Selection Considerations: AC Solar Power Collection
By Mark Sauls, EPEC Solutions
Utility interconnection agreements for community solar/distributed generation PV plants (typically 5 MW or less) require detailed specifications for plant size, location, and the inverters being utilized. The AC collection equipment for these projects, AC panelboards and switchboards, that “collect” the power are also denoted on the one line, but no other specifications are required. This leads many developers to select AC collection equipment with little investigation. This, in turn, has caused many projects to suffer operational problems for years afterward. This article explores selection considerations critical to making the right AC solar collection choices.
Solar developers/engineers usually utilize AC panelboards (UL 67) and AC switchboards (UL 891) for AC collection. This equipment is employed in various industries, and is readily available from a few well-known large OEM suppliers. These large OEM’s serve every industry (construction, power plants, residential, power plants) and offer equipment that is “one size fits all”. However, the solar industry is a unique industry with specific challenges.
Solar equals sun. Almost always, the switchboards/panelboards are subjected to direct sunlight, high ambient temperatures, and large temperature swings. Equipment operates at 100 percent in the daytime, then cycles off after the sun goes down. Without proper equipment selection, many projects suffer years of nagging problems including: nuisance tripping, loosening connections and circuit breaker failures. How to combat these common pitfalls?
The answer is found in selecting AC collection equipment designed specifically for outdoor applications. First, let’s examine enclosure color. The defacto standard for electrical equipment is ANSI 61 (medium gray). That medium gray paint finish, when subjected to direct sunlight, is 30 degrees Fahrenheit (F) warmer than the same enclosure painted white.
Nuisance tripping is the unwanted trip of over-current protective devices, (circuit breakers or fuses), utilized to protect inverter circuits feeding AC collection equipment. Operations & Maintenance (O&M) groups battle nuisance tripping issues for many years after the plant is installed.
What causes nuisance tripping? Many AC panelboards/switchboards utilize “thermal magnetic” trip units or fuses for circuit overcurrent protection. Both the thermal magnetic circuit breaker and fuse utilize a thermal element to protect the circuit. When current flows through the device, a thermal element heats up and either bends to cause a trip (thermal mag circuit breaker) or melts (fuse). Both technologies are subject to the ambient temperature inside the panelboard/switchboard. A generic ANSI 61 gray switchboard plus thermal element tripping can equal a disastrous O&M experience.
In addition to enclosure color, the type of circuit breaker should be considered. Electronic trip circuit breakers utilize the actual current (measured by current transformers inside the circuit breaker). Ambient temperature has no impact on these circuit breakers thus nuisance tripping goes away. However, it is still important to keep the overall temperature inside of the panelboard/switchboard below 150 degrees F as electronic trip circuit breakers’ internal circuits can be damaged above 158 degrees F.
When selecting AC collection equipment, bus bar material should be considered as well. Solar equipment is subject to wide temperature swings in each 24-hour period. These temperature swings cause expansion and contraction of the bus bar material. Aluminum has a higher thermal expansion coefficient (35 percent more than copper). Thus with daily temperature cycling, an aluminum bus can cause loose connections in this equipment much quicker than a copper bus solution.
Next, let’s explore system efficiency. Equipment selection can play a key role in the affordability of plant construction. The inverter circuits for community solar/DG plants are considered continuous loads. he National Electric Code (NEC 210.19A.1) states: “The minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load.”
If utilizing thermal magnetic circuit breakers or fuses for continuous loads, electrical conductors are required to be oversized by 25 percent. When utilizing a 100 percent rated circuit breaker, the NEC allows the conductor to match the load without derating, reference (NEC 210.19A.2). Only electronic trip circuit breakers carry a 100 percent rating; fuses and thermal magnetic circuit breakers are excluded. However, not all electronic trip circuit breakers are 100 percent rated, so developers should specify this rating. Also, make sure not only the circuit breaker but also the entire panelboard/switchboard assembly is 100 percent rated by the vendor.
A final consideration: since 2018, string inverter companies have provided equipment with a nominal 690-800VAC output voltage rating. Since most AC panelboard/switchboard providers rate equipment up to 600VAC, this causes many headaches for developers sourcing equipment. However, there are major advantages to harnessing power at higher voltages, with the most compelling being savings on cabling and AC collection equipment. These are all more affordable at the same power rating since this equipment is sized in amps. Increasing the voltage directly decreases the amps for the same amount of power. A full discussion on 690-800VAC will require another article. However, developers should utilize their equipment supplier’s ability to support those voltage levels when making their equipment vendor selection.
Mark Sauls is Vice-President of Sales/Operations at EPEC Solutions, Inc (www.epecsolutionsinc.com).
Q4 2024
