Operation and Control of Microgrid

Introduction

An electric grid is a network of synchronized power generators and consumers, which are connected by transmission and distribution lines. The demand for electricity is increasing day by day due to technological advancements. According to the demand, the size of the grid is also increasing rapidly in the past decade. However, the traditional centralized power grid has many drawbacks such as high operating cost, customer satisfaction, less reliability and security. The vulnerability of traditional grid exposed during the large block outs occurred in United States in the year 2003 and South snowstorm in the year 2008. To avoid all these problems, a new trend of generating power locally at distribution voltage levels by using renewable sources like solar photovoltaic cells, fuel cells, wind power, biogas, natural gas, combined heat and power (CHP) systems, natural gas and microturbines and also integration of these into the utility distribution network. It is called as distributed generation (DG) and energy sources are called as distributed energy resources (DERs). The major research work carried about the issues in DG by the group of International Council on Large Electric Systems (CIGRE) and the International Conference and Exhibition on Electricity Distribution (CIRED) in late 1990s and discussed by Stevens, et al 2007. According to the researchers, the following are the common attributes accepted universally,

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  It is not centrally planned by utility, not distributed centrally

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  It is smaller than 50MW

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  DG is connected to distribution system, which are typically of voltages 230/415V upto 145kV.

As like conventional sources, the electricity networks are also need transition from passive distribution networks with unidirectional electricity transportation to active distribution networks with bidirectional electricity transportation. The distribution networks without any DG units are passive and it becomes active when DG units are added which leads to bidirectional power flows in the networks. The active networks should have flexible and intelligent control with distributed intelligent systems. To obtain clean energy from DERs, active distribution networks should employ future network technologies leading to smartgrid or microgrid networks (Fini et al., 2016; Alhelou et al., 2018; Zamani et al., 2018; Alhelou et al., 2015; Njenda et al., 2018; Haes Alhelou et al., 2018).

Distribution generation provide less pollution and global warming due to renewable sources. It also provides the scope for co-generation, tri generation or CHP plants by using waste heat in Industries, domestic and commercial applications and thereby increases the overall efficiency of the plant as said by ANG, et al, in 2014. The location of DG sources are very near to its loads. Hence it reduces the transmission and distribution losses. It is possible to connect DERs to the utility distribution network or in the form of Microgrids because DG generating low voltages. DG also provides high energy efficiency and flexible installation than traditional generation. It also improves the performance of the grid in peak load and reliability of supply.

Micro-Grid

The concept of Micro-grid is proposed by CERTS. A definition was given by CERTS as Micro-grid is a system which composed of load and the Micro-power, while it can provide electricity and heat. The internal power of Micro-grid is mainly responsible for the energy conversion by power electronic devices. Relatively, the external of Micro-grid manifested as a single grid controlled unit, and it meet customer requirements and supply security in power quality.