Robust Distributed Cooperative Controller for DC Microgrids with Heterogeneous Sources
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ISSN:1598-6446 eISSN:2005-4092 http://www.springer.com/12555
Robust Distributed Cooperative Controller for DC Microgrids with Heterogeneous Sources Juwon Lee and Juhoon Back* Abstract: In a dc microgrid, a conventional solution for the primary control is droop control, which is achieved by imposing a virtual impedance on each converter. Its main drawbacks are the current sharing inaccuracy and the voltage deviation. To address these problems, we propose a robust distributed cooperative controller, which is an alternative to the droop mechanism. The controller is distributed in the sense that each one exchanges information with its neighboring converters, and hence no central controller is required. It is robust in the sense that parameters of power sources or load are not directly used to determine the controller parameters. A rigorous mathematical analysis is presented to guarantee the stability of the proposed controller, and simulation results are included to validate it. Keywords: DC microgrids, distributed system, load sharing, robust control.
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INTRODUCTION
A microgrid is a small-scale power system composed of distributed energy resources, storage devices, and loads. It can operate as a part of the main grid, and it can still provide power isolated from the main grid if needed. Because various renewable power resources can be combined with a microgrid, this system has received significant attention over the last few decades. For details, see the recent reviews [1–3]. Although most of the literature on this topic has been on ac microgrids, there is an increasing focus nowadays on dc microgrids [4–6]. Fig. 1 shows the structure of a dc microgrid consisting of photovoltaic panels, wind turbines, dc–dc converters, RLC filters, loads, and the main grid. Development of efficient controllers for microgrid has been an active research topic, and a widely-accepted solution is the hierarchical control structure consisting of tertiary control, secondary control, and primary control [7–9]. The tertiary control manages the power between the main grid and the microgrid. The secondary control works to satisfy the demand of the tertiary control. Its objective is to maintain the bus voltage of the microgrid at its reference value, which is generated from the tertiary control. The primary control controls the output voltage and current of each converter.
Energy Source 1
Converter 1
Trans. Line
Energy Source 2
Converter 2
Trans. Line
Energy Source N
Converter N
Trans. Line
Bus Local Load
Local Load
Local Load
Grid
Fig. 1. Typical configuration of a dc microgrids with multiple sources. In dc microgrids, the secondary and primary controls should satisfy two main objectives: dc bus voltage regulation and proportional load sharing. A common solution for the primary control is droop control, which imposes virtual impedance on each converter. In dc microgrids, this control method is achieved by linearly reducing the output voltage reference when the output current increases; see [10, 11] and references therein. Howe
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