Influence Of Electronic Power Transformer On Relay Protection In Distribution Network

In recent years,with the improvement of people's living standard and the enhancement of environmental awareness,the demand for power quality is higher and higher,and the demand for clean energy is bigger and bigger.The grid-connected renewable energy has great impact on the power supply quality.Conventional transformers are difficult to meet the control requirements of grid.The applications of power electronics in the transformer,greatly improve the performance of the power transformer.This new type of transformer is called electronic power transformer,power electronic transformer or solid state transformer.At present,the research of EPT has been overdeveloped from previous theoretical research to the development of industrial prototype.The access of EPT will change the power flow distribution and the grid structure of distribution network,a part of the protection may not work properly.Therefore,it is particularly important to study the influence of electronic power transformers on the protection of distribution network.According to the classical three stage electronic power transformer topology,mathematical models of input stage,isolation stage and output stage are established respectively.If the three-phase input voltage were asymmetrical,and the high voltage direct current capacitor would appear harmonic.The positive and negative sequence control strategy was introduced to the input stage,effectively suppressing the harmonic of DC in high voltage side.The output stage uses the positive and negative sequence control strategy to ensure that the output voltage is symmetric,when the three phase load is asymmetrical.The protection configuration and the principle of protection setting in distribution network is analysised,and the control strategy of EPT under the fault condition of power grid is expounded.This section focuses on the analysis of the effects of EPT,which connect to the radial overhead distribution network and the cable ring network,on the protection of the circuit outlet breakers,the circuit breakers at middle section,the branch lines and the transformer.A simulation model is built in MATLAB to verify the influence of EPT on the fault current.The EPT fault current limiting control strategy is put forward,of which the output side connect to the 10 k V distribution network.Fault current and voltage characteristics of electronic power transformer is analysised.The influence of EPT on the protection and the mutual cooperation of each protection of radial network and cable ring structure is emphatically analyzed.The fault current limiting control strategy is verified in the simulation software,and the influence on the fault current at the protection is verified.According to the two ways of parallel connection between EPT and conventional transformer,equivalent circuit diagram is established respectively.The fault currents of each protection are calculated and compared with the fault currents without EPT parallel connection.It is concluded that the factors affect the action of protection include short circuit impedance,the maximum output current of EPT and the impedance of conventional transformer.A simulation model is built to verify the conclusion.