Research On Control Strategy Of Microgrid Inverter Based On Virtual Oscillator

With the wide application of renewable energy power generation technology,the problem of insufficient inertia of power grid begins to appear,which affects the stable operation of power grid.Therefore,inverter control technology applied in low inertia power grid has become the current research hotspot.Virtual oscillator control(VOC)strategy is a novel inverter control technology,in which the inverter simulates the nonlinear oscillating circuit,and takes advantage of the inherent resonance properties of the nonlinear oscillating circuit to synchronize the parallel inverter.The inverter based on VOC has good angular stability in low inertia grid.However,the inverter based on the existing VOC strategy shows the characteristics of voltage-active power and frequency-reactive power droop,which can only synchronize with other types of AC power in resistive power grids,but not in inductive power grids.In addition,inverters based on the existing VOC strategy do not have virtual inertia,and a large number of connections to the grid will further reduce the overall inertia of the grid.Due to the existence of a large number of traditional power sources such as synchronous motors in the power grid,too low inertia of the power grid will reduce the angular stability of the power system.According to the above problem,this article is based on the virtual oscillator control technology,the micro inverter control strategy,this paper proposes a improved strategy of VOC,changed the traditional droop inductive of VOCinverter,the inverter can synchronization with other AC source in the inductive power grid,and gives the inverter virtual inertia,to enhance the robustness of the power system.Based on the VOC technology,this paper studies the control strategy of microgrid inverter,and puts forward an improved optimization scheme of VOC strategy,which changes the droop characteristic of VOC-inverter,and enable the inverter to synchronize with other AC power source,and gives the inverter a certain virtual inertia,which improve the stability of the power system.In this paper,the mathematical model of the main circuit and the mathematic model of the switching function of the single-phase voltage inverter are deduced,the design method of the filter is given,and the high and low frequency components of the H-bridge output voltage are analyzed respectively,which establishes the foundation for the analysis of the control strategy of the inverter.Then two control strategies of inverter simulating the characteristics of synchronous motor are introduced and the principle of angular stability is analyzed.The analysis results show that the stability region of the inverter based on the two control strategies power angle is(-90°,90°),and increasing the inertia of the power grid is beneficial to the inverter to keep the angular stability under strong disturbance.Aiming at the VOC strategy based on the dead-zone oscillating circuit,this paper first introduces the working principle and synchronization principle of the dead-zone nonlinear oscillating circuit,gives a sufficient condition for synchronization between parallel oscillating circuits,and introduces the realization method of VOC-inverter.This paper then introduces the existing problems of traditional VOC strategy and the improvement scheme of relevant literature.However,this improvement scheme still has two shortcomings,which cannot be used to inductive grid and reduce the inertia of power grid.Aiming at the two shortcomings,this paper further improves and optimizes VOC strategy.The improved VOC strategy is based on Van der Pol oscillation circuit,and a second-order generalized integrator(SOGI)with a small proportion coefficient at the current feedback end can rotate the current around 90° and produce inertia delay.Then the polar coordinate transformation method,the average method and the small signal analysis method are used to analyze the whole system.The analysis shows that the original voltage-active power and frequency-reactive power droop characteristics of VOC inverter are changed into voltage-reactive power and frequency-active power droop characteristics by the rotation action of SOGI,which enables the inverter to synchronize with other sinusoidal AC in inductive circuit.The inertia delay effect of SOGI gives virtual inertia to the inverter.Access to the grid can increase the overall inertia of the power grid and enhance the robustness of power systems.In addition,sometimes in order to make full use of the power from the front power supply of the inverter,the output power of the inverter needs to be controlled.This paper proposes a power control method of VOC-inverter.In order to solve the problem of excessive impulse current when VOC-inverter is incorporated into the power grid,this paper also designs a presynchronous circuit for improved VOC-inverter,which makes the inverter presynchronize with the power grid before being connected to the grid.Its structure is simple and does not need phase-locked loop.Finally,the correctness of the theoretical analysis and the effectiveness of the proposed method are verified by simulation and experiment.