What problems do harmonics create?

How do harmonics affect my site or facility?

Why are harmonics unknown or untreated in electrical distribution systems?

How can we wire electrical distribution systems for harmonics?

How can we treat harmonics?

Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz (50Hz for European power and 60Hz for American power). For example, if the fundamental power frequency is 60 Hz, then the 2^nd harmonic is 120 Hz, the 3^rd is 180 Hz, etc. In modern test equipment today harmonics can be measured up to the 63^rd harmonic. When harmonic frequencies are prevalent, electrical power panels and transformers become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When this happens, the power panel or transformer vibrates and emits a buzzing sound for the different harmonic frequencies. Harmonic frequencies from the 3^rd to the 25^th are the most common range of frequencies measured in electrical distribution systems.

Additionally, harmonics are caused by and are the by-product of modern electronic equipment such as personal or notebook computers, laser printers, fax machines, telephone systems, stereos, radios, TVs, adjustable speed drives and variable frequency drives, battery chargers, UPS, and any other equipment powered by switched-mode power supply (SMPS) equipment. The above-mentioned electronic SMPS equipment is also referred to as non-linear loads. This type of non-linear loads or SMPS equipment generates the very harmonics they’re sensitive to and that originate right within your building or facility. SMPS equipment typically forms a large portion of the electrical non-linear load in most electrical distribution systems. There are basically two types of non-linear loads: single-phase and three-phase. Single-phase non-linear loads are prevalent in modern office buildings while three-phase non-linear loads are widespread in factories and industrial plants.

In today’s environment, all computer systems use SMPS that convert utility AC voltage to regulated low voltage DC for internal electronics. These non-linear power supplies draw current in high amplitude short pulses. These current pulses create significant distortion in the electrical current and voltage wave shape. This is referred to as a harmonic distortion and is measured in Total Harmonic Distortion (THD). The distortion travels back into the power source and can effect other equipment connected to the same source.

To give an understanding of this, consider a water piping system. Have you ever taken a shower when someone turns on the cold water at the sink? You experience the effect of a pressure drop to the cold water, reducing the flow of cold water. The end result is you get burned! Now imagine that someone at a sink alternately turns on and off the cold and hot water. You would effectively be hit with alternating cold and hot water! Therefore, the performance and function of the shower is reduced by other systems. This illustration is similar to an electrical distribution system with non-linear loads generating harmonics. Any SMPS equipment will create continuous distortion of the power source that stresses the facility’s electrical distribution system and power equipment.

Harmonics are generally not an issue if you do not have any electronic SMPS equipment or non-linear loads in your building or facility. However, for the remainder of this discussion, we are assuming that you do.

In an electrical distribution system harmonics create:

Above we identified the problems directly affecting your electrical distribution system. In turn, these problems affect your entire site or facility in a number of different ways:

These are recommended ways to wire for the harmful effects that harmonics cause. However, these recommendations only keep the electrical distribution systems safe. These wiring recommendations do not eliminate or cancel high levels of harmonics.

1. Use double-size neutral wires or separate neutrals for each phase.

2. Specify a separate full-size insulated ground wire rather than relying on the conduit alone as a return ground path.

3. On a branch circuit use an isolated ground wire for sensitive electronic and computer equipment.

4. Segregate sensitive electronic and computer loads on separate branch circuits all the way back to the electrical panel.

5. Run a separate branch circuit for every 10 Amps of load.

6. Install a comprehensive exterior copper ground ring and multiple deep driven ground rods as part of the grounding system to achieve 5 ohms or less resistance to earth ground.

7. Oversize phase wires to minimize voltage drop on branch circuits.

8. Shorten the distance on branch circuits from the power panel to minimize voltage drop.

How can we treat harmonics?

In order to ensure the highest "Power Quality" for your building or facility, it is necessary to treat harmonics. Harmonic treatment can be performed by two methods: filtering or cancellation. A harmonic filter consists of a capacitor bank and an induction coil. The filter is designed or tuned to the predetermined non-linear load and to filter a predetermined harmonic frequency range. Usually this frequency range only accounts for one harmonic frequency. This application is mostly used when specified for a UPS or variable frequency drive motor in a manufacturing plant.

Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetics technology designed to remove high neutral current and the most harmful harmonics from the 3^rd through 21^st. The technique used in these transformers is call "low zero phase sequencing and phase shifting". These transformers can be used to treat existing harmonics in buildings or facilities. This same application can be designed into new construction to prevent future harmonics problems.

Listed below are four patents that are used in the design of these transformers.

1. Combined Phase-Shifting Directional Zero Phase Sequence Current Filter and Method for Using Thereof, 1995 (USA)

2. Zero Phase Sequence Current Filter with Adjustable Impedance, 1995 (USA)

3. Phase-Shifting Transformer or Autotransformer, 1996 (USA)

4. Phase-Shifting Transformer or Autotransformer with Low Zero Phase Sequence Impedance, 1998 (USA)

In closing, it is important to consult with a power quality expert before applying any of the recommendations or methods as discussed throughout this paper. A power quality consultant will be able to analyze the severity of the harmonics problem and design a plan tailored to your specific harmonics situation. If you know or think that your building or facility has a harmonic problem, please go to our "Power Problems" web page and register for your free 15 minutes of consulting.