| In recent years,with people’s attention and application of new energy and renewable energy power generation,the traditional power system has been transformed from centralized generation based on synchronous generator to distributed generation based on renewable energy,and the problems of inertia and damping loss have become more and more serious.Virtual Synchronous Generator(VSG)technology can provide a certain inertia and damping support for the power grid,and improve the power quality and operation stability of the system,which has become a research hotspot.In addition,due to the relatively small capacity of distributed power supply,in order to meet the power supply demand of large capacity load,parallel operation of distributed power supply is used to expand the capacity and improve the power supply reliability of the system.The operation conditions of the transformer network refer to all the grid conditions under which the distributed power source can be normally connected to the grid,that is,the grid frequency ranges from49.5Hz to 50.2Hz and the grid voltage ranges from 187 k V to 242 k V.This paper mainly studies the VSG power loop control parameters and the design method of virtual inductance under the condition of transformer network operation,and proposes an improved reactive power-voltage control strategy to solve the problems of voltage sag and reactive power distribution in parallel connection of VSG parallel island operation.Firstly,the mathematical model of the grid-connected inverter based on VSG control is established,and its circuit model is simplified by adding power loop cutoff frequency constraints.On this basis,the small-signal model of VSG parallel network is established.In order to enable the VSG to operate stably under the condition of transformer network operation,the constraints of phase Angle margin,sag coefficient and power loop cutoff frequency are added.Based on the above constraints,the robust control parameters of the VSG power loop are designed,and the Matlab/Simulink software is used to conduct a simulation experiment of a single VSG under the operating conditions of the substation to verify the effectiveness of the proposed robust control parameter design method.Secondly,in view of the power coupling phenomenon caused by the resistive-inductive circuit,the virtual inductance method is used for decoupling control of the VSG output power.In order to ensure that the VSG can always achieve power decoupling control under the changing operating conditions of the power grid,the coupling coefficient expression of the power loop is derived from the small signal model of the power loop,and the virtual inductance and the maximum coupling under the operating conditions of the substation are drawn.The relationship curve of the coefficient,the design method of the virtual inductance is obtained.The simulation model is built in Matlab,and it is proved that the virtual inductance selected by the proposed method can realize the decoupling control of VSG power loop under the condition of transformer network operation.Finally,taking the parallel system of two inverters as an example,the power loop control parameters of the VSG and the virtual inductance are matched and analyzed.Aiming at the problems of uneven reactive power distribution and voltage sag in parallel off-grid systems caused by line impedance mismatch,an improved reactive-voltage control strategy was proposed from the perspective of voltage compensation,and two sets of different capacities were built in Matlab.,The VSG parallel off-grid operation model with mismatched line impedance verifies the effectiveness of the proposed improved control strategy in improving the accuracy of reactive power distribution and improving the voltage quality at the parallel connection point. |
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