魏兴杰(1996), 男, 硕士, 研究方向为柔性直流输电(E-mail:
张英敏(1974), 女, 博士, 教授, 研究方向为高压直流输电
刘坤(1996), 男, 硕士, 研究方向为柔性直流输电
基于模块化多电平换流器(modular multilevel converter, MMC)的柔性直流电网在直流短路故障时电流峰值较高且上升速度极快,严重时会造成MMC闭锁从而导致系统大面积停运。为在短时间内限制故障电流对系统的影响,文中提出一种对半桥型MMC适用的故障限流组合控制策略,利用MMC自身的高度可控性,无须外加限流装置,即可达到故障限流效果,并降低对直流断路器的技术需求。首先,文中阐述了限流组合控制策略中2种不同的限流环节及其基本原理。其次,分别分析2种限流环节对直流故障电流、交流电流以及桥臂电流的影响,推导限流组合控制下的直流故障电流计算式。最后,在PSCAD/EMTDC平台搭建半桥型MMC四端直流电网模型进行仿真分析,结果表明所述限流组合控制策略能够有效限制直流故障电流,减小故障点近端换流器的功率和电压波动,降低交流电流和桥臂电流的过流峰值。
The flexible direct current (DC) grid based on modular multilevel converter (MMC) has a high current peak value and a very fast rising speed during a DC short-circuit fault. In order to limit the impact of fault current on the system in a short period of time, a fault current-limiting combined control strategy suitable for half-bridge MMC was proposed. By using the high controllability of MMC itself, the fault limit can be achieved without external current limiting device, and it can reduce the technical requirements for DC circuit breakers. Firstly, two different current-limiting links and their basic principles in the current-limiting control strategy are explained. Secondly, the influences of the two current-limiting links on the DC fault current, AC current and bridge arm current are analyzed respectively, and the calculation formula of DC fault current under current-limiting control is deduced. Finally, a half-bridge MMC four-terminal DC grid model is built on the PSCAD/EMTDC platform for simulation analysis. The results show that the current limiting combined control strategy can effectively limit the DC fault current and reduce the power and voltage fluctuations of the near-end converter at the fault point. It can also reduce the peak value of AC current and bridge arm current.
基于模块化多电平换流器(modular multilevel converter, MMC)的柔性直流输电技术具有有功和无功独立控制、不存在换相失败风险以及向弱交流系统和无源负荷供电等优势,在能源汇集和区域互联等方面具有较好的发展前景[
目前工程中实际使用的换流器主要采用不具备故障阻断能力的半桥型子模块,通过与高压直流断路器(direct current circuit breaker,DCCB)配合,及时隔离直流侧故障,但开断大容量的DCCB制造困难且成本较高,限制了高压大容量直流电网的发展[
文献[
基于此,文中提出一种适用于半桥型MMC直流电网的限流组合控制策略。首先,通过换流站外环控制,限制故障后交流侧馈入的有功功率,降低桥臂电流和交流电流峰值。其次,通过换流站的阀级控制,改变故障后换流器投入子模块的数量,旁路的子模块数目根据故障后的直流电压变化量自适应改变,从而进一步降低直流故障电流。最后,通过仿真验证文中策略限流的有效性。此策略无须在线路中附加限流装置,并能降低DCCB的开断电流,大大减少了系统的建设成本,加强了半桥型MMC对于故障过电流的抑制能力。
半桥型MMC的拓扑如
MMC拓扑
Topology of MMC
基于半桥型MMC的限流控制策略包括2个限流环节,即外环控制指令预设限流和直流电压变化自适应限流。对2个限流环节改进,可起到明显降低半桥型MMC直流侧故障电流水平的作用。
外环控制指令预设限流控制器如
外环控制指令预设限流控制器
Current limiting controller by predetermining the outer loop control instructions
故障发生后,由于系统功率和电压的波动使得外环比例积分(proportional integral,PI)控制器极易饱和,影响了控制器可控裕度。通过在外环PI控制器内预设清零指令(CLR),故障后将PI控制器清零,使得外环功率控制的输出
基于直流电压变化的自适应限流控制器如
基于直流电压变化的自适应限流控制器
Self-adaption current limiting controller based on DC voltage variation
由
由式(1)和式(2)可得,随着限流控制的运行,
综合以上两部分控制措施,文中所述基于半桥型MMC的限流组合控制策略原理如
组合限流控制策略原理
Schematic diagram of current limiting combined control strategy
此外,柔直电网直流侧的短路电流主要由故障点两侧的近端换流器提供[
发生直流侧短路故障后,子模块电容的放电电流为主要的故障电流,但实际上换流站的控制系统也会对故障电流产生影响[
计及限流环节的MMC控制结构如
计及限流环节的MMC控制结构
MMC control structure considering current limiting measures
交流侧三相电网电压平衡状态下,阀侧交流电压
由
MMC交流侧输出相电流基波幅值为:
式中:
式中:
双极短路故障是基于半桥型MMC直流电网最严重的直流侧故障类型,文中对此类故障进行分析。故障等效模型如
直流测短路故障等效模型
Equivalent model of DC side short circuit fault
式中:
根据2.1节分析可知,采用外环控制指令预设的方式能够减少来自交流侧的功率馈入,故文中忽略
由于直流系统的弱阻尼特性,电路中短路等效电阻
式中:
由式(10)分析可得,短路电流主要由3个衰减部分组成,在前1/4个衰减周期内,第一和第三部分主要形成电流的正向上升阶段,第二部分则为反向,限制电流上升。对于由系统参数决定的不同衰减部分的系数,第一部分的系数
综上,文中的限流组合控制策略流程如
组合限流控制策略流程
Flow chart of current limiting combined control strategy
为分析验证文中所述限流控制策略的效果,在PSCAD/EMTDC平台中搭建如
四端直流电网
Four-terminal DC grid
换流器及直流电网参数
Parameters of converters and DC grid
参数 | MMC1 | MMC2 | MMC3 | MMC4 |
额定直流电压/kV | 500 | 500 | 500 | 500 |
额定有功功率/MW | 600 | 800 | 800 | 600 |
子模块电容/μF | 15 000 | 10 000 | 15 000 | 15 000 |
桥臂电感/mH | 100 | 100 | 50 | 50 |
子模块导通电阻/mΩ | 1.361 | 1.361 | 0.908 | 1.361 |
桥臂子模块数目 | 250 | 250 | 250 | 250 |
平抗电感/mH | 150 | 150 | 150 | 150 |
交流电压/kV | 290 | 290 | 290 | 290 |
控制周期/μs | 100 | 100 | 100 | 100 |
线路长度/km | 200 | 200 | 200 | 200 |
线路电阻/(Ω·km-1) | 0.029 2 | 0.029 2 | 0.029 2 | 0.029 2 |
线路电感/(mH·km-1) | 0.82 | 0.82 | 0.82 | 0.82 |
系统采用主从控制策略,其中MMC4采用定直流电压控制,其余换流站采用定有功功率控制。直流电网正常运行时,MMC1和MMC3工作于整流状态,功率流入直流电网;MMC2和MMC4工作于逆变状态,功率流出直流电网。系统稳定运行后,在1.5 s时刻,MMC1出口(
故障前后MMC1和MMC2出口直流电流动态特性对比如
MMC1与MMC2直流母线电流
DC bus currents of MMC1 and MMC2
不同限流方法对直流线路电流的影响
Influence of different current limiting methods on DC line current
故障前后交流系统有功功率对比
Comparison of active power of AC system before and after fault
故障前后各换流站交流系统有功功率
AC system active power of each converter station before and after fault
故障前后MMC1交流电流
AC currents of MMC1 before and after fault
故障前后MMC1桥臂电流
Bridge arm current of MMC1 before and after fault
由
故障前后MMC1直流电压
DC voltage of MMC1 before and after fault
文中提出一种适用于半桥型MMC直流电网的限流组合控制策略,利用柔性直流电网的高可控性,通过2个限流环节的分别作用,实现了直流侧故障的限流,并得出以下结论:
(1) 基于直流电压变化的自适应限流环节能够在故障后实时更新子模块修正系数
(2) 通过在系统外环控制器中预设清零指令,能够减少故障后交流系统馈入的功率,从而降低功率波动以及交流电流和桥臂电流的过流峰值,减小对换流器内电力电子器件的技术需求。
(3) 相比于仅减投子模块的限流措施,文中所提限流控制策略能够降低故障切除后的电压波动,同时也能保证较好的直流电流、交流电流和桥臂电流的限流效果。
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