Abstract

Ideally, diesel hybrid autonomous power systems would operate with high penetration of renewable energy sources such as wind and photovoltaic to minimize fuel consumption. However, since these are inherently intermittent and fluctuating, the grid-forming diesel engine generator sets are usually required to operate with larger amounts of spinning reserve, often at low loading conditions what tends to increases operating and maintenance costs. Frequency stability is of great concern in “small” systems, such as mini-grids, where any individual generator in-feed represents a substantial portion of the total demand. There, the initial rate of change of frequency is typically larger and a lower value of frequency can be reached in a shorter time than in conventional systems with all generation supplied by rotating machines, possibly resulting in under-frequency load shedding and tripping of renewable energy generators. The first part of this paper, discusses some general concepts regarding frequency stability in a diesel hybrid mini-grid and how energy storage systems can be used to enhance system performance. Then, a particular technique based on a virtual synchronous generator is presented and its effectiveness is demonstrated with simulation results.

Keywords

Frequency Control; Mini-Grid; Energy Storage; Inverter

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1. Introduction

Historically, renewable energy sources (RESs) are integrated into autonomous power systems (mini-grids) already supplied by a diesel power plant with multiple diesel engine generator sets (gen-sets). With the addition of RES, it is more likely that the gen-sets will operate under low load conditions resulting in lower efficiency and potential of carbon build up and premature ageing of the diesel engines. Re-sizing of the gen-sets and implementation of appropriate cycling strategies, according to daily and seasonal variations of the mini-grid load and power output of the RESs to maintain optimum loading under various conditions, can help improve the RES contribution and displacement of diesel fuel [1]. Besides, one can also include shortor long-term energy storage systems (ESSs) and controllable (secondary) loads that can be used for improving stability and economic performance of the mini-grid. This can be done by a supervisory controller which should, at one level, (primary control) maintain grid stability and, at the other level, (secondary control) optimize the operation of the power sources, energy storage units and controllable loads, if applicable.

Figure 1 shows a schematic diagram of a supervisory controller of a diesel power plant. Each gen-set presents a primary controller, implemented in the governor, whose main function is to adjust the speed of the gen-set. Since most gen-sets employ a synchronous generator, the governor indirectly controls the frequency of the generated voltage. If the frequency is to be kept constant, an isochronous governor can be used. However, if multiple gen-sets are to operate in parallel, it is more convenient to use a droop based governor where the speed and frequency of a gen-set decrease as its output power increases. This behavior can be represented in steady-state by

Conflicts of Interest

The authors declare no conflicts of interest.

References