Research On Grid-connected Control Strategy Of Virtual Synchronous Generator Under Non-ideal Conditions

Most of the electrical energy generated by clean energy such as solar energy is connected to the power grid through inverters and other power electronic equipment.These power electronic equipment are different from traditional synchronous generators and lack the inertia and damping.The virtual synchronous generator(VSG)technology enables the inverter to have virtual rotational inertia and damping,thus simulating the operation of the synchronous generator.In actual operation,VSG must face frequent fluctuations on the photovoltaic side,and on the other hand,it must deal with non-ideal operating conditions such as grid voltage imbalance and harmonics.Therefore,it is necessary to study the control strategy of VSG in the non-ideal conditions.Based on this,the research content of this paper is as follows:(1)In order to cope with the problems of stable operation and limited frequency modulation capability of the VSG when the photovoltaic side fluctuates,an active standby VSG control strategy considering the photovoltaic side fluctuations is proposed.Design a control strategy with an additional minimum source voltage limit to prevent the photovoltaic power generation system from reducing the DC terminal voltage to an unstable region due to insufficient output power at the photovoltaic source.At the same time,it proposes an active standby control method to calculate the standby working point at the maximum power point of the system,so as to further enhance the ability of the photovoltaic power generation system to provide power support for the load end or participate in the frequency regulation demand.Finally,the correctness and effectiveness of the proposed control strategy are verified by simulation and experiments.(2)In order to deal with the problem of VSG grid-connected output current distortion and power fluctuation when the grid voltage is unbalanced,an improved VSG integrated coordinated control strategy considering multiple indicators is proposed.On the basis of using VSG balanced current control to obtain positive sequence reference current,it is proposed to realize the unified integration of negative sequence reference current expressions corresponding to the three objectives of current balance,constant active and constant reactive power,so as to design a comprehensive wave function expression.The derivation and discussion of the integrated volatility function realize that the VSG minimizes the comprehensive volatility under the premise of taking into account the three objectives and meeting the grid connection requirements.Finally,simulations and experiments verify thecorrectness and effectiveness of the proposed control strategy.(3)In order to cope with the problem that the VSG grid-connected current generates the same harmonic when the grid voltage has harmonics,an improved VSG control strategy based on the grid voltage full feedforward is proposed.An improved algorithm of inductor current reference value based on double quadratic generalized integration is designed to reduce the influence of harmonic content on obtaining inductor current reference value.In addition,the inertial integration link is used to replace the differential operation link in the grid voltage full feedforward control algorithm,so as to eliminate other subharmonics that may be introduced by the differential link,and select a suitable current regulator to achieve current tracking and harmonic suppression.Finally,the correctness and effectiveness of the proposed control strategy are verified by simulation and experiment.There are 64 figures,6 tables and 90 references in this thesis.