弱连接VSC在“外环-锁相环”时间尺度下的一类小扰动稳定性提升方法及等效性分析

Equivalence Analysis of A Class of Small-signal Stability Enhancement for Weak Grid Connected VSCs in“Outer-loop&PLL”Time-scale

大量理论研究发现电压源换流器(voltagesource converter,VSC)接入薄弱电网后,易发生由“外环–锁相环”交互所主导的小扰动失稳现象,且在多个实际工程中也发现此类稳定问题。为此,国内外学者提出了大量改进控制方法,其基本原理为在有功外环、无功外环与锁相环三者之间施加附加补偿控制,以提升弱连接VSC(weak-grid connected VSC,WG-VSC)并网稳定性。该文将这些方法归纳为WG-VSC在“外环–锁相环”时间尺度下的一类小扰动稳定性提升方法。试想,这些策略都能提升稳定性,那么它们是否存在某种等效性?此外,基于等效性原理,还能否推导出其他的改进控制措施?该文研究动机是期望从方法论的视角回答上述问题,进而为解决此类稳定性问题提供基本依据。该文创新工作是基于一种能够清晰展现“外环–锁相环”交互关系的等效锁相环模型,首次揭示了这一类改进控制方法在提升WG-VSC“外环–锁相环”时间尺度小扰动稳定性方面具有等效性。此外,通过观察分析现有补偿控制策略在拓扑结构和作用机理上的特点,该文还预测了其余多种新型小扰动稳定性提升控制策略,并基于等效锁相环(phaselockedloop,PLL)模型分析得出,通过特定的控制器设计方法,可使得此类补偿控制策略在WG-VSC系统小扰动稳定性提升方面具有等效性。最后,基于PSCAD/EMTDC的详细电磁暂态仿真模型验证了该文理论分析的准确性。

A large number of theoretical researches have found that as the voltage source converter(VSC)is tied to the weak AC grid,there is the risks of the small disturbance instability dominated by the"outer loop-phase locked loop"interactions.Such instability problems have also happened in many practical projects.To solve above problem,a lot of improving strategies have been proposed in the existing literatures.The basic principle is to exert an additional compensation control between the active outer loop,the reactive outer loop and the phase locked loop to improve the small signal stability of the weak grid connected VSC(WG-VSC).In this paper,these methods are summarized as a class of small disturbance stability improving strategies for the WG-VSC in the"outer loop-PLL"interaction timescale.If all these control strategies improve the small signal stability of the WG-VSC system,is there any influence equivalence in them?In addition,based on the equivalence principle,are there any other improving control strategies able to be derived?The motivation of this paper is to solve the above problem from the perspective of methodology,and then provide a theoretical basis for solving such stability problems.The innovation of this paper is to reveal for the first time that this kind of improving control methods have some equivalent effects on improving the small disturbance stability of the WG-VSC system in"outer loop&phase-locked loop"interaction timescale based on an equivalent phase-locked loop model.In addition,by observing and analyzing the characteristics of the existing compensation control strategies in terms of their topologies and mechanisms of action,this paper predicts a variety of other novel small disturbance stability enhancement control strategies.Based on the analysis of the equivalent PLL models,it is concluded that such compensation control strategies have equivalence for the small disturbance stability enhancement of the WG-VSC systems with certain specific controller design methods.Finally,the detailed electromagnetic transient simulation model based on the PSCAD/EMTDC verifies the accuracy of the theoretical analysis in this paper.