Microgrid

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A microgrid is a localized group of electricity sources and loads that normally operates connected to and synchronous with the traditional wide area synchronous grid (macrogrid), but can also disconnect to "island mode" — and function autonomously as physical or economic conditions dictate.[1]

In this way, a microgrid can effectively integrate various sources of distributed generation (DG), especially Renewable Energy Sources (RES) - renewable electricity, and can supply emergency power, changing between island and connected modes.

Control and protection are challenges to microgrids.[2] A very important feature is also to provide multiple end-use needs as heating, cooling, and electricity at the same time since this allows energy carrier substitution and increased energy efficiency due to waste heat utilization for heating, domestic hot water, and cooling purposes (cross sectoral energy usage).[3]

Definition

The United States Department of Energy Microgrid Exchange Group[4] defines a microgrid as 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 connected or island-mode.

The EU research project[5] describes a microgrid as comprising Low-Voltage (LV) distribution systems with distributed energy resources (DERs) (microturbines, fuel cells, photovoltaics (PV), etc.), storage devices (batteries, flywheels) energy storage system and flexible loads. Such systems can operate either connected or disconnected from the main grid. The operation of microsources in the network can provide benefits to the overall system performance, if managed and coordinated efficiently.

Types of microgrids

A typical scheme of an electric based microgrid with renewable energy resources in grid-connected mode

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.[6][7]

Community Microgrids

Community Microgrids can serve a few up to thousands of customers and support the penetration of local energy (electricity, heating, and cooling).[8] In a community microgrid, some houses may have some renewable sources that can supply their demand as well as that of their neighbors within the same community. The community microgrid may also have a centralized or several distributed energy storages. Such microgrids can be in the form of an ac and dc microgrid coupled together through a bi-directional power electronic converter.[9]

Remote Off-grid Microgrids

These microgrids never connect to the Macrogrid and instead operate in an island mode at all times because of economic issues or geographical 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.[6][10] Studies have demonstrated that operating a remote area or islands' off-grid microgrids, that are dominated by renewable sources, will reduce the levelized cost of electricity production over the life of such microgrid projects.[11][12]

Large remote areas may be supplied by several independent microgrids, each with a different owner (operator). Although such microgrids are traditionally designed to be energy self-sufficient, intermittent renewable sources and their unexpected and sharp variations can cause unexpected power shortfall or excessive generation in those microgrids. This will immediately cause unacceptable voltage or frequency deviation in the microgrids. To remedy such situations, it is possible to interconnect such microgrids provisionally to a suitable neighboring microgrid to exchange power and improve the voltage and frequency deviations.[13][14] This can be achieved through a power electronics-based switch[15][16] after a proper synchronization[17] or a back to back connection of two power electronic converters[18] and after confirming the stability of the new system. The determination of a need to interconnect neighboring microgrids and finding the suitable microgrid to couple with can be achieved through optimization[19] or decision making[20] approaches.

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.[6][21]

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.[6][10]

Basic components in microgrids

The Solar Settlement, a sustainable housing community project in Freiburg, Germany.

Local generation

A microgrid presents various types of generation sources that feed electricity, heating, and cooling to user. These sources are divided into two major groups – thermal energy sources (e.g,. natural gas or biogas generators or micro combined heat and power) and renewable generation sources (e.g. wind turbines, solar).

Consumption

In a microgrid, consumption simply refers to elements that consume electricity, heat, and cooling 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. When multiple energy storages with various capacities are available in a microgrid, it is preferred to coordinate their charging and discharging such that a smaller energy storage does not discharge faster than those with larger capacities. Likewise, it is preferred a smaller one does not get fully charged before those with larger capacities. This can be achieved under a coordinated control of energy storages based on their state of charge.[22]

Point of common coupling (PCC)

It is the point in the electric circuit where a microgrid is connected to a main grid.[23] 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 or economic constraints.

Advantages and challenges of microgrids

Advantages

A microgrid is capable of operating in grid-connected and stand-alone modes and of handling the transition between the two. In the grid-connected mode, ancillary services can be provided by trading activity between the microgrid and the main grid. Other possible revenue streams exist.[24] In the islanded mode, the real and reactive power generated within the microgrid, including that provided by the energy storage system, should be in balance with the demand of local loads.[25][26]

A microgrid may transition between these two modes because of scheduled maintenance, degraded power quality or a shortage in the host grid, faults in the local grid, or for economical reasons.[26][27] By means of modifying energy flow through microgrid components, microgrids facilitate the integration of renewable energy generation such as photovoltaic, wind and fuel cell generations without requiring re-design of the national distribution system.[27][28][29] Modern optimization methods can also be incorporated into the microgrid energy management system to improve efficiency, economics, and resiliency.[25][30][29][31]

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.[26] The most relevant challenges in microgrid protection and control include:

  • Bidirectional power flows: The presence of distributed generation (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.[26]
  • Stability issues: Interaction 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 islanding (stand-alone) modes of operation in a microgrid can create transient stability.[32][26] 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.[33][34]
  • 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.[26]
  • 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 phenomenon 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.[26]
  • Uncertainty: The operation of microgrids contain very much uncertainty in which the economical and reliable operation of microgrids rely on. 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).[26]

Modelling Tools

To plan and install Microgrids correctly, engineering modelling is needed. Multiple simulation tools and optimization tools exist to model the economic and electric effects of Microgrids. A widely used economic optimization tool is the Distributed Energy Resources Customer Adoption Model (DER-CAM) from Lawrence Berkeley National Laboratory. Another frequently used commercial economic modelling tool is Homer Energy, originally designed by the National Renewable Energy Laboratory. There are also some power flow and electrical design tools guiding the Microgrid developers. The Pacific Northwest National Laboratory designed the public available GridLAB-D tool and the Electric Power Research Institute (EPRI) designed OpenDSS to simulate the distribution system (for Microgrids). A professional integrated DER-CAM and OpenDSS version is available via BankableEnergy. A European tool that can be used for electrical, cooling, heating, and process heat demand simulation is EnergyPLAN from the Aalborg University in Denmark.

Microgrid control

Hierarchical Control

In regards to the architecture of microgrid control, or any control problem, there are two different approaches that can be identified: centralized[25][35] and decentralized.[36] A fully centralized control relies on a large amount of information transmittence between involving units and then the decision is made at a single point. Hence, it will present a big problem in implementation since interconnected power systems usually cover extended geographic locations and involves an enormous number of units. On the other hand, in a fully decentralized control, each unit is controlled by its local controller without knowing the situation of others.[37] 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.[25][26][38]

Primary control

The primary control is designed to satisfy the following requirements:

  • To stabilize the voltage and frequency
  • To offer plug and play capability for DERs and properly share the active and reactive power among them, preferably, without any communication links
  • 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.[39]

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[40] in which as a centralized controller, it restores the microgrid voltage and frequency and compensates for the deviations caused by variations of loads or renewable sources. The secondary control can also be designed to satisfy the power quality requirements, e.g., voltage balancing at critical buses.[39]

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.[39] 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.[29] 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.[32]

IEEE 2030.7

A less utility influenced controller framework has been designed in the latest Microgrid controller standard from the Institute of Electrical and Electronics Engineers, the IEEE 2030.7.[41] That concept relies on 4 blocks: a) Device Level control (e.g. Voltage and Frequency Control), b) Local Area Control (e.g. data communication), c) Supervisory (software) controller (e.g. forward looking dispatch optimization of generation and load resources), and d) Grid Layer (e.g. communication with utility).

Elementary control

A wide variety of complex control algorithms exist, making it difficult for small Microgrids and residential Distributed Energy Resource (DER) users to implement energy management and control systems. Especially, communication upgrades and data information systems can make it expensive. Thus, some projects try to simplify the control via off-the shelf products and make it usable for the mainstream (e.g. using a Raspberry Pi).[42][43]

Examples

Les Anglais, Haiti

A wirelessly managed microgrid is deployed in rural Les Anglais, Haiti.[44] 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.[45] 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.[46]

Mpeketoni, Kenya

A community-based diesel-powered micro-grid system was set up in rural Kenya near Mpeketoni called the Mpeketoni Electricity Project. Due to the installment of these microgrids Mpeketoni has seen a large growth in its infrastructure. Such growth includes increased productivity per worker with an increase of 100% to 200% and an income levels increase of 20–70% depending on the product.[47]

Stone Edge Farm Winery[48][49]

A micro-turbine, fuel-cell, multiple batteries, hydrogen electrolyzer, and PV enabled Winery in Sonoma, California.

See also

References

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