Microgrid Protection

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009

1045

Protection of Low-Voltage DC Microgrids
Daniel Salomonsson, Student Member, IEEE, Lennart Söder, Member, IEEE, and Ambra Sannino, Member, IEEE Abstract—In this paper, a low-voltage (LV) dc microgrid protection system design is proposed. The LV dc microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV dc microgrid protection system, knowledge from existing dc power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during dc faults. In contrast, an LV dc microgrid must be connected to an ac grid through converters with bidirectional power flow and, therefore, a different protection-system design is needed. In this paper, the operating principles and technical data of LV dc protection devices, both available and in the research stage, are presented. Furthermore, different fault-detection and grounding methods are discussed. The influence of the selected protection devices and grounding method on an LV dc microgrid is studied through simulations. The results show that it is possible to use available devices to protect such a system. Problems may arise with high-impedance ground faults which can be difficult to detect. Index Terms—Circuit transient analysis, DC power systems, power distribution faults, power distribution protection, power electronics.

I. INTRODUCTION

U

SE of distributed resources (DRs) in the electric power system at the distribution level opens new possibilities. A part of the distribution system with its sources and loads can form an isolated electric power system—a microgrid [1]. During normal operating conditions, the microgrid is connected to the ac grid at the point of common coupling (PCC), and the loads are supplied from the local sources and, if necessary, also from the ac grid. If the load power is less than the power produced by the local sources, the excess power can be exported to the ac grid. The sources used in a microgrid are often small ( 500 kW) and are based on renewable energy, for example, PV arrays, fuel cells, and microturbines. These sources produce power with different voltage amplitude and frequency than those used in the microgrid and, therefore, need to be interfaced through powerelectronic converters [2]. A microgrid is well suited to protecting sensitive loads from power outages and, in some cases, also disturbances, for example, voltage dips [3]. High reliability can be obtained by uti-

lizing the power-electronic interfaces of the DRs, together with fast protection systems. To operate the microgrid in island mode, an islanding detection system is necessary, which safely disconnects the microgrid when an ac grid outage occurs to prevent energizing the ac grid [4]. A low-voltage (LV) dc microgrid is most suitable to use where most of the loads are sensitive electronic equipment. The advantage of an LV dc microgrid compared to an LV ac microgrid is that loads, sources, and energy storage can be connected through simpler and more efficient power-electronic interfaces [5]. So far, LV dc microgrids have been used in telecom power systems, and power-system control and protection systems [6]. To ensure reliable operation of the LV dc microgrid, it is important to have a well-functioning protection system. As a starting point, knowledge from existing protection systems for high-power LV dc power systems, for example, in generating stations and traction power systems [6], [7] can be used. However, these systems utilize grid-connected rectifiers with current-limiting capability during dc faults. In contrast, an LV dc microgrid must be connected to an ac grid through converters with bidirectional power flow and, therefore, a different protection system design is needed. Short-circuit current calculations for LV dc systems have been treated in [8] and fault detection in [9]. However, the...
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