Abstract
Since microgrids should be able to smoothly operate in two distinct modes—grid-connected and islanded, their fault currents can widely fluctuate depending on the operational mode. When the microgrid is connected to the grid, the highest fault current, by far, is supplied by the utility grid. In this mode, the fault current contribution from distributed energy resources (DERs) is less than 20%. However, when the microgrid switches to the islanded mode, the fault current contribution from the utility grid is lost and DERs are the sole fault current sources. Thus, the overall fault current in the islanded mode is multiple times lower when compared to the grid connected mode. Moreover, most of the DERs are inverter-based, with limited fault currents, which further reduces the overall fault current in the islanded mode. With the rapid rise of the microgrid penetration around the globe, this phenomenon can adversely influence the relay protection, and thus the microgrid fault current needs to be precisely analyzed. Therefore, the main purpose of this paper is to thoroughly analyze the fault current differences in two distinct operation modes of a microgrid, and to consequently derive conclusions regarding the required improvements in fault calculations and relay protection analysis in emerging microgrids. A representative microgrid test bed is developed and modelled using the in-house developed software as well as in a state-of-the-art hardware-in-the-loop environment. Several different short-circuit faults were simulated and analyzed in both grid-connected and islanded modes. The results show that the fault currents significantly differ depending on the operating mode, and thus highly influence the protection system. Moreover, test results show that the fault calculation algorithms aimed at radial distribution grids, mostly used for microgrid fault calculations in the available literature, need to be further improved to provide precise and time-efficient results when the emerging microgrids are considered. These results provide a valuable insight into the current state of the microgrids’ fault calculation and protection and reveal several important directions for future research.
Highlights
Integration of distributed energy resources (DERs) into power systems is often performed through the microgrid concept, which is defined according to the US Department of Energy as “A group of interconnected loads and distributed energy resources (DERs) with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid and can connect and disconnect from the grid to enable it to operate in both grid-connected or islanded modes” [1]
Because of the ability of a microgrid to properly operate in both operation modes, its fault currents differ widely when a microgrid switches from one mode to another, and this presents a considerable challenge for the relay protection applications [6,7,8]
A thorough analysis of the fault current values in the emerging microgrids is performed. Both grid-connected and islanded modes are thoroughly analyzed, and high differences in fault currents depending on the operational mode are detected
Summary
In the islanded mode, the only fault current sources are DERs and the total fault current is significantly lower when compared to the grid-connected mode As it is discussed in [11,12] the fault currents of inverter-based resources widely differ from the traditional AC machines’ fault currents which complicates both their modeling and integration into the traditional procedures for fault calculations. To cope with the aforementioned challenges, a highly precise and robust fault calculation algorithm specially oriented towards microgrid’s structure needs to be developed, and an adaptive relay protection method that would adapt its settings in real time based on the fault calculation results shall emerge.
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