With four popular methods of addressing harmonics, remember to match the method with your problem. Each problem and possible solutions are unique.
So, you know you have a harmonic problem. Now, you're facing the difficult task of finding a practical solution. What can you do? Four of the most recommended solutions include: Increasing the size of the neutral conductor path; decreasing the load of delta-wye transformer; replacing the delta-wye transformer with a k-factor transformer; and installing a harmonic filter at the power source or equipment location.
The first three solutions help you cope with the problem; the fourth actually eliminates the problem. Lets' talk about each in detail.
Neutral conductor sizing. In Part 1 of this three-part article (June '99 issue), we discussed that harmonic currents affect the neutral conductor.Since these currents don't cancel out in a balanced 3-phase system (they add), i t's possible the neutral will carry more current than you anticipate. When this happens, the neutral conductor path overheats.
This is why you should double the size of the neutral conductor for feeders and branch circuits serving nonlinear loads. Office partition manufacturers have design requirements in place for doubling the neutral in their partition wiring. Increased neutral currents cause many electrical fires within office partitions. Some OEM partition wiring schemes include a separate neutral per phase conductor while others use a shared neutral doubled in size.
Increasing the size of the neutral conductor lowers its impedance, thus reducing harmonic problems. Even the National Electrical Code (NEC) warns you about problems with neutrals in Sec. 210-4(a), Multiwire Branch Circuits, FPN; Sec. 220-22, Feeder Neutral Load, FPN 2; Sec. 310-4, Conductors in Parallel, Ex. 4 FPN; Sec. 310-10, Temperature Limitations of Conductors; and Sec. 310-15 Ampacities, Note 10(c). This last section forces you to derate the ampacity of all conductors in a 4-wire circuit in a raceway by 20%.
Transformer loading. Transformers are more efficient when supplying linear loads. However, the majority feed nonlinear equipment, producing more copper losses in dry-type transformers than the fundamental current. These losses are associated with eddy currents and hysteresis in the core and skin effect losses in windings. The result is transformer overheating and winding insulation breakdown.
The NEC warns you about protecting transformers that feed nonlinear loads in Sec. 450-3, Transformer Overcurrent Protection, FPN 2; and Sec. 450-9, Transformer Ventilation, FPN 2. So, you can derate the transformer, but be careful. Most experts believe feeding a transformer to less than 60% of its kVA rating decreases the effects of any harmonic currents. If you can't alter the loading on it, then install one with a higher kVA rating.
K-factor transformers. These transformers differ in construction from standard dry-type transformers. They can handle harmonic currents at near capacity without having to be derated. Construction features include: * Electrostatic shield between the primary and secondary windings of each coil; * Neutral conductor lug size that's twice that of the phase conductor lugs; * Parallel smaller windings on the secondary to negate skin effect; and * Transposition of primary delta winding conductors (in large size units) to reduce losses.
A drawback to using this type of transformer is the K-rating of the transformer is only applicable to the harmonic content of the loads presently fed from it. If the harmonic content of the existing loads changes or if you install new nonlinear load equipment generating harmonic currents of a different frequency, then the k-factor unit's performance in mitigating the effects of the resultant harmonic currents will decrease.
Harmonic filters. A harmonic filter can eliminate the potentially dangerous effects of harmonic currents created by nonlinear loads. It traps these currents and, through the use of a series of capacitors, coils, and resistors, shunts them to ground. A filter unit may contain several of these elements, each designed to filter a particular frequency.
You can install filters either between the device your trying to protect and the load's power source, or between the device causing the condition and its power source.
There are two types of harmonic filters: passive filters and active filters. Passive filters are inexpensive compared with most mitigating devices. Internally, they cause the harmonic current to resonate at its frequency. This prevents the harmonic current from flowing back to the power source and causing problems with the voltage waveform. A disadvantage of the passive filter is that it cannot be perfectly "tuned" to handle the harmonic current at a significant frequency. Fig. 1, in original article, shows the different types of passive filter construction.
Active filters, on the other hand, can be tuned to the exact frequency of the harmonic current and do not cause resonance in the electrical system. They can also address more than one harmonic problem at the same time.
Active filters can also provide mitigation for other power quality problems such as voltage sags and power line flicker. (See Fig. 2, in original article.) They use power electronics to replace part of the distorted current sine wave coming from the load, giving the appearance you're using only linear loads. As a result, the active filter provides power factor correction, which increases the efficiency of the load.