00 hrs. EGAT Head Office

Lectured by Nitus Voraphonpiput, Ph.D.

Engineer Level 8 Technical Analysis – Foreign Power purchase Agreement

Branch Power Purchase Agreement Division

Electricity Generating Authority of Thailand

2

Objective

Providing basic concept of the HVDC transmission system to

attendee.

Introducing operation and control of the HVDC transmission System.

Discussing applications of the HVDC and its limitations.

3

Contents

1. HVAC vs. HVDC 2. HVDC Principle

Q&A for 15 minutes

Coffee break 10 minutes

3. Control of DC TransmissionQ&A for 15 minutes

4

1. HVAC vs. HVDC

Why use DC transmission?

This question is often asked. One response is that losses are lower, but is it true?

Reference [2] has been explained using Insulation ratio and Power capacity in order to proof this

statement.

5

1. HVAC vs. HVDC

Insulation ratio of HVAC and HVDC (Ref. 1-2)A given insulation length for an overhead line, the ratio of

continuous working withstand voltage factor (k) is expressed as, (note )

0.1 voltage withstandAC

voltagewithstandDC

(rms)

k

A line has to be insulated for over-voltages expected during faults, switching operations, etc.

Normally AC transmission line is insulated against over-voltages of more than 4 times the normal effective

(rms) voltage.

21 k

6

1. HVAC vs. HVDC

5.2 ground)-(phase Voltage AC Rated

levelInsulationAC

(rms)1 k

This insulation requirement can be met by insulation corresponding to an AC voltage of 2.5-3.0 times the

normal rated voltage.

For suitable converter control the corresponding HVDC transmission ratio is expressed as

7.1 ground)-(pole Voltage DC Rated

levelInsulationDC2 k

7

1. HVAC vs. HVDC

d

P

V

V

k

kk

K

2

1(rms)

voltagewithstandDClevel insulation DC

voltagewithstandAClevel insulation AC

Pole DCeach for requiredlength insulation

phase ACeach for requiredlength insulation)(ratioinsulation

Insulation ratio for a DC pole-ground voltage (Vd) and AC phase-ground (Vp) is expressed as

It can be seen that the actual ratio of insulation levels is a function of AC/DC voltage. Next, determine AC/DC

voltage.

8

1. HVAC vs. HVDC

Determine AC/DC voltage

Assumed resistances (R) of the lines are equal in both cases (HVDC and HVAC).

AC Loss = 3 x R x IL2 and DC Loss = 2 x R x Id

2

Let losses in both cases are equal, so that,

The power of a HVAC system and a bipolar HVDC system are as:

Ld II2

3

cos3 LPIVPowerAC dd IVPowerDC 2

9

1. HVAC vs. HVDC

dp VVcos

1

3

2

At the same power transfer,

So that,

1cos

2

3

2

cos3

d

P

dd

LP

V

V

IV

IV

PowerDC

PowerAC

It can be seen that HVDC requires insulation ratio at least 20% less that the HVAC which essentially reflects the cost.

Thus, insulation ratio (K) can be written as

cos

2.1

cos

1

3

2

2

1 k

kkK

10

1. HVAC vs. HVDC

Power CapacityCompared a double circuit HVAC line (6 lines) and

double circuit DC line of Bipolar HVDC.

Power transmitted by HVAC (Pac) and HVDC (Pdc) are

acac

LPdc PP

k

kkIV

k

kkP

cos

47.1

cos6

2

1

2

1

cos6 LPac IVP dddc IVP 6

On the basic of equal current and insulation, Id = IL, K=1:

11

1. HVAC vs. HVDC

For the same values of k, k1 and k2 as above and pf is assumed to 1.0, the power transmitted by overhead lines can be increased to 147%. The percentage line losses, which is inversion of the power transmit, are

reduced to 68%.

In addition, for underground or submarine cables, power transmitted by HVAC cable can be increase

294 % and line loss reduced to 34%.

Note: for cable k equals at least two.

12

1. HVAC vs. HVDC

From reference [3], losses are lower is not correct.

“The level of losses is designed into a transmission system and is regulated by the size of conductor selected. DC and AC conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will

generally result in lower losses but cost more.”

The reasons that HVDC have been used are:

1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length.

2. It is not practical to consider AC cable systems exceeding 50 km (due to VAR charging of the cable).

3. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is qui

te small.

13

2. HVDC Principle

The HVDC valve comprises the thyristors acting as controlled switch. In the ‘OFF’ state, the thyristor blocks

the current to flow, as long as the reverse or forward breakdown voltages is not exceeded.

It changes to ‘ON’ state if it is forward biased (VAK > 0) and has small positive ‘Gate’ voltage applied between the

Gate and the Cathode.

Anode (A)

Cathode (K)

Gate (G)

14

2. HVDC Principle

Thyristor switches between conducting state (ON) and non-conducting (OFF)

state in response to control signal (firing) as its characteristic.

The Gate voltage need not to be present when the thyristor is already in ON

state.

15

2. HVDC Principle

Rd

VT

Rd = VAK/ IA

VT

Anode (A)

Cathode (K)Anode (A)

Cathode (K)

Rr

Rr = VAK/ IA Ploss-ON state = VT.IA(avg.) + Rd.IA2

(rms)

Ploss-OFF state = Rr.Ir2

(rms)

iA

ir

16

2. HVDC Principle

ON-OFF state- ON state continues until current drops to zero, even reverse bias appears across the thyristor.

- The critical time to clear charge carriers in the semi-conductor

is referred as the turn-off time toff . If forward bias appears to soon, t < toff, thyristor can not OFF.

ON

OFF

OFF

VAK > 0 and VG >0

IA < 0

VAK > 0 and t < toff

t > toff

17

2. HVDC Principle

ON State OFF State

18

2. HVDC Principle

Rd = 10

Th1

Ld

Th3

Th4 Th230V220US

Vd

Id

Is

Vs

Single Phase Bridge Rectifier

19

2. HVDC Principle

Vs

Id

Is

Vd

= 30

Th3

Th4

Th1

Th2

Th3

Th4

Voltage waveform of inductor (Ld), VLd = Vd – Rd Id

Voltage waveform of resistor (Rd), VRd = Rd Id

20

2. HVDC Principle

Is

Vd

Id

50 Hz

100 Hz

150 Hz

200 Hz

250 Hz 350 Hz

300 Hz

100 Hz

Harmonics in the voltage and current waveform.

DC

DC

21

2. HVDC Principle

Even DC side does not have reactive power (Q), the reactive power still presents on the AC side. The reactive power

occurrence is caused by the delay angle () (or called firing angle) of the current waveform.

30

VS

IS

Vs Is

= 30

P = |VS| |IS| cos

Q = |VS| |IS| sin

time36020 ms

Phasor of fundamental component

22

2. HVDC Principle

Is

Vd

Id

50 Hz

100 Hz

150 Hz

200 Hz

250 Hz 350 Hz

300 Hz

100 Hz

Product of Vd and Id is (active) power (P).

Product of phasor VS and phasor IS is not the apparent power (S) . It represents the

active power (P) and reactive power (Q).

There are harmonic distortion power, which is a new term

caused by the higher harmonics (more than 50 Hz).

It is represented by D (distortion power).

Finally, S2 = P2 + Q2 becames S2 = P2 + Q2 + D2.

23

2. HVDC Principle

Lk

Rd

Ld

Ith2

Ith1

commutation

is overlap angle

Ith2

Ith1

Vd

Id

IsVs

It can be seen that if current is high, overlap angel is increased. In addition, if inductance is high, overlap angle is also increased.

The inductance Lk represents reactance on AC side (called

commutating reactance). Due to nature of an inductor, The

inductor current can not change suddenly. Thus, during turn-off

of the Th1 (and Th2) and turn-on of the Th3 (and Th4), both are in conducting state for a short time (overlap time). This phenomena occurs during commutation of

the thyristors.

Increasing Lk

Increasing Id

24

2. HVDC Principle

Is

Vd

Id

Vs

= 30

Th3

Th4

Th1

Th2

Th3

Th4

Inductor current can not suddenly be changed, thus there is a slope.

2

)cos(coscos

25

2. HVDC Principle

The impact of the overlap angle () is the reduction of the average dc voltage (Vd).

It decreases the harmonic content of the ac current (Is) and power factor of the AC side.

Vd

Id

Ideal case Vdo

dKdod IXVV2

KK LfX 2

Vd

Id

Voltage drop due to commutating reactance

is represented as DX

XK

Rd

Id

DX

DR

VT

VT and DR are very less compared to DX. Thus, there are usually neglected.

Overall voltage drop

Vd

26

2. HVDC Principle

Rd

IL

Th1

Ld

VA

Th2

Th3

Vd

VB

VC

IL

t

60 90 1200

Vd

coscos17.1 0dPd VVV

Natural commutation

VA = 2 VP sin t

VB = 2 VP sin t-120

VC = 2 VP sin t+120

d

d

R

L3-pulse converter

27

2. HVDC Principle

1.0

-1.0

0.5

-0.5

Rectifier

Inverter

60

45 18013590

0d

d

V

V

cos0

d

d

V

V

Positive average voltage

Negative average voltage

Inverter mode can be performed when firing angle is more than 90 degrees.

Rectifier mode can be performed when firing angle is less than 90 degrees.

Average voltage is zero when the firing angle is 90 degrees.

28

2. HVDC Principle

=60 =30

Vd

Id

29

2. HVDC Principle

VA, IA

Id

VB, IB

VC, IC

Th1 Th2 Th3 Th1 Th2 Th3

120

30

2. HVDC Principle

VA, IA

Id

Vd

=30=120

Inverter mode can be performed as long as the DC current continues flow.

Positive voltage

Negative voltage

Reversing phase sequence

31

2. HVDC Principle

t

IB IC IA

t

dVVA

VB

Lk

Id

Lk

Vk

IC

dkX

Xdd

ILD

DVV

2

3

cos0

IB IA

Vk

VA

VB

DX

32

2. HVDC Principle

t

dV

Vk

DX

The commutating reactance (Xk) results in

decreasing of DC voltage, but it increases DC voltage in inverter

mode.

It can also be seen that the overlap time will increase when DC

current is high and this can cause commutation failure in inverter mode.

Note: + < 180

The extinction angle () = 180 - -

IB IA IB IA

180180

dkX

Xdd

ILD

DVV

2

3

cos0

33

2. HVDC Principle

6-pulse converter

Vd+

Vd-

Vd+

Vd-

Vd

Vd= Vd+ - Vd-

=0

=0

-Vd-Vd+

The 6-pulse bridge consists of two 3-pulse bridges (positive and negative) connected in parallel.

34

2. HVDC Principle

6-pulse bridge HVDC

Vdr Vdi

Id

Id

The HVDC comprises two converters connected in anti-parallel through smoothing reactors and DC lines. One converter is operated in rectifier mode

to transmit power from the AC network to the other side whereas the other side converter is operated in inverter mode to receive power into the (other

side) AC network.

Smoothing reactor

Smoothing reactor

DC line

DC line

power

power power

Reactive power

Reactive power

35

2. HVDC Principle

30

VI.cos

I

I.sin

30

866.02

)2515cos(15cos

2

)cos(coscos

Rectifier Operation of the 6-pulse bridge converter

Assume = 15 and = 25

The converter operates in rectifier mode. It transmits active power while consumes reactive power.

36

2. HVDC Principle

145

VI.cos

II.sin

145

823.02

)25135cos(135cos

2

)cos(coscos

Inverter operation of the 6-pulse bridge converter

Assume = 135 and = 25

The converter operates in inverter mode. It receives active power while

consumes reactive power.

37

2. HVDC Principle

For convenience, the converter operated in inverter mode is often referred to extinction angle (). Thus

direct voltage in inverter mode (Vdi) are expressed as

dkX

Xdd

ILD

DVV

2

3

90,cos0

Actually, inverter is commonly controlled at constant extinction angle to prevent commutation failure. Therefore, it is not only for convenience, but also for

converter control purpose. It is important to note that voltage drop caused by commutating reactance (Dx) is now negative.

Xdd DVV cos0

38

is the control variable for rectifier and is the control variable for inverter.

2. HVDC Principle

Voltage vs. current (VI) characteristics at steady state

dN

d

I

I

0d

d

V

V

dN

d

I

I

0d

d

V

V

= 0

max < 180

1.0

-1.0

1.0

1.0

-1.0

1.0

= 0

= 0

Slope is DX

Rectifier

Inverter

Rectifier

Inverter

Incr

easi

ng

Incr

easi

ng

In

crea

sin

g

39

2. HVDC Principle

12-pulse bridge HVDC

VdrVdi

Id

VdrY VdiY

Y Y

Y

Y

Y

YId

The 12-pulse converter is required to improve harmonic current on AC sides. It comprises two 6-pulse converters connected in series. Harmonic current on AC sides are odd orders starting from 11th, 13th …. whereas even orders

present on the DC side (12th, 14th…). To achieve 12-pulse, phase displacement of 30 generated by Star (Y) and Delta () connection of the

transformers are employed.

40

2. HVDC Principle

Y Y

Y

IA

IA

IAY

IA

IA

IAY

Vd

VdY

VdVdYVd

Vd

Rectifier operation of the 12-pulse bridge converter

Assume = 15

and = 25

41

2. HVDC Principle

Y Y

Y

current

VdiVdrY Y

YId

Vdi

Vdr

Id

voltagemin <

min = 5 - 7

min <

min = 15 - 17

½ Rd

½ Rd

To ensure all thyristor valves are enough forward bias

to turn on.

To keep reactive power requirement on inverter side as

low as possible.

Voltage drop caused by line resistance (Rd) is taken into account and the VI characteristic presents operating point of the HVDC system.

decreasing

power

power power

Reactive power Reactive power

42

2. HVDC Principle

Detail Configuration of the HVDC

43

2. HVDC Principle

Alternatives for the implementation of a HVDC power transmission system

i) Mono-polar Configuration

ii) Bipolar Configurationa) Earth Return

b) Metallic Return

iii) Homo-polar Configuration

44

2. HVDC Principle

Alternatives for the implementation of a HVDC power transmission system (continued)

45

Can we use manual control for the rectifier (vary ) and the inverter (vary

)? If we can not do that, which side should

be controlled (rectifier or inverter) or control them both?

What is/are the control purpose(s)?

3. Control of the DC Transmission

46


Typical control strategies used in a HVDC system consists of:

Firing Control {Rectifier} Current Control (CC)

{Inverter} Constant Extinction Angle (CEA) Control {Inverter} Current Margin Control (CM)

{Inverter} Voltage Control (VC) Voltage Dependent Current Limit (VDCL)

Tap change Controls (TCC) Power Reversal

47


Firing ControlFunction of the firing control is to convert the firing angle

order (*) demanded fed into the valve group control system. There might be voltage distortions due to non-characteristic harmonics, faults and other transient disturbances such as

frequency variation. Thus, phase-locked loop (PLL) based firing system is generally applied.

Phase Detector

vA

vB

vC

PI Controller Voltage Controlled Oscillator

sin(.)

sin(.)

sin(.)

comparator

*

comparator

comparator

……

⅔

vo

-

verror

…

uA

uB

uC Gate firing

Ts

TsK

)1(

48


verror

uAvA

time

time

Firing Control (Continued)

2

timeFiring pulse of phase A

*

0

0

0

49


Current Control (CC)The firing angle is controlled with a feedback control

system as shown in figure. The dc voltage of the converter increases (by decrease *) or decreases (by increase *) to adjust the

dc current to its set-point (Id*).

*max

min

PIid*

id

-

+

Y Y

Y

Vdr

Current measurement

Firing Control

vA, vB , vC

6

6

IdTs

TsK

)1(

50

3. Control of the DC Transmission Constant Extinction Angle Control (CEA)

The firing angle of the inverter is controlled at minimum angle (min) to reduce reactive power requirement. This can be

achieved by using Gamma control (-control).

*max

min

PI*

-

+

Current measurement

Firing Control

vA, vB , vC

6

6

VdiY Y

Y

measurement

Valve voltage

51


VI Characteristic of the CC and the CEA

current

Vdi

Vdr

Id

voltage

* = min

*

VI Characteristic

current

VdiVdr

Id

voltage

* = min*= min

If AC voltage on rectifier side decreases, CC decreases * down to

min to increase DC current (Id), but there is no operating point (X). This problem

can be solved using CMC.

X

AC voltage decreasing

The intersection (X) is the operating point of the DC transmission line.

52

3. Control of the DC Transmission Current Margin Control (CMC)

A better way is to use the inverter to control current less than of the rectifier by an amount of current margin (Id) when the

rectifier can not perform CC.

*max

min

PI

*

-+Current

measurement

Firing Control

vA, vB , vC

6

VdiY Y

Y

id*

id

id = 0.1 to 0.15

+

- Control

Min

imu

m

sele

ctio

n

53


VI Characteristic of CC, CEA and CMC

current

Vdi

Vdr

Id

voltage

* = min

*

Combined characteristics of CC, CEA and CMC

*= min

If AC voltage on rectifier side decreases, CC decreases * down to min to increase DC current (Id), but

there is no operating point (X). This problem can be solved by CMC.

X

AC voltage decreasing

current

VdiVdr

Id

voltage

* = minX

Id IdCMC

CEA

CC

This method can maintain stable operation when AC voltage of both

sides are fluctuated.

54

3. Control of the DC TransmissionWhat will happen if AC network of the inverter side is too weak!

current

Vdi

Vdr

Id

voltage

* = min

*

In this range the intersection is poorly to define and both current controllers will hunt

between the operating points.

This problem can be solved by adjust VI characteristic of the

inverter to voltage control (VC) in order to avoid hunting between two

controllers.

X

IdCMC

CEA

Weak AC

current

Vdi

Vdr

Id

voltage

* = min*

IdCMC

CEA

VC

* > min

X

More Weak

55

3. Control of the DC Transmission Voltage Control (VC)

it is very effective when the inverter is connected to a weak AC network. The normal operating point X corresponds to a value

of higher than the minimum. Thus, the inverter (rectifier as well) consumes more reactive power compared to inverter with CEA.

*

max

min

PI

*

-+

Firing Control

vA, vB , vC

6

6

VdiY Y

Y

vdi*

vdi

- Control

Min

imu

m

sele

ctio

n

Voltage measurement

CMCM

axim

um

se

lect

ion

56


Voltage Dependent Current Limit (VDCL)Commutation failures can occur during an AC fault on the

inverter side. It results in continue conduction of a valve beyond its 120 conduction interval. The CC will regulate the DC current to its rated value, but in the worst case, two inverter valves may form

DC short circuit and continue conducting for a long time, which can cause valve damage. To prevent this problem, DC current

must be reduced. One possible to detect the AC side fault is the lowering of the DC voltage. This voltage is typically chosen at 40%

of the rated voltage.

Id

57


Voltage Dependent Current Limit (VDCL)The VDCL is a limitation imposed by the ability of the AC

system to sustain the DC power flow when the AC voltage at the rectifier bus is reduced due to some disturbance as well. The

VDCL characteristics is presented below.

current

Vdi

Vdr

Idmax

voltage

*

IdCMC

VC

X

Id-min

0.4

VDCLId

current

Vdi

Vdr

Id

voltage

*

Id

CMC

VC

X

Id-min

0.4

VDCLId

Idmax

VDCL VDCLVDCL

58


Voltage Dependent Current Limit (VDCL)

id*

vd

v

Vd

Ts1

1

Voltage measurement

i

Min

imu

m

selection

vd

CC

VDCL

iv

59

3. Control of the DC Transmission Tap Change Control (TCC)

When voltage of the AC system of the rectifier and/or of the inverter is fluctuated, transformer taps (both side) can adjust to keep the DC voltage within desired limits or suitable operating

point. Generally, the tap will be changed when the firing angle of the rectifier/inverter still reach its more than 10-15 minutes to

avoid interaction of other controls.

Example: if the firing angle () of the rectifier reaches minimum limit (min) for long time. It means that the AC voltage of the

converter is not appropriate. Thus, AC voltage of the converter must be reduced by tap changing of the converter transformer to

free the firing angle of the rectifier.

60

3. Control of the DC Transmission Power Reversal

The VI characteristic of power reversion is presented below (VDCL and VC are not included). The station 1 (rectifier) increases firing

angle () into the inverter region and the station 2 (inverter) decreases its firing angle () into rectifier region. This can be performed without altering

the direction of current flow.

current

V2di

V1dr

Id

voltage

* = min* X

current

V2dr

V1di

Id

voltage

* = min* X

61


Vdi

Y Y

Y

Y Y

Y Id

Firin

g

Co

ntro

l Min

.Min

.

Max. M

ax.

p*/vdCAE

CC

VC

VDCL

Power order

min

Vd*

Fir

ing

C

on

tro

l

CAE

CC

VC

VDCL

min

Vd*

id

Modulation Signal

id*

p*

Vdr

TCC TCC

Master Control

p

**

po

Vd, Id,,

62


CIGRE’s HVDC benchmark was simulated on ATP-EMTP with the typical HVDC control schemes, which the CC mode was employed

at rectifier and VC mode was applied at inverter. All simulation results are presented in normalized values.

Start Up HVDC

Rectifier Current Control Inverter Voltage Control

63


Start Up HVDC

Firing Angle () of Rectifier Firing Angle () of Inverter Extinction angle () is also shown

The HVDC started at 0.1 sec. The firing angle of rectifier started at 90 while the extinction angle of inverter started at 90.

64


Power Reversal

Firing Angle () of Rectifier and Inverter

DC Current

The HVDC started to reverse power flow direction at 0.5 sec. Firing angle of the rectifier increased (with a ramp rate) into inverter zone while firing angle of the inverter decreased (with a ramp rate) into

rectifier zone.

65


Power Reversal

The power flow direction of the HVDC reversed at 0.9 sec.

66


VDCL performance during 1-phase fault at AC network of the rectifier station.

1 –phase Fault at AC network of the rectifier station

cba V VV

67


Fault at AC network of rectifier station

REFIdI

diV

didREF VI I.u.p Degree )( Alpha_i )( r_Alpha ir

r

i

68


VDCL performance during 1-phase fault at AC network of the inverter station.

1-phase Fault at AC network of the inverter station

cba V VV

69


Fault at AC network of inverter station

REFIdIdiV

didREF VI I.u.p )( Alpha_i )( r_Alpha ir

i

r

Degree

70


Modulation signal is employed when a power system has a special requirement such as

frequency control, power oscillation damping, etc.

For example, the addition frequency control loop is included into HVDC control system to stabilize

frequency of the AC system.

71


Modulation Function of EGAT-TNB HVDC

72


Power Swing Damping (PSD) Function of EGAT-TNB HVDC

Thank you very much for your attention

74

References

1. Ani Gole, “HVDC Transmission Lecture Note”, University of Manitoba, 2000.

2. Jos Arrilaga, “High Voltage Direct Current Transmission”, 2nd , IEE-Press, 1998.

3. Dennis A. Woodford, “HVDC Transmission”, Manitoba HVDC Research Center, Canada, 1998.

4. Erich Uhlmann, “Power Transmission by Direct Current”, Springer Verlag, 1975.

5. Vijay K. Sood, “HVDC and FACTS Controllers”, Kluwer. 2004.

6. Edward Wilson Kimbark, “Direct Current Transmission” vol.1, Wiley-Interscience, 1971.

7. IEEE Transmission and Distribution Committee, “IEEE guide for planning DC links terminating at AC locations having

low short-circuit capacities”, IEEE, 1997. 8. กฤตยา สมสย, นิ�ทัศนิ� วรพนิพ�พฒนิ�, ว�ทัวส ผ่�องญาต�, “การจำ�าลองระบบส�งไฟฟ�า

แรงส!งกระแสตรงโดย ATP-EMTP”, สมมนิาว�ชาการระบบส�ง กฟผ่ . 2548.

lecture on basic concept operation and control of hvdc

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