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Publication Title | Control and Protection of Power Electronics Interfaced Distri- buted Generation Systems in a Customer-Driven Microgrid

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Control and Protection of Power Electronics Interfaced Distri-

buted Generation Systems in a Customer-Driven Microgrid

Fang Z. Peng, Yun Wei Li and Leon M. Tolbert

Abstract – This paper discusses control and protection of power electronics interfaced distributed generation (DG) systems in a customer-driven microgrid (CDM). Particularly, the following topics will be addressed: microgrid system configurations and features, DG interfacing converter topologies and control, power flow control in grid-connected operation, islanding detection, autonomous islanding operation with load shedding and load demand sharing among DG units, and system/DG protection. Most of the above mentioned control and protection issues should be embedded into the DG interfacing converter control scheme. Some case study results are also shown in this paper to further illustrate the above mentioned issues.

Index Terms—Distributed generation (DG); renewable energy source (RES); micro-source; microgrid; customer-driven micro- grid (CDM), power electronics; power converters; DG protec- tion; grid-connected operation; islanding operation; autonomous operation.

I. INTRODUCTION

Distributed generation (DG) is becoming an increasingly at- tractive approach to reduce greenhouse gas emissions, to im- prove power system efficiency and reliability, and to relieve today’s stress on power transmission and distribution infra- structure. In recent years, DG systems based on renewable energy source (RES) or micro-sources such as fuel cells, pho- tovoltaic (PV) cells, wind turbines, and micro-turbines are experiencing a rapid development, due to their high efficien- cies and low (or zero) emissions. The micro-source based DG also presents a challenge in terms of interaction to the grid, where the power electronic technology plays a vital role [1, 2].

The development of DG has lead to a more recent concept called microgrid [3], which is a systematic organization of DG systems. Compared to a single DG, a microgrid has more ca- pacity and control flexibilities to fulfill system reliability and power quality requirements. The microgrid also offers oppor- tunities for optimizing DG systems. A typical example is the combined heat and power (CHP) generation or cogeneration, which is currently the most important measure to improve energy efficiency. For the CHP applications, the heat produc- ing generation units and non-heat producing units in the mi- crogrid can be optimally placed with the siting flexibilities of DG units [4]. Furthermore, the microgrid can operate in grid- connected mode or autonomous islanding mode and benefit both the utility and the customers.

A customer-driven microgrid (CDM) is expected to fully

This work was supported in part by the National Science Foundation (USA) under CNS 0831165 and NSF-CNS-0831466, and National Science and Engineering Research Council (Canada) under Grant No. 355773.

F. Z. Peng is with Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824 USA (fzpeng@egr.msu.edu).

Y. W. Li is with Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB T6G 2V4 Canada (yunwei.li@ece.ualberta.ca).

L. M. Tolbert is with Department of Electrical Engineering and Comput- er Science, University of Tennessee, Knoxville, TN. (tolbert@utk.edu)

embrace all these benefits that a microgrid and DG can pro- vide. Since the micro-source based DGs are normally con- trolled and connected to the grid through power converters, by coordinating and controlling individual DG through the power electronics interface, the microgrid has significant control flexibility to fulfill system requirements in terms of efficiency, security, reliability, power quality, etc. In addition, a microgr- id with collective actions of DGs can provide many ancillary services to the upper-stream power system through proper control and communication.

Proper operation of a microgrid requires advanced local voltage control and power flow/sharing control. Moreover, system protection has to be carefully coordinated with each DG and short circuit current has to be limited through the proper control of DG units in a microgrid. To address the op- eration and control issues of a power electronics interfaced CDM system, in the following sections, we will discuss: mi- crogrid system configurations and features that are distinctive- ly different from today’s distribution systems, DG interfacing converter configurations and their control, DG power flow control during grid-connected operation, islanding detection and operation, load demand sharing among DGs, and sys- tem/DG protection. Case studies are provided to illustrate the operation and protection of a CDM system.

II. MICROGRID SYSTEM CONFIGURATIONS AND FEATURES

In a broader and more futuristic view, a microgrid is a tiny power system with a cluster of loads and distributed genera- tors operating together through an energy manager and flexi- ble ac transmission system (FACTS) control devices (such as power flow controllers, voltage regulators, etc.), and protec- tion devices. A microgrid itself can be a dc grid [5] or ac grid (or even a high frequency ac grid [6]). An ac microgrid can be a single-phase or a three-phase system. It can be connected to low voltage or medium voltage power distribution networks. This paper only considers ac microgrids that are connected to a distribution system of the utility power grid and serve as a part of the distribution system.

Fig. 1 shows an illustration of such a microgrid, where sev- eral DG units, including a PV system, a wind power system, two micro-turbine systems and a fuel cell system, are con- nected to the distribution feeders. The microgrid is then con-

nected to the mains grid through a separation device (normal- ly a static transfer switch, STS) at the point of common coupl- ing (PCC), which ensures fast disconnection of the microgrid from the utility in case of a utility fault. More simply put, a microgrid can be viewed as a distribution system with power generators and control devices. As shown in Fig. 1, an energy manager serves as the control center for the microgrid. This energy manager, together with the protection coordinator and power quality controller/monitor, maintain the reliable opera-

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