Study On Earth Fault Selection And Location In Distribution Network

As the development of economy and society in our country, there is a continously increase demand for electrical power, at the same time the power supply quality and reliability of distribution system is facing higher requirements made by people. Since the small current grounded system has a comparatively better reliability of power supply, it is widely used in the power system in our country. Although it has a comparatively better reliability of power supply, the fault current in the small current grounded fault is very tiny, which brings very big difficulty in the selection and location of fault line for grounded fault. Until now many power supply department still have to operate switchgear manually to find the fault line, making occurrence of unnecessary interruption to non-fault lines. As a result, the problem of single phase grounded fault line selection and location has not been solved very well, so make a timely and accurate fault line selection and location has magnificent significance to reduce economic loss and improve safety.The problem of small current grounded system single-phase grounded fault selection and location is researched in this paper. Based on through research into achievements reached by predecessor of this field, concerning the weakness in the signal of small current grounded system single-phase grounded fault causing the difficulty in fault selection and location. The principle of injected alternating current method and principle of injected direct current method are proposed in this paper to make fault selection and location.The principle of injected alternating current fault selection and location is that when a single-phase grounded fault happens, fault line's zero-sequence current is increased by plunging capacitors into fault system from the bus, which expand the differences between fault line and non-fault line's zero-sequence current manually. According to this principle, two methods, injected symmetric alternating current method and injected non-symmetric alternating current method, are included. For symmetric injected alternating current method, three-phase capacitors are plunged into fault system at the same time when single-phase grounded fault happens. For non-symmetric injected alternating current method, a capacitor of the leading phase of fault phase is plunged into the system when the fault happens. These two methods are analyzed and compared in detail in the paper, and a conclusion that non-symmetric injected alternating current method is better than symmetric injected alternating current method.The principle of injected direct current fault selection and location is when single-phase grounded fault happens; a half-wave direct current generation device at the leading phase of the fault phase is plunged into the system in order to inject half-wave direct current into the system. The half-wave direct current circuit exists only in the fault line, the fault selection and location can be made through tracking of half-wave direct current.These two methods can make system similar to large current grounded power supply system out of small current grounded power supply system, performing single-phase grounded fault selection and location in high accuracy and avoiding the problem of bad safety characteristic of large current grounded system. This paper proposes a method which is the same for fault selection and fault location of single-phase grounded fault for the first time, which makes magnificent significance on simplifying system and reducing investments. After the proposing of the method, calculation results manifest the effectiveness of this method, a model machine based on this method is made and run, the result of field measurement and long time run has proved the accuracy, effectiveness and reliability of this method in a further way.