Power Systems – HVDC / Dipti Khare

Reactive Power Compensation and Harmonic Filters for HVDC Classic © ABB Group Slide 1 09MR0163

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 Q 0,5

0,13

Classic

filter

converter unbalance

1,0

Id

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

q (=Q/PdN)

0.8 0.6

3

0.4

2

1: qexchng 2: qdc 3: qf 4: qac(limit)

0.2 0

1

-0.2

4

-0.4 -0.6 -0.8 0.00

0.20

0.40

0.60

p (pu)

0.80

1.00

1.20

Main components of a converter station Transmission Converter line or cable station

Converter station

Smoothing reactor Converter

AC bus

DC filter

Shunt capacitors or other reactive equipment

AC filters Telecommunication Control system

~~

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 i1 i1

+

i2

T/4

i1

Y

Y

Y

D

i2

T/2 3T/4

Phase current i2

i1 +

i2

[%]

In I1

10 5 5

S

11 13

17 19

2 3 25

i2

n

§Neglecting

[%]

In

i1 +

7

5 11 13

23 25

the commutating reactance

n §Rectangular

pulses

Harmonic generation Characteristic Harmonic currents on AC side of a converter Y-Y 6-pulse: i1 =

2⋅ 3 1 1 1 I d (cos ωt − cos5ωt + cos7ωt − cos11ωt + L) π 5 7 11

Y-? 6-pulse: i2 =

2⋅ 3 1 1 1 I d (cosωt + cos5ωt − cos 7ωt − cos11ωt + L) π 5 7 11

12-pulse:

i1 + i2 =

4⋅ 3 1 1 1 I d (cosωt − cos11ωt + cos13ωt − cos 23ωt + L) π 11 13 23

Id: d.c. current The fundamental current The n:th harmonic

© ABB Group Slide 13 09MP0163

In =

2⋅ 6 I1 = Id π

I1 n

n = 11, 13, 23, 25L

Characteristic AC-side harmonics §n

= 12k ? 1

k = 1, 2, 3, ...

§11

13

§23

25

§35

37

§47

49 §In = Kn * I 1/n §In

= harmonic current

§Kn=

reduction factor due to overlap §I1

= fundamental AC current

Characteristic converter ac harmonic currents Converter ac harmonic currents as a function of direct current (Id nom = 1500 A) 200 180 160 140 11

Amps

120 13

100 23

80

35

60 40 20 0

Direct current (A)

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 th th 5 and 7 distortion

3 th th (5 , 7 )

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

5 ,7 odd

6 even

Firing asymmetry

All

All

rd

th

th

nd

2 th 6

th

Converter ac harmonic currents as a function of direct current (Id nom = 1500 A) - non-characteristic harmonics 25

20 3 5

15

9

Amps

15

10

5

0 100

200

300

400

500

600

700

800

900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Direct current (A)

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

U d (n ) 1§48 2etc 2 = C + D − 2CD cos( 2α + u ) U dio 2 C=

cos((n + 1) ⋅ u / 2) n +1

D=

cos(( n − 1) ⋅ u / 2) n −1

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

Why do we need filters? To compensate the reactive power consumption of the converter (classic) §

Q 0,5

0,13

Classic

filter

converter unbalance

§

© ABB Group Slide 26 09MP0163

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

1,0

Id

AC Filter capacitors

Performance requirements Ø

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

Converter harmonic current generation

AC filters C1 L1

AC network impedance

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) Zmax

X

2

Zmin =

UL

Zmin φmax

⋅ n

Smax s.c. 2

Zmax =

R

φmin

§UL

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