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Solar Carport Design Optimization

Designing PV carports is an involved process, and key decisions—such as maximum system voltage, rapid shutdown requirements, inverter selection and location—must account for site-specific conditions. Here I discuss some of the main considerations that drive solar carport design, optimization and value engineering. 

Is the PV Carport a Building or On a Building?

From a National Electrical Code (NEC) perspective, one of the most consequential considerations is whether the authority having jurisdiction (AHJ) is going to treat the solar carport as a building or part of a building. 

 

OPTION A: NO, IT IS NOT A BUILDING OR ON A BUILDING.

If a PV carport is not on or in a building, there is a reduced risk of loss of life or property damage and fire-prevention requirements are less stringent. This is most likely the case for PV carports resembling elevated ground mounts, such as those installed at ground level over concrete parking lots.

Carport1.RBICourtesy RBI Solar

Rapid shutdown considerations. Rapid shutdown is an electrical code requirement intended to mitigate the risk associated with energized conductors during an emergency. As described in NEC Section 690.12:

“PV system circuits installed on or in buildings shall include a rapid shutdown function to reduce shock hazard for emergency responders.”

When PV carports are not considered as on or in buildings, 690.12 does not apply. In this scenario, module-level power electronics will not be required to control conductors within the array boundary.

Maximum system voltage considerations. NEC Section 690.7 limits the maximum PV system DC voltage when DC circuits are installed on or in a building. These requirements limit the DC voltage to 600 Vdc in residential applications and 1,000 Vdc in commercial and industrial buildings. Conversely, when DC circuits are not located on or in buildings, PV systems can operate up to 1,500 Vdc. 

Systems specified with 1,500 Vdc inverters provide some distinct advantages. All else being equal, increasing system voltage will allow for the use of smaller diameter conductors, reduce the percentage of voltage drop, and improve operating and financial efficiency. While designers may be tempted to go with this option where it is allowed, it is not always the best choice for every site.

Inverter AC voltage considerations. Most PV carports are behind-the-meter installations at facilities with a 480 Vac service. This 3-phase service voltage matches the nominal output voltage for 1,000 Vdc inverters, allowing for a simple interconnection. By contrast, 1,500 Vdc inverters have an atypical 600 Vac output, which will require a transformer at the point of interconnection (POI) to match the utility grid voltage. Transformers are expensive, long lead time pieces of equipment that will increase project cost and introduce conversion losses. 

As a general rule, our goal is to eliminate unnecessary transformers wherever possible, which means that many PV carport systems are not well-suited for 1500 Vdc inverters. The exception to the rule are large-scale solar carport applications with medium-voltage (MV) interconnections. In this scenario, the design will inherently require an additional transformer. Under these circumstances, it may make sense to specify 1,500 Vdc inverters and an AC collection system operating at 600 Vac.

OPTION B: YES, IT IS A BUILDING OR ON A BUILDING.

Some AHJs may treat any solar carport as a building. Also, some PV carports are installed on a building, such as the roof deck of a parking garage. Code requirements are more stringent in these scenarios because the risk to life and property is higher.

Carport2.RBICourtesy RBI Solar

Rapid shutdown considerations. If an AHJ considers a PV carport to be on or in a building, the system will need module level power electronics to comply with the most recent NEC rapid shutdown requirements. Means and methods for compliance include module-integrated power electronics, voltage-limiting DC converters, and microinverters. Complying with rapid shutdown will increase system cost, both due to the additional materials and an increase in installation time.

Maximum system voltage considerations. As discussed previously, when PV circuits are on a building other than a one- or two-family dwelling, the maximum DC voltage allowance is 1,000 Vdc. Inverter options are plentiful. All inverters designed for use in commercial behind-the-meter applications in North America will accommodate a 1,000-volt DC operating voltage limit. 

Inverter AC voltage considerations. The obvious advantage of using 1,000 Vdc inverters for PV carport systems on a building is the ease of interconnection. Many manufacturers offer inverters a 3-phase 480 Vac output voltage, which is the most common utilization voltage in commercial and industrial applications. While options are more limited for 3-phase 208 Vac or 240 Vac interconnections, it may still be possible to accommodate these interconnection voltages without a costly transformer. 

Inverter Selection and Location Considerations

Some solar carport design considerationssuch as limiting communication cable runsapply broadly and are covered in my previous post Design Recommendations For 1500-Volt String Inverters. Other considerationssuch as inverter selection and placementare somewhat unique to PV carport applications. 

Safety and O&M Convenience. PV carports are usually publicly accessed spaces where energized equipment must be properly guarded. As a result, designers may be tempted to take advantage of these elevated structures and locate inverters high up on the columns. The inverters, however, are the components most likely to require servicing and replacement over the life of the system. 

Installers and O&M technicians will need scaffolding or ladders to access inverters located more than six feet above grade. This is not only inconvenient but also introduces fall safety considerations. Wherever possible, we recommend locating inverters at ground level. This mitigates fall hazards and reduces complexity during installation, commissioning, maintenance or troubleshooting. 

Central vs. String Inverter. Conduit routing, along with site distances, will heavily inform the central versus string inverter decision. All else being equal, we find that string inverters provide the best value in carport applications. Because conduit in PV carports is embedded into concrete foundations, we prefer any configuration that does not require jumping PV source circuits between carport columns. 

System optimization will also consider inverter placement (distributed versus aggregated); inverter-integrated versus external DC combiners; balance of system (BOS) components selection; combiner box placement; and so forth. As part of the value-engineering process, the experienced team at Pure Power Engineering will evaluate these many options—as well as other subtle yet impactful engineering considerations—to produce the best possible solar carport design. 

For more tips on optimizing commercial- or utility-scale PV power systems, contact Pure Power Engineering to learn more about our value-engineered design and construction drawing services.

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