Self-tuning virtual synchronous generator control for improving frequency stability in autonomous photovoltaic-diesel microgrids

This paper investigates the use of a virtual synchronous generator(VSG) to improve frequency stability in an autonomous photovoltaic-diesel microgrid with energy storage. VSG control is designed to emulate inertial response and damping power via power injection from/to the energy storage system. The effect of a VSG with constant parameters(CP-VSG) on the system frequency is analyzed. Based on the case study, self-tuning algorithms are used to search for optimal parameters during the operation of the VSG in order to minimize the amplitude and rate of change of the frequency variations. The performances of the proposed self-tuning(ST)-VSG, the frequency droop method, and the CP-VSG are evaluated by comparing their effects on attenuating frequency variationsunder load variations. For both simulated and experimental cases, the ST-VSG was found to be more efficient than the other two methods in improving frequency stability.

An Improved Virtual Inertia Algorithm of Virtual Synchronous Generator

Virtual synchronous generator(VSG)simulates the first-order motion equation of a synchronous generator(SG)with the algorithm.VSG can improve the system voltage and frequency support capabilities of a microgrid or a weak grid.It is now widely applied at a high penetration level of distributed generation(DG)systems.However,because there is a contradiction between active power steady-state deviation of VSG and dynamic impact regulation,the VSG running in grid-connected mode with existing strategies cannot meet the steady and dynamic control requirements.Thus,an improved virtual inertial control strategy of VSG is proposed in this paper.The active power impact is reduced effectively under the circumstance of damping coefficient Dωequal to 0 and a large inertia,thus the dynamic characteristic of active power is improved and its steady-state characteristic is maintained.Firstly,based on the analysis of the damping coefficient effect on the system dynamic process,two forms of improved virtual inertia algorithms are put forward by cascading a differential link into different positions of the first-order virtual inertia forward channel.Then,by comparing the characteristics of the system with the two improved algorithms,the improved virtual inertial strategy based on differential compensation is proven to be better,and the design of its parameters is analyzed.Finally,simulation and experimental results verify the effectiveness of the proposed algorithm.

An Optimal Coordination Control Strategy of Micro-Grid Inverter and Energy Storage Based on Variable Virtual Inertia and Damping of VSG

The virtual inertia and virtual damping affect both the dynamic stability of the virtual synchronous generator(VSG)and the configuration of energy storage,but there is a conflict between them while selecting the virtual inertia and virtual damping.An optimal coordination control strategy of micro-grid inverter and energy storage based on variable virtual inertia and damping is proposed to mitigate this conflict.With the integrated optimal constraint of the VSG frequency variation and the energy storage capacity,the virtual inertia and damping of VSG are configured dynamically,adopting linear quadratic optimal control.It can suppress the oscillations of the active power and frequency to improve the stability of VSG and optimally configure the energy storage capacity of VSG simultaneously.The proposed strategy aims to improve the control performance of micro-grid inverter and the system economy.The simulation and experimental results verify the effectiveness of the strategy.

Overview on Resonance Characteristics and Resonance Suppression Strategy of Multi-Parallel Photovoltaic Inverters

Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the number of string PV inverters increases,which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system.Thus,the problems of multi-parallel inverters resonance have become a hot research topic.This paper is focused on the case of grid-connected string PV inverter systems,and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed.Firstly,the structure of string PV inverter and multi-parallel inverters system are presented.Secondly,discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO)system,Norton equivalent circuit and carrier synchronization.Thirdly,two strategies of^inverter level"and"system level"are provided to summarize the multi-parallel inverters resonance suppression strategy.Finally,the development tendency of resonance suppression strategy and conclusions are prospected.