Research On New Type Of Current Amplitude Based Differential Protection For Active Distribution Network

As the main protection of high-voltage transmission lines,traditional current phsor based differential protection has excellent performance,but it has high requirements for data synchronization on both sides of the protected line.Because there exists amounts of lines in the distribution network,including much more unmeasurable branches,the above differential protection cannot be widely used in the distribution network widely due to high investment cost.Fortunately,current amplitude based differential protection is one of the effective ways to solve the selective problem of active distribution network protection.Whereas,there is still room to improve the operation performance of the existing amplitude differential protection,especially for the active distribution network with multiple unmeasurable branches.In this case,it is urgent to study a more excellent performance criterion of the amplitude differential protection.In order to solve the problem of CT saturation resistance of the traditional amplitude based current differential criterion,this thesis proposes the complex amplitude current percentage differential criterion for the important trunk lines which do not contain the unmeasurable branches or the unmeasurable branches can be ignored approximately.The simulation results show that the new amplitude current based differential criterion proposed in this thesis has better performance in active distribution network with inverter DG,and achieves the protection goal of active distribution network under weak synchronization condition.In fact,most lines in the distribution network will be connected with unmeasurable load branches.The existing current amplitude based differential protection is one of the effective ways to solve the selective problem of active distribution network protection without significant dependence on data synchronization at both ends,but its performance is affected by practical engineering factors such as unmeasurable branches.In addition,it should be noted that the unmeasurable branches connected in the line often contain a large number of dynamic loads,such as induction motor and other motor type loads.In this thesis,based on the research idea from shallow to deep,the unmeasurable branch is considered as static load firstly.In this case,the characteristics of the amplitude difference of the currents on both sides of the active distribution network with unmeasurable branches under normal operation and different fault scenarios inside and outside the area are analyzed.On this basis,a new type of current amplitude differential protection criterion is proposed by using the amplitude difference of the positive sequence component of the current on both sides of the protected line before and after the fault to constitute the restraining value and operating value respectively.Simulation results show that the proposed criterion can adapt to different unmeasurable branch access capacity.When considering the unmeasurable branch as the large capacity load mainly composed of induction motor,the existing amplitude based differential protection criterion is difficult to meet the security and sensitivity of the protection at the same time.In order to solve the above problems,the characteristics of fault power effect of induction motor are analyzed,the variation law of current amplitude on both sides of the line with induction motor type unmeasurable branch before and after fault is revealed from the mechanism.On this basis,a new criterion of amplitude based differential protection is proposed: the differences between the amplitude of the two sides of the line before and after the fault constitute the restraint value and operation value respectively,and an adaptive operating characteristic curve is developed.The difficulty of identifying high resistance faults in the operating area and serious faults outside the operating area caused by the charging effect of induction motor is solved.The simulation results show that the criterion can effectively avoid the risk of protection mal-operation caused by the induction motor connecting to the line,and has the ability to withstand the fault resistance of 100 Ω of the whole line in the protection area.