Research On Protection Scheme In Active Distribution Network

The worldwide problem of energy problem and environmental pollution is more and more serious,traditional grid model of centralized power generation and long-distance transmission have the problem of high power loss and poor power supply stability.Distributed power(Distributed Generation DG)is finding wider and wider application in the power distribution network because of its advantages of cleaning performance,flexibility and adjustability.The access of distributed power supply has changed the unidirectional power supply mode of distribution network,and the flexible operation mode has also brought the grid fluctuations.Integrated distributed power continuously connected to the distribution network has an impact on the traditional three-step current distribution network protection.Through analysis of the working principle of distributed power supply,the equivalent model of inverter interfaced distribution generation(IIDG)was put forward.Active distribution network fault model was then established and the quantitative analysis of how distributed power access affects three-step current protection was implemented.Finally relay protection scheme of active distribution network was discussed,which guarantees the reliable operation of power distribution network.First,the paper introduced the related concepts of active power distribution network,and distributed power supply can be divided into three types: inverter interfaced distribution generation,synchronous electrical machine power and asynchronous electric machine power supply.In the three types of power supply,inverter interfaced distribution generation with good control and the ability to convert electric energy is widely used,becoming a main form of distributed power supply.In addition,the short circuit current model is based on inverter control mode,so the analysis of the influence on the distribution network is more complicated.In view of the different types of distributed power supply,the analysis of short-circuit current was carried on.Second,to realize the optimal efficiency of IIDG according to the low voltage ride through(LVRT)requirement stipulated by the distribution grid standards,a reactive current support strategy with optimized active current output was demonstrated by setting a voltage drop threshold value.The fault characteristics of IIDGs were discussed,and a short-circuit calculation model of distribution networks with IIDGs was established.Case studies in Digsilent show that the IIDG could remain grid-connected and output active power is maximized during voltage dips.The proposed short-circuit calculation model,taking into account the LVRT strategy,is effective for fault analysis of distribution networks with IIDGs,as well as scheme studies of protective relays in large-scale distribution networks.Third,the analysis of the influence of DG capacity and fault point location on three-step current protection and inverse time current protection setting were realized.Using the above proposed IIDG fault equivalent model based on the strategy of low voltage across,the setting methods of instantaneous trip current protection were improved.To solve the problem of IIDG's short circuit current is too small to identify,the overcurrent protection scheme based on voltage acceleration factor was put forward.The contrast analysis was carried out on the different ways to accelerate,and finally the best two methods were chosen.Considering the trend that active distribution network remains meshed structure,the operation of the over current protection close to the power delays too long,which is conducive to stability of the system,so a new protection scheme based on cooperation of the protections at both ends of the fault line was put forward.High permeability of the active power distribution network simulation proves that the protection scheme can solve problems caused by distributed generations in distribution network.