An Adaptive Carrier Frequency Optimization Method for Harmonic Energy Unbalance Minimization in a Cascaded H-Bridge-Based Active Power Filter

Cascaded H-bridge (CHB) based active power filters (APF) can compensate high-order harmonics in medium and high-voltage systems with relatively low switching frequency. This paper reveals that a severe dc capacitor energy unbalance can be induced by harmonic energy exchange if the frequency of APF output current coincides with the switching harmonic frequency of CHB cells. The mechanism of harmonic energy exchange is analyzed, and it is found that the dc capacitor energy unbalance is essentially affected by the carrier frequency of CHB. A noninteger ratio carrier frequency can be adopted to avoid the divergence of dc voltage, but there is still a significant low-frequency ripple on the dc voltage. This paper proposes an adaptive carrier frequency optimization method. By predicting the dc voltage ripple amplitude under different carrier frequencies, this method can adaptively select the optimal carrier frequency to minimize the dc voltage ripple. The proposed method is also proved to be robust against system parameter variations, and it can be implemented by a practical and simple linear computation method. Results obtained from simulation, experimental prototype, and field test are finally presented to verify the proposed adaptive carrier frequency optimization method.