Harmonic Distortion - Internal

Harmonic distortion is found in both the voltage and the current waveform. Most current distortion is generated by electronic loads, also called non-linear loads. These non-linear loads might be single phase loads such as point-of-sale terminals, or three-phase like variable speed drives.

As the current distortion is conducted through the normal system wiring, it creates voltage distortion according to Ohm's Law. While current distortion travels only along the power path of the non-linear load, voltage distortion affects all loads connected to that particular bus or phase.

Current distortion affects the power system and distribution equipment. It may directly or indirectly cause the destruction of loads or loss of product.

From the direct perspective, current distortion may cause transformers to overheat and fail even though they are not fully loaded. Conductors and conduit systems can also overheat leading to open circuits and downtime.

On three-phase systems, current distortion causes higher than expected currents in shared neutrals. A shared neutral is one that provides the return path for two or three-phases. Currents as high as 200% of the phase conductors have been seen in the field. This large level of current can easily burn up the neutral creating an open neutral environment.

This open neutral forces voltage swells and overvoltages to exist. These voltage conditions easily destroy equipment, particularly power supplies. Thus, indirectly, the current distortion damaged the loads.

One other indirect problem current distortion introduces is called resonance. Certain current harmonics may excite resonant frequencies in the system. This resonance can cause extremely high harmonic voltages, again possibly damaging equipment.

One additional comment about current distortion. When the current is non-sinusoidal, our conventional ammeters and voltmeters will not accurately respond. To accurately measure currents that are harmonically distorted, you must use a true-RMS meter. This applies equally to distorted voltages.

Voltage distortion, on the other hand, directly affects loads. Distorted voltage can cause motors to overheat and vibrate excessively. It can also cause damage to the shaft of the motor.

Even non-linear loads are prey to voltage distortion. Equipment ranging from computers to electronically-ballasted fluorescent lights may be damaged by voltage distortion.

If damage is occurring due to current distortion, except for high neutral current, then one solution is to reduce the distortion. This can be done through three methods. First, a passive filter can be used to reduce the current from one or two specific harmonics.

An active filter, the second method, reduces all the harmonic currents. It is more costly and complex to use, but it works better than passive filters.

The third method involves the use of transformers. Delta-wye transformers reduce certain harmonics, particularly what are called zero sequence harmonics. Zigzag transformers can also be used to reduce zero sequence harmonics, but without changing the system type between delta and wye.

If there is concern that these special transformers or the regular distribution transformers may overheat, then transformer derating or the use of K-rated transformers is recommended.

If high neutral currents are the culprit, then the first step to take is eliminate shared neutrals wherever possible. Where this can't be done, try oversizing the neutral wire so it won't overheat. If this doesn't work, then the distortion must be reduced as described above. Zigzag transformers also help reduce high neutral currents.

There are two ways to reduce voltage distortion. First, remember that internal voltage distortion is the result of the business's non-linear loads interacting with the wiring. The first way to reduce the distortion is to reduce the harmonic current. We already talked about how to do that.

The second way is to reduce the impedance of the wiring. This is done by increasing the size of the conductors.

Where the total voltage distortion is the sum of internal and external distortion, these techniques reduce the internal contribution.