Microgrid is a localized grouping of electricity sources and loads that normally operates connected to and synchronous with the traditional centralized electrical grid (macrogrid), but can disconnect and function autonomously as physical and/or economic conditions dictate. By this way, it paves a way to effectively integrate various sources of distributed generation (DG), especially Renewable Energy Sources (RES). It also provides a good solution for supplying power in case of an emergency by having the ability to change between islanded mode and grid-connected mode. On the other hand, control and protection are big challenges in this type of network configuration, which is generally treated as a hierarchical control.
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Definition
A formal definition from the U.S. Department of Energy Microgrid Exchange Group states: A microgrid is a group of interconnected loads and distributed energy resources (DERs) within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode.
From the definition from the EU research projects: microgrids comprise Low-Voltage (LV) distribution systems with distributed energy resources (DERs) (microturbines, fuel cells, PhotoVoltaics (PV), etc.), storage devices (flywheels, batteries) energy storage system and flexible loads. Such systems can be operated in both non-autonomous way(if interconnected to the grid) or in an autonomous way (if disconnected from the main grid). The operation of microsources in the network can provide distinct benefits to the overall system performance, if managed and coordinated efficiently.
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Types of microgrids
Campus Environment/Institutional Microgrids
The focus of campus microgrids is aggregating existing on-site generation with multiple loads that located in tight geography in which owner easily manage them.
Remote "Off-grid" Microgrids
These microgrids never connect to the Macrogrid and instead operate in an island mode at all times because of economical issue or geography position. Typically, an "off-grid" microgrid is built in areas that are far distant from any transmission and distribution infrastructure and, therefore, have no connection to the utility grid.
Military Base Microgrids
These microgrids are being actively deployed with focus on both physical and cyber security for military facilities in order to assure reliable power without relying on the Macrogrid.
Commercial and Industrial (C&I) Microgrids
These types of microgrids are maturing quickly in North America and Asia Pacific; however, the lack of well -known standards for these types of microgrids limits them globally. Main reasons for the installation of an industrial microgrid are power supply security and its reliability. There are many manufacturing processes in which an interruption of the power supply may cause high revenue losses and long start-up time.
Basic components in microgrids
Local generation
It presents various types of generation source that feed electricity to user. These sources are divided into two major groups - conventional energy sources (ex. Diesel generators) and renewable generation sources (e.g. wind turbines, solar).
Consumption
It simply refers to elements that consume electricity which range from single devices to lighting, heating system of buildings, commercial centers, etc. In the case of controllable loads, the electricity consumption can be modified in demand of the network.
Energy Storage
In microgrid, energy storage is able to perform multiple functions, such as ensuring power quality, including frequency and voltage regulation, smoothing the output of renewable energy sources, providing backup power for the system and playing crucial role in cost optimization. It includes all of electrical, pressure, gravitational, flywheel, and heat storage technologies.
Point of common coupling (PCC)
It is the point in the electric circuit where a microgrid is connected to a main grid. Microgrids that do not have a PCC are called isolated microgrids which are usually presented in the case of remote sites (e.g., remote communities or remote industrial sites) where an interconnection with the main grid is not feasible due to either technical and/or economic constraints.
Advantages and challenges of microgrids
Advantages
First of all, a microgrid is capable of operating in grid-connected and stand-alone modes, and handling the transitions between these two modes. So that it provides good solution to supply power in case of an emergency and power shortage during power interruption in the main grid
In the grid-connected mode, ancillary services can be provided by trading activity of microgrid and the main grid. In the islanded mode of operation instead, the real and reactive power generated within the microgrid, including the help of energy storage system should be in balance with the demand of local loads.
In islanding mode, there are intentional (scheduled) or unintentional in which intentional islanding can occur in situations such as scheduled maintenance, or when degraded power quality of the host grid can endanger microgrid operation or because of economical reason. On the other hand, unintentional islanding can occur due to faults and other unscheduled events that are unknown to the microgrid. Both of those situation can be dealt actively by using microgrid.
All of above mentioned points and by means of modifying energy flow through microgrid components, microgrid allows and facilitates integration of renewable energy generation such as photovoltaic, wind and fuel cell generations without requiring re-design of the distribution system. Modern optimization methods can also be incorporated into the microgrid energy management system to improve the efficiency, economics, and resiliency.
Challenges
Microgrids, and integration of DER units in general, introduce a number of operational challenges that need to be addressed in the design of control and protection systems in order to ensure that the present levels of reliability are not significantly affected and the potential benefits of Distributed Generation (DG) units are fully harnessed. Some of these challenges arise from invalid assumptions typically applied to conventional distribution systems, while others are the result of stability issues formerly observed only at a transmission system level. The most relevant challenges in microgrid protection and control include:
o Bidirectional power flows: The presence of DG units in the network at low voltage levels can cause reverse power flows that may lead to complications in protection coordination, undesirable power flow patterns, fault current distribution, and voltage control.
o Stability issues: Interation of control system of DG units may create local oscillations, requiring a thorough small-disturbance stability analysis. Moreover, transition activities between the grid-connected and stand-alone modes of operation in a microgrid can create transient stability. Recent studies have shown that direct-current (DC) microgrid interface can result in significantly simpler control structure, more energy efficient distribution and higher current carrying capacity for the same line ratings.
o Modeling: Many characteristic in traditional scheme such as prevalence of three-phase balanced conditions, primarily inductive transmission lines, and constant-power loads are not necessarily hold valid for microgrids, and consequently models need to be revised.
o Low inertia: The microgrid shows low-inertia characteristic that are different to bulk power systems where high number of synchronous generators ensures a relatively large inertia. Especially if there is a significant share of power electronic-interfaced DG units, this phenomenant is more clear. The low inertia in the system can lead to severe frequency deviations in stand-alone operation if a proper control mechanism is not implemented.
o Uncertainty: The operation of microgrid contains very much of uncertainty in which the economical and reliable operation of microgrids rely on that. Load profile and weather forecast are two of them that make this coordination becomes more challenging in isolated microgrids, where the critical demand-supply balance and typically higher component failure rates require solving a strongly coupled problem over an extended horizon. This uncertainty is higher than those in bulk power systems, due to the reduced number of loads and highly correlated variations of available energy resources (limited averaging effect).
Microgrid control
Regarding to architecture of microgrid control or any control problem there are two different approaches can be identified: centralized and decentralized. A fully centralized control relies on a big amount of information transmittence between involving units and then the decision is made at a single point. Hence, it will present big problem in implementation since interconnected power systems usually cover extended geographic and involves enormous number of units. The fully centralized control is currently considered as infeasible solution. On another hand, in a fully decentralized control each unit is controlled by its local controller without knowing the situation of others. The fully decentralized control is also irrelevant in this context due to strong coupling between the operations of various units in the system. A compromise between those two extreme control schemes can be achieved by means of a hierarchical control scheme consisting of three control levels: primary, secondary, and tertiary.
Primary control
The primary control is designed to satisfy the following requirements:
o To stabilize the voltage and frequency.
o To offer plug and play capability for DERs and properly share the active and reactive power among them, preferably, without any communication links.
o To mitigate circulating currents that can cause over-current phenomenon in the power electronic devices
The primary control provides the setpoints for a lower controller which are the voltage and current control loops of DERs. These inner control loops are commonly referred to as zero-level control.
Secondary control
Secondary control has typically seconds to minutes sampling time (i.e. slower than the previous one) which justifies the decoupled dynamics of the primary and the secondary control loops and facilitates their individual designs. Setpoint of primary control is given by secondary control in which as a centralized controller, it restores the microgrid voltage and frequency and compensate for the deviations caused by the primary control. The secondary control can also be designed to satisfy the power quality requirements, e.g., voltage balancing at critical buses.
Tertiary control
Tertiary control is the last (and the slowest) control level which consider economical concerns in the optimal operation of the microgrid (sampling time is from minutes to hours), and manages the power flow between microgrid and main grid. This level often involves the prediction of weather, grid tariff, and loads in the next hours or day to design a generator dispatch plan that achieves economic savings. In case of emergency like blackouts, Tertiary control could be utilized to manage a group of interconnected microgrids to form what is called "microgrid clustering" that could act as a virtual power plant and keep supplying at least the critical loads.During this situation the central controller should select one of the microgrid to be the slack (i.e. master) and the rest as PV and load buses according to a predefined algorithm and the existing conditions of the system (i.e. Demand and generation), in this case, the control should be real time or at least high sampling rate.
Examples
A wirelessly managed microgrid is deployed in rural Les Anglais, Haiti. The system consists of a three-tiered architecture with a cloud-based monitoring and control service, a local embedded gateway infrastructure and a mesh network of wireless smart meters deployed at 52 buildings.
Non-Technical Loss (NTL) represents a major challenge when providing reliable electrical service in developing countries, where it often accounts for 11-15% of total generation capacity. An extensive data-driven simulation on 72 days of wireless meter data from a 430-home microgrid deployed in Les Anglais, Haiti has been conducted to investigate how to distinguish NTL from the total power losses which helps energy theft detection.
Source of the article : Wikipedia
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