Microgrids offer an ideal solution to counter the risk of weather-related power disruption by bolstering operational resilience, reducing dependence on local utilities, and lowering energy costs. However, the functionality of a microgrid relies upon the success of its design.
Extreme weather is impacting millions of people around the world with increasing frequency. Over the past decade, power outages are up roughly 78% in the U.S. over the previous decade and the vast majority of major power outages were due to weather-related causes, according to Climate Central, a climate research news organization.
It’s devasting to see the impact of climate emergencies; look no further than recent hurricanes that struck both the U.S. and Puerto Rico. The climate challenges the world faces are not abstract issues. More than ever, the global energy supply and infrastructure must be able to withstand extreme conditions, because lives and livelihoods depend on it.
Resilience is about staying operational for our homes, communities, businesses, and institutions. And it requires preparation. The challenge is to predict what we need our electrical infrastructure to withstand.
1. At the Eaton Experience Center in Pittsburgh, a full-scale demonstration and testing facility, visitors can get a hands-on look at how microgrids support resilience. The controlled environment has a fully functioning building microgrid that interconnects with multiple onsite sources. Courtesy: Eaton
Today, microgrids (Figure 1) are progressively being used for business continuity in the face of extreme weather. In these applications, it is important that the microgrid equipment is installed to perform reliably and safely no matter how disastrous the weather may be. To this end, how do you determine how robust your microgrid needs to be?
It’s important to know the minimum code requirements for the operation and maintenance of a microgrid, and operators also need to know when to consider going beyond the code. The code requirements are important to ensure your microgrid system and distributed energy resource (DER) assets can withstand:
- ■ Extreme temperatures.
- ■ High winds.
- ■ Lightning.
- ■ Rain, water ingress, and flooding.
It’s also important to know the role that microgrid controls play in overall system resilience.
Protecting Energy Assets from Extreme Temperatures
Peak site conditions act individually or in concert to increase the internal operating temperatures in photovoltaic (PV) system enclosures and can stress components well beyond their Underwriters Laboratories (UL) design ratings. Common peak conditions include ambient operating temperatures approaching or exceeding 40C, internal heat gain due to direct solar radiance on the enclosure or reflected from the terrain, and geographical elevations above 3,300 feet.
These issues can be addressed by estimating the expected internal heating of the enclosure from solar radiance. To start, you can study local weather data, including record, daily, and average monthly temperatures. PV system designers often use 2% high or 0.4% high weather temperature data as the basis for system design, and size the PV system ampacities to minimum National Electrical Code (NEC) requirements without taking additional thermal rating factors into consideration.
On the cold side of the spectrum, electronic equipment such as inverters and controllers typically found in microgrid systems are commonly listed for a minimum ambient temperature of –40C (–40F). In environments where winter temperatures could drop below –40C, equipment is best located in heated indoor locations that maintain temperatures above –40C.
Ensuring Resilience in High Winds
According to the U.S. Department of Energy, wind is the most common damaging weather element. However, it is also the most complex force to understand and plan for, and varies greatly depending on the type of storm.
High winds can have a huge impact on the installed base of PV arrays, and it is up to system designers to interpret local building codes and standards to develop a mounting system that will withstand the wind loading of the given site. For example, your state or local level building codes will provide guidance on wind load calculations and limitations for a given area. These formulas take many aspects of the PV system and environment into consideration, including historical wind data, panel tilt, distance from roof or foundation, racking material selection, and bracing type.
There are also opportunities to protect other microgrid components, such as generators and battery banks, by ensuring they are enclosed within a reinforced structure. These structures will need to meet local building code requirements for wind bracing, structural engineering, rooftop weight, and more.
Provisions for Lightning
Lightning strikes can damage structures, while the surge generated can harm sensitive electronic equipment. Several codes and standards exist to help protect microgrid systems against the various types of lightning damage.
National Fire Protection Association (NFPA) 780, Standard for the Installation of Lightning Protection Systems, provides lightning protection system installation requirements to safeguard people and property from fire risk and related hazards associated with lightning exposure. For example:
- ■ The 126.96.36.199 code of the standard dictates that surge protection shall be provided on the direct-current (DC) output of the solar panel from positive to ground and negative to ground, at the combiner and recombiner box for multiple solar panels, and at the alternating-current (AC) output of the inverter.
- ■ 188.8.131.52 requires additional surge protection devices at the DC input of the inverter if the system inverter is more than 30 meters from the closest combiner or recombiner box.
Additionally, grounding is a fundamental technique for protecting PV assets against lightning damage. Damage can be prevented by following NEC articles 690.43, 690.45, and 690.47 for bonding and grounding. For ground-mounted solar PV arrays, the metal support structures installed in the ground serve as additional grounding electrodes. An insertion depth of 10 feet or more provides additional support for wind loading and meets NEC requirements for grounding electrodes.
Further, if the microgrid is connected to the utility grid when a lightning-induced fault occurs, there will be fault currents from the utility grid and the microgrid system. In accordance with NEC Article 705, the primary interconnection equipment must include a circuit breaker supervised by redundant protection relays.
Preparing for Rain, Water Ingress, and Flooding
Much like protecting against high winds, it is a critical first step to understand the environment your microgrid system is placed in to protect against water damage. Planning is essential and needs to address the following (at minimum):
- ■ Historical rainfall averages.
- ■ Proximity to 100-year flood plain.
- ■ Local building codes.
- ■ Drainage solutions.
- ■ Potential exposure to corrosive saltwater.
Aside from protecting your physical building structures from water, you must also ensure sensitive electronic components have appropriate enclosures for the environment. For instance, most components commonly found within a microgrid system have enclosures that are rated by the National Electrical Manufacturers Association (NEMA) 3R—indicating they will resist a degree of wind-blown rain. These enclosures include ventilation and drainage holes to allow for proper temperature control and allow any internal condensation to escape. They also will remain undamaged by the external formation of ice on the enclosure. Installations that could potentially be exposed to saltwater require NEMA 3X enclosures, which provide an additional level of protection against corrosion.
Intelligent Controls Maintain Microgrid System Stability
The brain of the microgrid system is the microgrid controller with standardized communications that enables easy system configuration, commissioning, and future adaptability to changing system assets. When disaster strikes, these controllers can quickly and accurately react to changing conditions to maintain power to critical loads. Once the controller is properly programmed, it can adjust energy production, storage, and consumption to maintain overall system stability, shave peak demand, shift loads, maximize renewable energy contribution, and more.
For example, the microgrid controller will automatically recognize an outage if the primary utility source is interrupted before transitioning energy production and storage assets into grid-forming mode to keep power flowing throughout your operations. The microgrid controller can also strategically prioritize the electrical load based on predetermined settings, keeping safety and other critical systems online as long as possible if generation assets are compromised.
Attention to Design
Without a crystal ball, it’s difficult to know what challenges the future may bring. Yet, it’s clear that the energy transition is charging ahead and the need for energy resilience is paramount.
2. At the Eaton Experience Center in Pittsburgh, a full-scale demonstration and testing facility, visitors can get a hands-on look at how microgrids support resilience. The controlled environment has a fully functioning building microgrid that interconnects with multiple onsite sources. Courtesy: Eaton
Microgrids (Figure 2) have emerged as an ideal solution to counter the risk of weather-related power disruption by bolstering operational resilience, reducing dependence on local utilities, and lowering energy costs. However, the functionality of a microgrid relies upon the success of its design.
Eaton has taken a lead role in developing microgrids for customers and at our own manufacturing and education centers. Here’s how Eaton designs microgrids to withstand Category 5 hurricanes:
- ■ Controls and switchgear enclosures are rated for extreme conditions, and the battery storage system is raised on concrete pads and rated for marine environments.
- ■ The solar PV system adjacent to our facilities uses strong racking that is secured deep in the ground (not on the roof), and the solar panels are tilted to withstand hurricane-force winds.
- ■ The generator power system is updated with paralleling switchgear and microgrid controls that enable an operator to cut off non-critical loads. That maintains operation even in the event of a generator going down, using the other generation and solar PV and battery system.
Developing a microgrid to withstand extreme weather events can be a challenge due to the breadth of involved equipment and knowledge required, but it’s also a necessity. Today, there are many qualified microgrid suppliers you can lean on for expertise and engineering support. At the end of the day, it is vital to consider the impact extreme weather events can have on each asset to ensure the microgrid can keep the power on when it matters most.
—Robert Kirslis is a senior microgrid application engineer for Eaton. He has more than 25 years of experience in the planning, maintenance, and operations of data centers, power plants, industrial facilities, and commercial properties.