Basic Concepts and Control Architecture of Microgrids

This chapter discusses the basic concepts and control structures of microgrids. Nowadays, distributed generation technology is becoming increasingly mature, and is deployed as active distribution networks working cooperatively with conventional power grids. In addition, the issues of exhaustible natural resources, fluctuating fossil fuel prices and the security of electricity have encouraged governments around the world to hold positive attitudes toward the development of emerging microgrids. Future microgrids will allow high renewable penetration and become building blocks of smart grids thanks to advanced communication and information technology. As the underlying scientific and engineering research questions are being answered, there is no doubt that microgrids will play an extremely important role in future electric power and energy systems.

Keywords

Centralized control; Control architectures; Decentralized control; Energy storage system; Microgrid concepts; Microgrid protection; Renewable energy resources; State estimation

1.1 Introduction

This chapter discusses the basic concepts and control structures of microgrids. Nowadays, distributed generation technology is becoming more and more mature, and is deployed as key elements of active distribution network working cooperatively with conventional power grids. In addition, the issues of exhaustible natural resources, fluctuating fossil fuel prices and security of electricity have encouraged governments around the world to hold positive attitudes toward the development of emerging microgrids. Future microgrids will allow high renewable penetration and become building blocks of smart grids thanks to advanced communication and information technology. As the underlying scientific and engineering research questions are being answered, there is no doubt that microgrids will play an extremely important role in future electric power and energy systems.

1.1.1 Concepts of Microgrids

Power generation in the traditional power grid is highly centralized, with power and energy flowing unidirectionally from large synchronous generators through a transmission/distribution network to end-users. However, the technological issues associated with traditional electric utilities, as well as the environmental problems caused by the combustion of fossil fuels, have stimulated research and development into new power system technologies. With the emergence of distributed energy resource (DER) units, e.g., wind, photovoltaic (PV), battery, biomass, micro-turbine, fuel cell, etc., microgrid technologies have attracted increasing attention as an effective means of integrating such DER units into power systems. However, there is no clear definition of a microgrid, and the concept varies in different countries and regions. Based on the European Technology Platform of Smart Grids [1], a microgrid is a platform that facilitates the integration of distributed generators (DG), energy storage systems (ESS) and loads to ensure that the power grid can supply sustainable, price-competitive and reliable electricity. Figure 1.1 shows a typical microgrid structure, comprising DGs, such as combined heat and power unit (CHP), microturbines, PV systems, wind power systems, fuel cells; a distributed energy storage (DES) facility such as battery banks, super-capacitors, flywheels, electric vehicles; flexible loads and control devices.

Microgrids can be classified as AC and DC types. AC microgrids can be integrated into existing AC power grid, but they require quite complicated control strategies for the synchronization process in order to preserve the stability of the system. On the other hand, DC microgrids have better short circuit protection and significantly improved efficiency. Furthermore, some synchronous units (e.g., diesel generators) and some non-synchronous units (e.g., micro-turbine machines) are usually connected in the same microgrid system. As the penetration level of more DC loads (especially Plug-in Hybrid Electric Vehicles) increases, hybrid AC/DC synchronous/non-synchronous microgrids via multiple bi-directional converters will become increasingly attractive. Figure 1.2 shows a typical system structure for a hybrid AC/DC microgrid that contains power electronic interfaces and multiple DER units.

Although many types of DG units are more sustainable, a high level penetration of renewable energy resources (e.g., wind, PV) in microgrids can make maintaining grid stability and delivering reliable power challenging due to intermittency and fluctuation issues. In such cases, a DES can play an essential role in improving stability, strengthening reliability, and ensuring security. Not only can DES units be used for smoothing the fluctuations from the output of DG units, but they can also contribute to the stable operation of microgrids. Advances in material science and power electronics technologies have facilitated the effective employment of new DES facilities.

The development of microgrids will bring many benefits but does present significant challenges. For instance, the voltage and frequency disturbance problems in unpredictable weather conditions when integrating renewable energy, monitoring and managing local power generation and loads, designing protection devices to cope with bi-directional power flow and so on. More research needs to be conducted to solve these problems.

1.1.2 Benefits of Microgrids

As mentioned, microgrids provide an effective way for integrating small-scale DERs in proximity of load into low-voltage distribution network. Microgrids can supply highly reliable power to a wide range of customers, both residential and commercial, such as schools, hospitals, warehouses, shopping centers, university campuses, military installations, data centers, etc. Various research stations (Arctic-based or space-based) can also utilize this technology to enhance their operation since it will provide an uninterrupted power supply. It is also useful for remote places having no or limited access to the utility grid. Further, it is beneficial for customers facing large power outages (for example, hurricane-prone areas). Microgrid technology can also be used in areas facing high stress and congestion in their transmission and distribution systems (for example, the northeastern US).

There are many benefits of implementing microgrids. They help facilitate the integration of distributed generation, most notably, renewable energy resources such as wind and solar. This helps curb the dependency on fossil fuels as a source for electricity, significantly reducing carbon emissions and pollution, and thus promotes energy sustainability. They also facilitate the use of highly efficient generators which utilize combined heat and power technology. They can increase the quality of power at the consumer side. With proper control, microgrids increase electrical reliability by decreasing outage occurrences as well as their duration. Utilities see microgrids as controllable loads, which can contribute to peak shaving during times of peak demand by reducing their own consumption via shedding of non-critical loads and delivering more power to the main grid utility. Microgrids can lower overall distribution system losses by implementing distributed generation located at the demand site eliminating the need for transmission lines and deferring the construction of new transmission lines to a later time. This also results in higher energy efficiency. By using renewable energy resources like wind and solar fuel costs can be reduced. There are also several economic opportunities for microgrids if they can participate in local electricity markets. They can offer several ancillary services to the main grid if properly incentivized to do so. Microgrids can provide active power support via frequency regulation, black start support, system restoration support, and load balancing services. Microgrids can be compensated for these services via fixed payments, payments for service availability, payments based on frequency of usage, and/or payments based on lost opportunity cost. This last is the revenue that the microgrid could have made but was not able to because it had to be available for the main utility grid even if it was not called upon [2].

1.1.3 Integration of Microgrid to Distribution Networks

Conventional DGs are usually directly interconnected to distribution networks at medium or high voltage levels. However, generators in microgrids (e.g., PV, wind turbines, fuel cells) have a relatively small installed capacity (e.g., a few hundred kWs). These generators should be connected to distribution networks at a low voltage level. In conventional power systems, loads are passive and power only flows from distribution substations to customers, but not in the opposite way. But power can flow in both directions between microgrids and the main grid.

In the US, the Federal Energy Regulatory Commission (FERC) provides oversight for constructing electric generation, transmission or distribution facilities. FERC permits various ways of integrating renewable energy resources to facilitate electricity market reform.

The technical requirements for distribution interconnections have been stipulated in IEEE 1547 “IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems”. IEEE 1547 is suitable for all distributed resource technologies, with aggregate capacity of 10 MVA or less at the...