Grid-connected Power Generation Systems Based On Virtual Synchronous Generator

Renewable energy generation systems are mostly connected to the grid through power electronic converters.However,the grid-connected power generation systems based on power electronics lack mechanical inertia,and exhibit weak damping characteristics to the power system.Large-scale integration of renewable energy generation systems decreases grid inertia,and weakens the support ability of the grid under dynamics and faults.Therefore,it is very necessary and urgent to improve the friendliness of gridconnected power generation systems and enhance their active support capability.Virtual synchronous generator(VSG)control enables the interface inverter of the gridconnected power generation system to emulate the inertia and damping characteristics of the synchronous generator,so that the compatibility,especially the ability to support the grid,of the grid-connected power generation system are improved effectively.Aiming at the control and application issues of VSG in grid-connected power generation systems,this work deeply studies the VSG control and modeling of the interface inverter,implementation of grid-supporting photovoltaic(PV)grid-connected power generation systems with VSG control,and the identification of VSG damping coefficient and the moment of inertia.Additionally,the low voltage ride-through(LVRT)control method for VSG is presented and the non-detection zone(NDZ)of over/under frequency(OUF)and over/under voltage(OUV)methods detecting VSG islanding are analyzed.The main content and innovation of this work are as follows:(1)Through analyzing the impacts of the key links of VSG control,including the synchronous generator model,the numerical solution for the model and the equivalent port,etc.,on the accuracy of the inverter fitting the synchronous generator characteristics,the implemtations of these links are determined.Accordingly,a strategy of VSG control is obtained.In order to model the external characteristicthe of the VSG-controlled inverter,an equivalent model,which is based on dividing the inverter from the equivalent port into two parts,is proposed.Additionally,the factors,which affect the accuracy of the equivalent model,are analyzed.The verification results show that the proposed VSG control strategy can enable the inverter to accurately mimic the characteristics of the synchronous generator,and that the equivalent model can precisely represent the dynamic and static behavior of the VSG-controlled inverter when the currents track their references accurately.(2)A grid-supporting PV generation system based on VSG is studied.The impacts of perturbation to parameters on system performance are analyzed,and the constraint of system stability is obtained.The analysis and verification results show that the gridsupporting PV generation system can actively support the frequency and voltage of the grid through emulating the characteristics of synchronous generator and its primary frequency/voltage regulation.Moreover,the influence of PV fluctuations and intermittent on the grid is reduced with the proposed grid-supporting PV system,by buffering catastrophe of the PV power and smoothing the power that the grid-supporting PV system feeds to the grid.(3)The quantitative relationship of the static error of active power(SEAP)of VSG,under the condition that the frequency reference is not equal to the grid frequency,with respect to the damping coefficient is established,and then a SEAP-based method for estimating VSG damping coefficient is proposed.To identify the moment of inertia of VSG,the fading-memory recursive instrumental variable method(FMRIVM)is presented based on the constructed transfer function of the output active power.It is demonstrated that the damping coefficient and moment of inertia can be identified through the presented methods.(4)For two abnormal operating conditions of low voltage at interconnection point and unintended islanding,a control method based on mode switching for VSG to implement LVRT is presented,and the NDZ of OUF and OUV detecting the unintended islanding of VSG are also developed.It is indicated with the analysis and verification results that the LVRT control method can capacitate the VSG to ride through the low voltage and can ensure the smooth switching from current control mode to VSG control mode.Furthermore,the obtained NDZ can evaluate the efficiency when OUF and OUV are applied to VSG.(5)The experiment platform based on the VSG control presented in this work is constructed and then the experiments for the grid-connected power generation system are carried out.The theory,method and analysis in this work are verified.