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Harmonics Technical Overview

Is There Really a Harmonics Problem?

Yes, there is a problem. In fact, there are so many difficult problems that hundreds, if not thousands, of theses, papers, articles, and case studies have been published on this subject. To define and limit harmonic problems, IEEE has published ANSI/IEEE Std. 519-1992 -- Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.

What Causes These Problems?

All electronic loads generate positive- and negative-sequence harmonic currents. Single-phase electronic loads, connected phase-neutral in a three-phase, four-wire distribution system, also generate zero-sequence harmonic currents.

Three-phase motor drives and single-phase lighting loads often account for a significant portion of a system’s 480V loads. Single-phase electronic office and data processing equipment typically accounts for more than 95% of a 120/208V power panel’s loads. At these levels, 100% total harmonic distortion of current (THDi) is common.

System Performance Issues

The injection of harmonic currents into an electrical distribution system, which is based on a non-mitigating conventional design, will normally produce the following unacceptable outcomes:

  • High total harmonic distortion of current (THDi)
  • High neutral currents
  • High harmonic current injection into the utility’s point of common coupling (ref. IEEE Std. 519-1992)
  • Increased voltage drop in feeder circuits, bus duct risers, and branch circuits

Power Quality Issues

In an Ohm’s Law relationship with the system’s harmonic impedances, the harmonic currents generate harmonic voltages. These voltages normally produce the following unacceptable power quality problems:

  • High total harmonic distortion of voltage (THDv) at the loads (ref. IEEE Std. 519-1992)
  • High neutral-to-ground voltage at the loads (ref. ITIC, formerly CBEMA)
  • Large neutral-to-ground voltage differentials at the office and data processing equipment, which are connected into a communications network (ref. EPRI)

Cost Issues

A detailed analysis of a conventional design will normally uncover many of the following undesirable financial issues:

  • High penalty losses, particularly in distribution transformers, 120/208V feeder and branch circuits
  • Low power factor
  • Additional ventilating and air conditioning costs
  • High power costs
  • Loss of productivity, quality, and revenue

How Big Are the Problems?

In the early 1980s, the proliferation of personal computers and conversion to electronic lighting ballasts produced harmonic problems in commercial office buildings. Maintenance electricians and designers soon discovered that single-phase electronic loads caused distribution transformers to overheat and "shared" neutral conductors to become overloaded.

Today, more than 95% of all 120/208V power panel loads, in modern office and data center environments, are electronic. Electronic motor drives are now routinely applied to ventilation fans and elevators at the 480V level.

What Were the Options?

To improve system performance and provide the best possible environment for the loads, a designer’s options have been limited to over-sizing distribution transformers and shared neutral conductors. As an alternative, branch circuits have been configured with a separate neutral conductor for each phase conductor. In either case, branch circuits have been underutilized or limited in their length as a means of reducing voltage distortion and neutral-to-ground voltage at the loads.

As an alternative to over-sizing conventional distribution transformers, many designers have specified K-Rated transformers. Unfortunately, their higher harmonic impedances actually cause an increase in voltage distortion.

Who Are the Stakeholders?

The implementation of these options did not resolve all of the system performance issues or any of the power quality and cost issues. Without the means to reduce harmonic currents, the facility’s stakeholders were subject to the following potential problems:

The Designer

  • Exceeds IEEE recommendations and requirements
  • Exceeds CBEMA /  EPRI recommendations
  • Power system and loads are incompatible
  • Exceeds budgetary goals
  • Loss of client confidence

The Facility Owner

  • High capital costs
  • Increased fire and safety risks
  • Premature apparatus failures
  • Significant tenant dissatisfaction

The Tenant

  • Higher power cost
  • Premature office equipment failures
  • Data corruption or loss
  • Computer and system lockups
  • Loss of productivity
  • Reduced quality assurance
  • Loss of customer confidence
  • Loss of revenue

Is There a Solution?

Yes, there is a solution. Harmonic current reduction will resolve all of the foregoing issues. The PQI Solution™ includes the consulting services of a professional engineer specialist who will provide guidance in the selection and application of PQI’s technically advanced, ultra-efficient e-Rated® products. We offer this service on a no-fee basis to system designers. If our recommendations are fully implemented, we guarantee compliance with the power quality requirements of IEEE Std. 519-1992 – IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. The proper selection of products and application of our engineered solution will result in the most cost-effective optimization of system and load efficiency and compatibility. Implementation of our recommendations will also result in an increase in our standard warranty from 10 years to 20 years.

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