Research On Fault Line Detection In Neutral Point Ineffectively Grounding Power System Based On Improved Phase-locked Loop

Safety, reliability, high-quality and economy are the basic operation requirements of power system. The operation of the power line is closely related to the national production of people’s life. Power system running in different ways is related to the safety of the power supply, reliability of the equipment running, etc. In low voltage distribution network, the small current grounding system is widely used in China. However, single phase-to-earth fault is prone to occur in small current grounding system. It occupies more than90%probability of all kinds of faults. When single phase-to-earth fault occurs, the voltage magnitudes of non-fault two phases are rise to1.732times than before. This will pose a threat to the insulation weak spots of power system and cause an accident. Therefore, the fault line should to be detected timely and accurately, so as to avoid adverse effects on the users.In neutral point ineffectively grounded power system earthed through arc suppression coil, when single phase-to-earth fault occurs, zero-sequence voltage and zero-sequence current are different compared with other grounding system which are used to realize fault line detection. However, there are some harmonic components in zero-sequence current except fundamental component. The compensation of arc suppression coil to fifth harmonic current is only0.04times than to fundamental current. The influence of compensation to the fifth harmonic current can be neglected. The fifth harmonic current in fault line has larger amplitude and opposite phase, which is useful for fault line detection.As zero-sequence current has little fifth harmonic current component, those existing methods fail to extract the fifth harmonic current accurately and other frequency harmonic currents come from error extraction have great influence on detecting sensitivity. In order to solve this problem, on the one hand, almost all kinds of fault line detection in recent years are analyzed, the reasons influencing the accuracy of the line detection is summed up and the steady and transient signals after fault are expounded in this paper. On the other hand, on the basis of analyzing the classical phase-locked loop, the improved phase-locked loop is proposed and the improved phase-locked loop’application in harmonic line detection is analyzed. Finally, using Matlab/Simulink tool, simulation have been carried out to verify the correct of the above line detection theory. The simulation confirms that the method can realize fault line detection timely and accurately.