13.1 INTRODUCTION TO MICROGRIDS

A microgrid is a group of interconnected loads and distributed resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid, enabling local power generation for local loads. It is comprised of various small power-generating sources that make it highly flexible and efficient. Microgrids can be connected to and disconnected from the grid, which makes it operable in both grid-connected or island mode. Its connection to the utility grid prevents power outages in the system; excess power produced can be sold to the grid. Hence, it provides an answer to the energy crisis and transmission losses.

The advantages of microgrid systems are numerous. First, it is very reliable in case of a blackout of any kind. This is important in places such as hospitals. Also, it combines a variety of energy sources including renewable energy, which makes it environment friendly. Since it is locally operated, it is more in touch with appliances in the community and can perform grid optimization and load priority, and use the heat produced by the plant, which previously went unused, to provide heat to homes. A very important factor is lower costs, which can be achieved because the community microgrid has the power to choose how much power it wants from the megagrid based on what it is producing.

The first power plant created, in 1882, could be called a microgrid since centralized grids had not yet been established. However, as centralized grids were created, the utility industry evolved to a state-regulated monopoly. The idea of microgrid was resurrected in the wake of a July 2012 India blackout that left three hundred million people without electricity. Technology was the key enabler of microgrid development.

The reciprocating engine, fuel cells, solar, and wind-farm development have reached a point where a small network of assorted generators can provide power to neighborhoods, retail areas, and even industrial facilities. In a microgrid-enabled future, you might not have to drive to the hydroelectric dam one state away to see where your electricity comes from. Instead, sources will be found in a refrigerator-sized microturbine behind a house, or in a wind farm on the outskirts of town. An experimental microgrid system in Japan keep a hospital running after an earthquake that caused weeks-long outages. As a result of successes like this, the US Department of Energy has invested significant research funds into ways to implement microgrids and the smart grid into industry.

Energy markets today are undergoing a transformation. The top-down monopoly system dominated by large-scale fossil and nuclear power plants that send power to passive consumers is shifting. Microgrid systems aim to be dynamic, bidirectional networks increasingly dependent upon distributed resources. Major trends in microgrids include empowering smart meters from back office to the field in real time, marrying renewable power sources, energy storage and smart grid, visualized distribution grid for planning ahead and emerging microgrid controller design elements.

13.2 TYPES OF MICROGRIDS

Microgrids are electricity-distribution systems containing loads and distributed-energy resources—such as distributed generators, storage devices, and controllable loads—that can be operated in a controlled, coordinated way, either while connected to the main power network or while islanded.

13.2.2 Classification by Purpose

Most microgrids can be classified into four purposes:

• Customer microgrids or true microgrids (µgrids) are self-governed and usually downstream of a single point of common coupling (PCC). Many of the most well-known demonstrations are of this type. They are particularly easy to imagine because they fit neatly into our current technology and regulatory structure. Just as a traditional customer has considerable leeway in the operation of the power system on its side of the meter, so the restrictions on the nature of a µgrid are relatively loose. For this reason, one would expect much of the early deployment of microgrid technology to be of this type.
• Utility or community microgrids or milligrids (mgrids) involve a segment of the regulated grid. There are also existing well-known examples such as Sonoma Community Microgrid Initiative (Santa Rosa, CA), Valencia Gardens Energy Storage Project (San Francisco, CA) and Long Island Community Mirogrid Project (East Hampton, NY). While technically not different from µgrids, they are fundamentally different from a regulatory and business model perspective, primarily because they incorporate traditional utility infrastructure. The corollary of this feature is that utility regulation comes much more significantly into play. In other words, any mgrid must comply with existing utility codes or accommodations must be made in the code. A typical community microgrid structure is shown in Figure 13.1 [1].
• Virtual microgrids (vgrids) cover distributed energy resources (DERs) at multiple sites but are coordinated such that they can be presented to the grid as a single controlled entity. Very few demonstrations of vgrids exist, but they have been proposed in literature. Note that, to be consistent with the definition above, the system must be able to operate as a controlled island or coordinated multiple islands.
• Remote power systems (rgrids) are obviously not able to operate grid-connected, isolated power systems, but involve similar technology and are closely related. From a research point of view, they are so close that they are commonly described as microgrids.

13.3 MICROGRID ARCHITECTURE

Figure 13.2 shows the architecture of a microgrid, which are made up of five major components:

1. Power Source

   Sustainable energy is derived from natural sources that replenish themselves over a short period of time—often called green power. The future electric grid will invariably feature rapid integration of alternative forms for energy generation as a national priority that requires new optimization for energy resources distributed with interconnected standards. A major component of a modern microgrid is its use of DERs.

   There are various types of power sources, usually selected based on cost and efficiency. For example, people in windy areas will favor a wind farm, while people in hot areas of the world will favor solar energy. Several technologies may be considered as probable sources for the generation of electricity. The technology most suitable for each location is unique to that location, depending on the particular application and feasibility as a result of physical location, availability, suitability, source of energy, and cost to deliver or produce energy. Possible energy options for a microgrid are solar photovoltaic cells, biopower, hydroelectric power, wind turbines, fuel cells, and internal combustion engines using natural gas.

   Renewable-energy options for the microgrid provide benefits such as remote utilization and storage of renewable-energy resources, enhancement of functionality of grid-connected renewable-energy systems (RES) to facilitate “give and take” of energy from the system, redistribution/relocation of unused power from a grid-connected RES, facilitating storage of grid-generated and renewable-energy resource–generated energy by backup storage technologies at the customer end, and tracking of interactions for billing and study. Enhancement of functionality of electric vehicles and plug-in hybrids and utilization of a vehicle's battery pack as grid-energy storage devices are also among the benefits.

   The proportion of electric energy or power supplied from wind turbines or other RES is usually referred to as the penetration. It is expressed using annual energy from renewable-energy-powered generators in kWh and total annual energy delivered to loads in kWh as a percentage. When fuel or CO₂ emission savings are considered, the term average penetration is used.

   (13.1)

   The term instantaneous penetration is used for other purposes, including system control. It is much greater than the average penetration.

   (13.2)
2. Energy Management System

   Transmission line provides power management system for microgrid to make it manageable. An inverter is used to change output of the transmission line to the level required by the customer.

3. Energy Storage System

   An energy storage system, such as batteries, is essential to allow the microgrid to balance the demand and supply of electricity, and also to make it accessible.

4. Electricity Loads

   Electricity loads are energy-consuming devices in the microgrid.

5. Main Grid/Utility Connection

   Microgrids are operated as standalone or interconnected mode. When operating in an interconnected mode, it either receives or supplies power to main grid.

13.4 MODELING OF A MICROGRID

A microgrid is basically an active distribution network made up of collection of DG systems and various loads at the distribution-voltage level. Microgrids are generally small, low-voltage, combined heat loads of a small community such as university or school campuses, commercial areas, industrial sites, housing estates, or a suburban locality.

In order to introduce the microgrid to the utility power system as a single controlled unit that meets local energy demands for reliability and security, the micro sources must have power electronic interfaces (PEIs) and controls to provide the necessary flexibility to the semiautonomous entity to maintain the dictated power quality and energy output.

Figure 13.3 shows a typical microgrid connection scheme. Microgrids are connected to a medium voltage (MV) utility “main grid” through the PCC circuit breaker. Micro-source and micro-storage devices are connected to feeders B and C through microgrid controllers (MCs). Some loads on feeder B and C are considered priority loads (i.e., need an uninterruptable power supply), while the rest are non-priority electrical loads. Central controllers (CC) include an energy management module (EMM) and a protection coordination module (PCM). The EMM supplies the set points for active and reactive power output, voltage, and frequency to each MC. This is done by advanced communication and artificial intelligence techniques, while the PCM answers to microgrid and main grid faults and losses of grid situations so that proper protection coordination of the microgrid is achieved.

The model of the microgrid varies for different configurations depending on the type of components used. One important aspect of microgrid modeling is power generation, such as diesel, wind, and solar PV systems.

Diesel engines are a speed-feedback system from a system-control point. After the operator gives a speed command by adjusting the governor setting, the engine governor, also working as a sensor, recognizes the difference between the actual speed and the desired speed and regulates the fuel supply to maintain the engine speed within range. This mathematical relation between fuel flow and mechanical torque is given as

(13.3)

(13.4)

where, φ(s) is the fuel flow, T(s) is the mechanical torque, I(s) is the input current, τ₂ is the time constant, K₃ is the current driver constant, and K₂ is the gain.

A PV cell can be represented as a diode in parallel with a constant current source and a shunt resistor. Different modeling approaches are followed in a PV system. A simplistic modeling of PV system output using a derating method based on the ratio of the actual insolation to the standard condition is given as

(13.5)

where f_pv is the PV derating factor,  Y_pv is the Pv array capacity, I_T is the global solar radiation incident on the PV array, and I_s is the standard amount of radiation used to rate the capacity.

Wind turbines can be equipped with squirrel-cage-induction generators, doubly fed (wound-rotor) induction generators, and direct-drive synchronous generators. Real power output is given by Equation 13.6:

(13.6)

where ρ is the air density (kg/m3), R is the turbine radius (m), Cp is the turbine power coefficient power conversion efficiency of a wind turbine, and V is the wind speed (m/s).

  The electrical power output is given by Equation 13.7:

(13.7)

where, n_o = η_m, and η_g η_m and η_g are the efficiency of the turbine and the generator, respectively.

The models developed for these components, and in a similar manner for the remaining components of a microgrid, are used in different applications such as load-flow algorithms and stability studies of microgrids.

For instance, in power-flow modeling of Ac microgrids, the nodes in the network can be generalized to two types: one as the PQ bus and one where the DG units are connected. Buses connected with loads in the system naturally belong to the PQ buses, and the characteristics of voltage and frequency dependency of these buses can be modeled as

(13.8)

(13.9)

where P_0i and Q_0i are the rated real and reactive power of the load; αi and βi are the exponent coefficients of real and reactive power; Δf is the frequency deviation; and Kpf, i and Kqf, i are constants to represent frequency dependency.

The mismatch for the dispatchable DG bus is given as

(13.10)

(13.11)

Combining the nodal and mismatch expression of PQ bus and dispatchable DG bus, the overall mathematical model of load flow for the overall system can be obtained.

13.5 CHAPTER SUMMARY

In this chapter, the microgrid system was discussed. The basic architectural configuration of a microgrid includes distributed resources, storage devices, utility connection, electrical loads, and power and energy management systems. Identifying site needs, consumer requirements, type and classification of distributed resource availability, and matching with demand and control strategy are among the basic required steps in the planning, design, and operation of a microgrid system. Modeling of a microgrid varies for different configurations depending on the type of components used. Typically, a microgrid is modeled as a low-voltage power system consisting of distributed resources, commonly solar, wind, and other resources, to supply loads to a small community.

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