Power quality analysis of variable speed drives
A study was conducted to evaluate the effects of harmonics generated by Variable Speed Drives (VSDs). A VSD with a technology known as Reduced Harmonics Technology (RHT) was considered and benchmarked against existing solutions in industry in terms of cost and effectiveness. The RHT VSD, like the standard VSD, uses a three phase rectifier but with significantly lower DC bus capacitor banks and an advanced motor control processor. Simulation results reveal that the RHT VSD model produced current harmonics of approximately 30% when compared to a standard VSD, without any additional mitigation solutions, produced current harmonics above 100%. The RHT VSD was also found to be less expensive than the equivalent rated standard VSD. Laboratory experiments reveal that the input current of the RHT VSD and the standard VSD are similar to the input current waveforms from the simulation of the RHT VSD and the standard VSD respectively. Simulation of the DC bus capacitance and the source impedance reveal that in a lower range of DC capacitance values (below C1 = 600μF), the size of the DC bus capacitance has more effect on the input harmonics than the source impedance. An increase in source impedance does not reduce input harmonics. In the above mentioned range of capacitance values, it was noted that, the DC bus capacitance dominate the source impedance in its ability to reduce input harmonics. When the DC capacitance was increased above C1 = 600μF, the source impedance has more effect on the input harmonics than the size of the DC bus capacitance. The simulation and experimental results show that there are higher order (above 13th order) harmonic frequency components appearing at the input of the RHT VSD when compared to a standard VSD. It is clear that there is a trade off, due to the effect of the motor control processor, between there being reduced harmonics at the lower orders (below 13th order) and there being an increase in harmonics at higher orders (above 13th order). It was also noted from the experiment that there is no notable difference in the harmonic content at the outputs of the two VSDs.