Research On Grounding Fault Monitoring Of DC System

DC system in Substation and power plant supplies power for protection, measurement, automation and control equipments. The healthy of DC system is critical to safe and reliable operation of power system. A single point grounding fault will not affect normal operation immediately. But it presents serious danger on system safety as a new grounding point may cause short-circuit fault or relay mal-operation. Now, there are two kinds of equipment for mornitoring grounding fault. One is portable equipment that can identify actual fault point. Its performance is not satisfactory as the results of influence of distributed capacitance and electromagnetic field. The other is online monitoring equipment that can only identify faulty branch. They are all designed as a centralized system and need to take the signals in through long wires from sensors installed at each branches.The combination of leakage current method and inproved dual-frenquency AC signal injection method to identify the branch with grounding fault is proposed,and a new type of online grounding fault of substation and power plant DC system monitoring system is designed. The leakage current method is used to find fault in single wire. It detects imbalance DC current between positive and negative wires to ascertain branch circuits with grounding fault. The inproved dual-frenquency AC signal injection method is used to detect fault with insulation reduction in both wires,and provide injected signal for portable fault location instruments when branch circuits with grounding fault are ascertained.When it injects a low frequency square wave signal to DC system,current waves of all branch circuits are being sampled, amplitude values of first and third harmonics can be got by FFT of sampled data,so grounding resistances of all branch circuits can be calculated,and then branch circuits with grounding fault are ascertained.The online grounding fault of Substation and power plant DC system monitoring system designed by author is made up of main processing unit, branch circuit deletion units and low-voltage power line carrier communication interfaces. Every branch circuit deletion unit is installed at the branch point,it provids online detection of DC leakage current and assertion of single wire fault;and it detect grounding fault of both wires by sampling current wave to calculate grounding resistance.main processing unit is installed on central control panel, provides human machine interface,receives and processes the data from branch circuit detection units by low-voltage power line communication interface.The simulation of inproved dual-frenquency AC signal injection method and low-voltage power line carrier communication interface test indicate that the system has high accuracy rate.