There are two conditions that occur which can warrant the trouble-shooting of a solar array. The first is no power output and the second is low power output – both of these are disconcerting situations, but the first of the two is more than likely the inverter or charge controller having a problem. Solar PV panels are so reliable that they almost never quit completely unless they are severely damaged by fire, impact or some other violent act.
The second situation, lower than expected output, is actually very common – but rarely the fault of the panels themselves. Here is a list of the factors that can lead to low output:
- Shading – always check for shading first. Most installers will make sure that there are no shading issues when they install a solar array, but solar arrays can last for more than 25 years and trees grow very quickly. After modules are cleaned of any debris or surface dirt and output is checked at the inverter, perform a site survey with a Solar Pathfinder or Solmetric – most reputable supply houses will have one or both to rent or loan. Using the output of the site survey, trim any trees or bushes that are causing the shading – output should go back to normal. If not then check the operating temperature of the array.
- Temperature – Solar panels do not like heat – the hotter they get, the lower their voltage drops. Most systems are designed to take this into consideration, but some series strings can be sized right at the lower voltage limit needed to start the inverter and keep it going through intermittent cloud cover. Check the string outputs based on the actual output as well as the designed output. If the voltage gets close to 200 VDC for a grid-connected inverter, then check the ambient temperature. If it’s higher than normal (a heat wave for instance) then you might have to add a module to each string or combine some strings to get a higher voltage so that the system can power through unusual summer heat. Also check to make sure that airflow under the array hasn’t changed. Fact: You can actually measure module temperature very accurately by checking voltage and using the module’s temperature coefficient to calculate – remember that an operating module is cooler than an unconnected one.
- Faulty Connections – If modules aren’t dirty or overheated, then there’s probably a bad connection somewhere. Some experts will insist that the connections are checked first, since a ground fault caused by a hungry squirrel or poor installation can be dangerous (lethal), but the fact is that a professionally installed array just doesn’t have bad connections. Wires should be secure, watertight, not pinched by any metal surface and junction boxes should be sealed. A ground fault condition will be most likely registered on the inverter – if it is, then the system is very dangerous and any further trouble-shooting should be conducted by a DC voltage specialist.
- Series Resistance – This is the most unlikely condition to occur. Solar arrays using central inverters are long strings of generators all the way down to the cell level. At each interconnection point, there is a chance for a bad connection. Cells are encapsulated in glass and silicone, terminals are enclosed in waterproof boxes and wires are connected by waterproof plugs. If moisture or extreme heat attacks any of these points, their resistance will increase and bring the output of the entire system down. By way of example, one manufacturer of solar panels had been soldering the module cells together with a solder that was of insufficient heat. Over time, the electrical resistance in these bad connections generated enough heat to burn through the top glass, eventually leading to the replacement of the cells of thousands of modules. However, this is very rare. Any series resistance is more likely to occur in a junction box, a connector, or a combiner box. Both junction and combiners boxes can be opened to inspect the connections: any metal surfaces should be free of oxidation and the screw terminals should be shiny.
These are the four basic conditions to check a poorly performing system. Once they are all determined to not be an issue, the inverter can then be inspected. Inverter trouble-shooting will be a follow up article.
[Photo credit: Dominica Amateur Radio Club]