Power electronic circuits in wind and photovoltaic (PV) power systems basically perform the following functions:
• Convert alternating current (AC) to direct current (DC) • Convert DC to AC • Control voltage • Control frequency • Convert DC to DC
These functions are performed by solid-state semiconductor devices periodically switched on and off at a desired frequency. Device costs in 2005 have declined to about a tenth of those in 1990, fueling an exponential growth in applications throughout the power industry. No other technology has brought about a greater change in power engineering, or holds a greater potential for bringing improvements in the future, than power electronic devices and circuits. In this chapter, we review the power electronic equipment used in modern wind and PV power systems.