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01 August 2017, Volume 38 Issue 8
    

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  • Editor's Letter on the "Special Issue on Multi-Terminal Direct Current Transmission & High Voltage Direct Current Grid”
    HAN Minxiao
    Electric Power Construction. 2017, 38(8): 1.
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  • Current-Breaking Test Method Based on LC Source for Hybrid HVDC Circuit Breaker
    DING Xiao, TANG Guangfu,HAN Minxiao,GAO Chong, WANG Gaoyong
    Electric Power Construction. 2017, 38(8): 2. https://doi.org/10.3969/j.issn.1000-7229.2017.08.001
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    ABSTRACT：  Current-breaking test is the core part of the operation tests for DC circuit breaker, whose effectiveness is directly related to the verification of the electrical performance of DC circuit breakers. This paper studies the current-breaking test method of hybrid HVDC circuit breaker for MMC-HVDC system, aiming to present the current-breaking test method and  test parameters for engineering feasibility. Firstly, based on the analysis of the principle and working condition of hybrid  HVDC circuit breaker, we reveal the its stress characteristics； then, on the basis of stress extraction, we present the LC source test circuit and parameters； finally, we verify the accuracy of the simulation circuit and test method through the short-circuit current-breaking test for a 200 kV circuit breaker, whose results can provide reference for the current-breaking test design of hybrid HVDC circuit breaker.
  • Hybrid HVDC Circuit Breaker Operation Test for VSC-HVDC
    CAI Licun,CHANG Zhongting,ZHANG Kun,XU Tao
    Electric Power Construction. 2017, 38(8): 10. https://doi.org/10.3969/j.issn.1000-7229.2017.08.002
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    ABSTRACT： Hybrid high voltage DC （HVDC） circuit breaker is one of the core equipment of DC power grid. In order to test and verify the rationality and validity of DC circuit breaker design, and accurately reflect the electrical, thermal and mechanical performance of DC circuit breaker, it is needed to study the operation test of HVDC circuit breaker. Taking the test standards of AC circuit breaker, converter valve and other similar products as reference, this paper studies the operation test item of HVDC circuit breaker. Firstly, we introduce the working principle of the test circuit used in operation test, and analyze the working conditions of related equipment at various moments. Then, we successfully complete a 83 kV prototype sample operation test, which can provide reference for HVDC circuit breaker with higher voltage level in the future.
  • DC System Impedance Scanning for MTDC
    CUI Kangsheng, WEN Jun, ZHAO Xiaobin, LI Rong, YAN Han, DUAN Ruimin
    Electric Power Construction. 2017, 38(8): 17. https://doi.org/10.3969/j.issn.1000-7229.2017.08.003
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    ABSTRACT：  DC system impedance scanning is one of the important means of harmonic suppression research and DC filter design on multi-terminal direct current （MTDC） transmission. This paper presents a DC system impedance scanning method for MTDC, in which the error from converter generated harmonic is excluded by improving impedance formula. Influence of various factors on the impedance scanning, such as amplitude, phase and frequency distribution of the injected harmonic source, is also studied. PSCAD/EMTDC simulation result shows that the presented method is effective and reliable for DC system impedance scanning of MTDC, which has reference value of DC engineering.
  • Fault Location Technique for Multi-Terminal VSC-HVDC System 
    ZHANG Ming, HE Jinghan, ZHANG Yizhi, LUO Guomin
    Electric Power Construction. 2017, 38(8): 24. https://doi.org/10.3969/j.issn.1000-7229.2017.08.004
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    ABSTRACT：  The fault location technique of multi-terminal flexible DC （MTDC） grid is one of the crucial techniques for the safe and reliable operation of DC power grid. This paper takes the MTDC grid based on two-level voltage source converter （VSC） as research object, and adopts the idea of “first predetermined section then position” to study its fault location technique. Under the impact of fault loop and big DC capacitor, the DC capacitor voltage change rate （ du/dt） at converter outlet is different in fault region or non-fault region. Based on this characteristics, we can determine fault region by comparing the   du/dt at VSC outlet. After the determination of fault region, we can use the discharge circuit formed by the short-circuit fault to derive the calculation formula of the fault position, and calculate the precise fault position by using recorded data of transient voltage of VSC DC capacitor. We build VSC-HVDC system in MATLAB/Simulink, simulate and verify the proposed method. At the same time, we compare the proposed method with the existing methods to verify the feasibility and effectiveness of the proposed method.
  • Analysis and Comparison of DC Fault Handling Strategies for Hybrid HVDC System
    LU Yi, LI Jihong, QIU Peng, XIAO Huangqing, LIU Gaoren, XU Zheng
    Electric Power Construction. 2017, 38(8): 33. https://doi.org/10.3969/j.issn.1000-7229.2017.08.005
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    ABSTRACT： The hybrid high voltage direct current （hybrid HVDC） transmission system based on line commutated converter （LCC） and modular multilevel converter （MMC） is a feasible solution for long distance bulk power transmission. In order to handle the DC short-circuit fault, two kinds of methods can be adopted. The first one is to use the sub-modules （SMs） which have the capability of DC-fault clearance in inverter side converter, such as full-bridge SMs and clamp-double SMs. The second one is to arrange high-power diode at the MMC DC port to cut off the current flow path after the fault. Through analyzing the physical mechanism and control flow of different DC fault handling strategies, this paper studies its feasibility and applicability. The typical testing system is built in PSCAD/EMTDC, the system response under DC fault is investigated, and the system transient characteristics under different strategies are compared. In addition, the non-block DC-fault-handling strategy based on full-bridge SMs is simulated and verified. The results show that this kind of solution is not suitable for the bipolar HVDC system and the DC fault current can not be blocked thoroughly.
  •  Suppression Method of DC Magnetic Bias Based on Multi-DC Power Support
     
    LONG Bing, LIU Yuanqi, TANG Huaishou, HE Yingfa, XIE Zhicheng
    Electric Power Construction. 2017, 38(8): 42. https://doi.org/10.3969/j.issn.1000-7229.2017.08.006
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     ABSTRACT： The conventional research of suppressing DC magnetic bias mainly focus on the development of DC magnetic bias blocking device, which cannot solve the DC magnetic bias fundamentally. This paper proposes a suppression method of DC magnetic bias based on multi-DC power support. Firstly, based on PSCAD/EMTDC, we establish a DC current distribution simulation model of transformer neutral point in Yichang power grid. Secondly, from the perspective of coordination running and power support between DC lines in the same transmission section, the transmission power of HVDC is reducd, which is operating in monopole-ground return mode, and the reduction of transmission power will be transferred by other HVDC projects in the same transmission section, to actively generate reverse unbalanced grounding current to neutralize the DC current in monopole-ground return line, which can reduce the amplitude of DC current at transformer neutral point in AC grid. Finally, the generation output of sending-end unit is optimized by genetic algorithm and then the static and transient stability of the sending-end grid after power transfer are verified based on PSASP simulation system.
     
  •  DC Fault Protection of VSC-HVDC Grid Based on Hybrid MMC and DC Switch
     
    ZHANG Junjie, XIANG Wang, LIN Weixing, WEN Jinyu
    Electric Power Construction. 2017, 38(8): 52. https://doi.org/10.3969/j.issn.1000-7229.2017.08.007
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     ABSTRACT： To transmit the bulk renewable power using overhead lines, this paper proposes a VSC-HVDC grid transmission system based on hybrid modular multilevel converter （hybrid MMC） and DC switch. According to the hybrid MMC composed of half bridge sub-modules and full bridge sub-modules, this paper analyzes its topology, basic operating principle and DC voltage operating region, proposes a control framework embedding three-channel control freedom for hybrid MMC, analyzes the DC fault ride through control strategy in detail, and then designs the fault clearing method and multiple restart sequence of the VSC-HVDC DC grid based on hybrid MMC. During the DC fault, hybrid MMC can control the fault current to be zero without tripping the IGBTs. Therefore, the DC grid remains continuous operation to provide reactive power to the AC system. During post-fault, if the system fails to restart after three restart attempts, the DC switches installed at the overhead lines will be tripped to isolate the fault path at zero DC current. After fault clearance, as the change of network topology, the power flow will be re-distributed. At last, the feasibility of the fault clearing strategy and restart sequence is validated in PSCAD/EMTDC.
     
  •  Comprehensive Coordination Control Strategy for VSC-MTDC System
     
    DENG Qi，ZHANG Yingmin，ZENG Qi，ZHANG Likui，LI Zhihan
    Electric Power Construction. 2017, 38(8): 59. https://doi.org/10.3969/j.issn.1000-7229.2017.08.008
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     ABSTRACT： This paper proposes a comprehensive coordinated control strategy, which is aimed at the different degrees of unbalance power in DC network. The margin droop control strategy, which is considering both the voltage droop on DC line resistance and the power margin of converter station in the design of operating characteristic curve of droop control, is introduced in the active-power controller, to ensure the stability of DC voltage when the unbalanced power is too large or the converter station out of operation. Transient simulation and analysis by establishing a simulation model of the five-terminal flexible DC transmission system in PSCAD/EMTDC show that not only the proposed control strategy can optimize the power flow when there is a large unbalance power in the DC network, but also the DC voltage has a strong rigidity, and the stability of the VSC-MTDC （voltage source converter multi-terminal direct current） system also has been improved.
     
  • Coordinated Control Strategy of VSC-MTDC System Under Different Power Dispatching Modes
     
    DENG Qi, ZHANG Yingmin, LI Zhihan
    Electric Power Construction. 2017, 38(8): 67. https://doi.org/10.3969/j.issn.1000-7229.2017.08.009
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     ABSTRACT：  Aiming at the real-time power flow distribution requirement of DC power system in three kinds of dispatching modes, this paper designs a coordinated control strategy for  voltage source converter multi-terminal direct current （VSC-MTDC）  system, which can change the value of droop coefficient and switch control mode to meet the DC power flow distribution. In order to avoid frequent switching of the control mode, a hysteresis comparator is added to the controller of the receiving end converter station. In the end, this paper constructs a four-terminal VSC-MTDC mode in PSCAD/EMTDC, and simulates the DC grid in different scheduling modes. The results show that, compared with the traditional control method with constant droop coefficient, this coordinated control strategy can meet the real-time power flow distribution requirements of the power system, and the transmission flexibility of the DC power grid is improved.
     
  •  Fault Characteristics and Control & Protection Strategy of Three-Terminal LCC-MMC Hybrid HVDC Transmission System 
     
    HONG Chao, SHI Bonian, SUN Gang, ZHANG Ye, YANG Jian, LIU Bin
    Electric Power Construction. 2017, 38(8): 73. https://doi.org/10.3969/j.issn.1000-7229.2017.08.010
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     ABSTRACT：  This paper studies the AC/DC fault characteristics and control & protection strategies of the three-terminal hybrid DC transmission system with line commutated converter （LCC）-modular multilevel converter （MMC） converters. Based on the analysis of the existing fault crossing control strategy, the coordination strategy of the minimum triggering angle control, the maximum modulation ratio of the MMC and the DC voltage deviation control of the inverter side LCC is proposed for the AC side fault. In order to solve the DC fault, the DC line fault current transient variable can be used to identify the fault location and break the fault by the DC circuit breaker by arranging the boundary condition of the current limiting reactor at both ends of the DC link, which can quickly isolate the DC link fault and narrow the scope of the impact of failure. Finally, a hybrid DC transmission system model is established in PSCAD/EMTDC, and the feasibility of the proposed strategy is verified by simulation. The results show that the proposed control strategy can improve the transmission power of the DC system and reduce the probability of the power transmission interruption when the AC fault condition is rectified. The DC current mutation rate protection scheme based on the DC circuit breaker can quickly isolate the fault, and improve power supply reliability.
     
  •  Compensation Droop Control of MMC-MTDC Considering Line Voltage Drop
     
    DONG Hui,SONG Pinggang, ZHOU Zhenbang
    Electric Power Construction. 2017, 38(8): 80. https://doi.org/10.3969/j.issn.1000-7229.2017.08.011
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     ABSTRACT：   The traditional error control and droop control cannot achieve the floating control of power and voltage in MMC-MTDC （modular multilevel converter  based multi-terminal DC transmission systems）. In order to make the system achieve the optimal operating state, this paper proposes the droop compensation control strategy. Firstly, we analyze the traditional control methods with fully considering the voltage drop caused by line impedance in the traditional control mode. Then, we compensate the voltage reference instruction of the droop curve for the voltage offset caused by the power deviation of the system. Finally, we construct a five terminal simulation model on PSCAD/EMTDC, and compare three control methods under the two conditions of steady state and transient state. The results show that the static deviation of power and voltage can be eliminated by changing the power of the system, and the voltage will not overshoot, which can ensure that the system is always running in the optimal state.
     
  • Startup-Stop Control Strategy and Dynamic Simulation Experiments of Multi-Terminal Hybrid HVDC System Based on LCC-VSC
     
    HU Jingen, JIA Xuantao, WANG Yao, WANG Ruyu, GAO Shilong, CAO Litan
    Electric Power Construction. 2017, 38(8): 86. https://doi.org/10.3969/j.issn.1000-7229.2017.08.012
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     ABSTRACT： In order to research the basic control strategy of line commutated converter （LCC）-voltage source converter （VSC） multi-terminal hybrid HVDC system, this paper sets up the dynamic simulation experimental platform of LCC-VSC three-terminal bipole hybrid HVDC, and studies the start-stop control strategy in different operation modes of hybrid HVDC system including LCC （rectifier）-VSC （inverter）, VSC （rectifier）-LCC （inverter） and VSC （rectifier or inverter） connected to LCC HVDC system, as well as the effect of multi-terminal hybrid HVDC when one terminal is deblocking or blocking online. Based on the experimental result, the main control functions and characteristics of the hybrid HVDC system are analyzed. The results show that the proposed control strategy can achieve stable start-stop and on-line switching of the hybrid HVDC system.
     
  •  Start-Up Strategy for LCC-LCC and FBMMC Series Based Hybrid HVDC System
     
    LU Yi, ZHAO Jian, QIU Peng, XUAN Jiazhuo, GUO Chunyi, LIU Wei
    Electric Power Construction. 2017, 38(8): 95. https://doi.org/10.3969/j.issn.1000-7229.2017.08.013
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     ABSTRACT：  This paper proposes a series hybrid HVDC system, whose rectifier side adopts the line commutated converter （LCC）,and the inverter side is composed by LCC in series with the full bridge submodule based modular multilevel converter （FBMMC）. Firstly, we construct the mathematical model of the hybrid HVDC system, design the corresponding coordinated control strategy to ensure the safe and stable start-up of the system, and proposes a three step start-up procedure for the hybrid HVDC system with LCC in rectifier side and LCC in series with FBMMC in inverter side （LCC-LCC+FBMMC）. Phase 1, both the LCC and FBMMC are locked, and the DC side voltage charging process of FBMMC is uncontrolled with a current limit resistor in series. Phase 2, the current limit resistor is in bypass and FBMMC in inverter side is unlocked； the rated DC voltage of FBMMC is gradually built with the DC voltage controller. Phase 3, both the LCCs in the two sides are unlocked, and the rated DC current and DC voltage are gradually built by the DC current controller in rectifier side and DC voltage controller in inverter side. So far, the start-up process is completed. Finally we establish the simulation model for LCC-LCC+FBMMC based hybrid HVDC system in the PSCAD/EMTDC, and verify the effectiveness of the proposed start-up strategy for hybrid HVDC system.
     
  •  Measures for Continuous Commutation Failures of Multi-Infeed HVDC System in Zhejiang Power Grid
     
    FU Junbo，LING Weijia，DENG Hui，WANG Xinbao，HUA Wen，YU Qiuyang
    Electric Power Construction. 2017, 38(8): 102. https://doi.org/10.3969/j.issn.1000-7229.2017.08.014
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     ABSTRACT：   Aiming at the multi-infeed HVDC continuous commutation failure problem caused by AC system fault in Zhejiang power grid, based on the essence of commutation failures, this paper establishes a typical systematic model to verify the effects of large capacity static var generator （SVG） and synchronous condenser on suppressing the commutation failure. At the same time, this paper adopts voltage-time area method to optimize DC control and protect system, in order to reduce the probability of DC continuous commutation failure. This method can control extinction angle to reduce the continuous commutation failure of multi-infeed HVDC during the grounding fault in AC system. At last, this paper verifies the improvement of voltage-time area method through system test.
     
  •  Power Coordinated Control Strategy for AC-Side Failure Ride-Through of VSC-MTDC System
     
    SHAO Bingbing, HAN Minxiao, GUO Shuying, MENG Zhaojun
    Electric Power Construction. 2017, 38(8): 109. https://doi.org/10.3969/j.issn.1000-7229.2017.08.015
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     ABSTRACT：   When AC-side failures of voltage source converter multi-terminal DC （VSC-MTDC） system leads to power grid voltage sag, this paper designs the coordinated control of the active and reactive power of each converter during the fault to maintain the voltage level of related side point of common coupling （PCC）. In the duration of the fault, the converter near the fault is switched from preference of active current control to preference of reactive current control and generates reactive power depend on the grid voltage sag. When added reactive power does not cause over-current on the AC side, the extra current capacity is used to maintain the active level before the fault. When added reactive power causes over-current on the AC side, this paper proposes a control strategy to avoid the over-current by changing the active power of the converter related to fault side. Meanwhile, this paper comes up with the control strategy based on the droop control to convert the power into control signals that are instructed by input voltage. With the changing of the active power of the converter near the fault, the active power is out of balance and the DC capacitors rises or falls due to the charge or discharge. At the meantime, other converters adjust their own active power instructions without the communication between stations automatically according to the changing of the DC voltage, which can attain the goal of self-adjustment and power automatic-distribution and realize autonomous decentralized control. In the end, this paper constructs the simulation model of the VSC-MTDC system in the PSCAD/EMTDC to verify the effectiveness of the proposed control strategy. 
     
  •  Impedance-Frequency Characteristic of MMC-MTDC System
     
    TIAN Yifan, XU Dong, HAN Minxiao, ZHENG Chao
    Electric Power Construction. 2017, 38(8): 118. https://doi.org/10.3969/j.issn.1000-7229.2017.08.016
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     ABSTRACT：  This paper investigates the frequency response characteristics of modular multilevel converter-multi-terminal direct current （MMC-MTDC） system under AC background harmonics. Firstly, we analyze the frequency-impedance characteristics of MMC-MTDC system to illustrate the reason of the DC network resonance caused by AC background harmonics. Secondly, we analyze the relation between the inductance value of smoothing reactor and the resonant frequency of MMC-MTDC system, which can be used as the basis for the reasonable selection of smoothing reactor to avoid the resonance of the DC network at AC harmonic complementary frequency. Finally, the analytical results and the relationship between resonant frequency of DC network and smoothing reactor are verified by the simulation model of a 4-terminal MMC-MTDC in PSCAD. The simulation results validate the correctness the theoretical analysis, and confirm that the resonant frequency of DC network can be changed by adjusting the inductance value of smoothing reactor, in order to avoid the resonance of the DC network caused by AC harmonic complementary frequency.
     
  •  A Small-Signal Modeling Method for AC/DC System Based on Transfer Matrix
     
    ZHANG Lidong, CAO Xin, HAN Minxiao
    Electric Power Construction. 2017, 38(8): 123. https://doi.org/10.3969/j.issn.1000-7229.2017.08.017
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     ABSTRACT： With the broadly application of high power electronic converters in power system, modeling and stability analysis of power system are becoming ever more challenging. Firstly, this paper proposes a small-signal modeling method for AC/DC power system based on transfer matrix concept, whose characteristic is explicitly defining a controlled object of a power system as the so-called Jacobian transfer matrix （JTM）. Secondly, this paper establishes a power system with an AC voltage source connected with a voltage source converter （VSC）. By applying JTM modeling method, it can be found that JTM is appropriate for low frequency stability of power system. In addition, with the increase of load angle, the zero of JTM moves to origin, and the trajectory corresponds to the power angle and voltage stability boundary in phasor method. Then, to study power system resonance, JTM modeling method is used to build a small-signal model of system. The resonance pole is provided with virtual damping by a high-pass filter HHP（s）. The result shows that, with the increased current control gain of HHP（s）, the resonance poles of JTM are shifted to the left half-plane and the system stability is improved. Meanwhile, the damping effect is applicable to any resonance frequency of AC system.
     
  •  MMC-HVDC Operation Under Unbalanced Grid Conditions and Its Protection Control
     
    DONG Wei1，HUANG Jingsheng1，SHI Enze2，WU Xuezhi2，JING Long3
    Electric Power Construction. 2017, 38(8): 129. https://doi.org/10.3969/j.issn.1000-7229.2017.08.018
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     ABSTRACT：  AC system imbalance will cause the fluctuation on output power and the imbalance of output current in modular multilevel converter （MMC）, and also lead the bridge arm current and sub module voltage to be greater volatility in modular multilevel converter-high voltage direct current transmission （MMC-HVDC）, which may endanger the security operation of the converter. This paper analyzes the influence of AC system unbalance and control parameters on the bridge arm current of MMC and sub module voltage, and constructs a 3-D function among unbalance component, control parameters, bridge arm current and sub module voltage. Then, through the first fitting method, we obtain the control parameter that makes the bridge arm current minimum in different situation and ensures the safety and stability of MMC at utmost. Finally, we establish the control system of MMC on stationary coordinate and verify the correctness of the theoretical analysis through simulation.
     
  •  Influence of Large Capacity VSC-HVDC Transmission System on Transient Stability of Grid
     
    LIN Zhangsui
    Electric Power Construction. 2017, 38(8): 136. https://doi.org/10.3969/j.issn.1000-7229.2017.08.019
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     ABSTRACT：  Based on the voltage source converter-high voltage direct current （VSC-HVDC） demonstration project in Xiamen, this paper analyzes the influence mechanism of large capacity VSC-HVDC transmission system on transient stability in the AC grid, and uses PSD-BPA simulation tools to analyze the transient stability of power grid after large capacity VSC-HVDC transmission system access. The results show that there is no significant effect of VSC-HVDC on the transient stability of AC grid under single fault, while the active power of VSC-HVDC and the short circuit ratio （SCR） both have effects on the transient voltages. The system voltage stabilization level can be improved by increasing power transmission of VSC-HVDC to AC grid, and the smaller the short-circuit ratio is, the more obvious the effect is. In view of the safety and stability of AC/DC hybrid power grid, corresponding measures and suggestions are put forward.
     
Administrator：State Grid Corporation of China
Sponsors：
State Grid Economic and Technological Research Institute Co. Ltd
China Power Engineering Consulting Group Co., Ltd
China Electric Power Research Institute
Editor in Chief: Zhang Quan
Associate Editor-in-Chief:KANG Chongqing, BI Tianshu, LI Hui
Executive Editor-in-Chief:YANG Weihong, SHENG Yi
Editorial Director:WEI Xihui
CN: 11-2583/TM
ISSN: 1000-7229
Address: Tower A225, SGCC, Future Science & Technology Park, Beijing 102209, China
Post Code: 102209
Editor’s Tel: 86-10-66602697
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E-mail：dljs@263.net
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Post Issue Number: 82-679
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