Design and manufacturing – Howard Network Transformers User Manual

Page 2

Howard Industries, Inc.

Laurel, MS 39440

www.howardtransformers.com

Network Transformer

94-10

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 specific requirements.

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

Energy’s minimum efficiency 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 five-legged core-form design provides excellent

mechanical strength that has been proven through rigor-

ous design verification testing and years of field 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 specifications. In addition, all network

transformer ratings, where applicable, are designed to

satisfy the minimum efficiency standards set by the U.S.

Department of Energy.

Core Construction

Cores are fabricated using high-efficiency 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 flux densities

well below saturation. Stress-relief annealing is employed

to maximize efficiency 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 specifications. 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 flow 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 finish is in ac-

cordance to the requirements of ANSI C57.12.40.

Design and Manufacturing