HARMONICS

 

 

TREATING HARMONICS
IN
ELECTRICAL DISTRIBUTION SYSTEMS

PRESENTED BY:
VICTOR A. RAMOS JR.
TECHNICAL CONSULTANT
COMPUTER POWER & CONSULTING CORPORATION

January 25, 1999

What are harmonics and what cause harmonics?

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?

 

What are harmonics and what cause 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 2nd harmonic is 120 Hz, the 3rd is 180 Hz, etc. In modern test equipment today harmonics can be measured up to the 63rd 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 3rd to the 25th 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.

What problems do harmonics create?

In an electrical distribution system harmonics create:

  1. Large load currents in the neutral wires of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Since only the phase wires are protected by circuit breakers of fuses, this can result in a potential fire hazard.

  2. Overheating of standard electrical supply transformers which shortens the life of a transformer and will eventually destroy it. When a transformer fails, the cost of lost productivity during the emergency repair far exceeds the replacement cost of the transformer itself.

  3. High voltage distortion exceeding IEEE Standard 1100-1992 "Recommended Practice for Powering and Grounding Sensitive Electronic Equipment" and manufacturer’s equipment specifications.

  4. High current distortion and excessive current draw on branch circuits exceeding IEEE Standard 1100-1992 "Recommended Practice for Powering and Grounding Sensitive Electronic Equipment" and manufacturer’s equipment specifications.

  5. High neutral-to-ground voltage often greater than 2 volts exceeding IEEE Standard 1100-1992 "Recommended Practice for Powering and Grounding Sensitive Electronic Equipment."

  6. High voltage and curent distortions exceeding IEEE Std. 519-1992 "Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems."

  7. Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing, and industrial) with a power factor less than 0.9.

  8. Resonance that produces over-current surges. In comparison, this is equivalent to continuous audio feedback through a PA system. This results in destroyed capacitors and their fuses and damaged surge suppressors which will cause an electrical system shutdown.

  9. False tripping of branch circuit breakers.

How do harmonics affect my site or facility?

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:

  1. Voltage distortion and voltage drop as mentioned in above items #3 and #6 cause the equipment connected to the branch circuit to draw more current to maintain the power rating (watts) of the unit. The bigger the current draw from the unit, the more it produces excess heat within the unit that was not factored for by its original design. In turn, the excessive heat causes premature component level failures within the unit. Additionally, you will experience computers locking up and other operational malfunctions that are unexplainable. Think about how many times we have experienced the "no problem found" syndrome with our computers! The excessive heat produced can directly contribute to downtime. Therefore, downtime is identified as any event that incurs or contributes to lost productivity, lost revenues, lost savings, and more importantly lost time. As we all have heard in the business world, "Time is Money".

  2. In special facilities such as call centers or data centers, the excessive heat produced due to the large concentration of monitors and PCs will also cost you money in energy dollars. The air computer room (CRAC) or building air conditioning system will run longer or harder, therefore requiring more energy to maintain the desired temperature.

  3. Telecommunications cabling is commonly run right next to power cables. If harmonics are above normal tolerances (more than 5% THD) as outlined in IEEE Standard 519-1992, then high frequency harmonics can be induced into phone lines and data cabling. The end result is noisy phone lines and unexplained data lose or data corruption in your LAN or WAN.

  4. Soon to be imposed utility regulations limiting harmonics per IEEE Standard 519-1992.

Why are harmonics unknown or untreated in electrical distribution systems?

First, one must understand that the electrical distribution system of most sites or facilities was never designed to deal with an abundance of non-linear loads. It’s a problem that has only recently begun to be recognized in the building industry. Within the last decade, the widespread use of computers and SMPS equipment is turning modern office buildings, factories, and industrial plants into high-tech computer environments. Even older buildings that are renovated are not retrofitted with modern harmonic treatment or cancellation. The end result is a building or facility unable to fully support today’s technology and the high-tech problems that it brings along with it. Obviously, given the problems harmonics can cause, it is imperative that today’s electrical distribution systems be designed for non-linear electronic loads, not just linear electrical loads. Unfortunately standard building codes and engineering designs do not meet the requirements of today’s technology. With the advent of newer SMPS equipment the harmonic problem will continue to get worse along with inadequate facility grounding. Grounding which is another subject is mentioned here because it too is seldom addressed or considered a problem area (See grounding paper under "Library" heading).

How can we wire electrical distribution systems for harmonics?

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 3rd through 21st. 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.

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