Study On The Faulty Feeder Selection And Protection Principle Of Radial DC Distribution Grid

With the large demand for distributed generation(DG)integration and the rapid development of power converter technologies,flexible DC distribution grid has been widely focused because it can improve the power quality and help DG integrate DC grid.Owing to the low loss and excellent dynamic and static characteristics,MMC is favored in DC projects.However,the fault characteristics of MMC-based flexible DC distribution grid are quite different from two-level VSC-based DC distribution grid.Thus,it raises a big challenge for the design of the protection method.Meanwhile,the fault current rises rapidly for pole-to-pole fault,which improves the quickness requirement of the protection method.The research on the relay protection principle of MVDC distribution line based on MMC are significant for improving the safety of flexible DC distribution grid and ensuring the reliable operation of the system.In this paper,we firstly analyze the fault characteristics of single-pole-to-ground(SPTG)fault and pole-to-pole fault.Based on characteristics of faulty feeder and faulty segment,we propose a faulty feeder selection method for SPTG fault based on S-transform and Pearson correlation coefficient,and a protection method for pole-to-pole fault based on STCFB Phase.After analyzing and simulating different types of fault,we propose the protection method and set the value.Finally,we construct the radial DC distribution grid and validate the protection method in this model.The details can be stated below:(1)A faulty feeder selection method based on S-transform and Pearson correlation coefficient for SPTG fault is proposed.Firstly,this paper analyzes the characteristics of current and voltage under SPTG fault:for SPTG fault,the waveforms of current fault component of positive and negative poles are negatively correlated for a faulty feeder;while they are positively correlated for a normal feeder.Secondly,we obtain the characteristic frequency band of the transient current fault component after extracting and analyzing the parameters of DC feeders.Then,we propose the faulty feeder selection method based on S-transform and Pearson correlation coefficient to identify the faulty feeder.Finally,this paper analyzes the fault component of different feeders and different segments,and proposes a method to identify the faulty pole and locate the faulty segment.(2)This paper proposes the comprehensive protection and location method for feeders and segments based on STCFB Phase difference.Firstly,we calculate the gradient of current fault component according to current gradient algorithm.Thus,we can identify whether there is a fault on the DC side.As there are multiple branches in the feeders of distribution grid,we install single-ended branch-line protection method to prevent the over-level strip.Utilizing the polarity correlation of voltage and current fault component on different feeders,we accurately identify the fault on different branches based on STCFB Phase.Meanwhile,we install pilot protection in each segment of the main feeder.The faulty segment of main feeder can be judged according to the polarity of the STCFB Phase on both sides of different segments.(3)Utilizing PSCAD/EMTDC,we established the ±10kV radial DC distribution grid.In this simulation model,we set different types of fault to validate the faulty feeder selection method and protection method,the simulation results validate the accuracy and reliability of our method.