Research On Steady State Operation Strategy And Fault Isolation Technology Of Photovoltaic System Through Flexible DC Transmission

In the current situation of achieving the goal of carbon peak and carbon neutrality,China’s solar energy utilization technology and application ushered in a new development opportunity,photovoltaic power generation will be one of the important means to achieve the goal of "double carbon" in China.But due to the constraints of photovoltaic absorption technology,the phenomenon of light abandonment is not uncommon.However,this issue can be effectively addressed by the gradually mature flexible DC transmission technology,which delivers electricity from areas with abundant photovoltaic resources to areas with high load demand,effectively solving the problem of power load imbalance.In view of this research background,the focus of this paper is on the steady-state operation strategy and fault isolation technology of the photovoltaic transmission system through flexible DC.The main work is as follows:(1)The topology of photovoltaic system via flexible DC transmission system is established,including the topology of photovoltaic system,the topology of photovoltaic DC-DC converter system,the topology of photovoltaic inverter system,and the topology of flexible DC transmission.On this basis,the working principles and operational strategies of various topologies are analyzed.Simulation models of photovoltaic system,photovoltaic DC-DC converter,photovoltaic inverter,sending-end converter station,and receiving-end converter station are built on the MATLAB/Simulink software platform,and the correctness of the models is verified through simulation results.(2)The control strategies of the photovoltaic inverter and the DC system in the photovoltaic flexible DC transmission system are proposed.Firstly,the principle and application of space vector pulse width modulation control algorithm and double closed loop grid-connected control strategy based on PI control are analyzed for the control strategy of PV inverter.Secondly,focusing on the inner loop control of the converter station in the DC system,an improved model predictive control strategy is proposed to achieve capacitor voltage equalization,circulation suppression,and AC current tracking.The improved strategy further reduces the target variable error by introducing PI tracking in the traditional model predictive control and adjusting the proportional coefficient and integral coefficient.Finally,the simulation results based on MATLAB/Simulink show that the strategy can achieve AC current tracking and capacitor voltage equalization,and effectively suppress the internal circulation of the converter station.(3)In view of the shortcomings of traditional DC circuit breakers in DC fault handling,a new Z-source DC circuit breaker topology is proposed.In the structure of the traditional parallel Z-source DC circuit breaker,the common ground between the power supply load is realized by adding a ground capacitor branch in which the diode in the new topological ground circuit is used for fault continuation,and the resistance is used to protect the diode.And by adding a diode on the output side of the power supply,it ensures that the current will not flow back to the DC power supply in the case of DC failure.Secondly,the performance of the new topology and parallel Z-source DC circuit breaker in power loss and fault isolation is compared,and the main parameters of the new topology,such as capacitance and inductance,are determined and applied to the DC line in the photovoltaic flexible DC transmission system.Finally,simulation results based on MATLAB/Simulink show that the new topology can not only suppress the amplitude of fault current,but also improve the speed of DC fault removal.