Research On Phase Sequence Exchange Stability Control Based On MMC

In the process of power system instability,when the power angle is set to a set angle(such as 120 degrees),the power electronic switch equipment is used to perform phase change sequence operation,and the difference of the power force angle between the two power sources is pulled back to a small angle,so that the system can be restored to stability again.This method is called phase change sequence technology.In this paper,based on the flexible characteristics of DC power modulation in power system,a commutated phase sequence topology based on modular multilevel converter(MMC)is proposed,and the method to improve transient stability control of power system is studied.The main research results of this paper are as follows:(1)A commutation sequence circuit topology based on MMC is proposed,which can not only realize the function of AC commutation sequence to improve the transient stability of the system,but also have the characteristics of flexible control of deceleration area,and realize the function of back-to-back DC transmission.(2)The switching process between different operation states of MMC commutation sequence circuit is studied,and the corresponding simulation model is built in PSCAD、This paper studies the effect of MMC commutation sequence circuit to improve the transient stability of power system from the aspects of deceleration area increment,system stability margin and transmission capacity.The analysis results show that compared with ordinary AC lines,the circuit with MMC commutation sequence device has larger deceleration area,higher system stability margin and stronger transmission capacity.(3)The optimal control method of MMC commutation sequence technology is studied.Different commutation sequence modes should be adopted under different working conditions.The PSCAD platform is used to simulate the transient process of three-phase short circuit fault in single machine to infinite bus system with MMC commutation sequence circuit.The results show that the method of starting MMC immediately after the fault is more efficient and more conducive to system recovery It is stable again.When the requirement of restoration stability time is strict,the 120°or 240° commutation sequence is preferred,while the 360° commutation sequence does not damage the original structure of the system,which can effectively avoid other problems caused by phase sequence exchange.It is better for the system which is less sensitive to restoration stability time.