Junction temperature of inverter IGBT and diode. (a) power factor (PF) = 1, var = 0; (b) PF = 0.9, var = 0.44 p.u.; and (c) PF = 0.9, var = – 0.44 p.u..

This project involves the power loss analysis of PV inverters [1], electrothermal modeling of PV inverters [2], and lifetime estimation of PV inverters [3].

The power loss distribution among inverter semiconductors varies with respect to different output power factor (PF). In IGBT-diode type of PV inverters, providing reactive power will reduce the conduction loss of IGBTs and increase the conduction loss of diodes which increases the diode thermal stress because the equivalent current that flows through the diodes increases as the PF decreases. The theoretical analysis of power loss distribution is researched in this project. The power loss distribution among inverter semiconductors are verified in PLECS simulation.

Semiconductor losses are typically cycling in a fundamental cycle. In addition to the average power loss, the power loss variation during a fundamental cycle also varies with power factor. The average losses determine the mean junction temperature (Tj). The power loss variation in a fundamental cycle determines the junction temperature variation (∆Tj). Both junction temperature and temperature variation greatly influence the lifetime of a semiconductor. This project analytically derives the conduction loss of the semiconductors and shows the results that the junction temperature variation of inverter diodes will increase as the output power factor decreases. The junction temperature variation of semiconductors is evaluated in PLECS simulation.

In addition to loading condition, the inverter filtering inductor will also change the actual inverter power factor and indirectly influence the current distribution of the semiconductors. This project also studies the effect of filtering inductor on the current distribution of semiconductors.