柔性直流输电系统低频振荡机理及抑制策略

Low-frequency Oscillation Mechanism and Suppression Strategy of Flexible HVDC Transmission System

柔性直流输电技术已经发展为超高压直流输电的一种主要方式。然而多个工业现场出现低频振荡现象,从而影响了供电的可靠性。现有理论从感应发电机效应和次同步控制相互作用角度解释了暂态短路等异常工况下低频振荡的原因,却无法分析正常功率调动时低频振荡的机理。基于此,该文从多个角度,系统地研究了柔性直流输电系统的低频振荡问题:1)通过数学模型定量地分析了其作用机理,MMC阻抗特性、开关频率、低频运行特征和环流的低通特性共同引起了低频振荡;2)采用环流抑制、开关频率优化和有源阻尼等多种控制手段,改进了现有的低频振荡抑制算法;3)搭建了双端±420kV柔性直流输电系统的基于实时数字仿真系统(real time digital simulation system,RTDS)的硬件在环仿真,利用典型柔性直流输电工程的参数,对比所提算法和传统算法的实现结果,验证工业现场的适用性。该文所提低频振荡机理能够解释低频振荡发生的各种原因,所提控制策略可以抑制低频振荡发生,提高柔性直流输电系统的可靠性。

Flexible DC transmission technology has developed into a main method of ultra-high voltage(HVDC)transmission.However,the low-frequency oscillation faults occuring in several industrial sites at the same time will affect the reliability of power supply.The existing theories explain the causes of low frequency oscillations under abnormal operating conditions such as transient short circuits from the perspectives of induction generator effects and subsynchronous control interactions,but they cannot analyze the mechanism of low frequency oscillations during normal power transfer.Based on this,this paper systematically studies the low-frequency oscillation problem in the flexible DC transmission system from multiple angles:(1)Quantitative analysis of a mathematical model is given to illustrate its mechanism of action:It is the MMC impedance characteristics,the switching frequency,the low-frequency operating characteristics,and the low-pass characteristics of the circulating current that cause the low-frequency oscillation;(2)Various control methods,such as circulating current suppression,switching frequency optimization and active damping,are used to improve the existing low-frequency oscillation suppression algorithms;(3)The RTDS hardware-in-the-loop simulation of a doubleended±420kV flexible DC transmission system is built.By using the parameters of the typical Flexible HVDC Transmission Project and comparing the implementation results of the proposed algorithm and the traditional algorithms,the applicability of the industrial site is verified.The low-frequency oscillation mechanism mentioned in this paper can explain various causes of low-frequency oscillations.The proposed control strategy can suppress low-frequency oscillations and improve the reliability of flexible DC transmission systems.