A synchronous-frame controller for a single-phase sine wave inverter

A synchronous frame controller for a single-phase PWM inverter with an LC output filter is presented. These types of inverters are often used in uninterruptible power supplies (UPS) where a sinewave output voltage is to be maintained. The advantages of a synchronous frame regulator are well known in three-phase controllers where regulated AC variables (voltage, current, flux) are “-rotated” into a frame synchronous to the fundamental output frequency where they appear as DC vector quantities. Such a controller can regulate the steady-state errors of the corresponding DC quantities to zero. While the familiar dq transformation is not applicable in a single-phase system, a frame synchronous to the output fundamental can be formed by recognizing that the filter capacitor current and the filter capacitor voltage are identically 90° out of phase. Thus, orthogonal quantities much like dq components exist, and can be used to create a rotating frame where a composite reference vector can be summed with a similar measured vector. Simulations of a synchronous controller and a conventional state-feedback cascaded controller are presented and compared. Preliminary experimental results are also presented