含柔性直流的交直流混联电力系统紧急频率控制研究

Research on Emergency Power Control of AC-DC Hybrid Power System with Flexible DC

在含柔性直流的交直流混联电力系统中,如何利用柔性直流提升交直流混联电力系统稳定性是亟须解决的关键技术问题。传统直流(LCC)进行紧急功率支援时,存在有功功率与无功功率难以独立解耦控制,且需要配置大量的无功补偿装置。针对此问题,将现有交直流混联系统中的常规直流LCC逆变站替换成柔性直流(VSC)换流站,根据LCC-LCC和LCC-VSC各自的优势,提出一种优先级紧急功率支援协调控制策略,即含柔性直流的交直流混联系统需要直流紧急功率支援时优先采用LCC-VSC对系统进行紧急功率支援,若功率缺额超过LCC-VSC的功率调节裕度时,剩余功率由LCC-LCC进行传输。与未采用优先级紧急功率支援协调控制策略相比,极大地缓解了潮流转移时交流输电线路过载负担,进行紧急功率支援后,交流系统的频率稳定性得到较大改善。通过搭建含LCC和VSC的7机39节点的交直流混联系统,验证了所提的控制策略在交流联络线发生N−1、N−2和LCC-LCC双极闭锁故障情况下的有效性。

The emergency power control of the DC system is used to balance the power shortage in the system,reduce the frequency deviation of the generator set,and improve the transient frequency stability of the system.In the AC/DC hybrid power system with flexible DC,using the flexible DC to improve the stability of the AC/DC hybrid power system becomes a pressing technical challenge.Traditional DC systems(line commuted converter,LCC)face difficulties in independently controlling active and reactive power during emergency power support,often necessitating the addition of numerous reactive power compensation devices.Therefore,the conventional DC inverter station LCC in the existing AC/DC hybrid system is replaced with a flexible DC converter station(voltage source converter,VSC).Based on the respective advantages of LCC-LCC and LCC-VSC,a prioritized coordination control strategy for emergency power support is proposed.When the AC/DC hybrid system with flexible DC needs DC emergency power support,the LCC-VSC is preferentially used to provide emergency power support to the system.If the power shortage exceeds the power regulation margin of LCC-VSC,the remaining power is transmitted by LCC-LCC.The fast power regulation characteristics of the DC system are used to rebalance the system power shortage.Firstly,the theory of DC participation in emergency frequency support is analyzed.Then,a priority hierarchical coordinated control strategy is designed.Finally,simulation results verify the effectiveness of the proposed scheme in realizing emergency frequency support.For the AC system N−1,N−2 fault,and LCC bipolar blocking three fault conditions in the AC/DC hybrid power system with flexible DC,the flexible DC is preferentially selected for emergency power support control.When an N−1 fault occurs,compared to situations without DC emergency power support,LCC-VSC is preferred for emergency power support.The transmission power burden of non-fault pole AC line 2 is greatly alleviated,the AC frequency fluctuation rate is reduced by 40%,and the system frequency is quickly restored and stabilized to 50 Hz.When an N−2 fault occurs,the system frequency offset exceeds 0.5 Hz without DC emergency power support.However,using the LCC-VSC as the primary emergency power support and LCC-LCC for additional support,the system frequency fluctuation rate is reduced by 65.9%,and the LCC-LCC and LCC-VSC transmission power values are within the allowable range.When LCC-LCC bipolar blocking,LCC-LCC VSC is preferred for emergency power support,and the remaining power is transmitted by AC lines 1 and 2.The system frequency fluctuation rate is reduced by 20%.Through the emergency power coordinated control of LCC-VSC and LCC-LCC,the system frequency is quickly restored to stability.After the emergency power coordinated control starts,the system frequency is basically restored to 50 Hz after 60 ms.When relying solely on LCC-LCC for emergency power control,the system frequency recovery time is significantly longer than the coordinated control of LCC-VSC and LCC-LCC.