AC Aggregation on Commercial Rooftops: Page 2 of 3
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AC voltage drop. Best practice is to limit the maximum voltage drop within the ac collection system to 1.5% or less to limit the opportunity for ac overvoltage errors. Though some inverter manufacturers recommend designing systems for an ac voltage drop of 1%, realizing this goal can be difficult or expensive in the field. Some designers push ac voltage drop as high as 2%, but anything higher is worrisome. The risks of operational headaches from nuisance tripping due to errors caused by high grid voltage outweigh any up-front cost savings. (See “Voltage Rise Considerations for Utility-Interactive PV Systems,” SolarPro, June/July 2012.)
Since troubleshooting ac overvoltage errors in the field is expensive, it is wise to take a conservative approach and keep ac voltage drop within acceptable limits by design. The goal is to specify conductors that are large enough to keep the ac voltage drop below 1.5% without incurring unnecessary costs associated with excessive size. When calculating the ac collection system conductor sizes based on a maximum voltage drop of 1.5%, apply a few restrictions: First, identify the inverter manufacturer’s maximum allowable size for the inverter output circuit conductor, and do not exceed this value. Second, limit the maximum size for the ac combiner output circuit conductor to 500 kcmil copper or 700 kcmil aluminum.
If you cannot limit ac voltage drop to 1.5% based on these parameters, you need to expand your design criteria. If ac voltage drop is greater than 1.5% between all of the inverters and the POI, evaluate voltage drop using paralleled conductors between the ac combiner and the POI; this will reduce voltage drop universally. If only a few inverters see an excessive amount of voltage drop, look for ways to address these specific circuits. For example, adding a splice box outside an inverter can allow for the use of a larger inverter output circuit conductor back to the ac collection panel. However, this also adds labor and material costs.
Optimizing an ac collection system based on voltage drop is an iterative process. If you decide to use paralleled conductors for the ac combiner output circuit, you may need to reevaluate the inverter output circuit conductors. If the voltage drop to some inverters is low, you may be able to reduce output conductor size, provided that you still meet the Code minimum requirements. To the extent that you can identify the right conductor size for each circuit, you can engineer the project for maximum value.
As detailed in Section 705.12, the Code allows for load-side or supply-side interconnections.
Load side. The rules in 705.12(B) govern interconnections on the load side of the service disconnecting means. Commercial services usually top out at 4,000 A. Depending on the size of the service and the utilization voltage, there may be sufficient backfeed capacity to interconnect a commercial-scale PV system under the 120% allowance [705.12(B)(2)(3)(b)]. In a 1,000 A switchboard with a 1,000 A overcurrent protection device (OCPD), for example, the 120% rule allows you to use a 200 A inverter interconnection breaker, which would allow for a 133 kWac PV system in a 3-phase 480 V application.
While switchboards all tend to look the same on the outside, they are highly customizable pieces of equipment. You cannot make any assumptions about what is going on under the cover, but must inspect each one individually. How is the distribution bus fed? Though end-fed and center-fed configurations are typical, you may encounter other configurations. Where is the utility section of the switchboard with the service drop conductors and the meter current transducers? Generally speaking, you cannot open this sealed utility section of the switchboard or make an interconnection inside it. Are there any supply-side feeds? I have seen people mistake a supply-side fire pump feed for a main breaker, overlooking the fact that the switchboard did not have a main disconnect. Are there open spots where you can add a PV breaker, or is there an unused breaker for the PV interconnection?
In some cases a switchboard inspection will identify a simple way to make a supply-side connection, which allows for a much larger PV system than a load-side connection does. In addition to capacity limitations, there are other reasons why a load-side connection might not be the best option for a project. It may be difficult and expensive to move a load breaker in order to locate the PV breaker at the opposite end of the busbar from the main OCPD. Meanwhile, interconnections at center-fed or multiple-ampacity busbars incur additional engineering costs. These considerations may favor a supply-side connection.