CONTROL STRATEGY IMPROVEMENT AND DIFFERENTIAL PROTECTION IMPROVEMENT OF FLEXIBLE LOW FREQUENCY TRANSMISSION SYSTEM

Due to the power electronic converter on both sides of the low frequency transmission line, the fault current is limited in amplitude and uncertain in phase under the influence of the converter control, which is difficult to adapt to the conventional relay protection. In this paper, two control strategies, negative sequence current suppression and negative sequence current injection, are used to analyze the adaptability of current differential protection on the low frequency side of modular multilevel matrix converter. Because of the difference of fault characteristics under different control strategies, it may lead to the rejection of conventional differential protection. The traditional current differential protection has the problem of low sensitivity. In order to improve the sensitivity of differential protection, the traditional differential protection is improved first, and then the control strategy is improved. Two control strategies of negative sequence current injection and negative sequence current absorption are proposed, and a high sensitivity current differential protection principle suitable for low frequency transmission systems is proposed. Finally, the corresponding electromagnetic transient simulation model is constructed in PSCAD/EMTDC, and the verification results show that the M3C low frequency negative sequence current injection proposed in this paper is suitable for different fault types.