The Reverse Zero-Sequence Current Compensation Method for Back-to-Back Active Power Conditioners

In this thesis, a novel reverse zero-sequence current (RZSC) compensation method for three-phase four-wire back-to-back active power conditioner (APC) is proposed. The objective of the APC is to achieve the active and reactive power transmission between two micro-grids. However, for the unbalanced voltage sag micro-grid system, the double-line frequency oscillation exists in the output active/reactive power of the APC. Therefore, this thesis utilizes the positive-/negative-/zero-sequence current to eliminate these oscillation, but the zero-sequence current produces the line frequency ripple voltage on the split capacitors of the dc-bus. The capacitor ripple voltage speeds up the aging process and reduces its life-time. Also, it may exceed the maximum operating voltage and damage the split capacitors, distort the output current of the inverter, or even cause the malfunction of the APC. Therefore, the RZSC method is proposed to compensate the impact of the ripple voltage to the split capacitors. The operation principle of the back-to-back APC under the unbalanced grid voltage is introduced, and the mathematical equation derivation of the proposed RZSC method is presented. Both computer simulations and hardware experimental results are presented to verify the performance of the back-to-back APC with the proposed RZSC method.