The prevalence of adjustable speed drives and other non-linear loads in industry has heightened awareness of the effects of harmonic current distortion on utility capacities and energy costs. It is not uncommon for utility companies to demand that harmonic-mitigating strategies be applied to larger drive installations. Let’s examine drive-induced harmonics in more detail, and look at ways that the problems they might cause can be reduced or eliminated.
Harmonics arise when current and/or voltage waveforms deviate from sinusoidal. The design of the front end (rectifier) section of an AC adjustable speed drive incorporates rectifying devices (typically diodes) to convert AC to DC, which then links by bus to the output section of the drive, where it is converted to a pulse-width modulated sine-wave for supply to the load. The DC bus incorporates a capacitor to smooth out the DC signal by damping the ripple in the line resulting from the rectification process. This capacitor does not draw current continuously, but only when discharging to the output section and when the rectifier section is forward biased; i.e when the instantaneous AC voltage is higher than the DC bus voltage. Because the current draw is not continuous, the wave form is disrupted and “noisy”; hence, rich in harmonics. The frequencies of these harmonics are calculated as multiples of the fundamental (50 or 60 Hz) frequency; for a six-pulse (i.e. six-diode bridge) rectifier, 5th, 7th, and 11th order harmonics predominate, but higher-orders also contribute.
There are several ways of reducing the amount of harmonic distortion in the supply line. Most are passive and serve to attenuate the flow of harmonics through them. Three of the more common methods, in order of increasing cost, are DC link chokes, AC line inductors, and passive filters.
DC link chokes, which are installed in the DC bus between the rectifier diodes and the DC capacitor, serve to smooth out current discontinuities and thus reduce harmonic content. Unlike AC reactors, they do not introduce additional voltage drop (unless they are incorrectly over-sized) and so do not affect the drive’s rated output. They are commonly provided as standard equipment in many manufacturers’ drives, typically adding an equivalent 3% impedance. One drawback is that the DC link choke does not protect the rectification section from incoming voltage spikes, so relying solely on them in a supply system subject to a lot of fluctuation is not advised. They can be used in conjunction with AC reactors to address this risk, but sizing is important so as not to affect the drive output too much.
AC line inductors, also known as AC chokes, introduce additional impedance into the supply line, helping to smooth out current discontinuities. Also, because impedance is proportional to frequency, the inductor reduces high-order harmonics more than lower-order ones. The reactor also serves to electrically de-couple the DC bus voltage from the AC supply, helping to ensure that the DC bus capacitor doesn’t cause flat-topping of the AC voltage waveform if the supply system is weak (i.e. has a high total impedance and/or lower capacity and is thus more susceptible to being dragged down). A fairly typical application involves adding a 3% impedance line reactor to the installation, assuming an input impedance of perhaps 1.5% for the line-side distribution equipment before reactor addition. One caveat with AC line reactors – while they reduce harmonics and protect the drive against incoming voltage spikes or surges, they also introduce voltage drop and thus can reduce the rated output of the drive. In high-reactance applications, careful sizing and drive integration are called for.
A third, increasingly less common, passive harmonic reduction method is the use of passive inductance/capacitance filters. They are intended to attenuate harmonics, or shunt the harmonic-current components (i.e. higher-frequencies) to ground. Several configurations of these filters exist, and all have limitations inherent to the use of capacitors, including contributing to DC bus over-voltage and introducing leading power factor to a drive which already operates near unity. Due in part to these limitations, drive manufacturers provide alternative 12-pulse, 18-pulse, or fully active rectifier sections in many of their larger drives. Higher-pulse rectifiers shift harmonic content to higher-order levels, thereby reducing overall harmonic distortion; for example, a 12-pulse rectifier will introduce essentially no 5th or 7th order harmonics, shifting the harmonic content to the 11th order and higher.
Other active, transistor-based methods of harmonics reduction exist, but they are topics for another day. If you have concerns or questions about any of the above, please let us know by visiting our Comments section, or contact us at simon.fan@vtdrive.com or visit us at www.vtdrive.com and www.vtdrive.tk. We’ll be glad to assist.
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