Download Design of AC Filter for HVDC Transmission Project

Design of AC Filter for HVDC Transmission Project Adopting Common
Station and Double-circuit on the Same Tower
HUA NG Ying, State Key Laboratory of HVDC, Electric Power Research Institute, China Southern Power Grid,
86-020-38120760, [email protected]
LI Xiao-li, State Key Laboratory of HVDC, Electric Power Research Institute, China Southern Power Grid,
86-020-38120766, [email protected]
XIN Qing ming, State Key Laboratory of HVDC, Electric Power Research Institute, China Southern Power Grid,
86-020-38120824, xinq [email protected]
XU Zheng, Depart ment of Electric Engineering, Zhejiang Un iversity, 86-0571-87952074,
[email protected]
ABSTRACT The HVDC transmission project adopting common station and double-circuit on the same tower
have two DC circuits operating independently and the same A C switch yard. The flexib le and various operation
modes put forward higher requirements for AC filter design. This paper analyzed the key issues of AC filter
design for HVDC trans mission project adopting common station and double-circuit on the same tower, such as
the DC operating modes, the computing method of maximal harmonic current, the system harmonic impedance
partition, filter types and parameter selecting, and AC filter switching strategy. The corresponding solutions are
approached. The results prove that the solutions are applicable in the HVDC t ransmission project adopting
common station and double-circu it on the same tower.
KEY WORDSHVDC TRANSM ISSION PROJECT, COMMON STATION AND DOUBLE-CIRCUIT ON THE SAM E
TOWER, AC FILTER DESIGN, OPERATION MODES, HARM ONIC IM PEDANCE
I. INTRODUCTION
AC filter design is a complex subject which is related to the system impedance scanning, DC reactive power control,
inverter harmonics calculation, the filter parameter tuning and setting selection. Its purpose is not only to consider the safety,
technical and economic factors, to allocate reasonable filter programs and develop appropriate switching strategy, but also to
meet the requirements of the system harmonic performance and device settings under a variety of operating conditions.
In recent years, HVDC transmission project has been widely used and in-depth study. AC filter design algorithms and
processes gradually become mature and have been verified by DC transmission projects. The calculation of the system
harmonic impedance and the impact on filter design is shown in [1]. It has been reported in [2] that harmonic impedance
equivalents have been partitioned in UHVDC transmission project from Xiangjiaba to Shanghai so as to relax the conditions
for filter design. A fast calculation of harmonic current which is applied for filter design in projects has been proposed in
references [3-4].In order to adjust and design filter parameter easily, a parameter tuning method that M ulti-tuned filter is
equivalent to multiple single-tuned filter has been described in [5]. DC Filter rating calculations-transients for
Yunnan-Guangdong ±800 kV DC Project is proposed in study on the key issues of AC filter design for ±800 kV DC
project [6].
±500kV DC power transmission project from right bank hydropower station in Xiluodu to Guangdong province is the
first±500kV DC power transmission project adopting common starting point and end point for both DC line, common
converter station and double circuits on the same tower[7] . The flexible scheduling and coordination put forward higher
requirements for AC filter design. As is shown in reference [8], based on the characteristics of the HVDC system, filter
design methods are analyzed and improved. In this paper, the key influencing factors of filter design for DC power
transmission project adopting common converter station and double circuits on the same tower are analyzed further. While
discussing how to choose the reasonable DC operating conditions, a variety of power distribution portfolio synthesis DC
double circuits harmonic current source. The reasonable operating modes, the maximal harmonic current combination picked
up among different power combinations, the partitioning process of the system harmonic impedance, filter types and
parameter selecting are discussed in detail. The corresponding solutions are approached. Calculation results show that the
solutions can be used for AC filer design of double line DC transmission project in a common station.
II. OVERVIEWAND FEATURES OF DOUBLE LINE DC TRANSM ISSION
Basic structure of DC transmission project adopting common converter station and double circuits on the same tower is
shown in Fig.1.
12-pulse unit
Line 1
Smoothing
Reactor
AC Bus in
Zhaotong
Station
Double DC
Line on
the Same Tower
AC Bus in
Conghua
Station
The
Conventer
The Earthed Pole
Line 2
DC
Filter
Reactive Power
Compensation
AC
Filter
Fig.1 The schematic of double Line and common converter station DC transmission project
Compared with single line DC transmission project,double Line and common converter station DC transmission
project gets lots of features, and two of them affect AC filter design most [7,9-10]:
1)Double line DC transmission project adopt common converter station, with coordination control, which DC field is
absolute.
2) Double line DC transmission project adoptcommon AC field, and AC filter needs to be controlled uniformly.
In feature 1, it means that power coordination control have two-dimensional selections, that is to say, under certain total
transmission power preconditions, double line transmission power combine in a variety of ways. In feature 2, it means that
reactive power exchange limit with the system is unchangedunder double line operating condition or single line operating
condition.
III. FUNDAM ENTAL OF AC FILTER DESIGN
A.
Performance calculating model
Performance calculating is used for examining the filtering effect of harmonics generated by inverter under different
operating conditions, ignoring the effect of system buses background harmonic. Performance calculating modelis shown in
Fig.2.
If
In
Un
Zf
Is
Zs
Fig. 2 S chematic of AC Filter Performance Calculation
B.
The steady state rating calculation model
Based on the performance calculation, the steady state rating calculation modelcalculates the maximum value of the
voltages and currents that may occur in the each element so as to examine whether the des igned filter parameters can be
manufactured in the actual process and to configure the appropriate arrester parameters based on the filter
economically.Calculation model is shown in Fig.3.
SW1
SW3
SW2
~
Ub
In
Zn
Zf
Fig. 3 S chematic of AC Filter S tatic Rating Calculation
IV. KEY ISSUES OF AC FILTER DESIGN ANALYZED
A.
Calculation condition selected
1.
DC operating modes combination
The operating modesof double line DC transmission project adopting common converter station are flexible, including
not only traditional single line operating mode, but also double line operating mode.On one hand, single line operating mode
includes:①Bipolar 100% voltage,②Bipolar 80% voltage,③Bipolar 70% voltage,④Unipolar metal 100% voltage,
⑤Unipolar metal 80% voltage,⑥Unipolar metal 70% voltage, ⑦Unipolar ground 100% voltage,⑧Unipolar ground80%
voltage,⑨Unipolar ground70% voltage. On the other hand, in addition to considering any combination of the nine modes
above, double line operating mode also need to consider a combination of bipolar asymmetric operation mode. Besides,
when AC filter designed, operating modeneed to considerthe maximum value and the minimum value of DC resistance and
the AC system voltage level of both ends(550kV、525kV、500kV、475kV)[14].
2.
DC power combination
The issue of power combination exists in the DC transmission project adopting common converter station and double
circuitsis that conveying the specific capacity of power between the double line DC power distribution in a certain operating
mode. For example, when double line DC transmits 3200M W active power in the bipolar 100% voltage mode, total power of
double line DC being the same with single line DC, there are a variety of combinations for double line DC transmission,
such as 10%+90%,15+85%,20%+80%,…,50%+50%.
Different combinationmay lead to different DC transmission losses and different reactive power consumption at both
converter stations. However, the reactive power exchange range between converter station and AC system is usually
determined. So, in a certain operating mode, when double line DC transmits specific capacity active power, the reactive
power compensation capacity needed will change in a large context. What’s more, switching strategies of AC filter and
checking the reactive power exchange range will become more complex. Calculation of operating conditions will increase
for power combinations.
B.
The formation of the harmonic current source in double line DC
The calculation of harmonic current is to provide various harmonic current source data in various operating conditions
and load levels. The harmonic voltages on the entrance of the DC line are assumed to be zero. Based on this characteristic,
double line DC transmission project adopting common tower is equivalent to two conventional single line DC transmission
project; Each DC line traverses all possible operating modes above by scanning power, considering the level of the DCpower
gradually increase to a certain value from the minimum level. Usually, the minimum power is 10% of the rated power, the
step is 5%. When double line DC is running in the 100% voltage operating mode, it will take the 120% overload into account.
When double line DC is running in the 70% or 80% voltage operating mode, it will take the 90% overload into
account.Harmonic current of each level is calculated step by step. Finally, two different operating modes compose to a new
one. As helical axis shown in Fig.4, the total power level gradually increase from the minimum. The orthogonal lines
perpendicular to the helical axis Ptotal points {P1, P2} is thepointofany combinationwith a certain totalpower of double line
DC. In order to retain a certain margin, given total power level in the corresponding operating mode, the largest
incompatibility current ofvarious power distribution portfolio was taken as harmonic current source for rating calculation of
the filter performance.The minimum consumption of reactive power will be taken to check filter switching strategy whether
to cause reactive power beyond the limit:
I Ptotal  n
Qd  Ptotal 









 P1  n  P2  n  

Ptotal  P1  P2 
max
I
I




Ptotal  P1  P2 
min
(1)

Qd  P1  Qd  P2  
(2)

Where P total stands for total transmission power; P1 stands for transmission power of Line 1; P2stands for transmission power
of Line 1; n stands for harmonic frequency ; Qd stands for consumption of reactive power.
P2
1.2
Ptotal
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
P1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Fig. 4Power combination of two circuits
Based on the above ideas,the two-dimensional problem of double line DC power combination is equivalent to a
one-dimensional problem. So, the power point needed for rating calculation of the filter performance reduce greatly.
For the following two situations, taking dual tuner AC filter for example, the results of filter rating calculation are
shown in Tab.1:
1)The largest incompatibility current of various power distribution portfolio was taken as harmonic current source for
rating calculation of the filter performance. This is called one-dimensional calculation method.
2) Take currentcorresponded to a variety of power distribution so as to find out rating calculation ofAC filter with
various power combinations.
The results in Table 1 show that the error of two results does not exceed 5%. That is to say, the solution that
two-dimensional problem is equivalent to a one-dimensional one is reasonablewhich has little effect on the rating calculation
of AC filter.
Tab.1 The rating of AC filter component
C.
The Component Rating
two-dimensional method
one-dimensional method
error
M ax. total voltage stress of C1 (kV)
519
523
0.8%
M ax. total current stress of L1 (A)
748
773
3.2%
M ax. total voltage stress of C2 (kV)
96
100
4%
M ax. total current stress of L1 (A)
1814
1878
3.4%
System harmonic impedance partition
The system harmonic impedance of the system is the basis for the AC filter design conditions from the converter station
AC bus to DC transmission system. System operating mode or changes in the structure of the network, has a very significant
impact on the system harmonic impedance[16-17].However, filter design needs to meet the performance indexof all the
operating conditions. So, generally all reasonable harmonic impedance points are packagedby the “envelope method”. Based
on the amplitude and frequency of the actual harmonic impedance, phase-frequency characteristic differences, fan-shaped
impedance diagram is used for being equivalent to low-order harmonic impedance. Round impedance diagram is used for
being equivalent to high-order harmonic impedance[13,16] , shown as in Fig.5-(a) and Fig.5-(b).
X
Z max
 max
O
 max
O
 min
Rmin
R
r
Rmax
 min
R
Z min
(a)Harmonic Impedance Sector
of the System
(b)Harmonic Impedance Circle
of the System
Fig. 5The envelope of system harmonic impedance
In this section, taking the 2nd, 3rd, 7th, 11th, 13th harmonic impedance for example, the harmonic impedance has impact
on the AC filter design. The greater system impedance phase angle is, the greater the voltage distortion caused by resonance
between AC filter and system will be, the harder filter performance will be to meet code requirements. At the same time, it
cause steady state rating of the components to be too large, resulting in equipment selection and arrester difficult to configure.
Table 2 shows the partition of 2nd harmonic impedance. Fig.6 shows the partition envelope of 2 nd harmonic impedance.
According to the round impedance diagram, AC filter performance results are shown in Table 3.
Harmonic
impendence circle
Original harmonic
impedance circle
Partition 1
Tab.2 The partition of 2nd harmonic impedance
M inimum impedance
M aximum impedance
M inimum
magnitude
magnitude
impedance angle
(p.u.)
(p.u.)
(p.u.)
M aximum
impedance angle
(p.u.)
0.0055
0.053
72.518
87.811
0.0055
0.014
81.802
87.811
Partition 2
0.015
0.022
78.083
84.826
Partition 3
0.023
0.053
72.518
81.806
Fig.6 The partition envelope of 2nd harmonic impedance
Tab.3 The results of 2nd harmonic distortion
Harmonic impedance circle
2nd Thd(%)
Original harmonic impedance circle
1.465
Partition 1
0.532
Partition 2
0.539
Partition 3
0.532
M aximum value for all partitions
0.539
Preliminary calculation shows that if impedance circle is combined, it will make filter design conditions too harsh and
difficult to design. However, partition can reduce the difficulty of the harmonic performance calculation results. So, it is
necessary to make reasonable partition of the harmonic impedance when designing AC filter.
D.
Filter type and parameter selection
1. Filter type selection
Double tuned filter can be equivalent to two parallel single tuned filters. The harmonics of two different frequencies can
be eliminated in this way. Besides, only one of the resonant circuit needs to withstand the full impact of impulse voltage.
Because double tuned filter has much advantage such as low loss, small footprint, low investment, easy to spare and
maintenance, it has been one of the most common forms in DC project [18,19] . It is necessary to consider the system condition
and reference to past experience in engineering design so as to filter out characteristic harmonic below 37 th. If the low order
harmonic current is too large and the harmonic impedance is not reasonable, C-type damping filter need to be configured [20]
(High pass filter, HP3) so as to filter out high order harmonic.In order to meet the requirements of reactive power
compensation, the parallel capacitor need to be configured. General Structure of filtersare shown in Fig.7.
C1
C1
L1
C1
C2
R1
L1
R1
R3
C2
L2
R2
（a）
L1
R1
（b）
（c）
Fig. 7 General S tructure of filters
2. Filter tuning point selection
Filter tuning point is proposed to be DT11/24、DT13/36, which is different from the double tuned filter type
configuration(DT11/13、DT24/36) in the early DC projects. The largest characteristic harmonics (11 th, 13th) can be filtered
out separately. Besides, the stress of filter elements and the effects caused by detuning can be reduced[6]. Because the
negative angle of system harmonic impedance (11 th, 13th) is rather large, optimal tuned frequency is considered in order to
avoid system resonance zooming harmonics. Filter becomes capacitive with tuning frequency, but the frequency should not
be too large in order to avoid poor filtering performance.
If the 2nd harmonic impedance angle is too large, and the filter becomes capacitive with low frequency, the filter will
produce resonant easily. So, the tuning frequency of HP3 should be as close as possible to 2 times on the basis of that the 3 rd
harmonic does not affect filtered off.
The shunt capacitor in series with a reactance tunes to 48th in order to filter out high order characteristic harmonic and
improve the filter performance.
3. Filter resistance selection
To meet the requirements such as wide range of filter, detuning low sensitivity, high-frequency damping, filters often
need to be added on a shunt resistor in the appropriate location. The basic principles of selection are as follow:
First, do not change impedance frequency characteristics near the tuning pointoverly, otherwise it will lead to
performance exceeded.
Second, the loss can’t be too large, to minimize the operational or manufacturing cost.
Usually, there are three installation locations for parallel resistance of the doubletuned filter.R1, R2 or R3 has different
impacts on thefilter impedance frequency characteristics [22] , shown in Fig.7:
1) By adding R1, filter high frequency impedance characteristics and the harmonic form factor indicators can be
improved.
2) By adding R2, the filter element stress of the parallel circuit can be reduced.
3) By adding R3, can combine the above two kinds of function, but the loss is large and if the resistance value is too
small may affect the filtering effect near the filter tuning frequency.
Generally, resistance is small, the effect more obvious, but if the resistance is too small, the current stress increases and
increase the loss, which may affect the performance of filter.
At low load operation of bipolar H VDC project, due to the parallel capacitor filtering high-order harmonic can not
input, in order to meet the telephone harmonic waveform factor requirements, the shunt resistance of double tuned filter
position generally choose R1.
4.The switching strategy optimization
Accordingto the performance and rating requirements, different load levels to determine the various operating mode to
filter combination switch, usually need to satisfy the system reactive power exchange range:
Qd  Ptotal   Qacdc  Q ftotal  Qd  Ptotal   Qdcac
Where
Q ftotal is the operation of filter is provided which the idle work capacity, Qdcac
the AC system without power, maximum reactive power
Qacdc
(3)
the maximum absorption of
communication system provides, for DC power
transmission project double-circuit HVDC, the same site, by formula (2) determination
If 2, 3 harmonic impedance angle is larger, the possibility of low order resonance, it is suggested that as early as
possible into HP3. Usually at low load and (3) may no longer meet the need to be taken, such as increasing the trigger angle,
input low voltage reactor and other related measures to achieve the balance reactive power.
A preliminary set the filter switching strategy for filter combination, different load levels in various working conditions
to rating calculation, check the configuration scheme meets t he requirements of performance and rating limits, if does not
meet the requirements will continue to adjust and optimize the parameters and the switching strategy, until meet index date.
V. CONCLUSION
This paper analyzes the influence of key problems of the filter design of double-circuit HVDC converter station, the
same address, for double-circuit DC operation mode, harmonic current, harmonic impedance calculation, filter type and
parameter selection are proposed solution:
a)
The reasonable selection of the DC operating conditions, various power combining the maximum incompatible current
as the filter rating calculation value of harmonic source, the power of two-dimensional problem reduction to the
one-dimensional problem, can avoid scanning running all the way and power point in order to reduce the calculation
amount of unnecessary.
b)
System harmonic impedance has important influence on the filter design, system design conditions for the partition to
choose reasonable when necessary;
c)
According to the system of basic data, can be further optimized structure, select filter tuning frequency and damping
resistor;
d)
M ake switching strategy is an important part of filter design, need to be adjusted and optimized continuously
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