Harmonic Distortion

General

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 as in 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 through they are not fully loaded. Conductors and conduit systems can also overheat leading to open circuits and downtime.

On three-phase wye 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 creates voltage swells and overvoltages. These voltage conditions easily destroy equipment, particularly power supplies.

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

There is one additional comment about current distortion. When the current is non-sinusoidal, our conventional ammeters and voltmeters will not respond accurately. 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 motor shaft.

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

Solutions

If damage occurs due to current distortion, except for high neutral current, then one solution is to reduce the distortion. There are three methods for this. First, a passive filter can be used to reduce the current from one or two specific harmonics. In the second method, an active filter 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. Zig-zag transformers can also be used to reduce zero sequence harmonics, but without changing the system type between delta and wye. In addition, they can help reduce high neutral currents. 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 is to eliminate shared neutrals wherever possible. Where this cannot 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.

There are two ways to reduce voltage distortion. Remember that internal voltage distortion is the result of the business' non-linear loads interacting with the wiring. The first way to reduce the distortion is to reduce the harmonic current. 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.

Standards

The standards covering Harmonics and Harmonic Distortion are set forth in:

  • ANSI C57.11: IEEE Recommended Practice for Establishing Transformer Capability When Supplying Non-sinusoidal Load Currents
  • IEEE Standard 519:
  • Recommended Practice and Requirements for Harmonic Control in Electric Power Systems
  • IEEE Standard 1001:
  • Recommended Practice for Interfacing Dispersed Storage and Generation

Harmonic Distortion

When the source voltage into a business is no longer sinusoidal, we say that it is distorted. Harmonic distortion implies that there are higher frequencies than just the 60 Hz involved in the power flow. These higher frequencies can disrupt, degrade, and damage equipment. But not just computer-based equipment. Voltage distortion affects even motors and lights, loads commonly thought to be rather immune to power disturbances.

If a neighbor draws a large amount of distorted current, then this current distorts the utility's source voltage. This source voltage, which is now distorted, is then fed to the energy customer. Loads within the business are now subjected to possible problems.

Solutions to a voltage distortion problem like this are a little complicated. If energy user A is adversely affected by distortion generated due to energy user B, the utility must be brought in to the picture. In general, the utility is responsible for providing customer A with appropriate power. Even though customer B is messing up the voltage, the utility may deal with it in one of two ways.

The utility may be able to work with the offending customer to resolve the problem. Preferably, the level of voltage distortion should be significantly reduced at this customer's site, thus reducing it on the utility's bus. This can be achieved through reducing the amount of total current or reducing the amount of distorted current.

In lieu of this, the utility may have to resolve the problem on their own. They can either increase the distribution cable size in hopes that reducing the impedance will be sufficient, or else they may have to provide harmonic current filters at their expense to reduce the level of distorted current.

If there is no other recourse, the energy user may have to deal with the distorted voltage. Unfortunately there are no cheap, easy answers to this one. The voltage must either be cleaned up or regenerated.

There are products which will clean up harmonically distorted voltages. They are large, costly, and a challenge to install and maintain. They are also new, with little history behind them as to their effectiveness.

Regenerating the voltage is also costly, but there are more choices with longer track records. Motor-generator sets and appropriate UPS will both provide low distortion.

One final comment. It is also very likely that the level of voltage distortion delivered to an energy customer from the neighbors is not detrimental. But due to internal non-linear loads, the voltage distortion worsens to a point of causing problems. In these cases, reducing the current distortion internally will reduce the added voltage distortion.

Links to Related Topics

Sags and Swells
Neighbor-Induced Transients