Voltage Oscillation on the AC Side of Series Active Filters and Its Suppression.

Attention has been paid to practical applications of series active filters and series active capacitors into power systems. These consist of both voltage-source PWM inverters and matching transformers (MTs) for series-connection to power systems. The purpose of the matching transformer is not only to isolate the PWM inverter from the source, but also to match the voltage and current ratings of the PWM inverter with those of the source. The output terminals on the ac side of the PWM inverter exhibit a high impedance because the impedance on the source side of the matching transformer is multiplied by N2 where the MT's turns ratio is 1: N. The PWM inverter consists of semiconductor switching devices such as MOSFETs or IGBTs. It is therefore essential to connect a small-rated LC passive filter for suppressing switching ripples appearing in the output voltage of the PWM inverter. Oscillation between a small-rated inductor and capacitor may occur because of the high impedance on the ac side of the PWM inverter, causing undesirable high frequency currents to flow into the power system.This paper proposes a new control scheme for the series active filter, aiming at suppressing the voltage oscillation between the small-rated inductor and capacitor. It is shown theoretically that the proposed simple current feedback can suppress the oscillation so that no undesirable current flows into the source. The proposed scheme is actually applied to a hybrid series active/shunt passive filter, and then a design example is shown. Digital simulation confirms that the proposed scheme can damp the oscillation effectively. In addition, experimental results demonstrate the effectiveness and practicability of the proposed control scheme.