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Howard Industries, Inc.

Laurel, MS 39440

www.howardtransformers.com

Network Transformer94-10

Catalog Section 94-10

Network Transformers

Issued: April 9, 2010

Transformer Design

Howard network transformers are designed with conser-

vative mechanical and electrical margins to withstand

the harsh environments encountered in today’s network

distribution systems. Core-and-coil designs are optimized

for the lowest procurement cost or lowest total owning

cost according to each customer’s specic requirements.

All designs are guaranteed to meet the U.S. Department of

Energy’s minimum efciency standards. General industry

standards applicable to Howard network transformer de-

signs include IEEE C57.12.00 Standard General Require-

ments for Liquid-Immersed Distribution, Power, and Regu-

lating Transformers, ANSI C57.12.40 American National

Standard for Secondary Network Transformers, Subway

and Vault Types (Liquid Immersed)—Requirements, IEEE

C57.12.90 Standard Test Code for Liquid-Immersed Distri-

bution, Power and Regulating Transformers and Guide for

Short Circuit Testing of Distribution and Power Transform-

ers, IEEE C57.93 Guide for Installation of Liquid-Immersed

Power Transformers, IEEE C57.98 Guide for Transformer

Impulse Tests, IEEE C57.100 Standard Test Procedure for

Thermal Evaluation of Oil Immersed Distribution Trans-

formers, 10 CFR Part 431, Department of Energy, Energy

Conservation Program for Commercial Equipment: Distri-

bution Transformers Energy Conservation Standards; Final

Rule, and 10 CFR Part 431, Department of Energy, Energy

Conservation Program: Test Procedures for Distribution

Transformers; Final Rule.

Core-and-Coil Design

Howard’s ve-legged core-form design provides excellent

mechanical strength that has been proven through rigor-

ous design verication testing and years of eld service.

Mechanical strength is achieved through the use of a rug-

ged steel mounting frame that provides solid support for

core/coil assembly.

Core-and-coil designs are optimized to provide the lowest

total owning cost or lowest purchase price according to

each customer’s specications. In addition, all network

transformer ratings, where applicable, are designed to

satisfy the minimum efciency standards set by the U.S.

Department of Energy.

Core Construction

Cores are fabricated using high-efciency grain-oriented

silicon steel that has been precision slit and edge con-

ditioned by the supplier. Step-lap joints are used to

minimize losses and exciting current, and to insure quiet

operation. Cores are designed to operate at ux densities

well below saturation. Stress-relief annealing is employed

to maximize efciency and establish the required rectan-

gular shape of each core loop. Prior to assembly each

core is carefully tested to ensure it meets dimensional, ex-

citing current and no-load loss specications. Amorphous

metal cores are available for those applications requiring

ultra-low excitation losses.

Coil Construction

High-voltage coil windings are constructed of copper

or aluminum magnet wire. Automatic wire tensioners,

computer-controlled traverse mechanisms and laser

alignment systems ensure that coils are wound tightly and

accurately. Low-voltage coil windings are constructed of

edge-conditioned full-width sheet conductor, available in

either copper or aluminum. Low-voltage sheet windings

provide the advantage of virtually eliminating axial forces

during short circuit.

Turn-to-turn insulation in the high-voltage winding is Form-

var® or extruded polymer coating. Main barrier and layer

insulation in both low-voltage and high-voltage windings is

thermally-upgraded craft, providing exceptional insulation

life. Insulation paper is coated with a thermoset epoxy

adhesive throughout the coil to produce excellent layer-

to-layer bonding. Strategically placed oil ducts provide

oil ow and adequate cooling throughout the windings.

The insulation system is designed to provide exceptional

impulse withstand capability.

Tank Construction

Network transformer tanks are of sealed construction,

including a sub-base and a welded main cover with

bolted (standard) or welded hand-hole cover. The sub-

base consists of steel bars parallel to the long axis of the

transformer with jacking areas located along the length

and width of the tank bottom. The copper-bearing steel

plate used to construct the tank is reinforced with side

wall braces, and all tank seams are continuously welded.

The completely sealed tank is capable of withstanding a

pressure of 7 psig without permanent deformation and

15 psig without rupture. Four lifting lugs are supplied and

arranged for lifting of the complete transformer includ-

ing the network protector, if attached. Tank grounding

provisions consist of copper-faced or stainless-steel pads

welded to the tank. Fastening hardware is composed of

corrosion-resistant steel. The tank exterior nish is in ac-

cordance to the requirements of ANSI C57.12.40.

Design and Manufacturing