Harmonics and VFDs

Q.  What is a harmonic?
A.  A Harmonic is a signal at a frequency that is a multiple of the fundimental or base frequency.
Odd mulitples (3rd, 5th, 7th,....) are known as odd order harmonics and are caused by symmetrical distortion or the waveform.
Even multiples (2nd, 4th, 6th,...) are known as even order harmonics and are caused be assymetric distortion. - the positive and negative halves of the waveform are distorted unequally.
Half wave rectifiers cause even order harmonics and full wave rectifiers cause even order harmonics.

Q.  What problems are caused by harmonics?
A.  Harmonics cause issues with other connected equipment, especially capacitive loads and rotating machines. If a distorted voltage waveform is conected to capacitors such as power factor correction capacitors, the current flowing will increase and cause higher losses in the capacitor. The impedance of a capacitor is inversely proportional to the frequency. If we apply line frequency to the capacitor, there will be a current flow through the capacitor of X Amps. If we apply an equal voltage across the capacitor with a frequency of 5 times line frequency, then the current through the capacitor will be five times higher than at line frequency. If the harmonic voltage is 10% line voltage at the fifth harmonic, then the current at the fifth harmonic is half of the line current. The addition of harmonics to the line voltage can cause a significant increase in the current through the capacitor and result in early failure.
In the case of an induction motor, the presence of harmonics causes torque fields running at the frequency of the harmonic. When the motor is operating at full speed, there is high slip at the harmonic frequencies. This causes high pwer dissipation in the rotors and with high resistance motors, (as found in submersible motors), can result in early rotor failure.
High harmonic current increase the I²R losses and cause in increase in supply losses. Transformers, fuses, cables etc must be oversized to allow for the increased heating losses caused by harmonics.

VFD Designs

In low cost VFDs, the input rectifier is directly connected to a large DC Bus capacitor bank. With this configuration, the charging current only flows while the instantaneous supply voltage is higher than the DC bus voltage. This means that the current only flows during the crest of the voltage waveform, a very high curent for a very short time. This causes high order harmonics of significant amplitude.
Better VFDs include either DC Bus chokes or AC line reactors to reduce the amplitude of the peak currents. The AC reactors or DC bus chokes cause the current waveform to change from a very high current for a few degrees, to a lower current for longer. The maximum conduction angle with a three phase rectifier, is 120 degrees.
The effect on the harmonics, is that the high amplitude current for a short time has significant harmonics to a very high order. As the conduction angle is lengthened, the order of the harmonics is reduced and the Total Harmonic Distortion is also reduced.
The harmonics produced by a three phase input VFD are primarily the 5th, 7th, 11th and 13th. The triplen harmonics (3rd, 9th, 15th) are not generated by a correctly operating three phase rectifier.
The THDi of a VFD without a DC Bus choke or AC line reactors is in the order of 81% and the THDi of a VFD with a DC bus choke or AC line reactors is in the order of 35%.
As the load is reduced, the THDi increases.

Harmonic Mitigation

There are nine approaches to harmonic mitigation:
1. Do Nothing
2. Add AC Line reactors or DC bus chokes.
3. Add Tuned Trap Filter.
4. Add Low Pass Filter.
5. Use 12 pulse or 18 pulse rectifier.
6. Use Active Front End.
7. Use parallel Active Filter.
8. Use an AUHF.
9. Use Harmonic mitigating supply transformer.