Eaton Corporation plc

08/11/2021 | Press release | Archived content

Blog: How to design safer solar PV systems

Blog series: For safety's sake

Safely connecting solar PV to electrical systems

Lawrence T. Conner, energy transition senior application specialist, Eaton, Jaska Tarkka, energy transition application engineer, Eaton, 08/11/2021

Solar is on the rise

The energy transition to a more sustainable, low-carbon future is accelerating, with renewables expected to provide 50% of our world's energy by 2050. By adopting a Buildings as a Grid approach, businesses and communities are leveraging this paradigm shift to become self-sufficient power producers that generate, store and consume their own renewable energy - in addition to helping balance the grid and selling excess energy back to the utility when possible.

This Everything as a Gridstrategy hinges on two layers of connectivity - local power system connection and interconnection with the utility grid. Solar photovoltaic (PV) systems are playing a large role in this transformation and it's important to know how to connect these systems to building infrastructure.

In our opinion, knowing what it takes to safely connect solar PV to building infrastructure is critical because its quickly becoming the norm. The California Energy Commission is advancing adoption of renewables, and recently added building standards that require solar PV systems in all new homes.

Codes and standards for safe PV system installation

There are a number of National Electrical Code (NEC) guidelines for the safe installation of PV electrical energy systems. As with any electrical application, the requirements of NEC Chapters 1 through 4 apply. The other primary Articles you should familiarize yourself with include:
  • NEC Article 690
    Article 690, consisting of eight parts, applies to PV electrical systems, array circuits and inverters, for PV systems, which may be interactive with other electrical power sources (the electric utility) or stand-alone with or without energy storage.
  • NEC Article 691
    Article 691 covers the installation of large-scale PV electric supply stations with an inverter generating capacity greater than 5000 kilowatts (kW)that are not under exclusive utility control.
  • NEC Article 705
    This Article was introduced in an earlier blog post and addresses how to connect additional power production sources to the existing premises' wiring system to operate in parallel with the primary source of electricity. Typically, the primary source is the electric utility while other local sources could include onsite energy storage, solar, wind, fuel cells or generators.

Together, these Articles are a fantastic starting point for understanding safe solar PV system installation but are not intended to serve as a design guide by any means. For one, the NEC is written to provide minimum requirements for fire and personnel safety. Additionally, every solar PV installation is different. This means you can often design a PV system that meets all minimum code requirements but isn't optimized for its environment, which creates uptime and production challenges. From our view, it is vital to consider going above code requirements when necessary to ensure the overall effectiveness and safety of a PV system.

Connecting solar PV to buildings: 4 design tips

1. Plan for peak conditions

Overcurrent protection devices provide vital functionality enabling cost-effective and reliable performance of PV systems. However, peak solar project site operating conditions are often not considered when sizing AC collection system components. This can lead to equipment overheating, nuisance tripping, system failure and reduced power generation during hot summer days when reliable power production is needed the most.

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 percent high or 0.4 percent high weather temperature data as the basis for system design and size the PV system ampacities to minimum NEC requirements without taking additional thermal rating factors into consideration.

This presents problems during the hottest summer days, when peak daily temperatures reach record levels. The IEEE C37.24 "Guide for Evaluating Effect of Solar Radiation on Metal-Enclosed Switchgear" is an excellent reference on this topic. For PV collection systems enclosures subjected to full sun exposure, the reflected solar gain and the direct solar gain can add up to 15°C to internal enclosure temperatures. This means the internal enclosure operating temperatures can exceed 50°C for an extended period (4 to 6 hours) during the peak of the solar day even in moderate climates. Effective thermal management is required to address this challenge.

2.Properly select overcurrent protection devices (OCPDs)

As discussed above, peak site conditions like solar radiance can often exceed UL equipment design ratings. For example, UL891 Switchboards, which utilize molded case circuit breakers and fused switches as OCPDs in enclosures, are UL Listed based on 40°C ambient with 65°C rise at maximum loading. For environments with internal enclosure temperatures above 40°C, you should apply techniques to reduce the heat rise in the enclosure. The following thermal management strategies can help prevent equipment overheating and OCPD nuisance operation:

  • Reference thermal rating factors published by manufacturers for OCPDs in service temperatures above 40°C

a.In our opinion, sizing OCPDs for 50°C ambient service is a best practice for solar PV applications

  • Recognize that OCPDs acting as string inverter AC collection devices will often be densely packed and highly loaded at the same time during the peak temperatures of the solar day, which impacts ambient service temperatures
  • It is also a good idea to size switchboard buses properly for system loading and consider upsizing to the next ampacity to reduce heat rise based on thermal conditions
  • Ensure equipment and OCPD terminal connections are UL rated for conductors applied at 75°C, even if 90°C rated conductors are applied

3. Size conductors for thermal conditions

Conductors are an important thermal management system that draw heat out of the OCPD during operation. As mentioned above, we recommend applying the conductors at 75°C ratings to match the OCPD terminal UL Listing. The conductors should also be sized per NEC 310 with applicable NEC conductor thermal rating factors applied. For example, the NEC 2020 Table 310.16 provides the allowable 75°C ampacities of insulated conductors based on 30°C ambient temperatures and Table 310.15(B)(1) provides thermal correction factors for ambient temperatures above 30°C. From our perspective, sizing cables for 50°C service in solar applications is a good way to reduce the temperature rise in the enclosure.

4. Go beyond the code to enhance safety

The NEC provides an exception [2020 NEC 690.9(D)] which eliminates the requirement for a main overcurrent protective device on the inverter side of the solar power transformer. The exception states that a power transformer with a current rating connected toward the interactive inverter output, not less than the rated continuous output of the inverter, shall be permitted without overcurrent protection from the inverter.

The elimination of the main OCPD on the secondary of the solar power transformer may provide economic benefits to the project cost, however, this approach increases arc energy hazards for operation and maintenance teams precisely where available arc fault energy is at its highest level.

We believe system designers may want to consider employing arc flash reduction measures at the low voltage side of the solar step-up transformer. To achieve this, you can incorporate an Arc Reduction VFI (AR-VFI) transformer design or add the main OCPD back into the design with an approved NEC 240.67 Fuse or NEC 240.87 Circuit Breaker to provide an arc energy reduction method for circuits 1200 amps and above.

It's important to understand that NEC installation requirements serve as a bare minimum in many cases. From our perspective, this is a great example why it is important to consider going beyond code requirements to ensure adequate levels of personnel and equipment safety are designed into solar PV installations.

Safe solar PV systems will accelerate a low-carbon future

Technologies that convert energy from the sun into electrical power have matured and are more cost-competitive, driving significant increases in renewable power generation around the world. Yet, adding solar installations to building electrical systems is complex and there are important safety considerations to keep in mind when designing PV systems that will supply reliable and safe power for years to come.

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