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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 WORDSHVDC 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 REFERENCES [1] ZHOUBaorong, JIN Xiaoming, WU Xiaochen, et al. 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