Microgrids are essential for maintaining business continuity during extreme weather events. Do you know the best practices for designing a microgrid that can perform reliably and safely, no matter how severe the weather may be?
With this in mind, here are five important design elements to consider when designing your next microgrid:
1. Consider adding reliability margins to equipment operating conditions
Peak site conditions can increase the internal operating temperatures in photovoltaic (PV) system enclosures, causing stress on components well beyond their UL design ratings. These include ambient operating temperatures approaching or exceeding 40°C (104°F), internal heat gain because of direct solar radiance on the enclosure or reflected from the terrain, and geographical elevations above 3,300 feet.
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 to calculate the maximum temperature of the site location. This is then used as the basis for system design and to 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 or heated outdoor enclosures that maintain temperatures above -40°C.
2. Do your research on high winds
Wind is the most common damaging weather element, according to the US Department of Energy. However, it is also one of the most complex natural forces to design against 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.
3. Protect power electronics and electrical components from lightning and surges
Everyone in the electrical industry is aware of the havoc lightning can inflict on sensitive electronic equipment, and several codes and standards exist to help protect electrical 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:
- 2.1 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 output of the inverter.
- 2.3 requires additional surge protection devices at the DC input of the inverter if the system inverter is more than 30 meters (98.4 feet) 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.
4. Safeguard against rain, water damage and flooding
To design against water damage and flooding, it is critical to understand the physical environment your microgrid system is installed in. Planning is essential and needs to address the following:
- 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 NEMA 3R — indicating they will resist a degree of wind-blown rain. These enclosures include ventilation and drainage holes for proper temperature control and to 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 4X enclosures, which provide an additional level of protection against corrosion.
It is also important to pay close attention to your equipment interconnection wiring. Pole-line construction can expose vital connections to the elements and hazards, which can damage your system. Underground equipment interconnections are better protected during extreme conditions, but this approach involves a significant investment.
5. Utilize intelligent controls to maintain microgrid stability
When disaster strikes, microgrid controllers react quickly and accurately to changing conditions to maintain power supply for 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 critical systems online as long as possible if generation assets are compromised.
It's all in the details: Attention to microgrid design is essential
It is impossible to predict when the next environmental disaster will strike, but you can implement microgrid design elements to ensure your microgrid remains functional when other power systems fail.
Developing a microgrid to withstand extreme weather events can be a challenge, but it’s also a necessity. Luckily, 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 operate as a harmonious system to keep the power on when it matters most.
Robert Kirslis is senior microgrid application engineer at Eaton.
