Research On Power Conversion And Protection Technology Of Space AC/DC Hybrid Microgrid

Solar space power station(SSPS)refers to a solar power system running in the orbit of a geosynchronous satellite.The large-scale development and utilization of space solar energy can effectively solve our country’s energy crisis and environmental problems while realizing the large-scale development and utilization of solar energy.The power transmission and management system technology is important in the technical researches of SSPS.It transmits and distributes the ultra-high power generated by photovoltaic cells to the microwave wireless energy transmission device and auxiliary operating equipment,so its reliability and efficiency directly affect to the safety of operation and the entire power station’s total energy conversion efficiency,but the power and voltage levels of existing spacecraft and satellite power equipment can’t meet the requirements of SSPS.Therefore,it is necessary to carry out targeted research and design on the transmission network frame,converter topology and control system,then focus on the characteristics analysis and protection measures after the system failure.In order to solve the above problems,this article first starts from the topology and uses the power electric transformer(PET)type hybrid AC/DC microgrid as the basis to design a space AC/DC hybrid microgrid with high voltage transmission between the grids and multiple voltage types within the grid.The microgrid serves as the transmission and distribution infrastructure of SSPS,which is mainly divided into three components:high-voltage side,medium-voltage side and low-voltage side.This article selects modular multilevel converter(MMC)as the rectifier on the high-voltage side to connect the grid-side and high-voltage DC buses within the microgrid.On the medium-voltage side,this article uses the isolated MMC-DC/DC to connect the high-voltage DC Bus and low-voltage DC bus.On the low-voltage side,this research selects MMC as AC-DC side energy exchange device to connect low-voltage DC bus and low-voltage AC bus.The low-voltage side is also equipped with input series output parallel dual active bridge(ISOP-DAB)converters for DC low-voltage equipment which is lower than rated voltage to exchange energy.After the topology’s design is completed,this paper constructs mathematical models of MMC,MMC-DC/DC,and ISOP-DAB.After simplification and state averaging,the dynamic model in a control cycle is obtained,which is designed for subsequent control schemes and provides the theoretical basis.Then this paper designs the control system of the two converters on the high and medium voltage side.It’s complicated to design the high-voltage side’s MMC control scheme considering that the dynamic model in the original static coordinate system is the AC quantity.This paper transforms it into a synchronous rotating coordinate system,so it becomes the DC quantity.After that the use of a proportional integral controller can achieve no difference adjustment,supplemented by the sub-module voltage equalization strategy of capacitor’s voltage sorting,the pulse width modulation(PWM)modulation is achieved through the nearest level control(NLC),and the stable conversion of energy is realized.The controller of isolated MMC-DC/DC on the medium-voltage side has to consider the dynamic response,return power suppression,capacitor voltage balance and other goals at the same time,so it’s complicated to design and inconvenient for practical application.In order to solve this problem,this paper introduces model predictive control(MPC)into the design of the control system of the MMC-DC/DC converter.Firstly,it starts from the mathematical model of the MMC-DC/DC converter and uses the discretized space state equation as the predictive model.When performing phase current tracking and return power suppression,the output side’s MMC is made to work in the unit power factor rectification state,so it can effectively suppress the return power while increasing the active power as much as possible.Then the circulating current suppression method based on the predicted duty cycle is proposed for the MMC-DC/DC converter,which is improved from the traditional method.The combination of the sub-module capacitor voltage sorting algorithm and the circulating current suppression can effectively suppress the circulating current and also promote the equalization of the capacitor voltage of the sub-module.The control strategy of the low-voltage side MMC inverter and the high-voltage side MMC rectifier is basically the same,but it needs to be noted that the current flows in the opposite direction to that of the rectifier side MMC,so the dynamic model also needs to be changed accordingly.In addition,considering that its low voltage leads to a less quality of bridge arm sub-modules,carrier phase shift modulation carrier phase shifting SPWM(CPS-SPWM)is used.In the design of the ISOP-DAB controller,this article focuses on solving the problem that the existing control scheme increases with the number of sub-modules,which causes the controller to adjust complex and the dynamic performance cannot be effectively guaranteed.Introduce model predictive control and adopt the combined control method of outer-loop proportional integral controller inner-loop model predictive control.The control objective function adopts the form of quadratic term that is convenient for derivation.The outer loop sets the target input current value of the inner loop to realize output voltage tracking.The inner loop calculates the optimal phase shift angle of each module,realizes the input voltage balance,and simulates and verifies the control.The validity and dynamic performance of scheme,and the two sub-module hardware experiment platform of ISOP-DAB are built,and the experiment verifies the validity of the proposed scheme.Finally,considering the safety requirements of space equipment,this article analyzes the fault characteristics of a power electronic or a diode after the open circuit and short circuit,and designs a submodule spare topology for protection.The possibility of line failures in the system is relatively small,and space radiation effects and single event effects may lead to the damage of power electronics and its anti-parallel diode will further affect the operation of the converter even the system.Therefore,this paper starts from the MMC submodules and DAB which are used in the designed space microgrid,carries out the characteristic analysis of power electronics and its anti-parallel diode after open and short-circuit faults,after that this paper combines designs the corresponding protection scheme using the characteristics and corresponding sub-module replacement strategies which is designed to realize the system recovery after failure.Simulations verify the effectiveness of the proposed topology and scheme.