| Microgrid provides a new way for distributed power sources to connect to the power grid on a large scale.However,with the increasing popularity of distributed power,the inertia of the power grid continues to decrease,which poses a serious threat to the stability of the power grid.Virtual Synchronous Generator(VSG)control provides inertia and damping support for distributed power generation by introducing the rotor motion equation of the synchronous machine,so that it has the external characteristics of traditional synchronous generators.In recent years,it has become a research hotspot in the field of new energy.The VSG control can be divided into two control loops,the active power loop and the reactive power loop.Normally,the control of the active power loop and the reactive power loop are coupled with each other.Especially in the medium and low voltage microgrid,the line impedance generally presents resistance-inductance,and the problem of the power coupling is particularly prominent.Aiming at the problem of single VSG power coupling in low-voltage microgrid,this paper adopts voltage feedforward,which is to introduce virtual impedance control into the basic control of VSG,based on the analysis of the power coupling mechanism.And further the virtual impedance expression is derived according to the control block diagram.Finally,the external characteristics of the VSG are used for simulation verification,which proves that the control has a good decoupling effect.Aiming at the problem of accurate power distribution when parallel VSGs are in steady state,the power distribution conditions are first derived.Then by changing the size of the virtual impedance value,the matching of the total impedance of each VSG to achieve power distribution is ensured.Finally,a calculation example is set to simulate the power distribution,which verifies the correctness of the power distribution theory.Aiming at the line impedance measurement problem encountered in this process,a microgrid line impedance measurement method based on the coordination and control of the microgrid central processing unit(MGCC)is adopted.The MGCC,smart circuit breaker,and micro-grid inverter in the micro-grid are used to cooperate with each other to calculate the line impedance in the micro-grid.Aiming at the problem of terminal voltage drop after virtual impedance control is added,the magnitude of the terminal voltage drop relative to the output terminal voltage is first derived.Then an improved virtual exciter based on the voltage amplitude estimator is proposed to compensate the voltage drop caused by the line impedance and the virtual impedance.Finally,the simulation verifies the effectiveness of the terminal voltage compensation control.In addition to the power coupling and distribution problems in the steady state,the virtual synchronous generator also faces the frequency optimization problem in the transient process.Different from the general synchronous generator,the virtual inertia and damping coefficient of the virtual synchronous generator can be changed according to the needs of the scene,so as to obtain better frequency characteristics,so that the system can transition to a stable operating state as quickly as possible.Therefore,for the VSG frequency optimization problem,firstly,the influence mechanism of virtual inertia and damping coefficient on the output active power and frequency transient process is analyzed.Then the virtual inertia and damping adaptive control methods are studied.Through the sensitivity analysis of the control parameters and root locus analysis,the selection of the relevant control parameters of the VSG is studied.Finally,the programming simulation is carried out,and the comparison shows that the inertia time optimization adaptive control has better optimization performance,which can reduce the frequency offset and the rate of change of frequency fluctuation.The transient process of frequency response is optimized,and the stability of frequency is improved. |
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