A robust 18-pulse rectifier for adjustable speed drive systems

Eighteen pulse rectifiers have been lately utilized to improve the input current distortion in adjustable speed drive (ASD). However, when a large capacitor is connected in the dc-side, the performance is significantly deteriorated. The input current THD decreases when the relationship between the dc output current and the short circuit current of the utility line (I_d/I_sc), decreases, i.e. for lighter loads or stronger utility lines the performance becomes worse. Similarly, the THD is also affected when the utility line suffers a voltage unbalance condition or voltage harmonic distortion. It is not uncommon to measure 1% to 3% voltage unbalance and/or 2.5% to 5% pre-existing 5th and 7th harmonic voltage distortion. In this paper, a combination of an 18-pulse rectifier and an active power filter (APF) to improve the input current distortion in ASD is proposed. The APF provides the 18-pulse rectifier with the ability to achieve a high performance over a wide spectrum of operating conditions. The approach also has the characteristic of maintaining a balance of performance, cost and reliability. Results from a 480 V/30 kW ASD prototype is discussed in the conference presentation.