d>
Below is a cache of http://www.usace.army.mil/publications/armytm/tm5-684/chap7.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
CHAPTER 7 TRANSFORMERS AND REGULATORS
TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
CHAPTER 7
TRANSFORMERS AND REGULATORS
Section I-CONSIDERATIONS
7-1. Voltage provisions covered.
This chapter provides maintenance and repair re-
quirements for transformers used in the transmis-
sion and distribution of electrical energy and for
voltage regulators. Requirements apply to units
having at least one medium-voltage winding and
generally providing three-phase service, although
single-phase units may be found for housing or
other small loads.
7-2. Defining transformer and regulator char-
acteristics.
A transformer utilizes electromagnetic induction be-
tween circuits of the same frequency, usually with
changed values of voltage and current. All trans-
formers covered in this chapter are constant-voltage
type. That is, they maintain an approximately con-
stant voltage ratio over loads from zero to the rated
output. Constant-current transformers are de-
scribed in chapter 6, section IV Transformers can be
classified in various ways, but their basic construc-
tion consists of windings, magnetic cores on which
windings are coiled, insulation, and any special con-
nections applying to the type of load.
a. Winding terminology. Winding terminology
given below is based on the voltage flow, rating, or
winding provisions.
(1) A primary winding has input from the
power source and a secondary winding supplies in-
put to the loads.
(2) A high-tension winding has a higher volt-
age than a low-tension winding. Most transformers
have high-tension primary windings and are there-
fore step-down transformers. If the same trans-
former utilized the low-tension winding as the pri-
mary winding it would be a step-up transformer.
(3) Most transformers have two windings,
which are electrically insulated from each other.
High-voltage power transformers found in
transmission-to-distribution substations may have
a single winding (autotransformers) or a tertiary
winding to eliminate voltage problems and/or to
supply a second load voltage economically.
b. Regulation. Transformers can maintain an ac-
ceptable voltage ratio of about a 2 percent voltage
drop from zero to rated output in most cases. Most
distribution transformers and smaller power trans-
formers have tapped windings, which permit ad-
justing the output voltage to broaden the range of
primary voltage inputs. The transformer will have a
manual tap changer, which can be operated if the
transformer is de-energized. However, on substa-
tions which serve varying loads, such as pumping
facilities, or on large installations with long primary
feeder lines, taps may not provide sufficient voltage
regulation and other means are necessary.
(1) Load-tap-changing
(LTC). This feature in-
stalled on a transformer provides automatic tap
changing under load, and normally varies the volt-
age to plus or minus 10 percent of the systems rated
voltage by changing tap connections using a motor-
driven, tap-changing switch.
(2) Voltage regulators. Sometimes voltage
regulation is needed and the system transformers
do not include the LTC feature. Voltage regulators
are used to supply the control for the variations in
load. A voltage regulator needs similar servicing to
that required for a power transformer. A step-
voltage regulator operates on the same principal as
the LTC mechanism. An induction voltage regulator
has a series winding and a shunt winding, and uses
a motor to rotate the shunt winding to either add to
(boost) or subtract from (buck) the series winding
voltage. The action provided is dependent upon the
voltage induced in the series winding and the re-
spective polarities of each winding (that is, the re-
spective instantaneous directions of currents enter-
ing the primary and leaving the secondary
terminals during most of each half cycle). The
switching mechanism in most new voltage regula-
tors is practically maintenance free, but many of the
older units require considerable servicing. The
manufacturers recommendations should be fol-
lowed for all maintenance and servicing require-
ments.
7-3. Transformer classification.
Transformers are generally classified by size, insu-
lation, and location.
a. Size. Transformers rated above 500 kVA are
classed as power transformers. Transformers rated
at 500 kVA or less are classed as distribution trans-
formers, as they usually have low-tension windings
of less than 600 volts. Instrument transformers,
covered in chapter 3, section VI, are not considered
distribution transformers since they do not serve
utilization loads.
b. Insulation. There are two types of insulating
classifications recognized by ANSI/IEEE C57.12.80. TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
Insulation classifications are affected by the insula-
tions temperature rating and by the method of cool-
ing needed to remove the heat from the transformer.
( 1) Liquid-immersed transformers. The core
and the coils are immersed in an insulating liquid. A
flammable mineral oii insulation is the most fre-
quently used liquid. Various less flammable liquids
are used to meet NEC code requirements. Only
flammable and less flammable liquids are accept-
able on military installations. Nonflammable-
insulated liquids, though available, are not consid-
ered environmentally acceptable. Polychlorinated
biphenyl (PCB) insulated transformers should have
been removed to meet OSHA requirements. Re-
placement of liquid-filled transformers in or near
buildings must take into account the latest appli-
cable NEC code restrictions, which might require an
existing installation to be modified or a different
type of insulation to be provided.
(2) Dry-type transformers. The core and coil are
in a gaseous or dry-compound insulating material.
c. Cooling classes. Distribution and small power
transformers are generally self-cooled. Other meth-
ods of cooling may be added to provide a greater
load capacity than would be available with a self-
cooled unit. Cooling methods include forced air-
cooling (fans) for liquid-immersed and dry-type
units; and forced air-cooling and/or forced liquid-
cooling for liquid-immersed transformers. Dry-type
units can be ventilated, nonventilated, or sealed.
Transformers may be provided with the cooling
equipment, may have provisions for adding cooling
equipment, or may be without future capability for
adding cooling equipment.
d. Insulation temperature ranges. Transformers
are designed to carry their normal rated load in
specific ambient temperatures with a maximum
stated temperature rise for normal life. If ambients
or temperature rises are exceeded under operating
conditions, the transformer life may be decreased. If
lower temperatures occur, the transformer life may
actually be increased. Overload capabilities of
transformers are indicated in ANSI/IEEE C57.91,
ANSI/IEEE C57.92, and ANSI/IEEE C57.96.
(1) Ambient temperatures. The ambient tem-
perature for an air-cooled unit should not exceed 40
degrees C; and the average temperature for any
24-hour period should not exceed 30 degrees C; with
a minimum ambient temperature for dry-type units
of not less than minus 30 degree C. These restric-
tions apply if the transformer is to provide its nor-
mal life expectancy.
(2) Insulation temperature ratings. Liquid-
immersed transformers are rated 65 degree C rise
or 55/65 degree C rise. Dry-type transformers are
rated 150 degree C rise, 115/150 degree C rise, or
80/115 degree C rise. The lower the temperature
rise the lower the rated full-load capacity.
(3) Altitude. The dielectric strength of trans-
formers, which depend in whole or in part upon air
for insulation, decreases as the altitude increases
due to the effect of decreased air density. This ap-
plies to liquid-immersed transformers as well as
dry-type transformers.
e. Location. Transformers can be classified by
their location, but only those which are most often
installed on military facilities are covered below.
(1) Outdoor transformer. This is a transformer
of weather-resistant construction, suitable for ser-
vice without additional protection from the weather.
Industry standards also classify transformers as in-
door units, which must be protected from the
weather.
(2) Pole-type transformer. An outdoor trans-
former which is suitable for mounting on a pole or a
similar structure.
(3) Pad-mounted transformer. This is a unit-
ized or compartmental-type transformer, with en-
closed compartments for medium-voltage and low-
voltage cables entering from below, and is mounted
on a pad. The terminology is confusing, and it is
recommended that this type of unit be called a pad-
mounted compartmental-type transformer.
__
(4) Station-type transformer. A unit designed
for mounting on a pad and installed in a substation,
more