Research On Operation Characteristics And Coordinated Control Of AC/DC Distribution Network

The development of DGs and DC loads brings great challenges to the traditional network.DC distribution network has become a research hotspot due to its better DG consumption ability and DC load adaptability.As the traditional AC distribution network has limited power supply capacity,small power supply range,and bad power quality,it is urgent to convert the power supply mode.Therefore it is necessary to study control and operation of DC distribution networks.In recent years,with the continuous development of demand response research,combining the demand response model with the control of VSC in DC distribution network has certain significance.The control of DC distribution network is divided into primary voltage regulation and secondary voltage regulation according to different control objectives and response time.The primary voltage regulation is used to quickly respond to system disturbances,and the system voltage is stabilized by adjusting the output of the controllable unit.The secondary voltage regulation is more inclined to the energy management.By setting an objective function and using power flow optimization,the economic and stability of the system are improved.Firstly,this paper studies the basic elements in DC distribution network,including topology,VSC,DGs,energy storage and loads.Then the simulation model is established,and the veracity of model is verified through PSCAD,a kind of software.In terms of DGs,through the equivalent circuit model of photovoltaic system,the characteristics of grid-connected Boost circuit and the principle and steps of MPPT control strategy,a simulation model of photovoltaic grid-connected system is established.And the photovoltaic properties and control effect of MPPT is verified.According to the simple equivalent circuit of battery,the characteristics Buck/Boost circuit and charging and discharging control strategy,the voltage and current control circuits for batteries were established respectively.According to the difference between converters of AC load and DC load,their control circuits are set respectively,and the simulation verify the effectiveness of the control strategy.After establishing the DG and load model,the primary voltage regulation for DC distribution network was studied.In order to meet the system's requirement for a self-regulating capacity,the U-P droop control is used as the primary voltage regulation strategy in this paper.Combining the principle of droop control with VSC control model,a corresponding control block diagram and control circuit are established.The droop coefficient is select according to the inversion of capacity.In order to prevent fault traversal from AC to DC system,the VSC will operate in the current-limit mode or exit mode.Through a seven-node two terminals DC distribution network,the controller and control strategy simulated to react to system disturbances under the normal operation mode,current-limit mode,and exit mode.Combining the demand response and the VSC,a secondary voltage regulation strategy for the DC distribution is proposed.The load aggregator is an agent of small and medium-sized demand response in the electricity market.The optimization model of load aggregators in macro-level and micro-level are established.Through a nine-node two terminals DC distribution network,the economy and stability of the control strategy combining the load aggregator and the VSC are verified.