Research Of Flexible Arc-suppression And Fault Line Selection Methods Of Earth-fault In Distribution Systems Based On Cascaded H-bridge Converter

With the expansion of the size and the increase of nonlinear loads in the distribution network,the earth fault current and harmonic content increase in neutral non-effective grounding grid,so that the arc of ground fault is difficult to self-extinguish.The heat and overvoltage producted by the arc may cause fire or other accidents,such as equipment breakdown.Further,permanent resistance grounding fault will generate step voltage witch are security risks.So arc suppression measures should be taken to achieve full compensation ground fault current for ground fault and suppress arc grounding overvoltage and use method to select and isolate faults selected feeder so as to resume normal operation of distribution network.To achieve full compensation for ground fault current,the hardware must compensate the inadequate of Petersen Coil whitch only fundamental current reactive power can be compensated.Then the corresponding implement and calculation method should be studied for compensating current.Therefore,the arc method witch uses the active inverter as a flexible means of arc is emerged on the basis of APF and SVG.A zero-sequence current was injected by single active converter to compensate the fault current in previous flexible arc-suppression method.Real-time generation voltage was used to calculate the compensation current.Whereas,zigzag transformer and boosting transformer were used for converter to integrate on power distribution network and faulty phase detection was necessary.In order to improve it,a novel method of earth-fault arc-suppression in distribution systems was proposed in this paper by phase separately flexible control based on three-phase cascaded H-bridge converter.When a single-phase-to-ground fault occurred,Real-time phase voltage was used to calculate the compensation current.Then,the compensation current was injected by three-phase cascaded H-bridge converter without zigzag transformer and boosting transformer.Therefore,the voltage value of fault phase was controlled to be zero and the fault arc was suppressed.Considering line parameters and load,an arc suppression optimal method of flexible control for distribution network is proposed.When a single-phase-to-ground fault occurred,the compensation current was injected by cascaded H-bridge converter.Through measuring phase voltage and zero voltage of the bus the grounding resistance and fault phase can be calculated.For higher grounding resistance,the voltage in the fault phase is controlled to be zero by adjusting the injection current according to the result of fault phase election,it means to adopt the voltage-based arc-suppression method.Otherwise,compensation current was injected continuously,it means to adopt the current-based arc-suppression method.After a ground fault arc is extinguished,two separate inter-harmonic currents are injected to the distribution network and the change of equivalent zero sequence admittance for each feeder is calculated to elected and isolate the faulted feeder and restore the normal operation of the distribution network.What's more,the impact of the injected current frequency and size of the active earth resistance for line selection method is analyzed in this paper.The simulation has verified the proposed principle is able to compensate fault current flexibly and effectively,and arc of ground faults is extinguished.What's more,faulty phase detection was unnecessary for single-phase or three-phase cascaded H-bridge converter to suppress the fault arc.At the same time,it suppresses arc flexibly and effectively and solves the shortage of current-based and voltage-based arc-suppression considering line parameters and load.