Question

Write the differences between characteristics harmonics and non–characteristics 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


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

It should also be noted that system unbalance (standing unbalance or ground faults), expressed as negative-sequence currents, can also reflect into the rotor as harmonic currents, which add to those noted

 

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.

With the addition of harmonic currents, standard design transformers must be derated to limit the temperature rise to be within the insulation temperature-rise rating


 

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.



 

Avoiding resonance problems

The best approach to avoid resonance problems is to install large capacitor banks at the main bus. This solution offers the following advantages:

More available reactive power to the system as a wholeEasier control of harmonic voltages and currentsLower capital costs, as large banks are more economical in terms of purchase costReactors can be added to shift the resonant frequency away from the characteristic harmonic frequency of the plant

Capacitors can also be combined with reactors to develop harmonic filters at the troublesome resonance harmonic frequencies. The resonant frequency at the capacitor bus

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.

These resonance risks and the harmonics themselves can produce the following problems for cable systems:

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 cubicle

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 harmonics.

The cable therefore has to be derated to compensate for additional heatcaused by the harmonic currents and the associated

 

Figure 3 shows the cable derating factors plotted against percentage harmonic current with the harmonic mix associated with a typical 6-pulse VSD. Due to the skin effect, more derating is required for large
 

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.