Reactive Power and Harmonic Filter - ABB
Reactive Power and Harmonic Filter - ABB
Reactive Power and Harmonic Filter - ABB
CONTENTS
AC Filters DC Filters
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
Classic
0,13
filter
converter
1,0
Id
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 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
priority restrictions are determined by limits in the reactive power compensation study for the different control modes.
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
q (=Q/PdN)
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)
AC bus
DC filter
~~
Harmonic Generation
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
i2
i1
T/4
T/2 3T/4
Phase current
i2
i1 +
i2
[%]
In I1
10 5 5 7 11 13 17 19 2 3 25
In i 2
[%] 5 11 13 23 25
Neglecting
i1 +
n Rectangular
12-pulse:
i1 + i2 =
2 6 I1 = Id
In =
I1 n
= 12k ? 1
11 23 35 47
k = 1, 2, 3, ...
13 25 37
49 In = Kn * I 1/n
In
= harmonic current
Kn=
= fundamental AC current
120 Amps
13
100
23
80 60 40 20 0
35
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 The characteristic harmonics will decrease with increasing commutation reactance
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 There is always some difference in the transformer reactance of each phase due to manufacturing tolerances The phase voltages are not exactly symmetrical, for example contain a small negative sequence component 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 system Negative sequence th th 5 and 7 distortion Transformer reactance Difference between Y/Y and Y/D Difference between phases Firing asymmetry
AC-side harmonics
3 th th (5 , 7 )
rd
DC-side harmonics
2 th 6
nd
5 ,7 odd All
th
th
6 even All
th
Converter ac harmonic currents as a function of direct current (Id nom = 1500 A) - non-characteristic harmonics
25
20
3 5
15
Amps
9 15
10
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Direct current (A)
The magnitude of non-characteristic harmonics is small comparing to the characteristic harmonics Most of them have a minor influence on the total harmonic distortion and filter design 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
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
Q 0,5
Classic
0,13
filter
converter unbalance
1,0
Id
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
AC Filter capacitors
Performance requirements
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 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 The requirements related to the a.c. bus voltage are commonly used disturbance criteria:
Individual harmonic voltage distortion Dn Total harmonic voltage distortion THD Telephone interference factor TIF (B.T.S. - EEI) 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%
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
Zmin =
Zmin
max min
UL Smax s.c.
2
Zmax =
UL
UL Smin s.c.
= harmonic number
Rad ius
AC filter types
4 1 10 Impedance (ohms)
L 1
Q 1 C1 = ( 1 ) 2 2 2 f1U n
3 1 10 100 10 1
R 1
L1 =
1 ( 2 f1n ) 2 C 1
R1 = 2f1n L1 / q
10
20
30
Harmonic number
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
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 0L variation (detuning)
q=
C 1
L 1
R 1
Q 1 C1 = ( 1 ) 2 f1U 2 n2 1 L1 = ( 2 f1n ) 2 C1
100
R1 = 2f1n L1 q
10 0 20 40 60
Harmonic number
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
3 1 10 Impedance (ohms)
L 1
100
C 2
L 2
R 2
10
10
15
Harmonic number
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
C1 L1 C2 L2 R1 R2
100
10
20
40 Harmonic number
60
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
C-type filter
Impedance (ohms)
C1 C2 R L
1 . 10
1 . 10
100
R=
L q C1
5 10 15 Harmonic number
20
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
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
Bus
C 1
C 1
L 1
L 1
C 2
L 2
C 2
L 2
Neutral Cnb
Bus
n n n
The large smoothing reactor plays a major role in mitigating harmonic current flow DC filter tuned to characteristic harmonics connected on the line side of the smoothing reactor, between pole and neutral bus 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
Pole bus
C 1
L 1
C 2
L 2
R 2
Neutral bus
0.1
10
20
30
Harmonic number
Triple-tuned filter
Pole Bus
1 .10
C1
R1 F1 L1
Impedance (ohms)
1 .10
100
10
C2 L2 F2 R2
10
20 30 Harmonic order
40
50
F3
L3
C3
Maximum use of high-voltage capacitor unit size Tuned to 3 harmonics for stringent Ieq requirement
Neutral Bus
C6p 2L3p
DC
Filter
DC
Filter
PT:
C6p L3p
Cnb
ETL
ETZ
DC
Filter
Re
DC
Filter
DC-filters
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