Early detection of photovoltaic system inverter faults

Photovoltaic (PV) systems are increasingly adopted as a source of green energy. The reliability of PV systems mainly depends on the reliability of the power MOSFETs of their inverter(s). It has been shown that short and open faults are the most frequent power MOSFET faults. They may compromise the inverter reliability, with consequent significant impact on the PV system energy efficiency. It has been proven that the likelihood of such faults is related to the value of the MOSFET ON-state resistance, which may increase over time due to aging mechanisms. When the value of such a resistance reaches a critical value, the likelihood of the MOSFETs subsequently failing as open or short becomes very high. In this paper, we first evaluate the effects of the MOSFETs' ON-state resistance increase on the harmonic components of the inverter input and output currents. Then, based on the obtained results, we propose an innovative strategy for the early detection (also referred to as condition monitoring [1]) of inverter faults, which enables to generate an alarm message when the ON-state resistance of any inverter MOSFET reaches the critical value. Upon the generation of such an alarm message, proper recovery strategies can be activated, to enable the online replacement of the affected transistors before they actually become faulty. Our detection strategy does not require to interrupt the inverter normal operation in the field and can be implemented using the microcontroller typically embedded within the control circuitry of PV systems.