Steady‐state stability of shaft torsional oscillation in an ac‐dc interconnected system with self‐commutated converters

Recent progress in power electronics technology makes it possible to consider applying self‐commutated converters using gate turn‐off thyristors (GTOs) to HVDC transmission systems. Since the self‐commutated converter can be operated stably without depending on ac‐side voltage, the magnitude and the phase angle of the converter output voltage can be controlled independently. Therefore, this type of converter will improve voltage stability at its ac side. On the other hand, shaft torsional oscillation of a thermal power plant caused by the interaction between the shaft‐generator system and the control system of the self‐commutated converter is still an open problem. In this paper, a linearized model for eigenvalue analysis of a power system, including HVDC interconnection with self‐commutated converters, is described to analyze the effect of the self‐commutated converter on the shaft torsional oscillation of a thermal power plant. Then, numerical results from the eigenvalue analysis of the shaft torsional oscillation are presented. Results obtained by the frequency response method are also reported. The numerical results make it clear that parameter regions of DC‐AVR and ACR control systems of self‐commutated converters exist where the shaft torsional oscillation may be caused. © 1999 Scripta Technica, Electr Eng Jpn, 128(4): 25–37, 1999     

柔直电网站内单相接地故障分析及过电压抑制

柔直电网换流站内单相接地故障的电磁暂态过程复杂且易产生过电压,严重影响系统的安全稳定运行。文中将换流站内单相接地故障分成交流故障区和直流故障区,首先采用复合序网络定量分析交流侧过电压机理及零序电压特征。然后,基于换流器控制器方程推导得到故障序分量从交流侧向直流侧传播的规律,得出零序电压是导致直流侧和其他换流站过电压的主要因素。此外,为了抑制直流侧过电压,提出一种零序过电压控制方法,并给出控制器参数的选择方案。最后,基于PSCAD搭建柔直电网仿真模型,仿真结果验证了故障机理推演的正确性及过电压抑制策略的有效性,提升了站内单相接地故障后的安全稳定运行能力。     

基于桥臂电抗器耦合的MMC-HVDC系统故障过流抑制方法

直流侧双极性短路故障作为模块化多电平换流器高压直流输电系统最严重的故障之一,会导致换流器桥臂发生严重的过流,威胁系统的安全。为了抑制该过流,文中提出了一种利用桥臂电抗器耦合抑制故障过电流的方法,将模块化多电平变流器同一相中上臂和下臂的电抗器耦合在一起来抑制短路电流。通过仿真从三个方面客观的评价了该仰制方法。结果表明,该方法可以有效降低损耗,双极故障时有效抑制故障电流的增加,为保护赢得时间。     

LCC-FHMMC混合直流输电系统受端交流系统故障穿越控制策略

送端采用电网换相换流器（LCC）、受端采用全半桥子模块混合型模块化多电平变流器（FHMMC）的LCC-FHMMC混合直流输电系统,当受端交流系统发生故障时,受端交流电压跌落,受端功率传输受阻,盈余的功率导致子模块电容过电压,甚至可能造成设备的严重损坏。为此,提出了一种基于FHMMC直流电压降压运行的受端交流系统故障穿越控制策略,使其直流电压始终低于逆变侧交流母线的电压有效值。同时,整流侧LCC保持常规的定直流电流控制,保证逆变侧的直流电流在额定值附近运行,从而实现了进入直流系统的有功功率与逆变器向受端交流系统输出的有功功率之间的平衡。最后在PSCAD/EMTDC仿真平台上对LCC-FHMMC混合直流输电系统受端交流系统发生的对称故障和不对称故障分别进行了仿真分析,仿真结果验证了所提控制策略能够快速有效地穿越受端交流系统故障,并抑制子模块电容过电压。     

基于虚拟阻抗的多落点混合级联直流系统故障电流抑制方法

多落点混合级联直流系统存在特有的模块化多电平换流器(MMC)功率盈余问题。当受端交流系统发生短路故障时,MMC过流、过压将引起MMC阀组闭锁,进一步可能导致系统功率中断。多落点混合级联直流系统整流侧采用电网换相型换流器(LCC)、逆变侧采用LCC与多台MMC级联。针对该系统提出一种适用于受端交流系统故障的故障电流限制方法,在逆变侧MMC控制中引入虚拟阻抗降低故障电流,无需额外添加设备。对虚拟阻抗的控制引入、计算以及投入实现过程进行了详细阐述,并在PSCAD/EMTDC中搭建模型进行仿真分析。结果表明,所设计的虚拟阻抗控制器可以实现故障电流的有效抑制,并防止功率倒送,从而实现混合级联直流系统的交流故障成功穿越和功率可靠传输。     

苏州同里±10 kV柔性直流配电系统直流故障特性研究

基于苏州同里±10 kV直流配电系统参数,针对直流单极接地和极间短路故障暂态特性进行了研究。采用示范工程系统结构及控制策略,利用PSCAD建立了电磁暂态模型,针对系统交流侧、换流器侧、直流侧和负荷侧进行了故障过电压、过电流研究,分析了接地电阻对直流侧电压和电流的影响。结果表明：系统直流侧发生单极接地故障时,交流侧出现持续直流分量,换流器不闭锁,DC-DC变换器高压侧电容放电;故障接地电阻对直流侧电压、电流影响大;极间短路故障产生严重过电流,将触发换流器过电流保护,导致换流器闭锁;故障电流是产生过电压的重要原因;电感元件两端过电压较大,极间故障对系统交流侧影响较小。     

Suppression of DC line current oscillation of HVDC transmission system using voltage source forced commutation converter

The conventional converter used in an HVDC system requires line voltage to turn off the current flowing through a thyristor or a mercury-arc valve. Hence, the HVDC system cannot supply electric power to the system which has no ac source, such as a generator or a synchronous condenser. Thus, the HVDC system has some difficulties in operating with the weak system. The conventional solution to avoid these problems is to install large-capacity synchronous condensers which require considerable construction costs and maintenance. Another solution is the application of a forced commutation converter. The recent development of high-power and high-frequency semiconductor devices with fast gate turn-off ability (e.g., GTOs and SI thyristors) suggests the application of a voltage-source-type forced commutation converter (VSFC) to an HVDC system. To ensure the promising characteristics of the overall system, further investigation is of course necessary. The performance of dc transmission into a remote load-only system without ac source has been investigated by means of computer simulation. In many runs of a simulation program of the HVDC system using VSFC, we noticed that long sustaining oscillation of dc line current and dc voltage occurs when a large disturbance is applied. The performance is worse when series dc reactance (DCL) is installed to suppress higher harmonics of dc line current. To improve the performance of the HVDC system using VSFC, an attempt was made to install the L-R parallel damper circuit in series of dc transmission line. Simulation results when DCL and L-R damper are installed in series show that suppression of higher harmonics is as effective as the installation of DCL only, and the damping of oscillation also is substantial to improve the dynamic performance of the whole system.     

多端直流输电系统中限流电抗器配置研究

多端直流电网的故障暂态发展极快,因此在实际工程中一般需要配置限流电抗器来抑制故障电流的上升。但是,限流电抗器的配置在限制故障电流的同时,也会对非故障的部分产生影响,因此文中对多端直流输电系统中限流电抗器的配置进行了研究。建立了多端直流输电系统的故障计算模型,并给出了包含断路器各动作阶段的系统状态量的计算方法;分析了多端直流输电系统中限流电抗器的配置需要考虑的问题;以四端直流输电系统为例,针对不同的限流电抗器配置情况下的直流系统故障过程进行了仿真计算,并根据计算结果分析了限流电抗器对系统最大故障电流、换流站闭锁情况、直流母线过压情况和过电流衰减时间等问题的影响。     

基于电池储能的MMC-HVDC系统的建模与仿真

由于交直流混联电网中的交流系统和直流系统之间存在有功功率耦合,因此基于模块化多电平换流器(MMC)的高压直流输电(HVDC)系统无法完全隔离故障时交直流侧之间的相互影响。文中提出将基于电池储能系统(BESS)的MMC应用于HVDC系统以实现交直流侧功率解耦控制的方法。首先分析了基于BESS的MMCHVDC系统的基本结构和工作原理。然后,基于数值积分和嵌套的快速仿真方法,推导了基于BESS的子模块和基于BESS的MMC换流器的戴维南等效电路。最后,搭建了±400 k V的两端MMC-HVDC系统对所提建模方法和系统的解耦效果进行了验证。结果表明,基于BESS的MMC-HVDC系统交流侧有功功率和直流侧功率可以彼此解耦,在交流或直流故障期间可以保持功率的正常输送。     

DC Fault Clearing Characteristics of a Bulk Power VSC HVDC Transmission System using DC Circuit Breakers

Development of wind power generation resources is considered in Japan. However, long‐distance transmission lines are necessary to utilize the wind power because most of the wind power resources are distant from load centers. A voltage source converter based high voltage dc transmission (VSC HVDC) system is an option to realize their effective use. A VSC HVDC system equipped with dc circuit breakers has been considered to clear the dc line faults promptly. However, the fault clearing characteristics by the dc circuit breakers have not been analyzed very well for the system with the long distance overhead transmission line. This paper focuses on a fault clearing in the VSC HVDC system with long‐distance overhead line by the dc circuit breaker using a new fault detection method. A 2.4‐GW bipolar VSC HVDC system with 600‐km overhead transmission line is modeled and its operation performance is verified by computer simulations. The simulation results demonstrate the detailed fault clearing characteristics with the long transmission line. Moreover, specifications required for the dc circuit breakers are considered in terms of operation delay, peak current, and absorbed energy.     

海上风电低频输电系统低频侧不对称故障控制策略

低频海底电缆发生不对称故障时存在模块化多电平矩阵换流器(M3C)功率器件电流越限、非故障相过电压风险,严重影响海上风电低频输电系统(LFTS)的安全运行。针对上述问题,在充分发挥M3C高可控性的基础上,提出了一种适用于海底电缆不对称故障的控制策略：根据故障严重程度动态调整M3C低频侧电压,防止非故障相过电压;各子换流器通过抑制负序电流限制短路电流上升,保护功率器件免受过电流危害;计及控制策略的影响,通过建立系统故障等值模型进行故障分析。在PSCAD/EMTDC中搭建了海上风电LFTS模型,仿真结果验证了所提控制策略和故障分析方法的有效性,其能够实现低频侧不对称故障下系统的稳定运行。     

含高比例分布式电源的直流配电系统故障恢复过电压机理及其抑制

针对直流配电系统故障恢复过程中出现的过电压问题,本文研究了该过电压的产生机理及其抑制方法。首先介绍了典型直流配电系统的拓扑结构,给出两种核心换流装备(两电平VSC和DC/DC换流器)在双极短接故障下的电流及电压故障特征。基于故障点隔离后的健全区系统等效电路,研究了故障恢复过电压机理及影响因素,揭示了故障存续期间各换流器电压跌落速度不一致是导致故障恢复过电压的根本原因。在此基础上,以保障换流器电压跌落速度相同和直流断路器开断容量为约束,提出各换流器出口限流参数匹配设计原则,同时兼顾限制故障电流和抑制故障恢复过电压效果。最后在PSCAD/EMTDC进行了仿真验证。     

天广直流换流阀异常导通EMTDC仿真分析

自2014年7月以来,天广直流发生了四次换流阀异常导通事件,造成主设备过电流或过电压,严重影响天广直流安全稳定运行。为查明换流阀异常导通原因,从极控系统、VBE系统、换流阀及其组件、TE板等方面开展理论分析、现场排查、仿真分析、物理试验,成功排除了极控系统、VBE系统、换流阀及其组件导致换流阀异常导通的可能性。通过TE板BTC回路分析以及加压试验,发现了TE板在高污高湿条件下误触发晶闸管的情况。在BTC动作门槛值与TE板异常个数关系分析的基础上,开展四次换流阀异常导通事件的EMTDC仿真分析,模拟了部分TE板BTC动作门槛值降低的情况,得到了与现场故障波形高度相似的仿真波形。结果表明即使部分阀片TE板表面较为清洁,其余TE板表面积污严重,在潮湿环境下也可以导致换流阀异常导通。     

政平换流站的无功功率控制

龙泉-政平±500kV直流输电系统自2003年6月正式投运,起着联系华中和华东两大区域电网的重要作用,研究其无功功率控制软件RPC的功能、结构和控制方法对其安全可靠运行和其他直流输电工程建设有着重要意义。为此介绍了政平换流站绝对最少滤波器等控制功能的控制模式。并简要说明了该站无功功率控制系统运行情况,指出需注意的问题以供同行业参考。     

双极MMC-HVDC系统站内接地故障特性及保护策略

针对双极柔性直流输电系统模块化多电平换流器(MMC)交直流出口接地故障,研究了换流器闭锁后的电压电流暂态特性,并推导了故障分量的数学解析式。研究结果表明,交流出口发生单相接地故障时,换流器闭锁后非故障相上、下桥臂分别出现了过电压和过电流现象,并且交流侧电流出现直流偏置导致故障相短路电流不存在过零点。直流出口发生单极接地故障,换流器闭锁后桥臂短路电流主要由交流系统注入的稳态电流和上下桥臂电抗间衰减的环流构成。针对交流出口发生单相接地故障这一特殊的故障特性,提出了一种选相跳闸保护策略,解决了故障电流不存在过零点时交流断路器无法正常断开的难题。最后搭建了张北四端环网结构柔性直流电网仿真模型,仿真结果验证了换流器出口故障特性分析的准确性以及所提选相跳闸保护策略的有效性和可行性。     

New control for HVDC system connected to large windfarm

HVDC consisting of self‐commutated inverters is able to be applied for power transmission connecting from a remote large windfarm to a weak AC system. Most self‐commutated HVDC is applied for connection between two AC systems that have synchronous power sources, but it is not suitable for a windfarm that consists of induction generators without synchronous power source. This paper presents new control for the self‐commutated HVDC system connected to a large windfarm of induction generators. The effect of the proposed control is shown by EMTP simulation. ©2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(4): 31–39, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20539     

配电网接地故障相主动降压消弧成套装置及其现场试验

中性点非有效接地配电网易发生间歇性弧光接地故障,产生过电压,危及人身设备安全,甚至引发电缆通道火灾。针对该问题,提出了接地故障相主动降压消弧控制原理与技术,研发了成套装置,通过灵活调控零序电压,控制故障点电压低于故障电弧重燃电压,强迫故障电弧自行熄灭,实现间歇性弧光接地故障的不停电消除,且有效抑制过电压。通过大量实验室测试、实际变电站实测与运行,验证了接地故障相主动降压消弧技术的正确性和可行性。     

电压源换相高压直流输电系统接地方式设计

针对电压源换相高压直流输电(voltage sourceconverter based HVDC,VSC-HVDC)系统分析了其不同接地方式的优劣性。通过建立VSC-HVDC的电磁暂态模型,对不同接地方式下的系统稳态谐波性能、内部交流母线故障特性以及直流线路故障特性进行了详细的仿真研究,并深入分析了故障机制。通过比较不同接地方式下的系统稳态及暂态工况,分析了不同接地方式的优劣性。作为结论,提出了滤波器中点连接于直流电容中点,同时直流电容中点通过高阻接地的VSC-HVDC系统接地配置方案对于提高系统的稳态及暂态性能是有利的。     

特高压直流输电系统换流站故障过电压研究

±800 kV特高压直流输电系统换流站内电容性和电感性组件较多,在发生短路故障时容易引起过电压现象。研究各种操作和故障情况下过电压的特性,保证系统的安全稳定运行非常重要。利用PSCAD仿真软件建立了±800 kV云南—广东特高压直流输电工程的模型,在换流站内选取了换流阀阀顶对中性母线短路故障和换流变压器阀侧单相接地两种典型故障工况进行了研究。结果表明阀顶对中性母线故障时非故障极线路过电压水平较高,在上组四个换流变压器阀侧绕组中高压端Y/Y绕组端子处单相接地时的过电压水平最高。     

海上风电经柔性直流联网系统受端交流故障穿越协调控制策略

针对海上风电经柔性直流联网系统受端交流故障导致的直流过电压问题,提出了直流过电压协调抑制策略。针对单极直流过电压,通过合理切换双极MMC控制模式,可使故障极MMC主动维持直流电压稳定。并设计了风电场精确减载控制策略,以保证非故障极MMC满载运行,从而降低单极MMC退出对受端交流电网的影响。针对双极直流过电压,设计了一种基于本地直流电压测量信息的风电场减载控制策略,即根据直流电压变化率及偏差量主动降低风电场有功出力,以抑制直流电压上升率及幅值。并提出了附加桨距角控制及其参数选取原则,使风电场与各换流站内电容共同维持直流电压稳定,提高系统故障穿越能力。最后,基于RTLAB OP5600实时数字仿真平台搭建了系统仿真模型。不同受端交流故障情况下的仿真结果表明,所提直流过电压协调抑制策略可保证直流电压在安全运行范围,维持系统安全稳定运行。