© ABB Group Slide 109MR0163

Reactive Power Compensation and Harmonic Filters for HVDC Classic

Power Systems – HVDC / Dipti Khare

CONTENTS

• Reactive Power Requirement

• Harmonic Generation

• Harmonic Control • AC Filters

• DC Filters

Reactive Power Requirement

Reactive Power Requirement

•HVDC converters absorb reactive power, approximately 50% to 60% of their active power.

•Harmonic filters are installed on the AC side for filtering the AC current and for generation of reactive power.

•The reactive power absorption of a converter increases with the transmitted active power. Also the need for filtering of harmonics is increased.

•The need for reactive power grows slowly at low power, and more pronounced at high power, whereas the filter needs behave in the opposite fashion.

•The reactive power compensation scheme has to take care of the unbalances for the AC system requirement, by switching of filters

0,13 filter

converterunbalance

1,0

0,5

Id

Q

Classic

Purpose of the Reactive Power Control

§ The purpose of the Reactive Power Control (RPC) is to control the properties in the AC network that are connected to the converter station. The RPC will also make sure that the required filters are connected to prevent excessive harmonics that may enter into the AC system.

§ These tasks are performed by switching of the filter banks.

Reactive Power Control•The reactive power balance of each side of the HVDC transmission will normally be performed by reactive power controller (RPC).

•Each RPC is located in the pole control level and operates independently from the RPC in the other end of the HVDC transmission.

•Switching of filter banks or sub-banks is ordered by the RPC or by protections.

•Switching priority restrictions are determined by limits in the reactive power compensation study for the different control modes.

Selection of AC filter configurations due to reactive power requirements

§ The a.c. filters, PLC-filters and shunt capacitor banks generate reactive power to compensate the reactive power consumption by the converter

§ The consumption of reactive power varies linearly with the active power, but the generation can only be changed in steps by switching in or out of filter banks. Therefore there will be a net interchange of reactive power with the network

§ Maximum size of the filter bank may also be influenced by the permitted voltage step size at the switching of a bank

Reactive power for typical AC filter switching sequence

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0.00 0.20 0.40 0.60 0.80 1.00 1.20

p (pu)

q (

=Q/P

dN

)

1: qexchng

2: qdc

3: qf

4: qac(limit)

1

2

3

4

Main components of a converter station

Converter station Transmissionline or cable

Converterstation

Converter

Smoothingreactor

DC filter

Telecommunication

Controlsystem

AC filtersShunt

capacitorsor otherreactive

equipment

AC bus

~ ~

Harmonic Generation

Characteristic Harmonic currents on the AC side of a converter

Idealized converter

§ The supply (AC) voltage is exactly symmetrical§ The direct current is perfectly constant without ripple (Infinite

smoothing reactor).§ The firing angles of each phase are perfectly equal§ The commutation impedances are equal in the three phases

Characteristic Harmonic currents on the AC side of a converter

T/4 T/2 3T /4

[%]

[%]

105

5

5 7 11 13 17 19 23 25

11 13 23 25

n

n

i1

i2

i + i1 2

S Ii + i

n

1 2

IIn

1

Phasecurrent

Y

Y

Y

D

i1

i2

i + i1 2

§Neglecting the commutating reactance

§Rectangular pulses

© ABB Group Slide 1309MP0163

Harmonic generation Characteristic Harmonic currents on AC side of a converter

Y-Y 6-pulse:

Y-? 6-pulse:

12-pulse:

)11cos111

7cos71

5cos51

(cos32

1 L+ω−ω+ω−ωπ⋅

= ttttIi d

)11cos111

7cos71

5cos51

(cos32

2 L+ω−ω−ω+ωπ⋅

= ttttIi d

)23cos23113cos

13111cos

111(cos34

21 L+ω−ω+ω−ωπ⋅=+ ttttIii d

Id: d.c. current

The fundamental current

The n:th harmonic

dIIπ⋅

=62

1

nI

I n1= L25,23,13,11=n

Characteristic AC-side harmonics

§n = 12k ? 1 k = 1, 2, 3, ...

§11 13

§23 25

§35 37

§47 49§In = Kn * I1/n

§In = harmonic current

§Kn= reduction factor due to overlap

§I1 = fundamental AC current

Characteristic converter ac harmonic currents

0

20

40

60

80

100

120

140

160

180

200

Am

ps

Direct current (A)

Converter ac harmonic currents as a function of direct current (Id nom = 1500 A)

11

13

23

35

Characteristic Harmonic currents on the AC side of a converter

Ø The inductive reactance of converter transformers gives a gradual transfer of current from one phase to another and so rounds the steps of the current waveforms

Ø The characteristic harmonics will decrease with increasing commutation reactance

Current Pulses with Overlap

Imperfections of the converter

Ø The odd 6-pulse harmonics that are supposed to cancel perfectly in a 12-pulse converter, may not do so because of some small difference in the reactance or in turn ratio between the wye-wye and wye-delta connected transformers

Ø There is always some difference in the transformer reactance of each phase due to manufacturing tolerances

Ø The phase voltages are not exactly symmetrical, for example contain a small negative sequence component

Ø There may be a spread in the firing angles for the different valves due to imperfections in the control system

Non-characteristic harmonics

Imperfections AC-side harmonics

DC-side harmonics

AC system Negative sequence 5th and 7th distortion

3rd (5th, 7th)

2nd 6th

Transformer reactance Difference between Y/Y and Y/D Difference between phases

5th, 7th

odd

6th even

Firing asymmetry All All

0

5

10

15

20

25

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Am

ps

Direct current (A)

Converter ac harmonic currents as a function of direct current (Id nom = 1500 A) - non-characteristic harmonics

3

5

9

15

Impact of Non-characteristic harmonics on a.c. side

Ø The magnitude of non-characteristic harmonics is small comparing to the characteristic harmonics

Ø Most of them have a minor influence on the total harmonic distortion and filter design

Ø However,if the short circuit impedance of the AC network is high, it could result in high distortion of the lower order non-characteristic harmonics(orders 2-7) on a.c. bus voltage due to parallel resonance between the network and filter banks

Characteristic DC side harmonics 12-pulse§n=12k k=1, 2, 3, ...

§12

§24

§36

§48 etc)2cos(2

21 22)( uCDDC

UU

dio

nd +α−+=

1)2/)1cos((

+⋅+

=n

unC

1)2/)1cos((

−⋅−

=n

unD

The characteristic harmonic voltage across a converter as a function of overlap angle u at firing angle of 15 deg.

The characteristic harmonic voltage across a converter as a function of overlap angle u at firing angle of 15 deg.

• As the overlap increases from a very low value at minimum current to a value in the range 15 – 25 o at nominal current, it can be seen for the 12th harmonic that the harmonic generation is high at low load operation, then decreases to a minimum before rising again to a value eventually greater than that at low-load operation

• For higher order harmonics, the maximum generation does not occur at full load. There are several local maxima, progressively increasing in peak magnitude, within the feasible range of overlap angle

Harmonic Control

© ABB Group Slide 2609MP0163

Why do we need filters?

§ To “clean up” the harmonics that are generated from the converter. These may otherwise cause…§ Increased losses / overload of system equipment

§ Telephone disturbances

§ Source of misbehavior of control equipment

§ To compensate the reactive power consumption of the converter (classic)

0,13filter

converterunbalance

1,0

0,5

Id

Q

Classic

AC Filter capacitors

Performance requirementsØ The basic requirement for the design of a.c. filter is a set of

interference disturbance criteria valid for the voltage of converter a.c. bus or in special cases for the currents in the outgoing a.c. lines

Ø It is difficult to specify limits on disturbing current (requiring very precise knowledge about the impedance of the a.c. network at all harmonics of interest) even though it would seem justified to specify limits on the disturbing currents in the outgoing a.c. lines

Ø The requirements related to the a.c. bus voltage are commonly used disturbance criteria:

Ø Individual harmonic voltage distortion Dn

Ø Total harmonic voltage distortion THD

Ø Telephone interference factor TIF (B.T.S. - EEI)

Ø Telephone harmonic form factor THFF (CCITT) (never used with TIF simultaneously)

Requirement specification

§ Voltage distortion§ Specified limits on Dn are in the range of

0.5% to 1.5% (most typically 1%)

§ Specified limits on THD are in the range of 1% to 4%

§ Telephone interference§ Specified limits on TIF are typically between

30 and 50

§ Required limit of THFF is typically 1%

Equivalent circuit for AC filter calculations

L1

C1

AC network impedanceAC filtersConverter harmoniccurrent generation

Network Impedance• Network harmonic impedance is of critical

importance to the design of the AC filters

• The a.c. network harmonic impedance varies with varying network conditions.

• It is customary to present limit curves (impedance envelope diagrams) for the network impedance in an R-X plane; make filter design manageable and easier

• Network harmonic impedance sector diagram

• Network harmonic impedance circle diagram

Sector limits for the AC network impedance (CIGRÉ WG 14.30)

§UL = line to line AC network voltage

§Smax s.c. = a.c.network maximum short-circuit capacity

§Smin s.c. = a.c.network minimum short-circuit capacity

§n = harmonic number

nSU

Zmin..max

2

L ⋅=cs

n⋅=s.c.min S

UZmax

2

Lφmax

X

R

Zmax

Zmin

φmin

ϕ =0- 80° el for n < 5 ϕ = ±75° el for 5≤ n<11 ϕ = ±70° el for 11≤ n ≤50

Circle limits for the AC network impedance

X

R

Rad

ius

Rmin

φmax

φmin

AC filter types

• Single-tuned filter

• High-pass filter

• Double-tuned filter

• C-type filter

Single-tuned bandpass filter

C1

L1

R1

0 10 20 301

10

100

1 103

1 104

Harmonic number

Impe

danc

e (o

hms)

§ Very low impedance in resonance frequency

§ Efficient filtering in a narrow frequency band

§ Single-tuned filter normally used for the largest harmonics, 11th and 13th

12

11 )2(

1Cnf

Lπ

=

qLnfR /2 111 ⋅π=

)11(2 22

11 nUf

QC −π

=

Quality Factor

§ The Quality factor is a measurement of the sharpness of a filter

§ Q-value normally lie in the range between 40 and 90 for tuned filter branchs

§ High Q-value filter is sensitive with the frequency variation (detuning)

RL

q 0ω=

High-pass filter, HP24

L1

C1

R1

0 20 40 6010

100

1 103

1 104

Harmonic numberIm

peda

nce

(ohm

s)§ Broadband filter to take care of all harmonics from the 23rd and

upwards, tuned to near the 24th harmonics

§ Q-value of the filter branch normally lie within the range of 2-10

§ This type filter can be designed with high Q-values for 11th and 13th with lower fundamental losses, but the parallel connected resistor is more expensive

1)2(1

21

1 CnfL

π=

qLnfR ⋅⋅π= 111 2

)11(2 22

11 nUf

QC −π

=

Double-tuned filter, 11/13

L2

L1

C1

C2 R2

5 10 151

10

100

1 103

Harmonic number

Impe

danc

e (o

hms)

§ Commonly used in modern HVDC station

§ At high system voltage the larger main capacitor is easier to optimize to a lower cost/kvar

§ lower reactive power generation in the filter branch to fullfil the requirement under lower power transmission

§ Each switched filter attenuates two harmonics to reduce filter branch types and facilitate filter redundancy

Double-tuned high pass filter

§ Tuned to 12th and 24th with relatively low resistive impedance for higher order harmonics; The efficiency of filtering at characteristic harmonics is not high,but it does cover the whole spectrum of interest with only one branch

§ Tuned to 24th and 36th with a lower resistive impedance for higher frequencies for stringent requirement on TIF or THFF

20 40 6010

100

1 103

Harmonic numberIm

peda

nce

(ohm

s)

R1L1

C1

C2 L2 R2

C-type filter

§ ‘Low-order high-pass’ filter for ‘3rd,5th,7th’ harmonic filtering

§ The lower L-C is series resonant at the fundamental frequency and so bypassed the resistor to greatly reduce the filter losses

L

C1

RC2

0 5 10 15 20100

1 .103

1 .104

Harmonic number

Impe

danc

e (o

hms)

)11

1(

)1

1(

222

2

21

1

ba nn

nUf

QC

−−

−⋅

π=

LnfC

b2

12 )2(

1π

=

qCLR ⋅=

1

12

1 )2(1

CnfL

aπ=

DC harmonic filter capacitor

Cancellation of harmonic currents between poles

Harmonic current - pole 1

Harmonic current - pole 2

§ In a balanced bipolar operation, the harmonic currents in the two pole conductors will partially cancel

§ lower level of disturbing current of bipolar operation than of monopolar ground return operation

Typical DC filter arrangement

§C§1

§L§1

§L§2§C

§2

§C§1

§L§1

§L§2§C

§2

§Cnb

§Ldc§Pole Bus

§Neutral Bus

n The large smoothing reactor plays a major role in mitigating harmonic current flow

n DC filter tuned to characteristic harmonics connected on the line side of the smoothing reactor, between pole and neutral bus

n Neutral bus capacitor providing a return pass for the harmonic current through stray capacitances in the converter transformers to ground

DC filters

• The filter types used on the d.c. side are essentially the same as those used on the a.c. side

• Double-tuned 12/24 filter• Triple-tuned 12/24/36 filter

• Hybrid passive plus active filter

Double-tuned 12/24 filter

L2

L1

C1

C2 R2

Neutral bus

Pole bus

0 10 20 300.1

1

10

100

1 103

1 104

1 105

Harmonic number

Impe

danc

e (o

hms)

Triple-tuned filter

10 20 30 40 501

10

100

1 .103

1 .104

Harmonic orderIm

peda

nce

(ohm

s)

C3

C1

L2

L1

R2

R1

C2

L3

F1

F2

F3

Pole Bus

Neutral Bus

§Maximum use of high-voltage capacitor unit size

§Tuned to 3 harmonics for stringent Ieq requirement

Modeling of DC side for harmonic calculation,bipolar

§DC Filter

§DC Filter

§DC Filter

§DC Filter

§Ldc

§L3p

§C6p

§2L3

p§L

3p

§Cnb

§ETL §ETZ

§PT

§Re

§C6p

§PT: Pole Line Transmission

§ETL: Electrode Line Transmission,

§ETZ: Electrode Line Transmission

© ABB Group Slide 4809MP0163

3GG Jingzhou station

Harmonics and Filter in Line Commutated Converter (HVDC Classical)

AC-filters

DC-filters

§www.abb.com/hvdc