Impacts Of Inverter-based DG And Induction-machine DG On Distribution Network Current Protection And Countermeasures

Distributed generation technologies are developed rapidly with their unique environmental and economic advantages.At the same time,the application of distributed generation technologies has been bringing significant changes to power system,and distribution system protection is considered to be one of the biggest technical obstacles.With the penetration of distributed generation in distribution network growing significantly,distributed generation is required with low voltage ride through in order to maintain the stability of power system.Hence,distribution network protection problems become a problem that can never avoid after distributed generation connected to the power distribution system.The distribution network is equipped with current protection generally.Therefore,it's necessary to study the impacts of distributed generation on current protection of distribution network and to find suitable protection schemes.The impacts of DG on the distribution system are mostly depending on their sizes and their electrical interfaces by which they are integrated to the distribution network.DGs are categorized into four different types as inverter-based DGs,synchronous-machine DGs,induction-machine DGs and permanent magnet synchronous generator DGs.The contribution of inverter-based DGs and permanent magnet synchronous generator DGs to fault current is investigated and whether each type can potentially impact the protection coordination is determined.Then the strategy is proposed to restrict DGs'contribution to fault current,and consequently,mitigating their impacts on the protection system's coordination,when LVRT is a requirement.The contribution of inverter-based DGs to fault current is mainly related to the control strategy.Based on the current research of inverter,a widely recognized modeling method is proposed.The mathematical model,LVRT strategy and the current limiting is invested,and the reasonable simulation model parameters are selected.The model is used to observe the change of the output current during the fault of the power grid.A response strategy dynamically adjust the reference current of the inverter in real time is proposed.Effectiveness of the proposed scheme is verified with theoretical calculation and simulation.Induction-Machine DGs is categorized into singly-fed induction generator and doubly-fed induction generator.Firstly,the output current of singly-fed induction generator during the grid fault is analyzed from the mathematical model and the simulation results.Because the time constant of the rotor is related with the external resistance of the rotor,the countermeasure to modify the external resistance of the rotor is put forward.Then,the crowbar resistance and output current of doubly-fed induction generator is analyzed according to mathematical model on the basis of the singly-fed induction generator.The theoretical calculation and simulation results show that there is no impact on current protection of doubly-fed induction generator.With clear theoretical principle and clear physical concept,the proposed method provides a novel feasible way to solve distribution network protection problems with DGs.