4366 LB McLeod Road  
Orlando, FL 32811  

Ph: 407-841-4405  
Fax: 407-841-4407  
 © Copyright 2005 TPS, Inc.  


by Powersmiths

Why use harmonic mitigating transformers?

While the concept of energy savings is much easier to relate to, many consider the negative impacts of voltage distortion to far outweigh the cost of increased system losses.

Click here to find the right transformer or PDU for your application.

Published transformer loss data can dramatically underestimate measured losses.

Today's transformer losses are being increasingly driven by electronic equipment like computers, UPS and variable frequency drives whose load profile drives both additional losses and unwanted distortion. Since transformer manufacturers test only under ideal (linear) conditions, as called for in present construction standards, there is a substantial gap between published loss data and actual losses incurred after installation.

Testing at Oak Ridge National Lab, a US DOE test facility, documented substantial increases in core and coil losses - almost triple what was expected - when a 112.5kVA transformer was feeding 60kW of 120V computer load. Years ago, equipment used to be predominately linear in nature, where current flows continuously, following the supply voltage, such as in motors and incandescent lighting.

Today, with the influx of electronic equipment from the computer in the office to the variable speed drive in the factory and in our HVAC systems, our electrical systems feed a growing portion of this type of load, which is nonlinear, drawing current in pulses rather than continuously. Electronic equipment and other nonlinear loads now make up most of the load on transformers in many facilities. Even in the average office, aren't we plugging in mostly computers, printers, scanners and other electronics to our 120V receptacles?

Transformer Losses Transformers have two major components that drive losses: the core and the coils. The typical core is an assembly of laminated steel, and core losses are mostly related to magnetizing (energizing) the core. These losses, also known as no-load losses, are present all the time the transformer is powered on - regardless of whether there is any load or not. Core losses are roughly constant from no-load to full-load when feeding linear loads. They represent a continuous cost, 24 hours/day, for the 25-year or more life of the transformer. The cost is substantial. A common 75kVA commercial transformer has in the range of 400 Watts in no-load losses (like leaving four 100 Watt light bulbs turned on). At $0.10/kWh, this represents a continuous cost of $350/yr or $8,750 over the 25 year expected life span of the transformer, eclipsing the purchase cost many times over. And this is just the cost of being turned on. This is just the base cost. The cost of powering the load itself is much more than this. Yes, it costs real money to run a transformer.

The Bottom Line:

* Endorsed by the DOE as an energy saving devise.
* Provides clean power to sensitive loads.
* Smaller footprint.
* Increases capacity on existing service.
* No de-rating of transformer size.

Frequently Asked Questions

  • What is IEEE-519?
  • What will happen to my loads and electrical system if I operate outside the IEEE
        519 guidelines?
  • What is the Maximum Possible Neutral Current?
  • Why do some people say neutral current can reach 3 times phase current?
  • What is a typical level of Neutral Current for an Office Environment?
  • Phase-shifting is great but what if I have a single nonlinear load, like one large     motor drive?
  • What are harmonics?
  • What causes harmonics?
  • What harmonics are most prevalent in today's electrical systems?
  • What are the effects of harmonics in an electrical system?
  • Why do neutral conductors need to be oversized?
  • How do I test for harmonics?
  • After collecting data how do I relate the measurements to an acceptable standard?
  • What strategies are available for harmonic reduction?
  • Will a K factor transformer reduce harmonics?
  • What are harmonic cancellation transformers?
  • What should I look for in a supplier?

    What is IEEE-519?

    It is the North American Standard that describes acceptable limits of harmonics in electrical systems. The proper reference is IEEE Std 519-1992, entitled "IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems".

    What will happen to my loads and electrical system if I operate outside the IEEE 519 guidelines?

    Perhaps nothing or maybe disaster! Systems that operate above the guidelines are operating in a less reliable environment. More problems occur than not. Some issues may be immediate while others occur over the long term.

    What is the Maximum Possible Neutral Current?
    In a system feeding single-phase (phase-neutral) loads using switching power supplies, such as 120V personal computers and other electronic equipment, the worst case scenario for neutral current is when the pulses drawn by the loads on each phase do not overlap. The total resulting current in the neutral conductor is the arithmetic sum, which is the root of the sum of the squares:

    The maximum is reached when the loads are balanced, so on a per unit basis, the result is: times the phase current.

    Any overlap of pulses between the phases, and typically there is some, reduces the amount of neutral current compared with the phase current.

    Why do some people say neutral current can reach 3 times phase current?

    Over the past ten years, there have been many articles published that mention neutral current, some mention 1.73 times at the maximum possible level, some say 3 times, others say it can be 1.73 or 3 without any back-up. We have described how neutral current can reach 1.73 times the phase current. The 3 times number comes from simply summing the current from the 3 phases into the neutral conductor (1+1+1 = 3). This reflects either a lack of understanding of the requirement to do the root of the sum of the squares, or contemplates that all loads draw only zero sequence 180Hz, not consuming any fundamental (or other) current, obviously not possible in our electrical systems.


    What is a typical level of Neutral Current for an Office Environment?

    Today's offices are filled with desktop computers and laser printers. Where these are the only loads fed from the local electrical panel, neutral current can be expected to be in the range of 1.3 to 1.5 times the phase current. In many offices, the workspace cubicles have a fluorescent task light that is plugged into the same system. Since the harmonic content of an average fluorescent lamp is lower than for a computer, it lowers the ratio of harmonics amps to RMS amps, and this dilution reduced the expected neutral current to about equal to the phase current. However, it should be noted that if the task lighting uses compact fluorescent lamps, which have a rich harmonic spectrum like a desktop computer, neutral current can be expected to be in the 1.3-1.5 times the phase current.

    Phase-shifting is great but what if I have a single nonlinear load, like one large motor drive?

    The phase-shift technique relies on using the harmonic currents produced by one load to cancel with the harmonic currents produced by a similar but phase-shifted load. Phase-shifting will therefore not work with a single harmonic producing load. Options for harmonic treatment include active filters, tuned filters, reactors, or moving the load to a dedicated feeder. Other treatments are possible. The choice will depend on the results of an analysis of many variables such as the electrical system characteristics, the load profile and duty cycle.

    What are harmonics?

    Harmonics are multiples of some base frequency, each having a frequency and amplitude. For example our electrical system is 60 hertz so the 3rd harmonic is 180 hertz and the 5th harmonic is 300 hertz. Each harmonic is sinusoidal and as such repeats through a system.

    What causes harmonics?

    Harmonic in our electrical systems today are caused by nonlinear loads such as personal computers and variable speed drives. These harmonics are introduced to the system through the way the load draws current.


    What harmonics are most prevalent in today's electrical systems?

    The loads that we place on our systems today are computers, laser printers, video monitors, electronic ballasts, variable speed drives and ups. Loads that are connected phase to neutral ( equipment typically plugged into a wall receptacle) produce an abundance of 3rd, 5th and 7th harmonic currents. Phase to phase and three phase loads ( variable speed drives for example ) that are 6 pulse produce mostly 5th and 7th harmonics.

    What are the effects of harmonics in an electrical system?

    The most common issue is extra heat due to the additional losses. Excessive neutral current will result in systems that have high density of single phase loads due to the triplens (3rd harmonic and its odd multiples) and the imbalance of the 60 hertz and the other non triplen harmonic currents such as 5th and 7th. In systems where the impedance is high voltage distortion will also become an issue.

    Why do neutral conductors need to be oversized?

    Single phase nonlinear loads carry a high amount of 3rd harmonic. The 3rd harmonic is in phase in all three phases so they add into the neutral. This can result in neutral current with a higher value than the phase current. The large conductor is designed to carry the additional expected current.

    How do I test for harmonics?

    The most thorough way to test for harmonics is to take measurements using a harmonic analyzer. Simple tests would include measuring neutral to ground voltage at the panels and at the wall receptacles. Take a simple meter and measure the neutral current. Also you can compare the RMS readings to the non RMS readings.


    After collecting data how do I relate the measurements to an acceptable standard?

    IEEE 519 is the North American recommended practice for harmonic levels. This practice recommend voltage distortion to be less than 5% thd and no individual voltage to greater than 3%. It also provides levels for current but only at the point of common coupling ( the point where the utility stops and in-house distribution begins).

    What strategies are available for harmonic reduction?

    Derating
    Capacitors ( LC filters )
    Phase shift ( change the pulse of the system )
    Lower the impedance for zero sequence to shunt
    Active
    Reactors

    Will a K factor transformer reduce harmonics?

    A K factor transformer is derated to survive in a harmonic rich environment. It will not cancel any harmonic.

    What are harmonic cancellation transformers?

    These transformers if designed properly will incorporate low zero sequence impedance and phase shifting, noise reduction and higher efficiencies. They are used in place of isolation and K rated transformers to cancel 3rd, 5th and 7th harmonic distortion.

    What should I look for in a supplier?

    Application support.
    Quality of manufacture
    Technical quality of product
    Application experience
    Installed base
    Agency approvals


    •