Field experience of power frequency overvoltages in wide-scale photovoltaic systems

Severe overvoltages have been detected in solar plants when switching the inverters off. These overvoltages damaged some pieces of low voltage equipment, in particular revenue meters and electronic devices associated with the inverters. The overvoltages take place when a switch upstream of the inverter (whether low or high voltage), in such way that one or several inverters become isolated from the rest of the grid. On the other hand, in large solar plants (several MW) fed by medium voltage busbars with no other load, it has been verified that overvoltages in the MV side reach dangerous values. Given that all systems are interconnected through their transformers, such overvoltages affect not only to the low voltage equipment, but to the medium or high voltage grid. The phenomenon is related to the interaction among the inverters until the anti-islanding protection trips. It must be pointed out that these islanding conditions are not considered in any standard. In any event, the problem is not the islanding detection, whose procedures are usually focused on a reliable detection when generation and consumption are balanced. The circumstances in which the overvoltage appears are the opposite, with a generation much higher than the load. Since standards are not applicable, it has been necessary to investigate this phenomenon in the field, testing several inverters in different solar plants. Several inverters, from different manufacturers, have been tested in wide-scale photovoltaic systems. The overvoltages, as well as the damages in meters, have been reproduced and recorded. This paper will present this new kind of overvoltage, specific of photovoltaic systems. The findings will be based on actual measurements in several wide-scale photovoltaic plants. Inverters used in solar plants behave as current sources, so they increase or decrease the voltage whatever is necessary to achieve the required current. It is this characteristic what creates an overvoltage when the net- work changes, reducing the load dramatically.