Research On Control Strategy Of Bidirectional AC/DC Converter In AC/DC Hybrid Micro-Grid

The AC/DC hybrid micro-grid can simultaneously provide power in the form of DC and in the form of AC,simplifying the topology of the new energy accessing micro-grid,reducing the power loss of the power electronic device,and improving the utilization rate of renewable energy.It has become a trend in the future development of micro-networks.The bidirectional AC/DC converter is an important device for AC-DC and micro-grid.Its main functions are: 1.Control the power flow between subnets and balance the power of the whole system.2.Stabilize voltage and AC frequency to improve power quality.Therefore,the control of the bidirectional AC/DC converter has an important influence on the operation of the AC/DC hybrid microgrid.This paper studies the control strategy of bidirectional AC/DC converter in AC/DC hybrid microgrid,mainly for the following functional requirements:1.The DC voltage can be stabilized under grid-connected conditions;2.The DC side voltage and the AC side frequency can be stabilized simultaneously under off-grid conditions;3.It can achieve bidirectional flow of energy and unit power factor operation;4.It can automatically determine the power which required to transmit according to the DC subnet and AC subnet required power;5.It can achieve coordination with the battery.The main contents of the paper include:1、The structure of AC/DC hybrid micro-grid is introduced,and four operation modes are analyzed.The general mathematical model and dq model of bidirectional AC/DC converter are established.The operation characteristics of AC/DC hybrid micro-grid are analyzed under grid-connected conditions and off-grid conditions.The operation modes of each unit during grid connection and off-grid are designed.Finally,three problems that should be considered when designing a bidirectional AC/DC converter are proposed.First,the direction of power transmission can be discriminated and achieve voltage and frequency stability.Second,therequired transmission power can be determined.Thirdly,it can coordinate control with the battery.2、The working principle and model of photovoltaic power generation unit and battery are introduced.Based on the analysis of the operation modes of each unit in the micro-network and off-grid mode,the control strategy of each unit is designed.Under grid-connected conditions,the DC-side photovoltaic unit adopts the MPPT control method,the front-stage Boost converter of AC-side photovoltaic unit adopts the MPPT control method,and the rear-stage grid-connected inverter adopts the grid voltage vector-oriented control technology(VOC).The battery adopts the control strategy of constant voltage charge and discharge.Under off-grid conditions,the DC-side photovoltaic unit still adopts the MPPT control method,and the front-end Boost of the AC-side photovoltaic power unit adopts a constant voltage control method to maintain the stability of the DC-side voltage,and the latter-stage inverter adopts direct power control(DPC)strategy.The battery still uses the mode of constant voltage charge and discharge.In addition,by adopting a method of setting a charge and discharge threshold range of the battery,the number of times of charge and discharge,the system harmonic content,and the life of the system are reduced.Coordinated control with a bidirectional AC/DC converter is also achieved.3、The control strategy of the bidirectional AC/DC converter is designed under the condition of grid connection.The three-loop control structure is proposed.The outermost loop adopts the improved coupling-droop control of P-Udc.There are three main advantages:(1)It can discriminate the direction and stabilize the DC bus voltage,achieving bidirectional flow of energy.(2)It can reduce the frequent fluctuation of the bidirectional AC/DC converter and improve the power quality.(3)Coordinated control with the battery can be realized.The second loop uses DC side current to control the amount of transmission power.The inner loop uses the AC side current to achieve tracking control,which can match the AC side and the DC side power in real time,so as not to cause an overcurrent impact at the interface of thebidirectional AC/DC converter.Secondly,the coordinated control algorithm of battery and bidirectional AC/DC converter under grid-connected conditions is designed,and five modes of bidirectional AC/DC converters are analyzed.Finally,the current outer loop and current inner loop controller are designed.4、The control strategy of the bidirectional AC/DC converter is designed under off-grid conditions,and the three-loop control structure is also proposed.Unlike the grid-connected conditions,the outer loop can no longer adopt the improved coupling-droop control of P-Udc.By analyzing the DC and AC side droop characteristics,the four requirements for the bidirectional AC/DC converter under off-grid conditions are proposed.The bidirectional droop control adopted in the outer ring,by adopting the normalized method to processes the DC bus voltage and the AC side frequency so that the DC bus voltage,the AC side frequency and the active power can be expressed in the same coordinate system.Improved bidirectional droop control is obtained by setting control thresholds.The improved bidirectional droop control can measure the DC side voltage and the AC side frequency at the same time and make a judgment on the power transmission direction.In addition,it can coordinate control with the battery.Aiming at the coordinated operation of the battery and the bidirectional AC/DC converter under off-grid conditions,the four modes of bidirectional AC/DC converters are analyzed,and the control algorithm is designed.