Reliability Evaluation Of Medium And Low Voltage DC Distribution Network Based On Power Electronic Devices And Equipments

With the development of power electronics and the popularity of renewable energy generation,on the one hand the existing distributed generators which are driven by renewable energy are mainly DC power,on the other hand more and more loads in life have been powered by DC.Traditional AC distribution networks are stretched to their limits.A future trend is DC distribution networks.It is of great significance for the planning and operation of DC distribution network to research the reliability and economy of DC distribution network for the purpose of satisfying the reliability of customers and the economic of power supply companies.Based on this,this paper takes a demonstration project of DC distribution network in an industrial park in Shenzhen as an example to study the reliability and economy of medium and low voltage DC distribution network in three typical topologies.The main contents of the paper are as follows:(1)In order to provide a basis for reliability assessment and economic loss calculation,an iterative algorithm using equivalent load conductance is put forward to calculate DC power flow by redefining the node type of DC distribution network with low power and numerous sources.This method is used to calculate the common DC distribution network topology of hand in hand,which is proved to be operational and effective.Considering the renewable power generation connected to the DC distribution network,a set of evaluation indices of DC distribution network are proposed in this paper.Combined with the traditional reliability indices,the reliability of DC distribution network can be evaluated comprehensively.(2)The reliability of DC distribution network is proposed from component level to equipment level and system level.IGBT modules are the core components of the key equipment for DC distribution network.Taking the Infineon's IGBT modules as an example,the lifetime and failure rate of the IGBT modules are evaluated based on the life model.On this basis,the reliability models of the key equipment of DC distribution network such as converter station,DC transformer and DC circuit breaker are established.Combined with IEEE Std 493 and other standards,the use of part count reliability prediction method to predict the reliability of key equipment's parameters,providing the original reliability parameters for the DC distribution network reliability assessment.(3)The Monte Carlo method has been used to evaluate the reliability of three typical topologies of DC distribution network,then this paper has researched the influence of the location and dispersion of renewable energy sources on the reliability level of DC distribution network.Based on reliability calculation results,the economics of the three topologies are calculated and the topology scheme of DC distribution network is optimized synthetically.The results show that: the relationship of the reliability level of the three typical topologies is that ring type is best,the next is hand in hand,radial type is the worst,which is consistent with the traditional AC distribution network.The reliability level of three topologies are improved after the renewable energy sources connect grid,among which the radial increases most,followed by hand in hand,ring dose not has obvious improvement.This shows that increasing the number of power supplies to improve the reliability of DC distribution network is limited,it is extremely important to optimize the layout and quantity of power supply.Renewable energy discard power frequency index and renewable energy discard power duration index have large differences when the renewable energy sources connect grid at the medium voltage and low voltage bus,respectively.Taking photovoltaic-energy storage power station as an example,its discard power frequency and duration are relatively low when connecting grid at the low voltage bus.According to the renewable energy sources rated voltage to choose a reasonable grid-connected location is of great significance for improving the reliability level of renewable energy source power generation.The economics of different topologies are related to not only operating life,discount rate,but also system reliability indices.The renewable energy sources grid-connected improve the reliability of different DC distribution network topologies to varying degrees,so the impact on the size of the economy is different.