New Problems And Solutions Of Relay Protection Under The Situation Of AC/DC Hybrid Power Grid

With the development of high power DC projects and the application of hybrid AC/DC network in China,the structure of the power grid becomes complex and the coupling degree of AC/DC network increases,hybrid AC/DC network faces a series of problems,including overvoltage in the converter station caused by improper protection and unpredictable onver-limited short circuit current caused by uncertain conditions and power electronic units.To overcome the shortcomings of the existing arrester based last breaker protection,this paper proposes two kinds of improved arrester based last breaker protection strategy and the tripping strategy of arrester based last breaker protections based on dynamic weight correction D-S evidence theory to improve the correction rate of the last breaker.On the other hand,we propose a short circuit current prediction method based on dynamic dual time window and least square method as well as a flexible trip strategy for uncertain over-limited short circuit current.The specific contents are as follows:First,considering the situation that the results of three criteria are inconsistent,the result of multi-criteria are combined and the reliabilities are corrected by using the weight adjustment method of adjacent historical data,which is able to improve the ability of last circuit breaker protection.Finally,an example is given to illustrate the effectiveness of the improved strategy in improving the security performance of the system.Then we propose a novel method of short circuit current prediction based on the dynamic dual time window to self-update and extend the short circuit current in the ultra fast prediction and satisfy the precision demand.Based on the short circuit current prediction model,we fit the existing data by the least square method to acquire the current wave form and lay the foundation for the following flexible tripping strategy.Finally,we put forward a flexible tripping strategy for uncertain over-limited short circuit current based on the predicting results.By properly adjusting the final export time of protection and checking the arcing area of the circuit breaker,we can directly trip the circuit breaker near the zero crossing point of current.