Stability Enhancement Of Low-inertia Grid Through Improved Coordination Of Multiple VSGs System

The high penetration of renewable energy sources(RES)is reducing the strength of a modern grid due to the lack of dependency on a conventional synchronous generator.A synchronous generator provides adequate inertia to a microgrid to achieve a smooth transition under any disturbance,which makes the overall microgrid robust.The inverter control based on virtual synchronous generator(VSG)can simulate the characteristics of a synchronous generator in a microgrid,which can overcome the adverse effects caused by the shortage of synchronous generator in a power grid.However,the coordination of multiple VSGs in a low inertia grid is challenging,such as a higher rate of change of frequency(RoCoF),uneven distribution of active power,irregular inertia and damping coefficient,reactive power sharing error,and incoordination of transient behavior between parallel VSGs.Based on the above problems,theoretical analysis,simulation modeling,and experimental research are carried out in this dissertation.Innovations and research achievements are as follows:1)The power flow equation and small-signal analysis model of grid-connected VSG are established to address a series of problems caused by power coupling in a low-voltage power grid.The small-signal model is extended to the multi-VSG system.The dependency of a droop and damping factor on a static and dynamic behavior during power oscillation is analyzed.It is determined that the minimum damping coefficient should be set for the VSG to avoid vibration.At the same time.the damping coefficient of all VSGs should be greater than the droop coefficient.2)A novel augmented transient control scheme,namely bouncy emergency control,has been introduced.This control uses a variable emergency gain to enhance or reduce the power contribution of individual VSGs on a microgrid during a disturbance.The transient analysis of a proposed controller is verified through the Lyapunov stability analysis.3)A method of predicting virtual impedances of parallel VSGs based on the virtual transient impedance(VTI)technique is proposed.Then the method of Q-V droop control is improved by correcting the excitation voltage of a VSG through a virtual impedance,to solve the problem of uneven distribution of reactive power caused by different rated power.line impedance and output impedance in the multi-VSG system.The effectiveness of the proposed method is verified through qualitative small-signal stability analysis and transient analysis based on the Lyapunov direct method.The another approach,virtual parallel inductor(VPI)technique,is proposed to reduce the reactive power sharing error in a parallel VSG system.It is based on open-loop Q-V control.The relationship of virtual parallel inductor with the output reactive power is derived to modify the equation of traditional Q-V droop control.The effectiveness of the proposed VPI technique is validated through small-signal stability analysis and simulation results.4)A VSG based robust control strategy for decentralized charging and discharging of distributed electric vehicles(EV)is proposed.The control of the virtual synchronous motor(VSM)in charging mode and the virtual synchronous generator(VSG)in discharging mode is adopted based on dq-decoupled independent active and reactive power control.It provides the artificial inertia and damping,the primary regulation of frequency and voltage to the microgrid.The simulation and experimental results show that the VSG based robust control strategy can improve the supportability of frequency and voltage,and enhance the stability of microgrid.