Research On Rely Protection Of Distribution Network With Distributed Generation

With the development of society,distributed generation(DG)plays an increasingly important role in human production and life.However,the topology of the distribution network is changed when DG is connecting to the distribution network.Once the distribution line is shorted,reverse short-circuit current or infeed current is provided by DG,leading protective device to mis-operate or refuse to work.Therefore,it is significantly important to analyze the influences on distribution network protective device by DG and to develop feasible protection methods.To mitigate the influences on distribution network protective device by DG,the calculation formula of phase voltage of the position where DG is connecting to is deduced in normal operation status and short-circuit status of distribution network in the thesis.And MATLAB/Simulink simulation model of distribution network with DG is built.Based on these facts,the influences on three phase short-circuit current by DG connecting to the distribution network from different position and owning changing capacity are researched.And influences on the protective devices by DG are presented.Based on influences on distribution network by DG and grid-connected control strategy,the reasons why traditional adaptive current instantaneous trip protection cannot be applied in the distribution network with DG are analyzed:When DG is connecting to the distribution network,PQ control strategy is applied.Once the line is shorted,DG is seen as voltage controlled current source(VCCS).Because the resistance of VCCS is infinite in theory,setting current is greater than short-circuit current when adaptive current instantaneous trip protection is applied to calculate setting value.And fault cannot be cut off by protection devices.Aiming to this problem,an adaptive current instantaneous trip protection method based on short-circuit current compensated by linear function is proposed after consulting a large number of references.Short-circuit current is compensated by linear function in the method.And method ensures that two phase short-circuit current is greater than setting current within protection range.And short-circuit current is less than the setting current beyond protection range.And operating condition of protective device is satisfied.Independent variable of compensation function is determined by fault location.And it is equivalent to introducing distance protection.Reliability of protection device is satisfied.Also,the free setting of the protection range can be realized by changing compensation function.And the controllability of the protection range is enhanced.Finally,the improved method has been verified by MATLAB/Simulink simulation model.The simulation result proved that reliability of protection device was satisfied by short-circuit compensation method when distribution network was shorted at two lines within protection range.And reliability of protection device was not satisfied beyond protection range.Therefore,changes of topology of the distribution network are adapted by short-circuit compensation method.And influences on distribution network by DG are eliminated.