Research On Modeling And Power Control Of Virtual Synchronous Generator Parallel System

In recent years,with the rapid development of renewable energy,a large number of renewable energy is connected to the grid through power electronic equipment.With the rapid development of renewable energy in recent years,a large number of renewable energy sources are connected to the power grid through power electronic equipment.Different from the mechanical inertia of the traditional synchronous generator,the converter has a fast response speed,but the inertia is small,and a certain control strategy and energy storage device are needed to compensate.Virtual synchronous generator(VSG)has great inertia because it simulates the external characteristics of synchronous generator,which greatly supplements the inertia demand of renewable energy access to power grid.The application of VSG in grid-connected inverter has also become a research hotspot and difficulty.The main research object of this paper is the grid-connected inverter controlled by VSG.The power coupling of VSG,multi-VSG power frequency oscillation and pre-synchronization are studied.Aiming at the power coupling problem of VSG in medium and low voltage microgrid,the traditional power decoupling method based on virtual inductance(VI)cannot achieve steady-state decoupling due to its limited voltage compensation ability.Although the power decoupling method based on q-axis voltage drop(QVPDC)achieves steady-state decoupling,it reduces the ability of reactive power fluctuation suppression.This paper proposes a power decoupling method based on virtual inductance and virtual capacitance(VIVC).Firstly,by introducing virtual capacitance on the basis of virtual inductance,the reference internal potential of VSG is modified by output voltage feedback,which improves the voltage compensation ability.By setting the steady-state coupling coefficient to 0,the virtual inductance and virtual capacitance pairs that can achieve steady-state decoupling are selected.Then,by calculating their dynamic coupling coefficients,the virtual inductance and virtual capacitance satisfying the steady-state coupling coefficient of 0 and the minimum dynamic coupling coefficient at each working point are calculated offline,and the selection table is generated to realize the online rapid selection of virtual inductance and virtual capacitance.Through the HIL experimental verification under different working conditions,the results show that the VIVC method can achieve steady-state decoupling,and has good reactive power fluctuation suppression ability and dynamic decoupling ability.Aiming at the problem that the modeling accuracy of multi-VSG grid-connected system is not high enough and the influence of reactive power loop coefficient cannot be analyzed,the traditional multi-VSG grid-connected power frequency admittance model(OPFA)does not consider power coupling,which has the problem of lack of accuracy in the case of large line impedance,and cannot use this model to analyze the influence of VSG reactive power loop on the power frequency response of the system.This paper proposes a power frequency admittance model(CPFA)considering power coupling.Firstly,the VSG power frequency admittance model is established.By establishing the VSG power small signal model,the power coupling is incorporated into the calculation of the second admittance,and the original power frequency admittance model is modified.The difference between different models is analyzed by Bode diagram.SIMULINK simulation verification is carried out in the case of large and small inertia and strong and weak power grids.The results show that the CPFA proposed in this paper has higher accuracy when the line impedance is large,and the reactive power loop coefficient has little effect on the power frequency response of the system.Aiming at the problem of inrush current caused by the large gap between VSG output voltage and grid voltage in the process of VSG grid connection,this paper studies the traditional PI regulation based on phase angle difference compensation(PI),PI regulation based on virtual power(P_v-PI)and first-order inertia regulation based on virtual power(P_v-JD)pre-synchronization method.The pre-synchronization principle and process of the above several pre-synchronization methods are analyzed,and the influence of their parameter changes on the pre-synchronization performance is analyzed.Through SIMULINK simulation,it is found that the PI method is longer than the pre-synchronization method based on virtual power,and the P_v-JD method is faster than the P_v-PI method.The change of damping ratio will also affect the pre-synchronization performance of the P_v-JD method.