Howard Network Transformers User Manual
Howard Industries, Inc.
ISO-9001 Certi ed
Catolog Section
94-10
Network Transformers
Introduction
Howard network transformers are designed and built
according to the most exacting engineering standards to
provide many years of outstanding performance and reliability in the most demanding utility network applications.
Product scope includes capacities from 300 kVA through
2,500 kVA with high-voltage ratings from 2.4 kV through
34.5 kV and low-voltage ratings through 600 Volts.
Network transformers are typically used to supply
power to grid-type secondary distribution systems in
areas of high load density, such as are found in large
cities and are designed for either vault-type or subway-
Catalog Section 94-10
Network Transformers
Issued: April 9, 2010
Copyright © 2010 Howard Industries, Inc.
Figure 1: Network Transformer
type applications. Vault-type network transformers are
designed for installation in above-ground dry vaults, where
occasional submersion may occur. Subway-type network
transformers are designed for installation in subsurface
vaults, where frequent or continuous submerged
operation is likely. Subway designs may also be used in
vault-type applications.
Howard Industries, Inc.
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www.howardtransformers.com
Laurel, MS 39440
Design and Manufacturing
Network Transformer94-10
Transformer Design
Howard network transformers are designed with conservative 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 specic requirements.
All designs are guaranteed to meet the U.S. Department of
Energy’s minimum efciency standards. General industry
standards applicable to Howard network transformer designs include IEEE C57.12.00 Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating 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 Distribution, Power and Regulating Transformers and Guide for
Short Circuit Testing of Distribution and Power Transformers, 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 Transformers, 10 CFR Part 431, Department of Energy, Energy
Conservation Program for Commercial Equipment: Distribution 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 rigorous design verication testing and years of eld service.
Mechanical strength is achieved through the use of a rugged 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 specications. In addition, all network
transformer ratings, where applicable, are designed to
satisfy the minimum efciency standards set by the U.S.
Department of Energy.
Core Construction
Cores are fabricated using high-efciency grain-oriented
silicon steel that has been precision slit and edge conditioned 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 efciency and establish the required rectangular shape of each core loop. Prior to assembly each
core is carefully tested to ensure it meets dimensional, exciting current and no-load loss specications. 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 Formvar® 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 layerto-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 subbase 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 including 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 accordance to the requirements of ANSI C57.12.40.
Howard Industries, Inc.
Laurel, MS 39440
www.howardtransformers.com
Copyright © 2010 Howard Industries, Inc.
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Catalog Section 94-10
Network Transformers
Issued: April 9, 2010
Network Transformer 94-10
High-Voltage Switch and Terminal Chamber
The high-voltage switch is a uid-immersed rotary type
switch located in the high-voltage switch chamber, with
an adjacent terminal chamber located above. An optional
single-chamber design is available. The switch has three
operating positions, OPEN, CLOSED, and GROUND, clearly
indicated on the switch indicator plate. A mechanical
stop is provided to prevent unintentional operation of the
switch and to allow an electrical interlock to prevent operation if the transformer is energized. Several different
types high-voltage cable entrances are available.
Figure 2: High-Voltage Switch and Terminal Chamber
Network Protector Provisions
A secondary throat and support brackets are provided on
the low-voltage termination side of the tank that are suit-
able for mounting a low-voltage network protector.
Table 1: Factory Testing
Catalog Section 94-10
Network Transformers
Issued: April 9, 2010
Figure 3: Network Protector Provisions
Copyright © 2010 Howard Industries, Inc.
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Howard Industries, Inc.
Laurel, MS 39440
www.howardtransformers.com
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