Analysis of the frequency‐based control of a master/slave micro‐grid

Abstract

Renewable energy penetration in the low-voltage grid faces several limitations due to the current grid topology. Master/slave micro-grids could help solving these issues, by offering additional services to the grid, such as the power management of the distributed power energy sources.In some cases, the power produced by the distributed energy sources exceeds the local consumption of the low-voltage grid. The consequent reverse power flow can be either dangerous (for the medium voltage) or impossible (for a micro-grid with limited storage). The droop characteristic of commercial inverter can be exploited to avoid this behavior. However stability problems can arise due to a low phase-locked-loop (PLL) bandwidth. This paper investigates the stability of this solution depending on the bandwidth of the PLL of the distributed power generation sources. I. INTRODUCTIONThe distributed power generation systems (DPGS) became more and more important over the last few years. Their increasing popularity has been driven by the use of renewable energy sources and the rising costs of the energy distribution, leading to modifications of the electric grid. Among the renewable energy sources employed in this framework, photovoltaic (PV) is receiving a great attention. PV systems can be divided into island-grid (or stand-alone), where the converters supply local loads, and grid-connected, where the power produced by the panels is transferred to the electrical grid.In general, grid-connected systems have been preferred to stand-alone ones, since the mains can compensate the energy production oscillations due to the variable nature of the PV source. As a consequence a great number of distributed low power sources have been connected to the LV grid in the last years, leading in some case to grid stability problem such as: transformer reverse power flow, voltage rise, unexpected islanding operations, sympathetic tripping, etc.[1], [2].To partially mitigate these problems, standards have recently imposed to grid-connected inverters to vary their output power in function of the instantaneous value of grid voltage and frequency (CEI-021 [3]). In fact, when in a LV grid the amount of energy from installed distributed sources is comparable to the loads' consumption, particular care must be taken to manage conditions of high imbalance between power generation and energy demand. Therefore, grid-connected systems have to limit their output power to face the cases of low local consumption. For PV systems an example is that of summer months in south European regions, when to an high PV energy production may not correspond an equivalent local demand [4]. This paper investigates the stability of a micro-grid where a frequency control is adopted to manage the power balance [5]- [7]. The basic assumption is that the frequency of the micro-grid can be changed (fully-decentralized micro-grid or smart-transformer micro-grid) to interact with the distributed sources. The novelty of the proposed approach is that standard current-controlled grid-connected ...

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