Protecting power factor correction capacitors from overvoltage generated by non linear drives using parallel resonance filter

Abstract

The speed of an AC motor primarily depends on the frequency of the AC supply. To control the speed of an AC motor, Variable Frequency Drives (VFDs) are widely used. VFDs offer energy efficiency, smooth start and stop functions, and many other advantages. However, when these drives are used at higher ratings, typically above 10 KVA, they generate harmonics that are introduced into the line supply. These harmonics distort the sine wave of the AC supply, effectively increasing the line voltage, and sometimes causing it to exceed the load's sustaining voltage. The voltage of the generated harmonics depends on the drive pulses used in VFDs. In 6-pulse VFDs, the 5th harmonic is particularly prominent. This research article presents a practical solution to these problems. The RMS voltages of the lower odd harmonics are detected by the circuit. Parallel resonance for higher harmonics is provided by a high-quality factor coil. Due to this parallel resonance, harmonic currents do not enter the power factor correction capacitor but remain in the line voltage, thus protecting the power factor correction capacitor from overvoltage generated by VFDs. The RMS voltages of the harmonics are detected by a PIC microcontroller and displayed on an LCD, with resonance provided specifically for the 5th harmonic.