08/11/2021 | Press release | Archived content
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.
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.
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:
a.In our opinion, sizing OCPDs for 50°C ambient service is a best practice for solar PV applications
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.
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