Power quality analysis of variable speed drives
Date
2014-09-03
Authors
Abraham, Amit
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Abstract
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.