08-16-2017, 11:56 PM
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ABSTRACT
LV dc microgrid is used to interconnect distributed resources and sensitive electronic loads. To ensure reliable operation of the LV dc microgrid, it is important to have a well-functioning protection system. 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 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.
1. INTRODUCTION
Use 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. 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. .A microgrid is well suited to protecting sensitive loads from power outages and, in some cases, also disturbances, for example, voltage dips. High reliability can be obtained by utilizing the power-electronic interfaces of the DRs, together with fast protection systems.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 interface. To ensure reliable operation of the LV dc microgrid, it is important to have a well-functioning protection system. In this paper, a protection-system design for LV dc microgrids is proposed. The influence of the protection system on the LV dc microgrid during faults is studied.
II. LOW-VOLTAGE MICROGRID
An LV dc microgrid is well suited for naturally demarcated power systems, for example, office buildings with sensitive computer loads or rural power systems, but also electric vehicles and ships. Since ac distribution is widespread and not all sources and loads benefit from having a connection to dc,it is reasonable to consider a mixed ac/dc microgrid as in the example in Fig. 1. The dc microgrid is denoted as Zone 1, the ac microgrid is Zone 2, and the ac grid is denoted as Zone 3. A mixed ac/dc microgrid can typically be used in systems up to a few megawatts, and the issue regarding how to interconnect different sources, loads, and energy storage with the ac grid has earlier been treated in [14]. However, the resulting design was partially based on the assumption that no loads were connected directly to the dc bus. The main components used in an LV microgrid are: sources, converters, energy storage, and loads.
A. Sources
Sources used in an LV microgrid can be of various types. Photovoltaic arrays and fuel cells produce dc voltage and, therefore, are suitable to connect to a dc power system via a dc/dc converter. Microturbines are also preferably connected to a dc power system due to their high-frequency output voltage which requires conversion. Similarly, wind-turbine generators produce voltage with varying frequency. To connect the wind turbine to a dc bus, only one converter is required. Internal combustion engines (ICE), for example, diesel engines, commonly used for standby-power generation, are preferably connected to an ac power system.
B. Converters
Both ac/dc and dc/dc converters are used in the LV microgrid, where ac/dc converters are used to interconnect the ac microgrid and the dc microgrid. These converters need to generate sinusoidal ac voltages and currents, and be able to control the bidirectional power flow. Furthermore, the converters must have galvanic isolation, and be able to handle grid disturbances, suchas voltage dips with unsymmetrical voltages [15]. Finally, the converters should have high efficiency. Different dc/dc converters will be used to connect different sources and loads to the dc microgrid. DC/DC converters canbe built simpler compared with ac/dc converters, which results in lower cost and higher efficiency [16].
C. Energy Storage
The availability or the transient response of some DRs, such as wind power, solar cells, and fuel cells require them to be combined with other energy sources or energy storage. Furthermore, energy storage can be used for power-quality (PQ) improvement, load leveling, or emergency power supply [17]. Commonly used storage techniques are batteries, (super)capacitors, and flywheels [18]. Batteries and capacitors can be directly connected to the dc bus, but flywheels are connected through a
machine and a converter.