【作者】 吴琼；

【导师】 郑涛； 肖远清；

【作者基本信息】 华北电力大学(北京) ， 工程硕士（专业学位）， 2020， 硕士

【摘要】 柔性直流配电网相比于交流配电网,控制更为灵活、谐波含量少、供电质量更好,但故障特性也较为复杂,分析柔性直流配电网直流故障下的故障特性具有明显的工程价值。作为保障柔性直流配电网安全稳定运行的关键,直流配电网保护始终是电网发展的核心环节和主要难点之一。本文针对柔性直流配电网发生的各种直流故障进行故障特性的研究分析,并据此提出适用的保护及故障隔离方案。本文研究的主要内容如下:1)推导柔性直流配电在单极接地故障、双极短路故障以及直流断线故障下的等效电路模型,分析各种故障的故障机理,得出故障电流的解析表达式,总结出故障特征。小电流接地的系统中,单极接地故障下电流无明显变化,极间电压保持不变,故障极电压降为0,非故障极电压升高为两倍;双极短路故障会出现严重过流现象,危害较大;直流断线故障下电网电流降为0,无电能输送;2)针对基于具有直流故障自清除能力换流器的柔性直流配电系统,重点研究系统双极短路故障时的故障特性,并利用其故障电流迅速增大的特点,提出基于直流电流过零检测的保护方案。方案通过识别线路两端出现直流电流过零时刻的异同,确定故障线路并完成故障隔离。该保护方案原理简单且易于实现,能够快速识别故障线路。通过大量仿真验证了该保护方案的可行性。3)基于FBSM-MMC的柔性直流配电网大多利用MMC闭锁来切断故障电流,但其闭锁会导致整个直流配电网的短时停电,降低了供电的可靠性,迄今尚无有效的解决方案。对此,本文提出一种将MMC主动限流控制与直流电网保护相结合的方案,故障发生后首先利用MMC主动限流控制将换流器输出直流电流限制在1.2倍额定电流附近,进而根据各条线路两端直流电流是否具有同步性过零特征实现故障线路的识别。在此基础上,进一步提出基于直流断路器与快速机械开关协同配合的故障隔离方案。通过断开故障关联的直流断路器,并控制相应换流站的输出直流电流降低为0,使得故障线路上的机械开关亦能快速开断,从而实现故障隔离。搭建四端环型柔性直流配电网进行故障仿真,验证了方案的有效性。更多还原

【Abstract】 Compared with AC distribution network,flexible DC distribution network has more flexible control,less harmonic content and better quality of power supply.But the fault characteristics of DC network are more complicated.The analysis of DC fault characteristics has obvious engineering value.As the key to ensure the safe and stable operation of flexible DC distribution networks,DC distribution network protection has always been one of the core links and major difficulties in the development of power grids.This paper analyzes the fault characteristics of various DC faults occurring in flexible DC distribution networks.Paper proposes an appropriate protection and fault isolation scheme.The main contents of the study are as follows:1)The equivalent circuit model of unipolar ground fault,bipolar short-circuit fault and DC disconnection fault is derived.The fault mechanism of various faults is analyzed,the analytical expression of fault current is obtained,and the fault characteristics are summarized.In a system with low current grounding,there is no significant change in current with unipolar grounding faults.The voltage between the poles remains unchanged.The voltage of the faulty pole drops to 0 and what of the non-faulty pole rises twice.A severe overcurrent phenomenon will occur in a bipolar short-circuit fault with greatly harm.When a DC disconnection fault occurs,the grid current drops to 0 and no power is transmitted;2)The paper focuses on the fault characteristics of the flexible DC power distribution system based on DC fault self-cleaning converters in the event of bipolar short-circuit fault.With the characteristics of fault current which increase rapidly,a protection scheme based on the detection of DC current zero-crossings at both ends of the line is proposed.The scheme locates the short-circuit fault by identifying the differences of DC current zero-crossings quickly and reliably,and removes the fault rapidly.The feasibility of the proposed protection scheme is verified by a large number of simulations3).FBSM-MMC blocking is mostly used to cut off the fault current in the distribution network which based on the FBSM-MMC.But the blocking will cause power outage of the whole network for a moment,which is not conducive to the reliability of the power network.Aiming at the problem,a protection scheme combined by the control of MMC and the protection of DC network is proposed.After a short-circuit fault occurs,the output DC current of the inverter will be limited to 1.2pu by the control of MMC.And then the faulty line is identified according to the synchronous zero-crossing characteristic at both ends of each line.Furthermore,a fault isolation scheme which contains the cooperation of DC circuit breakers and high speed switches is proposed.The switch in the faulty line can be turned off quickly with the associated DC breaker disconnected and the associated MMC’s output current reduced to 0.A four-terminal ring-shaped flexible DC power distribution network was built for fault simulation,which verified the effectiveness of the scheme.更多还原