A Series-Connected 24-Pulse Rectifier Using Passive Voltage Harmonic Injection Method at DC-Link

In this paper, a voltage harmonic injection method at the dc-link is proposed to reduce input current harmonics of the series-connected 12-pulse rectifier. The proposed method uses an auxiliary circuit to generate the voltage harmonics and injects the voltage harmonics into the dc link of the series-connected 12-pulse rectifier so that the input voltages of the 12-pulse rectifier can be shaped as 24 steps. The formation processes of the input voltage are analyzed based on the switching functions; and the turn ratio of injection transformer is optimally designed from the perspective of minimizing the total harmonic distortion (THD) of the input voltage. After using this method, experimental results show that the input voltages of the isolated transformer are 24-step waves with an experimental THD of around 3.34%; the input line currents of the proposed rectifier are close to sinusoidal waveforms with an experimental THD of around 2.65%. More importantly, the kVA rating of the injection transformer is only about 2.3% of the load power, which means that the cost of harmonic reduction is very low.