Simulation Analysis of Overvoltage of Single Phase Grounding Fault in Paralleled 12-Pulse Rectifier System
In order to solve the problem of overvoltage of single phase grounding fault of paralleled 12-pulse rectifier system, firstly the common mode wave problem of monopole in DC side at fault is analyzed. Secondly, the transient overvoltage of the system at the AC side single phase fault is analyzed by simplifying the charging and discharging circuit of capacitances on DC side. Finally, the theory of frequency extinguishing arc is used to simulate the intermittent arc grounding overvoltage. It is proved that the capacitance on DC side also affects the AC overvoltage in the system that provides a basis for the better suppression of overvoltage in the project. The correctness of the conclusion is verified by PSCAD/EMTDC simulation software in the analysis process. Introduction In recent years, DC power supply system has been widely used in long distance DC transmission, as well as the electrified transportation system including aircraft, ships and high speed rail, due to more easily grid-connected, smaller loss of line, higher reliability and better economy. In order to rectify high quality voltage of DC side, many different phase three-phase windings are often combined, and the 12 or 24 pulse rectifier generator is a common way of the AC and DC hybrid power system [1,2]. The multi pulse rectifier synchronous generator can reduce the ripple of DC voltage and effectively improve the power quality of DC side. Single phase to ground fault is common fault in power system, and single phase grounding inevitably causes overvoltage. Normally, the voltage of the non fault phase will rise to line voltage when the single phase grounding occurs, but in fact, the overvoltage is often higher than the line voltage level [3]. There are two main reasons for the overvoltage over the line voltage, one is the existence of distributed capacitance in the system, the oscillation overvoltage is produced in the process of charging and discharging, and the second reason is that the single-phase grounding fault is not fully metallic grounding, but the arc grounding, and the higher overvoltage is produced by arc reigniting. Single phase grounding faults sometimes generate 3.5pu overvoltage [4,5], which will not only damage the insulation of the line, but also burn the electrical equipment. At present, the research on single-phase grounding fault of AC distribution network is very mature, but the research on AC power system with rectifier load is less. In both DC power system and electrified transportation systems, the distance of the AC network to the rectifier load is very short, and the distributed capacitance is mainly concentrated on the DC side, which is different from the existing of distributed capacitance on AC side. The charge and discharge of the capacitor need to pass through the circuit of the rectifier bridge. In this paper, the paralleled 12-pulse uncontrolled rectifier system is studied, and the overvoltage of the single phase grounding fault is analyzed and calculated. The simulation software PSCAD/EMTDC is used to verify conclusion, a basis for the study of overvoltage in this kind of system in the future is provided. Common Mode Fluctuation of Monopole on DC Side The paralleled 12-pulse rectifier system can be powered by a transformer or synchronous generator, and the two sets of windings have 30 degree difference [6], as shown in Figure 1. Taking the uncontrolled rectifier bridge as an example, there are some distributed capacitances on the positive