Operation Optimization Strategy Of Multi-terminal Flexible DC Distribution Network

In order to deal with the increasingly serious environmental pollution and energy crisis,renewable energy source(RES)has been vigorously developed in the world.Compared with the traditional energy,RES has the advantages of less environmental pollution,flexible power generation and s o on.However,the integration of RES into the distribution network will bring a series of impact on the operation,control and protection of the power grid.And most of the distributed power supply produces DC power.Though wind power is a kind of AC power with the characteristics of randomness and volatility,which need AC/DC/AC converters,appropriate storage devices and complex control system to integrate to the AC network.The traditional AC distribution network is difficult to meet the demand for the integration of RES,and the development of flexible DC technology provides a solution to this problem.Flexible DC distribution network has a stronger compatibility of distributed RES.It can reduce the converter units for the RES integration into the network which takes DC power as direct or intermediate output,so as to improve the energy utilization rate of RES.Flexible DC technology has the advantages of independent control of power,non reactive power compensation,and power supply to the passive network which has good application prospects in the construction of urban power grid in the future.At present,the research of DC power distribution network and its engineering application at home and abroad is at the beginning stage.Research on operation control technology of multi-terminal multi-voltage grade DC power distribution network has great significance to improve the safety and reliability and save the running cost of DC distribution network.The characteristics of randomness and volatility of the distributed power supply increase the complexity of the distribution network operation.The integration of distributed power supply may cause the bidirectional power flow in the distribution network.The lack of reasonable and effective operation control strategy may cause that the line power flow and node voltage go beyond the limits.This paper proposes an optimal operation strategy for the multi-terminal flexible DC distribution network with high permeability of RES.Main work includes:1)Structure and control strategy of DC distribution network.Firstly,the ring six terminals DC distribution network including wind power,photovoltaic,energy storage and gas turbine is selected as the research object.According to the influence of the integration of RES on the operation performance of the system,the capacities of wind power,photovoltaic are determined,and the network element characteristics are analyzed.Then,according to the system structure,master-slave control mode is selected as control strategy for system level scheduling,and an optimization method of converter station operation strategy is proposed.The converter control mode and control references are added to the optimization variables.2)Dealing with the uncertainty of RES.The chance constrained programming is applied to the optimal operation strategy of DC distribution network with RES,and the particle swarm optimization algorithm combined with stochastic simulation is used to solve the problem.3)Charging and discharging strategy of energy storage system.In the study,the energy storage system is configured by the Power Grid Corp.Maximal revenue of Power Grid Corp is taken as the goal.The advanced control strategy is used to charge when the purchase price of power is low and discharge on the contrary.4)The multi time scale optimal operation model of DC power distribution network is established.By adopting the multi time scale energy managemen t strategy,the scheduling is divided into the day-ahead plan and the day rolling correction.The scheduling objectives,constraints and scheduling processes in different time scale are discussed in detail,and the multi time scale optimal scheduling model of DC power distribution network is constructed.5)Example calculation and analysis.A numerical example is provided to verify the effectiveness of the proposed optimization strategy.