Power Factor Correction Capacitors for Multiple Parallel Three-Phase ASD Systems: Analysis and Resonance Damping

Today's three-phase Adjustable Speed Drive (ASD) systems still employ Diode Rectifiers (DRs) and Silicon Controlled Rectifiers (SCRs) as the front-end converters due to structural and control simplicity, small volume, low cost, and high reliability. However, the uncontrollable DRs and phase controllable SCRs bring side-effects by injecting high harmonics to the grid, which will degrade the system performance in terms of lowering the overall efficiency and overheating the system if remain uncontrolled or unattenuated. For multiple ASD systems, certain harmonics in the entire system can be mitigated by phase-shifting the currents drawn by SCR-fed drives. In addition, optimizations of the firing angles and power levels further reduce the harmonic distortions. However, compromises between the harmonic distortion level and the resultant power factor are encountered in the optimization. To improve the power factor, passive capacitors can be installed, which yet can trigger the system resonance. Hence, this paper analyzes the resonant issues in multiple ASD systems with power factor correction capacitors. Potential clamping solutions are summarized. Simulations are carried out, while laboratory tests on a two-drive system are provided to demonstrate the harmonic mitigation scheme in multiple ASD systems.