The Valve Area Protection Feature Analysis Of HVDC Transmission

As a bridge of AC and DC transformation, inverter is the core element of high-voltage direct current transmission system, thus it is particularly important to protect the inverter effectively. However, the FPT and DPT of actual DC project have found that when outside converter buses fault, the misoperation of inverter protection will caused DC block, and while the inside faults happen in different time, the valves will response different and even appear commutation failure, cause the inverter fault feature different, and then resulting in the protection action of converter showed certain discreteness. This paper takes the discreteness analysis and the behavior analysis of inverter protection action as the main content to implement research, and then on the basis, discusses using the asymmetry degree of busbar voltage to block the outside faults, avoiding the misoperation of inverter protection.This paper firstly analyzes the fault partition and the measuring device configuration of actual DC. Based on PSCAD/EMTDC simulation platform with the actual control protection model and parameter, implements simulation experiment in different positions of inverter protection area, and analyses the corresponding fault characteristics when the fault occurs in different positions of inverter protection area. According to the different fault characteristics, this paper introduces the relevant protection configuration of inverter protection area, and does a statistic of the main protection and backup protection corresponding to different positions inside the protection area.Based on the response characteristics of inverter bus fault, this paper have done a statistic of the influence laws of fault to inverter commutation and protection action. Through the theoretical derivation of the commutation process of converter valves, we have found out the reason why the outside AC faults cause the inverter protection action discrete: Different fault types and different fault phase cause the change of corresponding phase line voltage amplitudes and the zero offset angles in different converter valves, also cause the change of different valve commutation ability, and form the difference between the easiness or difficulty of commutation failure. So the fault occurred in different types, different time or different transition resistance will cause different characteristics of the commutation failure in the converter valve, also cause the discreteness of the results.At last, Through the asymmetric degree to measure the severity of the asymmetric fault in the converter bus outside the protection zone, combined with the asymmetric degree and inverter commutation failure, corresponding law protections, it is found that the greater the degree of asymmetry is, the more serious the corresponding converter commutation failure occurs, and more serious of the protection misoperation. With the asymmetric degree is reduced to a certain extent, inverter commutation failure won’t happen again, so as to the protection misoperation, and the maximum asymmetry degree of the converter commutation failure inside is less than the minimum asymmetry degree of the nonoccurrence of inverter commutation failure when converter bus fault. Asymmetry degree is quoted from this phenomenon as the inverter protection action criterion, to distinguish the area fault, external fault, and avoid DC blocking led by inverter protection action which is caused by external fault.This paper is based on actual DC engineering model to do the research on valve area fault characteristics and protection, and the method to prevent the atresia in external fault misoperation is put forward, which has good reference value to actual DC engineering fault analysis and protection optimization.