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Extreme weather is impacting millions of people around the world with increasing frequency. The National Climate Assessment reports the rate and severity of heat waves, heavy downpours and major hurricanes has increased in the U.S. over the last two decades. The rising impact of these natural disasters makes it clear the climate challenges we face are real. More than ever, our global energy and infrastructure must be able to withstand extreme conditions.
Microgrids are progressively being used for business continuity in the face of extreme weather. To this end, it is important that the equipment supporting microgrids is installed to perform reliably and safely no matter how disastrous the weather may be.
This article will explore the minimum code requirements (and when you should consider going beyond the code) to ensure your microgrid system and distributed energy resource (DER) assets can withstand:
Peak site conditions act individually or in concert to increase the internal operating temperatures in PV system enclosures and can stress components well beyond their UL design ratings. Common peak conditions include ambient operating temperatures approaching or exceeding 40°C, internal heat gain due to direct solar radiance on the enclosure or reflected from the terrain, and geographical elevations above 3,300 feet.
You can address these issues 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 Electric 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 -40°C (-40°F). In environments where winter temperatures could drop below -40°C (-40°F), equipment is best located in heated indoor locations that maintain temperatures above -40°C.
NFPA 855 can be referenced for guidelines supporting safe energy storage installations and most manufacturers provide a temperature range for their batteries. It is important to follow these recommendations, as operation outside of an acceptable temperature range can lead to energy storage systems not working as intended, aging more quickly or even causing a complete failure that can result in fire and explosion.
Another important consideration is the environment within electrical houses or rooms. For example, IEC 62271-1 specifies normal service conditions for indoor switchgear—including guidance that ambient air temperature must not exceed 40°C (104°F) and its average over a 24-hour period must not exceed 35°C (95°F). The code also covers other important aspects of indoor operation such as humidity and contamination prevention.
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.
In addition to protecting PV modules and racking systems from winds, there are opportunities to protect other microgrid components such as generators and battery banks by ensuring they are enclosed within a reinforced structure. These structures will also need to meet local building code requirements for wind bracing, structural engineering, rooftop weight and more.
Most importantly, it is vital to ensure the foundation and support structures used for any microgrid component are rated for the intended load and potential environmental conditions. Oftentimes, this can be a challenge when attempting to retrofit an existing rooftop with solar PV modules and racking. Structurally reinforcing an existing rooftop is often cost-prohibitive, so ground-mounted PV installations on a reinforced concrete pad designed for the local environment are common.
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.
NFPA 780 provides lightning protection system installation requirements to safeguard people and property from fire risk and related hazards associated with lightning exposure. For example:
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. More information on NEC Article 705 can be found in a previous blog post here.
Surge protection devices are also critical to protect the electronic components of a PV or wind system as well as connected equipment, including battery banks and inverters. NEC Article 242 describes overvoltage protection, including the various types of surge protective devices. The correct device (AC or DC) and the voltage rating must be selected for each application. When properly selected and installed, these devices should minimize any equipment damage from currents and voltages that have not been fully diverted to ground by the grounding system.
Much like protecting against high winds, it is a critical first step to understand the environment your microgrid system is placed in. Planning is essential and needs to address the following (at minimum):
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 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.
It is important to note the NEMA 3R designation is specific to a limited range of mounting positions and further protection may be required in unique installation scenarios. In more extreme environments, NEMA 4 and 4X enclosures may be required to provide a dust- and water-tight seal to protect against windblown particles, rain, splashing water and hose-directed water.
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 life safety and other critical systems online as long as possible if generation assets are compromised.
Microgrids 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.
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.
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