A Virtual Synchronous Machine to Support Dynamic Frequency Control in a Mini-Grid That Operates in Frequency Droop Mode

Miguel Torres,Luiz A C Lopes

doi:10.4236/epe.2013.53025

Abstract

This paper addresses the problem of dynamic frequency control in a diesel-based mini-grid. It is shown that a virtual synchronous machine (VSM) can support dynamic frequency control by adding virtual inertia and damping to the system. However, it is found that the typical formulation of damping power does not work properly when the grid forming gen-set operates in droop mode because of the unknown stabilization value of the grid frequency. As a solution to this problem, an estimator for the stabilization frequency that works in conjunction with the damping function of the VSM is proposed. Theoretical and experimental results provide evidence of a satisfactory performance of the proposed VSM with estimator for different values of the gen-set droop factor. The estimated stabilization frequency converges in approximately 2 s and the maximum frequency deviation during the transient is reduced in 34%, on average.

Highlights

Frequency stability is of more concern in “small” power systems, such as mini-grids, where any individual generator in-feed represents a substantial portion of the total demand
This paper addresses the problem of dynamic frequency control in a diesel-based mini-grid
As a solution to this problem, an estimator for the stabilization frequency that works in conjunction with the damping function of the virtual synchronous machine (VSM) is proposed

Summary

Introduction

Frequency stability is of more concern in “small” power systems, such as mini-grids, where any individual generator in-feed represents a substantial portion of the total demand. When a diesel-based mini-grid operates with a reduced number of gen-sets or a high penetration of inertia-less distributed power sources, frequency stability can deteriorate [7]. In this regard, virtual synchronous machines (VSM), which can be defined as inverters that emulate a synchronous generator (SG) or a desired characteristic of it, have been proposed as a solution to address stability issues in power systems with large fraction of inverterconnected distributed generators [8,9,10,11,12].

Concept of Virtual Synchronous Machine

Estimation of the Stabilization Frequency

Experimental Results

Conclusion