Question

What are the adverse effects of harmonics?

Answer 1

 

1. Motors and generators

Generators and motors are adversely affected by harmonics in the networks to which they are connected. Typical effects are:

Increased heating due to iron and copper losses at the harmonic frequenciesHigher audible noise emission as compared with sinusoidal excitationHarmonic currents in the rotor

The harmonic currents noted above are caused by harmonics in the stator winding, which will produce harmonic currents in the rotor, e.g., 5th- and 7th-order stator harmonics will produce 6th-order rotor harmonics, while 11th- and 13th-order stator harmonics will produce 12th-order rotor harmonics.

These rotor harmonic currents will result in increased rotor heating and pulsating or reduced torque

 

2. Transformers

The stray-loss factor for copper conductors varies as the square of the load current and the square of the frequency, and will therefore vary with the harmonic mix in the power supply. Although the percentage contribution to distortion by higher harmonics decreases as the harmonic frequency rises, its heating effect, even if the harmonic percentages are low, could rise substantially.
The harmonics generated by nonlinear loads such as variable-frequency drives (VFD) will impose non-sinusoidal current on the power transformers that supply such loads, resulting in a substantial increase in losses and temperature rise.

 

3. Capacitors

Any capacitance in an AC network can produce a risk of resonance with the inductive parts of the network. Although electrical networks are designed not to have any resonances at fundamental frequencies, when the multiple frequency effects of harmonic distortions are considered, there is always the possible risk of system resonance.

These and other effects of harmonics on capacitors and capacitor banks are as follows:

Resonance imposes considerably higher voltages and currents in capacitors.The capacitor bank acts as a sink for higher harmonic currents, which increases the heating and dielectric stresses.The losses in a capacitor are proportional to the reactive output (kVAR), which, in turn, is proportional to the frequency. These losses are increased, and the overall capacitor life is shortened with increasing harmonics.

 



4. Power cables

Power cables are inherently capacitive and, as noted above for capacitor banks, their capacitance can produce a risk of resonance with the inductive parts of the network

Cables involved in system resonance may be subjected to voltage stress and corona.Increased heating due to higher rms current, skin effect, and proximity effect. The skin effect will vary with the frequency and conductor size.
Power cable conductors commonly lie very close to one another, and therefore the high-frequency currents in the outer skin of one conductor influence the spread and behavior of high-frequency currents in the skin of the adjoining conductors, giving rise to a “proximity effect.”
The skin effect and proximity effect are proportional to the square of a frequency. Cables therefore have to be derated if there is significant harmonic distortion, particularly if ITHD is greater than 10
Feeder cable to the drive cubicl
The power cable feeder to any variable speed drive (VSD) drivecarries 60 Hz fundamental or sinusoidal current plus the harmonic currents produced by the drive. The selected feeder size needs to be based on the heating from the total rms current (fundamental plus harmonics) and the skin effect of the higher order
5. Electronic equipment
Power electronic equipment is susceptible to mis-operation if there are significant levels of harmonic distortion. Some of the control systems for power electronic devices use zero crossing detection to control switching.
Harmonic distortion can result in shifting of the voltage zero crossing points, and these changes can be critical for many types of electronic control circuits. Also, if incorrect switching occurs, more harmonics can be produced, compounding the problem.
6. Fuses
Fuses suffer a derating factor because of the heat generated by harmonics. Fuses can therefore malfunction under the influence of harmonics. These effects must be considered so that the fuses can be derated correctly.
Also, the unwarranted and frequent blowing of fuses is an indication of the presence of unexpected harmonics or changes in the harmonic mix in that system