Microgrids
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Background
Historically, power grids in the U.S. and Canada were built according to the traditional, centralized structure of generation, transmission, distribution, and end user. Power is generated at large power plants, usually located away from the major population centers, with electricity transported across long distances to the end user. Although this worked well for over a century, the system is now under strain due to aging assets, an energy-hungry modern society that needs uninterrupted supplies and changing climatic conditions that are instigating a rise in the frequency and intensity of extreme weather events.
Utilities upgraded, modernized, strengthened and maintained the distribution grid, but largely kept the same structure and layout. Due to the integration of weather-based green energy resources in the current electricity grids, and technology advancement in communication, control and operation, the industry and governments are now looking at modern approaches to manage grids. The answer to tackling many of the problems faced by electrical distribution systems may lie within the microgrid, which offers a way to integrate disparate resources, build reliability and resiliency into the system, reduce grid congestion and peak loads, and provides a number of economic and environmental benefits.
What is a Microgrid?
The term “microgrid” is used to describe a number of concepts involving distributed generation (DG). However, the industry-accepted definition, from U.S. Department of Energy (DOE), describes a microgrid as: “a group of interconnected loads and distributed energy resources (DER) within clearly defined electrical boundaries that act as a single controllable entity with respect to the grid, and that can connect and disconnect from the grid to enable it to operate in both grid-connected and ‘island’ mode.”
Like the traditional, centralized electric grid, microgrids generate, distribute, and regulate electricity to customers, but do so locally and on a much smaller scale. Microgrids provide “grid-interactive solutions” to utility grid flexibility and resiliency challenges associated with meeting the demand for continuous supply of electric power.
Importantly, microgrids are not a complete replacement for utility distribution infrastructure. Instead, microgrids form a self-contained organization of DGs and load management that is capable of self-balancing, when necessary, within an isolatable portion of utility or non-utility infrastructure. Individual microgrids usually operate in a grid-tied mode, with bi-directional power flow between the microgrid and the surrounding system. The ability to separate from the grid provides a backup or emergency operation mode, as well as the opportunity for greater DER investments and coordination. Microgrids may contain dispatchable and non-dispatchable renewable generation, controllable loads, energy storage, electric vehicle-to-grid (V2G) charging/discharging, and advanced grid modernization technologies, such as advanced metering infrastructure (AMI) and distribution automation.
What is Not a Microgrid?
Virtual Power Plants (VPPs) and microgrids can serve overlapping use cases but remain distinct concepts. VPPs manage individual DER as aggregated groups to provide various services to the grid; the aggregation services of a VPP can be tied to a single point of reference (e.g., limiting voltage constraints at a distribution node). There is an overlap in the technologies used, with VPPs and microgrids both possessing the ability to incorporate demand response, renewable distributed energy generation, and local storage. Both types of control require direct communication with local resources and aggregation management. However, there are some important differences:
- VPPs are always tied to the grid and do not offer islanding functionality.
- VPPs do not require grid-forming resources, while microgrids must have DER that can control voltage and frequency.
- Resources managed by a VPP may operate over a wide geographical area; microgrids must operate within an electrically interconnected boundary of the distribution system.
- Microgrids usually operate DER at a local level, whereas VPPs can be linked with wholesale markets.
- VPPs can operate with few legal requirements, but microgrids require comprehensive political, social and legal requirements.
- VPPs require smart metering, communication network and associated technologies, while microgrids can depend only on switches, inverters and basis controllers.
Microgrid Design and Operation Fundamentals
Design and operation of each microgrid type comes with a unique set of technical requirements. The inherent ability of microgrids to intentionally energize portions of the grid during outages brings safety challenges to the design, including back feed during unintentional islanding, and effective grounding. This section presents a broad view of components and considerations that impact the microgrid.
- Voltage (Control/Support), Frequency (Control/Support), and Power Quality – Ability to black start, microgrid voltage, frequency, and power quality regulation within standard (ANSI, IEEE, NEC) limits during grid-connected, islanded, and transitional operating states.
- Protection Equipment and Coordination – Ability of protection equipment within the microgrid to effectively respond to fault conditions on the microgrid, isolate, and protect equipment and people during a fault event. This includes the ability to differentiate between intentional and unintentional islanding events. Effective grounding during grid-connected and islanded states, as well as having the ability to detect fault conditions during islanding operation and black start is critical to the system protection scheme.
- Communication – The communication infrastructure for a microgrid requires reliable connectivity between the microgrid controller and critical DER. Considerations must be made for loss of conventional telecommunication infrastructure during times of crisis. Definition and agreement upon a communication interface between the microgrid controller and utility control infrastructure (DERMS, SCADA, etc.,) may be needed. Cybersecurity must also be addressed.
- Dispatch – External microgrid control functionality, such as interactions between microgrid controller and utility control infrastructure, and allowable DER and non-DER dispatch strategy – particularly when entering and exiting island operations – should be clearly defined.
As a group of DER that connect to the grid, a microgrid is beholden to the same interconnection requirements as individual DER. However, the variability of site configurations and combination of different DER types make microgrid interconnections challenging. Interconnection requirements and related analysis normally covers several topics, including impacts on the following:
- Utility voltage and voltage regulating equipment
- Overcurrent protection
- Effective grounding
- Islanding prevention
- Harmonics
- Voltage flicker
- Load rejection overvoltage
- Installation above flood levels or use of submersible designs
- Telemetry and metering equipment
Many of these integration issues will also apply when the microgrid is operating in standalone (islanded) mode, but the circuitry, control modes, and power flows may be quite different than when grid connected. When a microgrid is in standalone mode, several additional issues arise that must be studied:
- Voltage control
- Frequency control
- Electric supply and demand balance
- Synchronization
- Black start capability
- Protection settings
- Unbalance
StorageWiki Microgrids Content
| Description | Access | Status |
|---|---|---|
| Microgrid Implementation Types Database | P94G & P174C | Periodically updated |