A Novel Unbalance Loads Compensation Method Based on Ultra-Capacitor

The unbalance loads in the distribution network have a great influence on the power quality and the safety of the transformer. To compensate the unbalance loads, power storage device is needed to buffer the unbalanced power. In this paper, the ultra-capacitor is used as the power storage device. First, the STATCOM is used to transfer the unbalance power from the AC side to the DC capacitor. The DC voltage of the STATCOM contains the second harmonic voltage, when the unbalance power is transferred. The second harmonic voltage is considered to cause the third harmonic contained in the output current of the STATCOM. To solve the problem, the control strategy based on notch filter is used. Second, the bi-directional DC-DC converter transfers the unbalance power into the ultra-capacitor. And the ultra-capacitor contains the second harmonic voltage. To transfer the total unbalance power into the ultra-capacitor will result in power oscillation between the AC side and the ultra-capacitor. Therefore, the control strategy is presented to make sure that the second harmonic amplitude of the voltage of ultra-capacitor should be proportional to the amplitude of second harmonic voltage in the DC-link voltage of the STATCOM. With the control method, the unbalance power is compensated by the ultra-capacitor and the DC side capacitor of the STATCOM. The ultra-capacitor compensated most of the unbalance power as the capacity is extremely greater than the DC capacitor of the STATCOM. The simulation results demonstrate that the proposed compensation system structure is correct and the control strategy is effective.