| Microgrid is emerging as a cost-effective solution for the integration of distributed generation technology in the recent decades.However,considering the high penetration of distributed generation represented by photovoltaic（PV）,microgrid is an electrical system having a low inertia and a lack of frequency support.Virtual synchronous generator（VSG）can effectively improve microgrid inertia by simulating the characteristics of synchronous generator.Therefore,this paper proposes a new VSG control strategy for PV power generation in standalone microgrid to improve frequency stability.The specific research contents are as follows:Firstly,the topology and basic model of PV-diesel-storage standalone microgrid are introduced,and the mathematical model and working principle of diesel generator,battery energy storage and photovoltaic power generation system are analyzed in detail,which provides an effective theoretical basis for the subsequent improvement of micro power control.Secondly,in order to make the PV power generation system have a certain margin to participate in frequency regulation,a PV reserved power tracking method based on off-line fitting is designed.The data points of the sudden change of slope of the PV reserved curve under different irradiance are selected as the segment points,and the measured data are used for linear fitting to obtain the fitting parameters and establish a four-segment linear function of PV reserved power.The PV array automatically locates the reserved power function of the voltage range according to the PV voltage to obtain the approximate reserved power.On this basis,a variable step voltage tracking strategy is adopted to prevent power fluctuation.In order to prevent the PV system from operating in the downhill section,a saturation limiter is set for the PV voltage in the control so that the maximum PV operating voltage does not exceed the voltage corresponding to the PV maximum power point.According to the approximate linear function of the uphill section of the PV P-V characteristic curve and the maximum power curve function obtained based on off-line fitting,the intersection point obtained by simultaneous equations can be approximated as the PV maximum power point voltage.The simulation model is used to verify the effectiveness and reliability of the proposed PV reserve power tracking method under different irradiance.Then,aiming at the delay and noise problems of conventional VSG,a new VSG control strategy based on voltage offset is designed.This strategy analogizes the PV voltage to the power angle according to the similarity of the PV array P-V characteristic curve and the power angle characteristic curve of the synchronous generator.The PV array voltage directly responds to the grid frequency,and modifies the predefined reserved power-voltage curve through the PV voltage deviation,thereby the inertial response is generated spontaneously without grid frequency information,which effectively improves the frequency stability of the system.Finally,the simulation model of autonomous microgrid with PV,diesel generator and battery energy storage is built,and the influence of virtual moment of inertia on inertial response is analyzed.Simulation results show that the inertial response of this strategy can effectively increase the frequency nadir of the system by 0.03 Hz.In the scenarios of different irradiance and load changes,the proposed VSG control strategy is compared with the conventional VSG control and that PV does not participate in frequency regulation,and the superiority of the frequency regulation performance of the proposed VSG control strategy is verified.The simulation results show that the steady frequency change rate of the system using the proposed VSG control strategy is reduced by 0.03 Hz compared with the PV without frequency regulation,and the transient frequency change rate is reduced by 0.01 Hz compared with the system using the conventional VSG control strategy,and the transient process is also more stable.That is,the proposed strategy has better frequency modulation performance.The simulation results of different PV permeability level models show that the higher the PV permeability level in the microgrid,the better the frequency response performance. |
PDF Full Text Request