Data Bulletin

7400DB0301

July 2003

Nashville, TN, USA

Harmonic Mitigating Transformers

Application Guide

Class 7400

ABOUT THIS DOCUMENT This document is a guide for applying Harmonic Mitigating Transformers

(HMT). The Low Voltage Dry Type Distribu tion Transformer Produ ct

Support Group is also available as a resource. If the harmonic issue is

severe, contact Square D for support. Square D’s Power System Analysis

group can provide a complete analysis of your electrical distribution system

and make recommendations to match your specific application criteria.

INTRODUCTION Harmonic Mitigating Transfo rmers (HMT) accompli sh harmonic mitigation

(attenuation) (see “What are Harmonics?” on page 6) by providing good

source impedance and through sine wave recombination. Sine wave

recombination occurs within the transformer itself at the nodes, the

connection points, of the windings. Sine wave recombin ation also occurs

when the sine waves are phase shifted using multiple transformers. All of

these methods may help reduce harmonics in electrical distribution

systems.

While not intended as a complete technical analysis, this bul letin will

highlight the appropriate application of HMTs. It is interesting to note that

none of this is new technology. In variable frequency drive applications,

harmonic mitigation has been accomplished with transformers for more than

50 years. Additional information regarding some of the theory of harmonic

mitigation using transformers is included in th e appendices.

INDUSTRY TRENDS Harmonic mitigation is a topic of high interest among North American

industry users and manufacturers. Howe ver, the European community has

established guidelines to control the maximum level of harmonics that

equipment can introduce onto a system. Computer and business equipment

manufacturers, in response to the European Community guidelines, created

power supplies that do not burden an electrical system with harmonics. It is

likely that North America will soon follow the limited harmonic path.

STANDARDS The standard for evaluating transformers serving non-linear loads is IEEE

C57.110. This standard recognizes the effects of harmonic attenuation

(reactive effect on harmonic levels) and harmonic diversity (cancellation

effect of multiple non-linear loads). C57.110 is often ignored by some HMT

vendors.

EXCEPTIONS • IEEE Standard 1100-1992 (the Emerald Book) is not appropriate for

evaluating transformer loading with non-linear loads. Any such

references should be ignored.

• IEEE Standard 519-1992 is not intended for use with in a facility. The

standard sets the maximum harmonic voltage distortion level allowed at

the point of common coupling between the utility and the customer. The

point of 519 is to insure that one customer’s power usage profile does

not negatively impact another customer.

Harmonic Mitigating Transformers 7400DB0301

July 2003

PROPER APPLICATION OF THE

TRANSFORMER METHOD OF

HARMONIC MITIGATION

The installation of any Square D HMT transformer -- delta-wye, delta-

zigzag, or wye-zigzag – provides an effective means of preventing triplen

, 9

, 15

, etc.) harmonics from passing to the source side of the

transformer. The standard conservative design practice of using delta-wye

transformers automatically results in electrical distribution systems with

some inherent harmonic mitigation (see Figure 1 ).

With a standard delta-wye transformer installed, the triplen harmonics are

not trapped in the transfo rme r, but nor do they pass through the

transformer. The sine waves induced on the windings are recombined at the

nodes of the delta into new sine wave shapes that do not contain triplen

components (see “Combining Sine Wave T heory” on page 8).

The transformer’s reactance causes some attenuation of the harmonics and

limits the crest factor allowe d downstream (see “Source Impedance” on

page 8).

Figure 1: Standard Delt a-w y e Tra nsforme r Fe e din g a Compu te r Pa nel From a Distri bu tion Pan el bo ard.

Computer

Panel

208Y/120

480V

Panel

Delta -Wye Transformers

‘Cancel’ 3rd, 9th, 15th Harmonics