Resonance reduction for AC drives with small capacitance in the DC link

Pulse Width Modulated AC drives equipped with small DC-link capacitor are becoming an attractive solution for electric drive applications with moderate requirements for shaft dynamic performance. However, when these drives are fed from a weak grid a resonance between the line side impedance and the DC-link capacitor appears. Due to this resonance, the THD and the partially weighted harmonic distortion of the line currents are increased, which may rise compatibility problems with the AC line harmonic standards. By using vector control the motor drive is transformed into a constant power load from the DC-link point of view, leading to instability of the entire drive system. This paper proposes a novel DC-link voltage compensation method, which can reduce the amplitude of the above mentioned oscillation, while the large DC-link voltage variation, caused by the rectification can be eliminated from the motor current. The method has been experimentally validated and the results show excellent performance. Moreover, the proposed compensation method can be applied for any kind of load connected to a three-phase passive rectifier equipped with small capacitor.