The distortion in the waveform in the load current of any nonlinear device causes similar changes in the voltage waveform relative to the harmonic impedance of the source network. This voltage distortion affects both the current and voltage for all other loads connected to that system.
The common effects of such harmonic distortion are as follows:
- Motors and generators
- Power cables
- Electronic equipment
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 frequencies
- Higher audible noise emission as compared with sinusoidal excitation
- Harmonic 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.
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 above.
Generators can also produce harmonics and, in particular, triplen harmonics that can circulate through adjacent Wye-grounded transformers when generators are directly connected to a load bus. The use of the Delta-connected generator transformers can control this.
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 or the transformer needs to be replaced with a special “K”-rated transformer.
Special K-factor transformer nameplate
The “K” factor has been established by Underwriter Laboratories (UL) to define the ability of a transformer to serve varying degrees of nonlinear load current without exceeding the rated temperature rise.
The “K” factor is based on the predicted losses as specified in ANSI/IEEE C57.110 [S14].
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.
To avoid or minimise such problems, capacitor banks can be tuned to reject certain harmonics by adding reactance.
In most industrial harmonics power systems, the primary objective for installing capacitors is to meet the utility power factor requirements as expressed in its tariff rates. Additional benefits are better voltage regulation and lower losses.
Commonly used locations are shown in Figure 2 below.
Figure 2 – Typical one-line diagram for an industrial system
Any capacitor bank can be a source of parallel resonance with the system inductance.
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 whole
- Easier control of harmonic voltages and currents
- Lower capital costs, as large banks are more economical in terms of purchase cost
- Reactors 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 can be calculated by:
- fr is resonant frequency
- fs is system frequency, 60 Hz
- kVAsc is three-phase system fault level in kVA
- kVAc is three-phase capacitor-bank rating in kVA
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) drive carries 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 heat caused by the harmonic currents and the associated skin effect.
Figure 3 – Cable derating vs. harmonics with six-pulse harmonic current distribution
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 conductors.
Most cable manufacturers have in-house computer software and can assist the users if the specific harmonic data is available for the drives under consideration.
In terms of the application of cables for variable speed drives, or other significant harmonic sources, the following recommendations can be made:
- Use three-conductor and not single-conductor cables.
- Avoid large conductors to minimize the losses due to skin effect. The conductor size should not exceed 350 kcmil.
- Use shielded cables for equipment rated above 600 V.
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.
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.
Source Reference: Electrical Engineering Portal (EEP)