The U.S. power delivery system’s complex network of substations, transmission lines, and distribution lines are not designed to withstand or quickly recover from damage inflicted simultaneously on multiple power system components. We’ve seen this in recent years during weather-related events such as Hurricane Irene and Superstorm Sandy.
The number and duration of power outages in the U.S. continue to rise, driven primarily by weather-related incidents. The average outage duration in the U.S. is 120 minutes and climbing annually. The outage duration total in the Midwest is 92 minutes per year and 214 minutes in the Northeast. By contrast, outage duration for the rest of the industrialized world is less than 10 minutes per year and getting better. For example, Japan averages only four minutes of interrupted service each year. The growing prevalence of physical and cybersecurity threats also pose significant challenges for organizations’ mission-critical operations in ensuring reliable access to power supplies.
Historically, when a disaster strikes the result is infrastructure improvements to address the specific cause of each power failure. Many times, planning fails to anticipate future emergencies. A large earthquake, nuclear explosion, or terrorist attack could cause suffering and disruption over a much larger area than a hurricane. Establishing safe haven enclaves to serve as bases of rescue and recovery could go a long way to address the human and economic impacts of future, unanticipated events.
When an outage strikes, the effects often stretch far beyond the initial impact zone. Regional outages inhibit the ability to protect those in danger and provide basic needs such as food, sanitation, and shelter. We could recover more quickly if islands within each area could maintain power and serve as centers for critical services and recovery.
Standby and backup diesel generators are often the only power source available. However, backup generators pose some problems:
- They typically serve only the buildings they are attached to, so nearby buildings do not get power.
- They often have less than 72 hours of diesel fuel in their tank. Fuel deliveries may be significantly delayed.
- They are often sized for the maximum load and do not use fuel efficiently when loads are much less.
To optimize available generation and make power available to a larger area, microgrids offer a viable solution during sudden power outages.
A microgrid can isolate itself via a utility branch circuit and coordinate generators in the area, rather than having each building operating independently of grid and using backup generators. Using only the generators necessary to support the loads at any given time ensures optimum use of all the fuel in the microgrid area.
A microgrid can integrate a number of features beyond backup diesel generators. Features include:
- alternative energy sources such as wind and solar
- gas turbines and central plants providing combined heat and power
- energy storage in batteries and electric vehicles
The microgrid senses loads and fault conditions and can reroute power to as many critical areas as possible given any situation. In that way, it is “self-healing.”
Thus, we define a microgrid as comprising four key elements:
- local electricity generation
- local load management
- ability to automatically decouple from the grid and go into “island mode”
- ability to work cohesively with the local utility
Backup generators only support loads immediately attached to them and they usually come into action during utility power outages. On the other hand, a microgrid consists of onsite generating sources that may include different combinations of diesel generators, gas turbines, fuel cells, photovoltaic and other small-scale renewable generators, storage devices, and controllable end-use loads that enable a facility to operate in a utility-connected mode as well as island mode, thereby ensuring energy reliability.
There are two key challenges to making microgrids work: utility interconnection and microgrid controls.
While having the capability to operate in island mode is a defining feature of a microgrid, the local electricity generators within it are usually connected to the utility grid. This allows a facility to purchase energy and ancillary services from the utility, and sell locally generated electricity back to the utility grid during times of peak demand. When the microgrid is operating with the utility grid, the utility is responsible for frequency and voltage stability. The microgrid control system needs to operate the generators and loads within it in order to maintain consistent power flow.Microgrids should be coordinated with utility grid management to minimize risk of transmission disruption or danger to line workers and others exposed to power currents. Therefore, a utility-microgrid interconnection agreement allowing two-way power flow needs to be developed for each microgrid.
A successful microgrid must have intelligent methods to manage and control all loads. Energy sources have defined output capacity and if overloaded, will severely distort the voltage output or completely shut it down. When a microgrid separates from all of the generation capacity of the grid and relies solely on local generation, management of all loads must be established to properly balance the power generation capacity. This is extremely critical whether a given site has multiple generation sources or simply more load demand than available local power generation. As local generation capacity is ramped up, the loads are also brought online in an intelligent predefined strategy. Typically, critical loads come first and other loads are adjusted to never overload available generation capacity. There are many approaches to controlling loads, at a building feeder level, circuit level, or discrete level. However, the load manager must be able to turn off power quickly and when restoring power, know the capacity so the generator isn’t overloaded.
Microgrid control is relatively easy when all generation resources within it are in close proximity, such as a central utility plant on a college campus. In a distributed microgrid where generation sources (e.g., backup generators) are connected to distribution circuits spread across a large geographic area, voltage and frequency regulation is extremely important. Generators of different sizes and response behaviors can’t simply be hooked up and synchronized over a large area. Such a grid would be unstable as generators on the distribution circuit react to one another by picking up and dropping their share of the load. A supervisory strategy needs to be employed with central controls to ensure stability. The effects can be minimized in a newly designed grid, but most microgrids will be cobbled together with existing generators that have a wide variety of vintages and behaviors.
Microgrid design and operation require extra focus on safety. If the utility is down in one area, it does not necessarily mean that all branch circuits will be blacked out. Safety dictates that everyone be aware of the possibility that microgrids could re-energize loads under the microgrids’ control.
Campuses, military bases, and waste water treatment plants are good candidates for a distributed microgrid. They often have a common mission and are managed by the same organization—facilitating coordination. Often, they also have central plants that can be used for base loads over a broad area with distributed generators. These types of generators are used when loads are too low to justify running the central plant or to support the central plant over a larger area.
Applicability and Benefits
A microgrid approach makes sense for many organizations, primarily those that have a high demand for energy in their facilities and where loss of critical operations poses a significant risk of revenue loss, data loss, or safety and security. Microgrid candidates include:
- military bases where power shutdown would pose unacceptable security risks
- federal facilities, including research laboratories, where wavering energy reliability could mean loss of data and millions of dollars in lost time
- hospitals that need to seamlessly deliver patient care, regardless of weather or other conditions
- large data centers that are the heart of most organizations’ business operations
- research-driven colleges and universities that need to safeguard and maintain years of faculty work
- local governments that need to offer operational assurance to large businesses in their district, as well as attract new companies for stronger job creation
- commercial campus settings where 24/7 power reliability is crucial for protecting long-term investments such as research and development
- a densely populated urban area, such as Manhattan, where concentration of energy use is high and significant scale justifies connecting multiple buildings as part of a microgrid network
- instances where bringing in new electrical lines to meet a facility’s power requirements will be cost- and time-prohibitive to the organization and local utility
Distributed microgrids provide additional benefits to utility operators by integrating renewable resources in distribution circuits. Today, utility distribution circuits are not designed to absorb large amounts of distributed or renewable generation. If microgrid operators can integrate renewables such as rooftop solar while providing frequency and voltage stability, their jobs becomes easier.
One can envision that a resilient and robust utility infrastructure of the future can be built out of interconnected microgrids at universities, hospitals, industrial parks, and neighborhoods. Individual microgrids would be nominally connected to form a single utility grid but could also isolate from the grid and operate independently in case of disruptions. Moreover, this would enable easier integration of distributed and renewable generation.
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