云广特高压直流送端孤岛运行超低频振荡与措施

大容量特高压直流输电系统与配套大型水电站构成的直流送端孤岛运行系统存在着较严重的调速器控制稳定问题。云广特高压直流送端孤岛系统也存在水电机组调速器与直流控制不协调而出现的超低频振荡现象。分析了电网超低频振荡过程中所有机组的同调特征,提出了电网超低频振荡分析模型。基于频域分析法指出了水轮机调节系统在超低频范围内存在负阻尼频带,水轮机调节系统的时间常数越大、调速器前向通道增益越大,负阻尼越显著。提出了通过退出部分机组一次调频功能或协调机组一次调频与直流频率限制控制器(FLC)动作死区设置,以直流FLC调频为主的直流孤岛频率控制策略能有效抑制孤岛系统的超低频振荡。基于实时数字仿真器(RTDS)的机网协调控制实时仿真结果验证了该策略的有效性。

Ultra-low-frequency Oscillation and Countermeasures in Yunnan-Guangdong UHVDC Sending-end System in Islanded Operating Mode

There exists serious governor control instability in the islanded operation system of large capacity UHVDC with its matching hydraulic power stations. Yunnan-Guangdong UHVDC islanded system also has ultra-low-frequency oscillation(ULFO)phenomenon because of the inconsistent control features between UHVDC and frequency regulation of generators. The ULFO analysis model is put forward based on the complete coherence characteristics of all generator rotors. There exists a frequency band of negative damping in the hydraulic turbine control system by frequency domain analysis. The larger value of turbine time constant, the more serious negative damping exists in the frequency control system. And if the forward gain of governor becomes larger, the negative damping will be more remarkable. An effective frequency control strategy for UHVDC islanded system frequency control is proposed by withdrawing a part of primary frequency control functions of generators or coordinating the dead zone values of governors and UHVDC frequency limitation controller(FLC). By the proposed strategy, islanded system frequency is mainly controlled by UHVDC FLC, which can successfully suppress the ULFO problem. Verification simulations are carried out and the simulation results prove the effectiveness of the proposed frequency control strategy.