Research On Effects Of Packet Switched Network QoS On Reliability Of Line Current Differential Protection

Line current differential protection is a relay protection method which is often used in the production of electric power,and it is the main protection that must be configured for for transmission lines of 220 kV and above voltage level.Its reliability is related to the quality of the transmission channel.And with the acceleration of smart grid construction,the service types of power communication network is shifting from low-rate TDM services to high-rate IP services,and SDH/MSTP technology has been unable to meet the demands.As the packet-based PTN technology conforms to the development trend of“IP,intelligent,broadband,and flat”of the network,it has become an inevitable choice to use this technology to form the next generation power communication transmission platform.However,whether PTN can carry the relay protection communication business is still inconclusive,so it is necessary to verify it through sufficient theoretical validation and sufficient simulation data.In addition,sufficient experimental verification should be carried out when conditions permit.And then theoretical,simulation and experimental data should be combined to fully verify the correctness of the conclusions.The existing researches on the impacts of packet switched network QoS on the reliability of line current differential protection were mostly qualitative analysis through simulations and tests,and the existing quantitative analysis were carried out in a specified conditions,and there is almost no comprehensive reliability analysis model.This paper analyses the protection principle and operation criterion,and a reliability evaluation model of current differential protection based on the commonly used QoS parameters of PTN in packet switching network on current differential protection is established.Among the commonly used QoS parameters,the asymmetric delay is extremely difficult to eliminate,and it will directly affect the synchronization process,so this paper focuses on the quantitative analysis method of the effect of channel asymmetric delay on the action under different fault conditions including fault types,fault resistances,fault locations,and calculates the minimum reliability of current differential protection under the influence of asymmetric delay using an actual example,and verifies the feasibility and correctness of the proposed model and method at the same time.The numerical simulation results show that the effects of asymmetric delay on line current differential protection have a significant correlation with the fault conditions.Under the given action criterion,the following conclusions can be drawnas long as the asymmetric delay is always less than the maximum value which can ensure that the protection does not malfunction: when the fault occurs in the area,the greater the grounding resistance is,the greater the rejection probability is;when the grounding resistance is not more than 300?,the probability of rejection is tiny;when the fault occurs outside the zone,the probability of fault operation is the largest when the grounding resistance is about 100? under A phase ground fault occurs;when there is no fault,differential current and braking current are small so that asymmetric delay will not cause fault operation.Consider of all fault conditions comprehensively,the lowest reliability occurs when phase A is grounded,and the most important factor affecting reliability is the probability of fault operation when the fault occurs outside the zone.Taking measures to reduce asymmetric delay or change the setting value of protection criterion reasonably can improve the reliability of the system under the same conditions effectively.