Research On Optimization And Coordination Of Inverse-time Overcurrent Protection For Distribution Network Feeders

The distribution network has a wide range of points,is directly connected to users,has variable operation modes,few electrical parameter measurement points,and has poor observability of topology structure and fault characteristics.Compared with the transmission grid,the distribution network fault is more complicated,and a considerable part of it has weak fault characteristics,which makes the passive fault observation at a single location of the feeder outlet and the simplified feeder protection based on the significant difference between the fault and non-fault characteristics.There are serious difficulties in setting and level difference coordination.With the continuous decentralized access of a large number of inverter-type DGs(Distributed Generation),DG's active participation in regulation is expected to positively improve the fault characteristics observed at the protection installation.Try to make the relay protection action more sensitive through the active participation of DG under the fixed protection setting and cooperation mode,that is,through the active regulation of DG to aggravate the differentiation of fault and non-fault characteristics,so as to make the protection and cooperation under the rigid setting It has better performance for complex faults.The main work and achievements of the thesis are as follows:(1)Analyzed the characteristics of the feeder phase-to-phase short circuit and the neutral point effective ground fault of the distribution network.On this basis,the inverse time overcurrent protection characteristics of the distribution network were studied,and the inverse time limit of distributed power access was obtained through simulation analysis.The influence of over-current protection provides theoretical foundation and technical support for the subsequent integration of distributed power sources to play an active role and the level difference of traditional inverse time characteristic protection.(2)A distribution network protection scheme based on photovoltaic power PV actively participating in the adjustment of the electrical parameters of the fault loop is proposed.The scheme first identifies the mode of the PV,and adjusts the PV to reduce the output during the fault and directly disconnect Means such as setting low voltage on the grid and grid-connected points to increase the fault current flowing through the protection installation to achieve the purpose of quickly removing the fault,and have minimal changes in distributed power access,low dependence on communication,and convenient coordination with traditional protection.(3)The fault characteristics of the low-resistance grounding system are studied.Based on the traditional inverse time overcurrent protection characteristics,a kind of protection is proposed to realize the coordinated protection by judging the power flow direction of the circuit breaker of the transfer line,and then adjusting each protection action time limit step by step.Method,real-time adjustment of the protection level difference setting value,this method does not need to rely on the remote communication between the two stations,and the protection coordination between the two stations can be realized only by adjusting the power direction and time level difference of the transfer line on the power transmission side.And it can be applied to the normal scale state and the transfer mode,to ensure the selectivity between the protections,to remove the fault in the shortest time,and to reduce the scope of the fault power outage.The research work in this paper is helpful to improve the existing protection performance of the distribution network,through the photovoltaic power source actively participates in the electrical parameter adjustment of the fault circuit,aggravates the fault characteristics,and improves the problem of weak faults in the system;small resistance based on inverse time characteristics The load transfer operation mode of the grounding system,by identifying the transfer mode and real-time adjustment and coordination of the protection level difference,helps to reduce the scope of the fault power outage,and has strong applicability to the actual distribution network operation.