LCC-MMC混合高压直流输电系统直流线路故障保护方案研究

Research on Protection Scheme of DC Line Fault in LCC-MMC Hybrid HVDC System

该文提出一种适用于LCC-MMC(line commutated converter and modular multilevel converters)混合高压直流系统的直流线路故障保护方案,其中系统整流端为常规线控换流器(line commutated converter,LCC),逆变端为基于半桥子模块的模块化多电平换流器(half-bridge modular multilevel converters,HB-MMC)。文中根据LCC侧与MMC侧的故障等效电路,对两侧故障电流进行数学分析,得出通过控制LCC触发角和MMC子模块闭锁可限制故障电流的方法。为保证完全清除故障,提出一种改进的混合直流断路器模型,利用反并联晶闸管串联电容支路,进行电流的转移和能量的吸收,减小主开关关断压力。改进后的直流断路器模型成本低、损耗小,且故障清除能力强,适用于LCC-MMC系统单极接地和双极短路故障的故障保护。经分析,最终形成包括基于直流电抗器电压变化率的故障检测、LCC与MMC故障保护协调控制与混合直流断路器故障清除的整体保护方案,并在PSCAD中建立仿真模型,仿真结果验证了所提方案的可行性。经方案评估分析,所提保护方案在成本与损耗方面具有一定优势。

This paper proposed a DC line fault protection scheme suitable for LCC-MMC(line commutated converter and modular multilevel converters)hybrid HVDC system,in which the rectififier adopted an LCC and the inverter adopted modular multilevel converters based on half-bridge modular multilevel converters(HB-MMC).According to the fault equivalent circuit on the LCC side and the MMC side,the fault currents on both sides were mathematically analyzed,and the method to limit the fault current by controlling the LCC trigger angle and MMC sub-module blocking was obtained.In order to ensure that the fault was completely cleared,an improved hybrid DC circuit breaker model was proposed,which used an anti-parallel thyristor series capacitor branch to transfer current and absorb energy to reduce the main switch off pressure.The improved DC circuit breaker model had low cost,low loss,and strong fault clearing ability,and was suitable for the fault protection of LCC-MMC system single-pole grounding faults and double-pole short-circuit faults.After analysis,an overall protection scheme including fault detection based on DC reactor voltage change rate,coordinated control of LCC and MMC fault protection and fault clearing of hybrid DC circuit breaker was finally formed.A simulation model was established in PSCAD.The simulation results verified the proposed feasibility of the scheme.Assessed by the scheme analysis,the proposed protection scheme has some advantages in terms of cost and loss.