Oliver George works hardest when the sun shines. He's an electrical foreman at Pennsauken, NJ-based SunEdison, which bills itself as North America's largest solar energy service provider. He does commissioning and troubleshooting on solar power systems from 50 kW to 8 MW, installing and troubleshooting the photovoltaic panels, junction boxes and inverters, plus the wiring in between. For much of this he uses just one instrument: A Fluke 117 Electrician's Multimeter.
Multiple values to measure
A solar power installation has a wide variety of voltages and current to measure: DC voltage and current from individual panels and the series strings into which they are connected, AC voltage and current from the inverters that change the DC from the solar panels to utility-grade AC, plus such things as leakage from any of the numerous parts in the system. The 117 handles all of them.
Output voltage from individual panels can be anywhere from 30 to about 40 volts, while string voltages range from about 450 to 550, says George; typical values are 480, 208 or 277 volts, depending on the bus voltage in the building. "Time of day has a variance on that," he explains. "In what we call commissioning all the voltages possible on the array are measured and recorded, so that when a service technician comes back to find a difficulty with the system he has something to go on: what was seen at this time, what he's finding." George checks voltages to ground and between positive and negative, as well as output current.
Checking for leakage
Another important measurement on a solar installation is leakage. Any installation with hundreds of volts DC on it that sits out in the weather has to be checked carefully for leakage paths. "If you put a little nick in a wire and you're not paying attention to what you're doing," he says, "and then it rains and you've got these groups of wires together, I've seen that burn out. With arcs, once it starts it's just going to eat things up until somebody stops it. As long as there is light available."
George recalls one incident in particular. "I had one incidence … where we had a leak" on a 50 kW system, he says. Reading from positive to ground, his 117 told him there was trouble. "In the particular combining box that it was in I was able to track it down to one or two strings that were causing the problem." He walked around but couldn't see it, because it started underneath the panel. It turns out there was a manufacturing defect in one panel, but unfortunately it wasn't corrected right away. "About 45 days later we were back," he says. The leak had acted like an arc welder whenever the sun was up. "It cut about a one-foot hole in a metal seam roof."
Other problems can come from mistakes in wiring "and when someone goes and miswires a string and they reverse wire things so you've got opposing voltages and currents, and you can imaging at that point, it just doubles. It just skyrockets and then starts arcing."
And, of course, weather can add to safety problems. "When it's rainy and wet," George says," "you don't want to be fooling with the string lines themselves, making connections or breaking them. If you're wet you're going to get it."