Technical Collection
Use and Maintenance of
ELVIM Oil-immersed
Distribution Transformers
Building a New Electric World
Use and Maintenance of
ELVIM Oil-immersed Distribution Transformers
The transformer is an electrical machine that allows the transmission and
distribution of electric energy simply and inexpensively, since its efficiency is
greater than 95%.
Through the brief description of the use and maintenance of the oil-immersed
distribution transformers, the present technical leaflet provides useful
information for the engineers, who are involved in the selection, purchasing,
installation, operation and maintenance of transformers.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 1
Contents
SECTION A: USE OF TRANSFORMERS
A.1
Transformer Types
A.1.1
Classification of transformers according to the use
page 4
A.1.2
Classification of transformers
according to the cooling method
page 5
A.1.3
Classification of transformers
according to the insulating medium
page 5
Classification of transformers
according to the construction of the magnetic circuit
page 6
A.1.4
A.2
A.3
A.4
A.5
A.6
ELVIM Distribution
Transformers
A.2.1
General characteristics
page 7
A.2.2
Advantages of ELVIM distribution transformers
page 7
Transformer
Manufacturing
Features
A.3.1
Magnetic circuit
page 8
A.3.2
Windings
page 8
Transformer
Components
Transformer tests
Transformer
electrical
characteristics
A.3.3
Metallic parts
page 9
A.3.4
Assembly
page 9
A.3.5
Cooling medium
page 9
A.4.1
Tank
page 10
A.4.2
Cover
page 10
A.4.3
Lifting lugs
page 10
A.4.4
Rollers
page 10
A.4.5
Draining and sampling oil valve
page 10
A.4.6
Neutral earthing link
page 10
A.4.7
High voltage bushings
page 11
A.4.8
Low voltage bushings
page 11
A.4.9
Low voltage connectors
page 11
A.4.10
Tap changer
page 11
A.4.11
Voltage selector
page 11
A.4.12
Transformer thermometer
page 11
A.4.13
Oil conservator
page 12
A.4.14
Buchholz relay
page 12
A.4.15
Air dehumidifier
page 12
A.4.16
Filling valve
page 12
A.4.17
Oil level indicator
page 12
A.4.18
Rating plate
page 13
A.4.19
Tank earthing point
page 13
A.4.20
Accessories of sealed type transformers
page 13
A.5.1
Type tests
page 14
A.5.2
Routine tests
page 14
A.5.3
Special tests
page 15
A.6.1
Rated power
page 16
A.6.2
Temperature rise
page 16
A.6.3
Ambient temperature
page 16
A.6.4
Altitude of installation
page 16
A.6.5
Short-circuit impedance
page 16
A.6.6
No-load losses
page 17
A.6.7
Load losses
page 17
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 2
A.6
Transformer
electrical
characteristics
A.6.8
Rated voltage
page 17
A.6.9
Vector group
page 17
A.6.10
Frequency
page 18
A.6.11
Noise
page 18
A.6.12
Efficiency
page 18
A.6.13
Short-circuit current
page 18
A.6.14
No-load current
page 18
A.7
Transformer standards
page 19
A.8
Tolerances
page 19
A.9
Transformer
operation
A.9.1
Overloading
page 20
A.9.2
Parallel operation
page 21
A.9.3
Load distribution of transformers in parallel operation
page 21
A.10
Transformer order form
A.11
Transformer
selection
A.12
ELVIM
transformers series
page 22
A.11.1
Electrical utilities
page 23
A.11.2
Industrial users
page 23
A.12.1
Single-phase transformers, from 5 to 50 kVA, 20/0.231 kV
page 24
A.12.2
A.12.3
Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV page 26
Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV,
with low losses
page 28
Three-phase transformers, from 250 to 1600 kVA,
20-15/0.4 kV
page 30
Three-phase sealed type transformers,
from 25 to 1600 kVA, 20/0.4 kV
page 32
A.12.4
A.12.5
A.13
Examples
A.13.1
Calculation of transformer efficiency
page 34
A.13.2
Calculation of voltage drop
page 34
A.13.3
Parallel operation of transformers
page 35
A.13.4
Transformer selection
page 35
SECTION B: TRANSFORMER INSTALLATION AND MAINTENANCE
B.1
Dimensions of transformer installation area
page 36
B.2
Instructions for transformer installation
page 37
B.3
Instructions for transformer maintenance
page 37
B.4
Instructions for thermometer connection
page 38
B.5
Instructions for the connection of the Buchholz relay
page 39
B.6
Instructions for the connection of the air dehumidifier
page 40
Services of Schneider Electric
page 41
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 3
SECTION A:
Use of Transformers
A.1 Transformer types
The transformers are classified into various categories, according to their:
(a) use,
(b) cooling method,
(c) insulating medium,
(d) core construction.
These categories are presented in the following subsections.
A.1.1 Classification of transformers according to the use
According to their use, the transformers are classified into the following categories:
(a) Distribution transformers
(d) Test transformers
They are used in the distribution networks in order
to transmit energy from the medium voltage (MV)
network to the low voltage (LV) network of the
consumers. Their power is usually ranging from 50
to 1600 kVA.
They are used for the execution of performance tests
with high or ultra-high voltage.
(b) Power transformers
They are used in the high-power generating stations
for voltage step up and in the transmission substations
for voltage step up or step down. Usually their power
is bigger than 2 MVA.
(c) Autotransformers
They are used for voltage transformation within
relatively small limits, for connection of electric energy
systems of various voltages, for starting of AC
(alternative current) motors, etc.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 4
(e) Special power transformers
They are used for special applications, e.g. in furnaces
and in welding.
(f) Instrument transformers
They are used for the accurate measurement
of voltage or current.
(g) Telecommunication transformers
They are used in telecommunication applications
aiming at the reliable reproduction of the signal in
a wide range of frequency and voltage.
A.1.2 Classification of transformers according to the cooling method
The identification of oil-immersed transformers according to the cooling method is expressed by a four-letter
code. The first letter expresses the internal cooling medium in contact with the windings. The second letter
identifies the circulation mechanism for internal cooling medium. The third letter expresses the external cooling
medium. The fourth letter identifies the circulation mechanism for external cooling medium. For example, if the
internal cooling medium is mineral oil, which is circulated with natural flow, and the external cooling medium is air,
which is circulated with natural convection, then this cooling method is coded as ONAN (Oil Natural Air Natural).
In power transformers, various cooling methods are used including oil circulation by pumps, or forced air
circulation by fans, or both of the above. As a result, the following cooling methods exist:
ONAF: Oil Natural Air Forced.
OFAN: Oil Forced Air Natural.
OFAF: Oil Forced Air Forced.
OFWF: Oil Forced Water Forced.
Combinations like ONAN/ONAF, ONAN/OFAN or ONAN/OFAF are also applicable.
A.1.3 Classification of transformers according to the insulating medium
According to their insulating medium, the transformers are classified into the following categories:
(a) Oil-immersed type transformers
The insulating medium is mineral oil or synthetic (silicon) oil.
(b) Dry type transformers
The cooling is implemented with natural air circulation and the windings are usually insulated with materials of H
or F class. The materials of H class are designed in order to operate, in normal conditions, under temperatures up
to 180ºC and the materials of F class under temperatures up to 155ºC.
(c) Resin type transformers
The resin type transformer is a dry type transformer insulated with epoxy resin cast under vacuum.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 5
A.1.4 Classification of transformers according to the construction of the magnetic circuit
The construction of the magnetic circuit of the three-phase transformers can be done, alternatively, as follows:
(a) With three legs (vertical limbs)
(b) With five legs (vertical limbs)
The magnetic flux of one leg must close through the
other two legs and the flux also flows through the
windings of the other phases, namely the transformer
has non free return of the flux.
Free return of the flux through the external legs.
There are two different technologies for stacking the sheets of the magnetic material of the core:
(a) Stack core
(b) Wound core
The layers of the sheets of the magnetic material are
placed one over the other and the vertical and the
horizontal layers are over lapped.
The magnetic circuit is of shell type and the sheets are
wound.
Two different materials are used for core construction:
(a) Silicon steel sheet
(b) Amorphous metal sheet
The silicon steel sheet that is used for the core
construction is an alloy consisting of 97% iron and 3%
silicon. This material has crystallic structure. The
silicon steel sheets have thickness from 0.18 up to 0.5
mm. There are also silicon steel sheets for operation in
high magnetic induction (Hi-B).
The amorphous metal sheet that is used for the core
construction is an alloy consisting of 92% iron, 5%
silicon and 3% boron. This material has not crystallic
structure. It has 70% lower no-load loss than the silicon
steel. The thickness of the amorphous metal sheet is
0.025 mm, namely it is about 10 times thinner than the
typical thickness of the silicon steel sheet.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 6
A.2 ELVIM Distribution Trasformers
A.2.1 General characteristics
At the industrial site oil of Schneider Electric AE, ELVIM distribution transformers are manufactured, with voltages
up to 36 kV, having oil as cooling medium and the following technical characteristics:
Single-phase transformers from 5 up to 500 kVA.
Three-phase transformers from 25 up to 2000 kVA.
A.2.2 Advantages of ELVIM distribution transformers
A potential transformer user has a lot of reasons to choose ELVIM distribution transformers that are manufactured
by the industrial site of Schneider Electric. Some of the most important reasons are the following:
More than 30 years of experience in transformer
manufacturing (the manufacturing site is active
since 1969). As a result, the best techniques and
methods are used during transformer design and
manufacturing.
The application of the ISO 9001 quality assurance
system in combination with the very careful
monitoring of the whole industrial process lead in
the manufacturing of high quality transformers.
The application of the ISO 14001 environmental
management system assures the protection of the
environment and the reasonable use of natural
resources during the transformer production.
The use of the best materials for the transformer
construction. The reliability of the suppliers of the
transformer materials is systematically monitored
and checked.
All the transformer offers are treated very carefully
in order to finally give an offer, which fully
satisfies the needs of the transformer user.
The offer is technically complete and represents
the optimum technical and economical solution for
the specific transformer application.
The wound core technology that is followed has
the following advantages, in comparison with the
stack core technology:
Lower magnetization current.
As a result, the transformer has lower current
harmonics (better quality), lower consumption
of reactive power and lower magnetization
current.
Less noise.
The high automation of the industrial process has
dramatically decreased the delivery time. For
example, special transformers can be delivered
within 3 weeks.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 7
A.3 Transformer Manufacturing Features
A.3.1 Magnetic circuit
The wound core technology is followed and magnetic
materials with low losses are used.
The magnetic circuit is of shell type and the cores
are wound.
The production procedure of the wound core is as
follows: the magnetic material is slit into sheets of
standard widths.
Next, the sheets are cut to predetermined lengths.
Next, the sheets are wound on a circular mandrel
and a circular core is created.
Annealing treatment follows in order to recover
the core’s physical and electrical properties.
The quality control department checks the quality
of the wound core.
Figure 1 shows one wound core.
Figure 1: Wound core.
A.3.2 Windings
The type of coil is rectangular concentric winding.
For the low voltage coil, copper sheet or copper
rectangular wire is mainly used.
The high voltage coil is constructed from copper wire
or copper rectangular wire. The combination of copper
sheet in low voltage with copper wire in high voltage
plus coated press paper with epoxy resin as interlayer
insulation, increases the coil’s ability to withstand
short-circuit.
CORES
Important points during the production procedure are
the following:
Coil heat treatment at 100ºC so that epoxy resin is
polymerized giving an extremely compact product,
All coils pass through quality control.
Figure 2 shows the assembled active part
(cores and coils) of one three-phase wound core type
transformer.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 8
COILS
Figure 2: Transformer active part.
A.3.3 Metallic parts
For the construction of transformer metallic parts,
the following basic mechanical equipment is used:
CNC machines for cutting, punching and bending
of steel sheets.
Different types of welding machines (i.e. MIGMAG, TIG, and electrode) for the welding of the
metallic parts.
Modern painting shop for the painting of the
metallic parts. The usual painting procedure
includes the following steps: sandblasting,
decreasing-phosphatizing Fe, painting with 4 coats
(two primer coats and two final color coats) with
total thickness of 160 Ìm. This painting procedure
results in a durable corrosion protection and
therefore lengthy life expectancy.
Machines for construction and welding of
corrugated panels and tanks. Certified technicians
and welders are the operators of these machines.
Stud welding equipment for stud welding on the
transformer cover so that secure insulator
placement is achieved.
Equipment for oil leak detection of the transformer
tanks.
A.3.4 Assembly
For the transformer assembly, the following basic
equipment is used:
two vacuum chambers, in which the transformers
are filled with oil,
one crane of 35 tons and two cranes of 5 tons,
machines for the processing of transformer oil, so
that the oil obtains the appropriate characteristics,
according to the international standards.
one drying chamber to dry the active parts in order
to remove the moisture, which is absorbed by the
transformer insulating materials during the
production procedure,
A.3.5 Cooling medium
Transformer oil according to IEC 296 specifications
is used as cooling medium. The initial filling of
transformer with oil is done under high vacuum in
order to assure the high penetration of oil everywhere
and to remove air bubbles or moisture that could cause
dielectric failure of coil.
Oil can also be filled later on without vacuum under the
prerequisite that the oil level covers the active part and
the oil has been filtered. In agreement with the
customer, the oil can be supplied from Schneider
Electric or another company provided that the oil is
according to the given standard.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 9
A.4 Transformer Components
A.4.1 Tank
The transformer tank consists of the bottom plate,
frame, and the tank sides.
The tank sides are made of corrugated panels in order
to increase the total cooling area.
The tank of sealed type transformers (without oil
conservator) is filled with oil and is sealed.
The corrugated panels do not allow the creation
of significant increase of pressure internally,
which is caused by the increase of oil temperature
during transformer’s operation.
The transformer tank has two earthing points.
The rolling system or the base skid is welded to the
tank bottom plate.
A.4.2 Cover
There are two lifting lugs on the tank cover,
which are used for lifting and carrying the transformer.
On request, the thermometer pocket and the
thermometer with two electrical contacts are placed
on the cover.
Moreover, a neutral earthing link is also placed on the
cover. A pressure relief device is usually placed on the
cover of the sealed type transformers.
A.4.3 Lifting lugs
The lifting lugs are used for lifting and carrying the transformer.
A.4.4 Rollers
The transformers up to 160 kVA are usually manufactured as pole-mounted. The transformers above 160 kVA are
equipped with bi-directional rollers.
A.4.5 Draining and sampling oil valve
In the lower part of the tank side there is a draining and sampling oil valve, which allows the oil sampling in order
to test the oil dielectric strength.
A.4.6 Neutral earthing link
This link ensures the neutral earthing of the three-phase winding with the transformer tank.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 10
A.4.7 High voltage bushings
For medium voltage of 6, 10, 20, 30 kV, porcelain bushings according to DIN 42531 are used. Alternatively, on
request, plug-in bushings can be used.
A.4.8 Low voltage bushings
Low voltage bushings of 1 kV series, according to DIN 42530, are used in the low voltage.
A.4.9 Low voltage connectors
Low voltage connectors, according to DIN 43675, are used.
A.4.10 Tap changer
The applying medium voltage to the primary winding
of transformer is not stable and depends upon the
transformer position in the distribution network.
Therefore, taken the primary voltage as granted, the
tap changer is used in order to keep the secondary
voltage of the transformer as stable as possible.
The tap changer is placed into the transformer tank.
The control interface of the tap changer is placed on
the cover. The handling of the tap changer must be
done when the transformer is out of voltage, as
follows: initially, the handle of the tap changer is pulled
upwards so that the pin is released and entered into
the fixed annulus.
Then we turn the handle right or left so that the pin is
placed to the desirable tap position. If it is desirable to
switch from one position (e.g. position 1) to another
(e.g. position 5), then the handling is implemented
step by step, through all intermediate positions
(e.g. positions 2, 3, 4).
The taps positions are inscribed on the rating plate of
the transformer.
For example, when the transformer is designed to
operate in two voltage levels, e.g. 20 kV and 15 kV,
then using a 5-position tap changer, the regulation of
the primary voltage can be ±2x2.5 % for medium
voltage 20 kV (i.e. voltages 19.0, 19.5, 20.0, 20.5, and
21.0 kV) and ±2x3.3 % for medium voltage 15 kV (i.e.
voltages 14.0, 14.5, 15.0, 15.5, and 16.0 kV).
A.4.11 Voltage selector
The voltage selector (changeover switch) is used for
the change of the transformer operating voltage from
one voltage level to another (e.g. from 15 kV to 20 kV
and vice-versa) in proportion with the voltage of the
network that the transformer is connected. The
handling of the voltage selector is the same with the
handling of the tap changer, the only difference is that
the annulus has two positions (e.g. 15 kV or 20 kV).
For example, if we want a 20-15/.4 kV transformer to
operate with primary voltage 19.5 kV, we set the
voltage selector at the 20 kV position and the tap
changer at the -2.5% position.
A.4.12 Transformer thermometer
The thermocouple of the thermometer is set at the
higher oil layer, in order to measure the maximum oil
temperature. The electrical contacts of the
thermometer are regulated to the desirable
temperatures and are connected to the protection
circuit for alarm and tripping of the circuit, when the
corresponding temperature limits are exceeded.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 11
A.4.13 Oil conservator
During the transformer oil temperature variation, and
consequently the oil volume variation, the oil
conservator undergoes this oil volume fluctuation. The
oil conservator is equipped with an oil level indicator
with two marks: the first mark shows the oil level
at -20ºC and the second the oil level at +20ºC.
Transformers with oil conservator are usually equipped
with an air dehumidifier and a Buchholz relay.
A.4.14 Buchholz relay
The protection of the oil-immersed transformers from
internal faults, which cause the development of gases
or strong oil leakage, is implemented with Buchholz
relay, which is installed between the transformer tank
and the oil conservator. In case of gases creation (as a
result of internal fault) or lack of oil, the first float is
moved downwards and the alarm contact is activated.
If the gases are sufficient (i.e. the internal fault is
significant), then the second float is moved
downwards and the trip contact is activated. The trip
contact is also activated in case of strong oil flux to the
oil conservator after short-circuit or internal fault.
Moreover, the Buchholz relay provides protection
from oil leakage.
A.4.15 Air dehumidifier
The air dehumidifier is placed on the oil conservator.
Because of contraction and expansion of oil volume,
the air passes through the dehumidifier towards and
from the oil conservator. The air dehumidifier contains
SiO2 crystals (silica gel), which absorb the air
moisture.
The silica gel absorbs the moisture until its color is
yellow. When it is full of moisture and it changes its
color and becomes soft blue-white, it must be dried
or it must be replaced. Drying is achieved by heating it
at temperatures between 120ºC and 150ºC until its
color becomes yellow again.
The silica gel should have the following colors:
Yellow (silica gel is fully dry).
Soft blue-white (silica gel is full of moisture).
A.4.16 Filling valve
The transformers are equipped with a filling valve, in order to have the ability to fill the transformers with mineral oil.
A.4.17 Oil level indicator
For the sealed type transformers (without oil
conservator), the oil level indicator is placed on the
tank side or on the transformer cover. For the
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 12
transformers with oil conservator, an oil level indicator
of tube type (glass transparent tube) or magnetic type
is placed on the oil conservator.
A.4.18 Rating plate
According to the international standards, all the
transformer data are mentioned on the rating plate:
type of transformer, power in kVA, phases, frequency,
short-circuit impedance, vector group, type of cooling,
windings material, serial number, year of
manufacturing, core and windings weight, oil weight,
total weight, maximum ambient temperature, winding
temperature rise, oil temperature rise, rated voltage of
the primary winding, rated voltage of the secondary
winding, rated current of the primary winding, rated
current of the secondary winding, no-load losses, load
losses, positions of tap changer and positions of
voltage selector (if one exists).
A.4.19 Tank earthing point
Two tank earthing points are placed near the bottom of the tank (one earthing point in diametric opposite direction
with the other earthing point), in order to have the ability for tank earthing.
A.4.20 Accessories of sealed type transformers
The sealed type transformers are usually equipped with a pressure relief valve and thermometer or DGPT2 relay.
The DGPT2 relay has an overpressure switch, thermometer with alarm and trip contacts and oil indicator with
contact for the trip of the circuit.
ELVIM Oil-immersed Distribution Transformer
Buchholz relay
High voltage
bushings
Oil conservator
Oil level
indicator
Air dehumidifier
Tap changer
Low voltage
bushings
Lifting lug
Thermometer
Rating plate
Tank
Rollers
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 13
A.5 Transformer tests
The transformer tests are classified, in accordance with the specification IEC 76, as follows:
Type tests.
Routine tests.
Special tests.
A.5.1 Type tests
The type tests, which are performed on one transformer from every transformer type, are the following:
(a) Temperature rise test
(b) Lighting impulse test
The procedure of the temperature rise test is
performed according to IEC 76-2. With this specific
test, the following tasks are implemented:
a) the determination of the temperature rise of the oil,
and
b) the determination of the average temperature rise
of the windings.
The procedure of the lightning impulse test is
performed according to IEC 76-3. With this specific
test, the transformer’s withstand against overvoltages
is checked. These overvoltages are caused from:
a) traveling waves (that are caused from thunders)
of transmission lines,
b) sudden on/off switching of breakers,
c) short-circuits at the substation area.
A.5.2 Routine tests
The routine tests are performed on every transformer separately.
The routine tests include:
(a) Measurement of winding resistance
(c) Measurement of short-circuit impedance
The procedure of the measurement of windings
resistance is performed according to IEC 76-1. During
this test the resistance of each winding is measured
and the temperature is recorded. The test is performed
with DC (direct current). The measurement of the
resistance of the windings is performed using a
resistance bridge.
The measurement of short-circuit impedance is
performed according to IEC 76-1. The short-circuit
impedance, which is expressed as a percentage of the
rated voltage, represents the transformer’s
impedance.
The international standards require the short-circuit
impedance to be calculated at the reference
temperature of 75ºC.
(b) Measurement of the voltage ratio and
check of phase displacement
(d) Measurement of load losses
The measurement of the voltage ratio is performed
according to IEC 76-1.
The objective of the specific test is to compare the
measured values of the transformer ratio with the
respective guaranteed values.
For the transformer, the turns ratio is equal to the
voltage ratio of primary and secondary winding,
namely:
U1
U2
=
N1
N2
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 14
The measurement of load losses is implemented with
the secondary winding short-circuited and by
increasing the voltage of the primary winding till the
current of the primary winding reaches its nominal
value. The load losses are calculated at the reference
temperature of 75ºC.
(e) Measurement of no load current and
no-load losses
The measurement is performed according to IEC 76-1.
The no load current represents the real value of current
that is required to magnetize the magnetic core.
The no-load losses represent the active power that is
absorbed by the transformer core when it is applied
rated voltage and rated frequency in the one winding
(e.g. secondary) and the other winding (e.g. primary)
is open-circuited.
A.5.2 Routine tests (continue)
(f) Dielectric routine tests
The dielectric routine tests are the following:
Induced voltage dielectric test
Applied voltage dielectric test
The duration of the test, according to IEC 76-3,
is 1 min.
With this specific test, the following are checked:
a) the insulation between MV and LV windings,
b) the insulation between the tested windings and the
tank, and
c) the insulation between the tested windings and the
magnetic circuit.
The procedure of the measurement is as follows.
Three-phase voltage, twice the rated voltage, is
induced to the transformer for 1 min. However, due to
the doubling of the voltage, the magnetic induction is
also doubled, resulting in transformer saturation and,
consequently, there is a danger for the transformer to
be destroyed. In order to avoid saturation, the
frequency is doubled, so that the magnetic induction
remains constant. Finally, during this test, the volts per
turn and therefore the volts per layer are doubled.
With this test, the dielectric strength between turns
and layers is verified.
(a) MV windings
The LV windings are short-circuited and grounded with
the transformer tank. Then, single-phase voltage is
applied to the MV windings, this voltage is determined
by the voltage of the MV system, in which the
transformer is going to be connected.
(b) LV windings
The MV windings are short-circuited and grounded
with the transformer tank. Then, single-phase voltage
is applied to the LV windings, this voltage is
determined by the voltage of the LV system, in which
the transformer is going to be connected.
A.5.3 Special tests
The special tests are not included in the category of type or routine tests and are executed after agreement
between customer and manufacturer. The special tests are the following:
(a) Dielectric special tests
(e) Measurement of the harmonics of the
no-load current
(b) Determination of capacitances of
windings-to-earth and between windings
(c) Short-circuit withstand test
According to this test, the transformer is subjected to
successively short-circuits of 0.5 sec duration and the
transformer must withstand these short-circuits. Since
this test requires high power, it is executed in special
test centers. For example, in Greece, the Tests
Research and Standards Center of Public Power
Corporation executes this test.
( f) Measurement of insulation resistance
and/or measurement of dissipation factor
(tan‰) of the insulation system capacitances
(g) Radio interference voltage
(h) Measurement of zero-sequence
impedance
(d) Determination of sound levels
The transformer is energized at no-load and at rated
voltage and rated frequency, so the noise peripherally
to the transformer can be measured. The test is
performed in accordance to specification NEMA
TR - 1/1974.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 15
A.6 Trasformer Electrical Characteristics
A.6.1 Rated Power
The rated power, Pn, of the three-phase transformer is calculated by the following formula:
Pn= Un In √3
where Un is the rated voltage and In is the rated current of the transformer.
A.6.2 Temperature rise
The temperature rise is the maximum rise when the
transformer operates at the primary rated voltage,
secondary rated current and rated frequency.
Transformer typical characteristics:
The average temperature rise of the winding
is 65 K.
The top oil temperature rise is 60 K.
A.6.3 Ambient Temperature
The rated power of the transformer is typically
calculated for the following conditions:
Maximum ambient temperature of 40ºC.
Average daily ambient temperature of 30ºC.
Average annual ambient temperature of 20ºC.
On request, transformers operating under different
ambient temperature conditions can be produced.
A.6.4 Altitude of installation
The rated power of the transformer is valid for installation altitude up to 1000 m. If the transformer is going to be
installed in an altitude higher than 1000 m, this should be mentioned in the transformer specification.
A.6.5 Short-circuit impedance
The short-circuit impedance is the percentage of the
primary rated voltage that has to be applied at the
transformer primary winding, when the secondary
winding is short-circuited, in order to have the rated
current at the primary winding.
The short-circuit impedance is very important,
because it represents the transformer’s impedance.
The higher the short-circuit impedance, the higher the
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 16
voltage drop. The lower the short-circuit impedance,
the higher the short-circuit current, in case of shortcircuit. Based on the short-circuit impedance, the
following are determined: the voltage drop due to the
transformer loading, the distribution of loads in case of
transformers parallel operation, and the short-circuit
current.
A.6.6 No-load losses
The no-load losses include losses due to no-load current, hysteresis losses and eddy current losses in core
laminations, stray eddy current losses in core clamps and bolts, and losses in the dielectric circuit.
Table 1 presents the 3 lists (A’, B’, C’) of no-load losses for transformers from 50 to 2500 kVA, according to
CENELEC HD 428.1 S1/1992.
List A’
Rated
No-load
power (kVA) losses P0 (W)
50
100
160
250
400
630
630
1000
1600
2500
190
320
460
650
930
1300
1200
1700
2600
3800
List B’
List C’
Noise
Lw (dB)
No-load
losses P0 (W)
Noise
Lw (dB)
No-load
losses P0 (W)
Noise
Lw (dB)
Short-circuit
impedance (%)
55
59
62
65
68
70
70
73
76
81
145
260
375
530
750
1030
940
1400
2200
3200
50
54
57
60
63
65
65
68
71
76
125
210
300
425
610
860
800
1100
1700
2500
47
49
52
55
58
60
60
63
66
71
4
4
4
4
4
4
6
6
6
6
Table 1: Lists of no-load losses according to CENELEC HD 428.1 S1/1992.
A.6.7 Load losses
The load losses include losses due
to load currents and eddy current
losses in conductors due to leakage
fields.
Table 2 presents the 3 lists (A, B, C)
of load losses for transformers from
50 to 2500 kVA, according to
CENELEC HD 428.1 S1/1992.
For example, a transformer has a
combination of losses of A-C’, if its
load losses belong to list A, and its
no-load losses belong to list C’.
List A
Rated
Load
power (kVA) losses Pk (W)
50
100
160
250
400
630
630
1000
1600
2500
1100
1750
2350
3250
4600
6500
6750
10500
17000
26500
List B
List C
Load
losses Pk (W)
Load
losses Pk (W)
Short-circuit
impedance (%)
1350
2150
3100
4200
6000
8400
8700
13000
20000
32000
875
1475
2000
2750
3850
5400
5600
9500
14000
22000
4
4
4
4
4
4
6
6
6
6
Table 2: Lists of load losses according to CENELEC HD 428.1 S1/1992.
A.6.8 Rated voltage
The rated primary voltage (input voltage) is the
voltage at which the transformer is designed to
operate. The rated primary voltage determines the
basic insulation level (BIL) of the transformer,
according to international standards (IEC 76). The BIL
is a basic transformer characteristic, since it indicates
the transformer ability to withstand the overvoltages
that can appear in the network. The calculation of the
winding insulation is based on the basic insulation
level.
The rated secondary voltage (output voltage) is the
voltage at the terminals of the secondary winding at
no-load, under rated primary voltage and rated
frequency.
The vector group determines the phase displacement
between the primary and the secondary winding.
The three primary or secondary windings can be
connected with different ways in order to have a
three-phase transformer.
These connections are the following:
D (d): delta connection for high voltage
(low voltage) winding
Y (y): star connection for high voltage (low voltage)
winding
Z (z): zigzag connection for high voltage
(low voltage) winding
N (n): the neutral exists in high voltage
(low voltage) winding for connection outside the
transformer.
A.6.9 Vector group
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 17
A.6.10 Frequency
The frequency at which the transformer is designed to operate is 50 Hz or 60 Hz in accordance with the network
frequency.
A.6.11 Noise
The transformer noise is due to the magnetostriction of the sheets of the magnetic circuit.
In general, a transformer operating at low magnetic induction has low noise level.
A.6.12 Efficiency
The distribution transformers are very efficient
machines since their efficiency is greater than 95%.
The power efficiency of any electrical machine is
defined as the ratio of the useful power output to the
total power input. The efficiency can be defined by
simultaneously measuring the output and the input
power. However, this measurement is expensive and
difficult, especially for large machines. Moreover, in
case of high efficiency machines (e.g. transformer),
higher precision can be achieved, if the efficiency is
expressed through the losses. Consequently, the
transformer efficiency is calculated by the following
formula:
S cos Ê
n=
S cos Ê + losses
where S is the transformer load in VA, losses are the
losses in W and cos Ê is the power factor.
The transformer efficiency is increased with the
decrease of transformer losses.
The transformer losses are divided into no-load losses
and load losses. The no-load losses are constant, while
the load losses are proportional to the transformer
load. Consequently, the efficiency of the transformer
is calculated by the following formula:
n=
S cos Ê
S cos Ê + NLL + LL(S/SB )2
where NLL are the no-load losses, LL are the load
losses and SB is the rated power of the transformer
in VA.
A.6.13 Short-circuit current
The short-circuit current is composed of the
asymmetrical and the symmetrical short-circuit
current. The amplitude of the first peak of the
asymmetrical short-circuit current is equal to Î√2 times
the value of the symmetrical short-circuit current.
The factor Î√2 depends on the ratio of Ux /Ur, where
Ux is the voltage drop in the reactive components of
the transformer and Ur is the voltage drop in the
resistance components of the transformer.
Table 3 presents the values of the factor Î√2 versus
the ratio Ux/Ur.
The symmetrical short-circuit current, IK , is expressed
as a function of the rated current In. If the secondary
winding is short-circuited and the nominal current is
applied at the primary winding, the following equation
holds:
IK
In
=
100
UK
where UK is the short-circuit impedance.
The asymmetrical short-circuit current stresses
mechanically the transformer, while the symmetrical
short-circuit current stresses thermally the transformer.
ELVIM transformers are designed and tested to
withstand short-circuit currents according to IEC 76-5.
Ux / Ur
Î√2
1
1.5
2
3
4
5
6
8
10
15
25
50
1.51
1.63
1.75
1.95
2.09
2.19
2.28
2.38
2.46
2.56
2.66
2.77
Table 3:
Values of the factor Î√2 versus the ratio Ux /Ur
A.6.14 No-load current
The no-load current represents the current that the transformer absorbs, when rated voltage is applied to the
primary winding and the secondary winding is open-circuited. The no-load current is expressed as a percentage
of the value of the rated current.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 18
A.7 Transformer Standards
The transformer manufacturing is based on the
international standards as well as on specific
customer needs. From time to time, some of the
standards may be modified and in that case they are
republished.
A list of transformer standards, according to IEC,
is shown in Table 4.
Number Standard
Description
1
2
3
4
5
6
7
8
Power transformers: general
Power transformers: temperature rise
Power transformers: insulation levels and dielectric tests
Power transformers: ability to withstand short circuit
Bushings for alternating voltages above 1000 V
Loading guide for oil-immersed power transformers
Dry-type power transformers
Loading guide for dry-type power transformers
IEC 76-1
IEC 76-2
IEC 76-3
IEC 76-5
IEC 137
IEC 354
IEC 726
IEC 905
Table 4: Transformer standards according to IEC.
The above standards are related with the electrical
characteristics and the accessories of transformers.
The IEC 76 standard describes the electrical
characteristics and the transformer tests that are
related with the dynamic, thermal and electrical
withstand of transformers. The DIN standard defines
the transformer accessories and the CENELEC
standard defines the lists of transformer losses and
short-circuit impedance.
A.8 Tolerances
Constructional reasons result in deviations between
the measured parameters and the values that are
defined by the specification of the transformer user
(i.e. the guaranteed values).
Table 5 presents the tolerances that are applied to
certain items, according to IEC 76-1.
πtem
Tolerance
Voltage ratio
The lower of the following values:
a) ±0.5% of guaranteed voltage ratio
b) ±1/10 of the measured short-circuit impedance on
the principal tapping
±10% of the guaranteed short-circuit impedance
+15% of the guaranteed no-load losses
+15% of the guaranteed load losses
+10% of the guaranteed total losses
(load and no-load)
+30% of the guaranteed no-load current
Short-circuit impedance
No-load losses
Load losses
Total losses (load and no-load)
No-load current
Table 5: Tolerances on certain transformer items according to IEC 76-1.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 19
A.9 Transformer Operation
A.9.1 Overloading
The rated overloading of transformer depends on the
transformer’s previous load or the corresponding oil
temperature at the beginning of the overloading.
Examples of the permissible duration and the
respective levels of the acceptable overloadings are
shown in Table 6.
Previous continuous
loading
(% of rated power)
Oil temperature
(°C)
For example, if the transformer is loaded with 50% of
its rated power continuously, then the transformer can
be overloaded to150% of its rated power for 15
minutes or to 120% of its rated power for 90 minutes.
Duration (min) of overloading
for specific levels of overloading
(% of rated power)
10%
min
20%
min
30%
min
40%
min
50%
min
90
60
30
15
50
55
180
75
68
120
60
30
15
8
90
78
60
30
15
8
4
Table 6: Permissible duration and level of acceptable overloading.
Moreover, it should be noted that the oil temperature
is not a safe measure for the winding temperature,
since the time constant of the oil is 2 to 4 hours, while
the time constant of the winding is 2 to 6 minutes.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 20
Therefore, the determination of the permissible
duration of the overloading must be done very
carefully, since there is a danger for the winding
temperature to exceed the critical temperature of
105ºC, without this being visible by the oil
temperature.
A.9.2 Parallel operation
The parallel operation of two or more transformers is
feasible, when the following requirements are met:
Their vector groups should be the same and the
connection should be implemented with the
corresponding terminals U-u, V-v, W-w. In other
words, the transformers must have the same
inherent phase angle difference between primary
and secondary terminals, the same polarity and the
same phase sequence. It should be noted that, in
case that the vector groups are not the same, the
parallel operation of transformers of groups 5 and
11 is permitted, if the connection is implemented
according to Table 7.
The ratio of their rated power should be less
than 3:1.
Their voltage ratio should be the same
(the permitted tolerance is according to IEC 76-1,
Table 5, ¨ A.8).
Their short-circuit impedance should be the same
(the permitted tolerance is according to IEC 76-1,
Table 5, ¨ A.8).
Transformer group
for parallel operation
Group of existing
transformer
Connection between phases
High Voltage
Low Voltage
R S T
5
11
r s t
5
U V W
x y z
11
U W V
w v u
or W V U
or v u w
or V U W
or u w v
11
U V W
u v w
5
U W V
z y x
or W V U
or y x z
or V U W
or x z y
Table 7: Parallel operation of transformers of groups 5 and 11.
A.9.3 Load distribution of transformers in parallel operation
If the parallel operated transformers have the same
voltage ratio but different short-circuit impedance,
then the load is distributed among them in such a way
that each transformer accepts a specific level of load
for which the short-circuit impedance becomes the
same for all the parallel operated transformers.
When none of the parallel operated transformers is
permitted to be overloaded, the transformer with the
minimum short-circuit impedance must operate
maximum under its rated power.
where Pi is the load that is distributed to the i
transformer, Pni is the rated power of the i transformer
UKi is the rated short-circuit impedance of the i
transformer and UK min is the minimum rated shortcircuit impedance of the n parallel operated
transformers.
Finally, the total power of the n parallel operated
transformers is:
n
Consequently, the load distribution is given by the
following equation:
Pi = Pni
UK min
UKi
,
™
i=1
(Pi )
UK min
UKi
n
<
™P.
i
i=1
An arithmetic example of the load distribution of
transformers in parallel operation is given in ¨ A.13.3.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 21
A.10 Transformer order form
The transformer order form includes the following data:
Customer
Sales engineer
Transformer type
three-phase
single-phase
Frequency (Hz)
50
60
Installation
indoor
outdoor
Altitude
≤1000 m
>1000 m
Cooling
ONAN
other
Transformer type
oil
dry type
Oil conservator
yes
no
Rated power (kVA)
Rated primary/secondary voltage (kV)
Transformer dimensions (mm)
Taps
length
width
± 2x2.5%
height
others
Short-circuit impedance (%) at 75°C
Vector group
No-load losses (W)
tolerance acc. to IEC
other tolerance
Load losses (W)
tolerance acc. to IEC
other tolerance
Maximum temperature rise of winding
65 K
other
Top oil temperature rise
60 K
other
Maximum ambient temperature
40°C
other
Painting type
RAL 7033
other
Accessories
Buchholz relay
DGPT2
air dehumidifier
oil indicator
pressure relief valve
thermometer
rollers Distance between rollers (mm)
Quantity (items)
Unit price (;)
Payment method
Order date
Delivery date
Comments
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 22
A.11 Transformer selection
The selection of the most appropriate transformer starts with the definition of the proper and detailed
specification. The special needs of each project specify the special characteristics or accessories that are needed.
The evaluation of the alternative transformer offers depends on the transformer user. The economic evaluation
method of the transformers by the electrical utilities and industrial users is presented below.
A.11.1 Electrical Utilities
The electrical utilities evaluate the transformers based
on the criterion of the total owning cost, TOC (;),
which is calculated from the following equation:
TOC = BP + A* NLL + B* LL,
where BP (;) is the transformer sales price,
A (;/W) is the no-load losses factor, NLL (W) are the
no-load losses, B (;/W) is the load losses factor, and
LL (W) are the load losses.
Among alternative transformer offers,
the economical optimum is the one with the minimum
total owning cost.
The values of the parameters BP, NLL, and LL are
determined by the transformer manufacturer.
The values of the parameters A and B are determined
by the electrical utilities.
A.11.2 Industrial Users
The procurement of transformers by the industrial users is based mainly on the transformer sales price and
secondary on the transformer losses. An arithmetic example for the determination of the most economical
transformer for an industrial user is presented in ¨ A.13.4.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 23
A.12 ELVIM Transformers Series
In this paragraph, five indicative ELVIM
transformers’ series are presented.
A.12.1 Single-phase transformers, from 5 to 50 kVA, 20/0.231 kV
General Description
Single-phase distribution transformers, 50 Hz.
IEC 76 standard is followed.
The cooling is implemented with natural circulation (ONAN) of mineral oil
according to IEC 296.
Sealed type transformers.
Outdoor installation.
Pole-mounted.
Rated primary voltage 20 kV, rated secondary voltage 231 V at no-load.
The top oil temperature rise is 60 K and the average temperature rise of the winding is 65 K.
Tolerances of losses and short-circuit impedance according to IEC 76.
The transformer painting type is RAL 7033.
Basic Equipment
3-position tap changer with ± 5 % tappings at 20 kV.
LV and HV bushings.
Valves for filling, filtering and oil sampling.
Pole-mounting elements.
Rating plate.
Electrical Characteristics
Rated Power (kVA)
5
10
15
20
25
30
50
No-load losses (W) (*)
55
55
70
85
105
120
180
Load losses at 20 kV (W)(*)
150
320
485
650
725
800
1350
Voltage drop
cosÊ =1
3.04
3.23
3.26
3.28
2.94
2.71
2.74
cosÊ = 0.8
3.99
4.00
4.00
4.00
3.97
3.93
3.93
4
4
4
4
4
4
4
cosÊ =1
96.06
96.39
96.43
96.46
96.79
97.02
97.03
Efficiency (%) 100% cosÊ =0.8
95.12
95.52
95.58
95.61
96.02
96.31
96.32
cosÊ =1
96.42
96.96
97.04
97.08
97.34
97.53
97.56
75% cosÊ = 0.8
95.56
96.23
96.33
96.38
96.69
96.93
96.96
at full load (%)
Rated short-circuit impedance (%)(*)
Load
Load
Order Details
(*)
Rated power
Load losses
Short-circuit impedance
Altitude of installation (if the altitude exceeds 1000 m)
Rated voltages
Primary tappings
No-load losses
Special accessories
Rated frequency
Ambient temperature
Transformers with different losses and short-circuit impedance can be manufactured, on request.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 24
C
General Arrangement of Single-phase Transformers, from 5 to 50 kVA, 20/0.231 kV
A
B
1. Filling plug DIN 42553 form “D”
2. Drain and sampling valve DIN 42551 NW22
3. Lifting lugs
4. H.V. bushings
5. L.V. bushings
6. Rating plate
7. Off-load tap changer
8. Transformer base
9. Pole mounted elements
Dimensions (mm)
Rated power (kVA)
A (mm)
5
10
15
20
25
30
50
620
620
690
690
730
730
805
B (mm)
540
540
630
630
640
640
770
C (mm)
930
1050
1020
1020
1020
1020
1035
Total weight (Kg)
115
140
155
165
210
225
295
Due to evolution of standards and materials, the present manual will bind us only after confirmation from our
transformer design department.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 25
A.12.2 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV
General Description
Three-phase disrtibution transformers, 50 Hz.
IEC 76 standard is followed.
Transformers with oil conservator.
The cooling is implemented with natural circulation of mineral oil according to IEC 296.
Indoor or outdoor installation.
Ground-mounted.
Rated primary voltage 20 kV, rated secondary voltage 400 V at no-load.
Vector group Dyn11.
The top oil temperature rise is 60 K and the average temperature rise of the winding is 65 K.
Tolerances of losses and short-circuit impedance according to IEC 76.
The transformer painting type is RAL 7033.
Basic Equipment
5-position tap changer with ± 2 x 2.5 % tappings at 20 kV.
LV and HV bushings.
Oil conservator with oil level indicator.
Thermometer with oil level indicator and electrical contacts.
Buchholz relay.
Air dehumidifier.
Valves for filling, filtering and oil sampling.
Bi-directional rollers.
Rating plate.
Electrical Characteristics
Rated power (kVA)
No-load losses
250
(W)(*)
400
500
630
800
1000
1250
1600
610
850
1000
1200
1450
1750
2100
2550
No-Load losses at 20 kV (W)(*)
4450
6450
7800
9300
11000
13500
16400
19800
Voltage drop
cosÊ=1
1.94
1.78
1.73
1.65
1.55
1.52
1.48
1.41
cosÊ=0.8
4.92
4.82
4.79
4.74
4.68
4.66
4.64
4.59
at full load (%)
Rated short-circuit impedance
6
6
6
6
6
6
6
6
cosÊ=1
98.02
98.21
98.27
98.36
98.47
98.50
98.54
98.62
100% cosÊ=0.8
Load
Efficiency (%))
(%)(*)
97.53
97.77
97.85
97.96
98.09
98.13
98.18
98.28
cosÊ=1
98.37
98.53
98.58
98.66
98.74
98.77
98.81
98.87
75% cosÊ=0.8
97.97
98.17
98.24
98.33
98.43
98.47
98.51
98.59
Load
Order Details
(*)
Rated power
Vector group
Short-circuit impedance
Altitude of installation (if the altitude exceeds 1000 m)
Rated voltages
Primary tappings
No-load losses
Special accessories
Rated frequency
Ambient temperature
Load losses
Details of cable boxes (on request)
Transformers with different losses and short-circuit impedance can be manufactured, on request.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 26
C
General Arrangement of Three-phase Transformers, from 250 up to 1600 kVA, 20/0.4 kV
D
D
12.
13.
14.
15.
16.
17.
18.
Transformer tank
Tank cover
Lifting lugs
Roller DIN 42561
Draining and sampling valve DIN 42551
Neutral earthing link
High voltage bushing DIN 42531
Low voltage bushing DIN 42530
Low voltage connector DIN 43675
Tap changer
Thermometer with two electrical
contacts
Oil conservator
Buchholz relay
Air dehumidifier
Filling valve DIN 42553
Oil level indicator
Rating plate
Tank earthing point
B
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
A
Dimensions (mm)
Rated power (kVA)
250
400
500
630
800
1000
1250
1600
1475
1700
1735
1710
1855
1960
1940
2155
B (mm)
905
1005
1005
1050
1195
1290
1270
1450
C (mm)
1530
1490
1720
1815
1890
1895
2085
2095
A (mm)
D (mm)
Total weight (Kg)
520
670
670
670
670
820
820
820
1100
1380
1700
1940
2380
2650
3200
3760
Due to the evolution of standards and materials, the present manual will bind us only after confirmation from our
transformer design department.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 27
A.12.3 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV, with low losses
General Description
Three-phase distribution transformers, 50 Hz.
Combination of losses A-C’, CENELEC HD 428.1 S1/1992.
IEC 76 standard is followed.
The cooling is implemented with natural circulation of mineral oil according to IEC 296.
Transformers with oil conservator.
Indoor or outdoor installation.
Ground-mounded.
Rated primary voltage 20 kV, rated secondary voltage 400 V at no-load.
Vector group Dyn11.
The top oil temperature rise is 60 K and the average temperature rise of the winding is 65 K.
Tolerances of losses and short-circuit impedance according to IEC 76.
The transformer painting type is RAL 7033.
Basic Equipment
5-position tap changer with ± 2 x 2.5 % tappings at 20 kV.
LV and HV bushings.
Oil conservator with oil level indicator.
Thermometer with oil level indicator and contacts.
Buchholz relay.
Air dehumidifier.
Valves for filling, filtering and oil sampling.
Bi-directional rollers.
Rating plate.
Electrical Characteristics
Rated Power (kVA)
250
400
500
630
800
1000
1250
1600
No-load losses (W)(*)
425
610
750
860
940
1100
1350
1700
Load losses at 20 kV
(W)(*)
3250
4600
5500
6500
8700
10500
13300
17000
cosÊ=1
1.37
1.22
1.17
1.11
1.26
1.22
1.24
1.24
cosÊ=0.8
3.33
3.25
3.22
3.17
4.49
4.47
4.48
4.48
4
4
4
4
6
6
6
6
Voltage drop
at full load (%)
Rated short-circuit impedance (%)(*)
Load
Efficiency (%)
cosÊ=1
98.55
98.71
98.77
98.85
98.81
98.85
98.84
98.84
100% cosÊ=0.8
98.20
98.40
98.46
98.56
98.52
98.57
98.56
98.56
Load
cosÊ=1
98.81
98.95
98.99
99.05
99.04
99.07
99.07
99.07
75% cosÊ=0.8
98.52
98.69
98.73
98.82
98.80
98.85
98.84
98.84
Order Details
(*)
Rated power
Vector group
Short-circuit impedance
Altitude of installation (if the altitude exceeds 1000 m)
Rated voltages
Primary tappings
No-load losses
Special accessories
Rated frequency
Ambient temperature
Load losses
Details of cable boxes (on request)
Transformers with different losses and short-circuit impedance can be manufactured, on request.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 28
C
General Arrangement of Three-phase Transformers, from 250 to 1600 kVA, 20/0.4 kV, with low losses
D
D
12.
13.
14.
15.
16.
17.
18.
Transformer tank
Tank cover
Lifting lugs
Roller DIN 42561
Draining and sampling valve DIN 42551
Neutral earthing link
High voltage bushing DIN 42531
Low voltage bushing DIN 42530
Low voltage connector DIN 43675
Tap changer
Thermometer with two electrical
contacts
Oil conservator
Buchholz relay
Air dehumidifier
Filling valve DIN 42553
Oil level indicator
Rating plate
Tank earthing point
B
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
A
Dimensions (mm)
Rated power (kVA)
A (mm)
250
400
500
630
800
1000
1250
1600
1580
1710
1705
1790
1950
2030
2120
2300
B (mm)
880
900
1020
1000
1140
1260
1350
1300
C (mm)
1480
1560
1580
1670
1740
1780
1880
1950
D (mm)
Total weight (Kg)
520
670
670
670
670
820
820
820
1150
1500
1750
2100
2400
2800
3200
4050
Due to the evolution of standards and materials, the present manual will bind us only after confirmation from our
transformer design department.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 29
A.12.4 Three-phase transformers, from 250 to 1600 kVA, 20-15/0.4 kV
General Description
Three-phase distribution transformers, 50 Hz.
IEC 76 standard is followed.
The cooling is implemented with natural circulation of mineral oil according to IEC 296.
Transformers with oil conservator.
Indoor or outdoor installation.
Ground-mounted.
Rated primary voltage 20 and 15 kV, rated secondary 400 V at no-load.
Vector group Dyn11.
The top oil temperature rise is 60 K and the average temperature rise of the winding is 65 K.
Tolerances of losses and short-circuit impedance according to IEC 76.
The transformer painting type is RAL 7033.
Basic Equipment
Voltage selector.
5-position tap changer with ± 2 x 2.5 % tappings at 20 kV and ± 2 x 3.33 % tappings at 15 kV.
LV and HV bushings.
Oil conservator with oil level indicator.
Thermometer with level indicator and contacts.
Buchholz relay.
Air dehumidifier.
Valves for filling, filtering and oil sampling.
Bi-directional rollers.
Rating plate.
Electrical Characteristics
Rated power (kVA)
No-load losses
250
(W)(*)
400
500
630
800
1000
1250
1600
575
810
930
1000
1180
1360
1720
1950
Load losses as 20 kV (W)(*)
4000
6350
7500
9300
10800
12800
13500
17400
Voltage drop
cosÊ=1
1.77
1.75
1.67
1.65
1.52
1.45
1.25
1.26
cosÊ=0.8
4.82
4.81
4.76
4.74
4.66
4.62
4.49
4.49
at full load (%)
Rated short-circuit impedance
6
6
6
6
6
6
6
6
cosÊ=1
98.20
98.24
98.34
98.39
98.52
98.60
98.80
98.81
100% cosÊ=0.8
97.76
97.81
97.94
98.00
98.16
98.26
98.50
98.51
Load
cosÊ=1
98.52
98.56
98.65
98.70
98.81
98.87
99.02
99.03
75% cosÊ=0.8
98.15
98.21
98.31
98.38
98.51
98.59
98.77
98.79
Load
Efficiency (%)
(%)(*)
Order Details
Rated power
Vector group
Short-circuit impedance
Altitude of installation (if the altitude exceeds 1000 m)
Rated voltages
Primary tappings
No-load losses
Special accessories
Rated frequency
Ambient temperature
Load losses
Details of cable boxes (on request)
(*) Transformers with different losses and short-circuit impedance can be manufactured, on request.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 30
C
General Arrangement of Tree-phase Transformers, from 250 to 1600 kVA, 20-15/0.4 kV
D
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
Transformer tank
Tank cover
Lifting lugs
Roller DIN 42561
Draining and sampling valve DIN 42551
Neutral earthing link
High voltage bushing DIN 42531
Low voltage bushing DIN 42530
Low voltage connector DIN 43675
Tap changer
Voltage selector
Thermometer with two electrical
contacts
Oil conservator
Buchholz relay
Air dehumidifier
Filling valve DIN 42553
Oil level indicator
Rating plate
Tank earthing point
B
D
A
Dimensions (mm)
Rated power (kVA)
250
400
500
630
800
1000
1250
1600
1530
1650
1873
1758
2025
1990
2135
2240
B (mm)
925
1035
960
1005
1225
1230
1280
1470
C (mm)
1520
1530
1718
1820
1890
1890
1910
2080
A (mm)
D (mm)
Total weight (Kg)
520
670
670
670
670
820
820
820
1100
1560
1800
2100
2550
2800
3200
3760
Due to the evolution of standards and materials, the present manual will bind us only after confirmation from our
transformer design department.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 31
A.12.5 Three-phase sealed type transformers, from 25 to 1600 kVA, 20/0.4 kV
General Description
Three-phase distribution transformers, 50 Hz.
IEC 76 standard is followed.
The cooling is implemented with natural circulation of mineral oil according to IEC 296.
Sealed type transformers (without oil conservator).
Indoor or outdoor installation.
Ground-mounted.
Rated primary voltage 20 kV, rated secondary 400 V at no-load.
Vector group Yzn5 (up to 160 kVA) and Dyn5 (from 250 kVA to 1600 kVA).
The top oil temperature rise is 60 K and the average temperature rise of the winding is 65 K.
Tolerances of losses and short-circuit impedance according to IEC 76.
The transformer painting type is RAL 7033.
Basic Equipmemt
5-position tap changer with ± 2 x 2.5 % tappings at 20 kV.
LV and HV bushings.
Valves for filling, filtering and oil sampling.
Oil level indicator.
Bi-directional rollers.
Rating plate.
Pressure relief device.
Thermometer with two electrical contacts.
Electrical Characteristics
Rated power (kVA)
No-load losses
25
(W)(*)
40
50
63
100
180
230
320
125
160
380
250
400
630
800 1000 1600
110
170
460
650
930 1270
1350 1700 2300
Load losses at 20 kV (W)(*)
700
985 1100 1350 1750
2100 2350
3250
4600 6500
8600 10500 13600
Voltage drop
cosÊ=1
2.84
2.51
2.26
2.20
1.81
1.75
1.54
1.47
1.32
1.21
1.25
1.22
1.03
cosÊ=0.8
3.96
3.87
3.77
3.75
3.57
3.54
3.43
4.63
4.53
4.46
4.48
4.47
4.33
4
4
4
4
4
4
4
6
6
6
6
6
6
at full load (%)
Rated short-circuit impedance (%)(*)
Load
Efficiency (%)
cosÊ=1
96.86 97.19 97.50 97.55 97.97 98.05 98.27 98.46 98.64 98.78 98.77 98.79 99.02
100% cosÊ=0.8
96.11 96.52 96.90 96.96 97.48 97.58 97.85 98.09 98.30 98.48 98.47 98.50 98.77
Load
cosÊ=1
97.38 97.64 97.91 97.95 98.29 98.36 98.54 98.70 98.84 98.97 98.98 99.00 99.18
75% cosÊ=0.8
96.75 97.07 97.41 97.45 97.87 97.96 98.18 98.37 98.56 98.71 98.73 98.75 98.97
Order Details
Rated power
Vector group
Short-circuit impedance
Altitude of installation (if the altitude exceeds 1000 m)
Rated voltages
Primary tappings
No-load losses
Special accessories
Rated frequency
Ambient temperature
Load losses
Details of cable boxes (on request)
(*) Transformers with different losses and short-circuit impedance can be manufactured, on request.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 32
C
General Arrangement of Three-phase Sealed Type Transformers, from 25 to 1600 kVA, 20/0.4 kV
Alternative position
of rating plate
D
D
B
1. Corrugated panels
2. Tank earthing point
3. Filling valve DIN 42553
4. Draining and sampling valve
5. Lifting lugs
6. High voltage bushings
7. Low voltage bushings DIN 42530
8. Rating plate
9. Thermometer with two electrical contacts
10. Tap changer
11. Pressure relief device
12. Rollers
13. Neutral earthing link
14. Oil level indicator
A
Dimensions (mm)
Rated power (kVA)
A (mm)
25
40
50
63
100
125
160
250
400
630
800
1000
1600
850
870
900
930
950
1000
1130
1370
1530
1820
1870
1900
2260
B (mm)
680
680
680
680
680
680
770
855
895
1160
1240
1220
1415
C (mm)
1140
1140
1140
1200
1260
1275
1275
1270
1350
1350
1460
1570
1600
D (mm)
520
520
520
520
520
520
520
520
670
670
670
820
820
Total weight (Kg)
365
435
450
500
640
705
825
1050
1450
1950
2220
2600
2740
Due to the evolution of standards and materials, the present document will bind us only after confirmation from our transformer design
department.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 33
A.13 Examples
A.13.1 Calculation of Transformer Efficiency
Let us assume that a three-phase transformer, 630 kVA, 20/0.4 kV, has 1200 W no-load losses and 9300 W
load losses. Determine the transformer efficiency at full load (case 1) and at 75% load (case 2) for power factor
1.0 and 0.8.
Case 1: full load
Case 2: load 75%
The efficiency at full load and for power factor equal
to 1.0 (cos Ê=1.0) is calculated as follows:
The efficiency at load 75% and cos Ê=1.0 is:
Ë1 =
=
S cos Ê
=
S cos Ê + NLL + LL(S/SB)2
630000*1.0
= 0.9836 = 98.36 %
630000*1.0 + 1200 + 9300*(1.0)2
Ë3 =
The efficiency at load 75% and cos Ê=0.8 is:
The efficiency at full load and cos Ê=0.8 is:
Ë4 =
Ë2 =
472500*1.0
= 98.66 %
472500*1.0 + 1200 + 9300*(0.75)2
472500*0.8
= 98.33 %
472500*0.8 + 1200 + 9300*(0.75)2
630000*0.8
= 97.96 %
630000*0.8 + 1200 + 9300*(1)2
A.13.2 Calculation of Voltage Drop
Let us assume that a three-phase transformer, 630 kVA, 20/0.4 kV, has 9300 W load losses and 6% short-circuit
impedance. Determine the voltage drop at full load (case 1) and at 75% load (case 2) for power factor 1.0 and 0.8.
The voltage drop is given by the following equation:
Udrop = S (er cos Ê + ex sin Ê) + 1 1 ( S )2 (er sin Ê - ex cos Ê)2 , where
2 100 SB
SB
er = LL = 9300 = 0.014762 = 1.4762% and ex =
630000
SB
U2k - e2r =
0.062 - 0.0147622 = 0.05816 = 5.816 %
Case 1: full load
Case 2: load 75%
For cos Ê = 1 , sin Ê=0.
For cos Ê = 1, the voltage drop is calculated as follows:
Udrop = S (er cos Ê + ex sin Ê) +
SB
Udrop = (0.75) * (1.4762 * 1 + 5.816 * 0) +
+ 1 1 ( S )2 (er sin Ê - ex cos Ê)2 =
2 100 SB
+ 1 1 (0.75)2 (1.4762 * 0 - 5.816 * 1)2 = 1.202 %
2 100
= 1.0*(1.4762*1 + 5.816*0) +
+ 1 1 (1.0)2 (1.4762*0 - 5.816*1)2 = 1.645 %
2 100
For cos Ê = 0.8, the voltage drop is:
Udrop = (0.75) * (1.4762 * 0.8 + 5.816 * 0.6) +
For cos Ê = 0.8, sin Ê =
1 - (cos Ê)2 = 0.6.
Udrop = (1.0) * (1.4762 * 0.8 + 5.816 * 0.6) +
+ 1 1 (1.0)2 (1.4762 * 0.6 - 5.816 * 0.8)2 = 4.741 %
2 100
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 34
+ 1 1 (0.75)2 (1.4762 * 0.6 - 5.816 * 0.8)2 = 3.543 %
2 100
A.13.3 Parallel Operation of Transformers
Let us assume that three transformers operate in parallel. The first transformer has 800 kVA rated power and 4.4%
short-circuit impedance. The rated power and the short-circuit impedance of the other two transformers is 500
kVA and 4.8%, and 315 kVA and 4.0%, respectively. Calculate the maximum total load of the three transformers.
Among the three transformers, the third transformer
has the minimum short-circuit impedance,
i.e Uk, min = 4.0 %.
The maximum total load of the three transformers is:
The load of transformer 1 is:
Uk, min
Pn,1 = P1
= 800 4 = 728 kVA.
Uk,1
4.4
The three transformers have total installed power:
The load of transformer 2 is:
Uk, min
Pn,2 = P2
= 500 4 = 417 kVA.
Uk,2
4.8
The load of the transformer 3 is:
Pn,3 = P3
Uk, min
= 315
Uk,3
4 = 315 kVA.
4
Ptot = Pn,1 + Pn,2 + Pn,3 = 728 + 417 + 315 = 1460 kVA.
P = P1 + P2 + P3 = 800 + 500 + 315 = 1615 kVA.
From the above, it is concluded that the maximum total
load (1460 kVA) represents the 90.4% of the total
installed power (1615 kVA).
It should be noted that, in order the maximum total
load to be equal to the total installed power, the
transformers must have the same short-circuit
impedance.
A.13.4 Transformer Selection
Let us assume that an industrial user wants to buy a 630 kVA transformer. The transformer will operate with 60%
average loading, 8 hours per day, 200 working days per year. Two transformer manufacturers submit two
different offers to the industrial user. The first manufacturer offers a transformer with 1200 W no-load losses and
8700 W load losses at a sales price of 5870 :. The second manufacturer offers a transformer with 940 W no-load
losses and 6750 W load losses at a sales price of 7045 :. Considering that the depreciation of the transformer
purchase investment is going to be done in 5 years and the energy charge is 0.075 :/kWh, calculate the
economical optimum offer.
The comparison of the two offers will be based on the annual total owning cost, which is the sum of the annual
buying cost and the annual usage cost. An approximation of the annual buying cost can be found by dividing the
sales price with the years of depreciation. An approximation of the annual usage cost can be calculated based on
the annual charge due to the transformer operation (annual charge for load losses and no-load losses).
Manufacturer A
Manufacturer B
The annual buying cost (:) is:
The annual buying cost (:) is:
OC1 =
5870 :
= 1174 :
5
OC2 =
7045 :
= 1409 :
5
The annual charge (:) for the no-load losses is:
:
NLLC1 = 8,760 h * 1.2 kW * 0.075
= 788.4 :
kWh
The annual charge (:) for the no-load losses is:
:
NLLC2 = 8,760 h * 0.94 kW * 0.075
= 617.58 :
kWh
The annual charge (:) for the load losses is:
:
LLC1 = (200 * 8 h) * 0,62 * 8.7 kW * 0.075
= 375.84 :
kWh
The annual charge (:) for the load losses is:
:
LLC2 = (200 * 8 h) * 0,62 * 6.75 kW * 0.075
= 291.6 :
kWh
The annual total owning cost (:) is:
The annual total owning cost (:) is:
TOC1 = OC1 + NLLC1 + LLC1 = 2338.24 :
TOC2 = OC2 + NLLC2 + LLC2 = 2318.18 :
As a result, although the transformer sales price of the second manufacturer is 20% more expensive (i.e. 1175
: more expensive), the transformer of the second manufacturer is finally more economical, since its annual
total owning cost is 0.9% less (i.e. 20.06 : less). From the above, it is concluded that the cheapest
transformer is not always the most economical. In particular, the difference at the annual total owning cost
could be more than 0.9%. This will happen, if we consider more years for the depreciation (instead of the
current assumption of 5 years), or if we use the transformer more (instead of the current assumption of 60%
average loading, 8 hours per day, 200 working days per year).
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 35
SECTION B:
Transformer Installation and Maint
B.1 Dimensions of transformer installation area
When the transformer is going to be installed inside an electrical room (indoor installation), particular attention
should be paid to the calculation of the dimensions of the installation area as well as to the ventilation of the
installation room. The ventilation of the electrical room influences the cooling, and consequently, the transformer’s
life. The distance between the walls of the room and the transformer end points must be from 50 to 60 cm.
Calculation of air resistance
For the calculation of the dimensions of the
openings for the input and output of air in the
electrical room, the calculation of the air
resistance is required.
For the air resistance, the symbol W is used in
the sequel. The value of the air resistance
depends on the existence or not of lattices,
meshes and venetian blinds.
If there are no lattices, meshes and venetian
blinds in the input and output openings of the
air, then the minimum air resistance is :
Wmin = 4.
For each lattice, the value WL=1 is added to the
value of Wmin .
For each adjustable venetian blind, the value of
WV = 3 is added to the value of Wmin .
For example, for a transformer installation room with
two meshes (one in the input and one in the output of
air), the minimum air resistance is:
W = Wmin + 2 WM = 4 + 2 x 1.5 = 7.
The lowest possible temperature in the transformer
electrical room is achieved with the following ways:
the opening for the output of the hot air is placed in
the highest possible location, and
the opening for the input of the cold air is placed in
the lowest possible location.
For each mesh, the value WM =1.5 is added to
the value of Wmin .
Calculation of cross-section area of the input and output openings
The cross-section area of the opening for the input of
air, F1 (m2), is calculated by the following formula:
104W ,
F1 = 4.25 . V .
100
H . t3
where V is the total transformer losses (kW), W is the
air resistance, H is the height (m) of the opening for the
output of air from the horizontal symmetry axis of
transformer (Figure 3), and t is the temperature rise
(°C) of the transformer room.
The cross-section area of the opening for the output of
air, F2 (m2), should be 10% to 15% larger than the
cross-section area of the opening for the input of air
(F1).
Gravel
Pit for oil
Figure 3: Dimensions of transformer installation room.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 36
enance
B.2 Instructions for transformer installation
The transformer is delivered at the industrial site of
Schneider Electric at Inofyta, Greece.
The responsibility for the safe transportation,
unloading, and connection to the network belongs to
the transformer user. The substation must be
constructed after study and design from a certified
engineer and the relevant authorities (e.g. electrical
utility, etc) must approve the substation design.
Under the transformer, there should be an oil collection
tank, which has on its upper part a metallic mesh and
gravel. The oil collection tank must have the
appropriate volume, so that in case of leakage all the
quantity of the transformer oil can be collected within
the oil collection tank. The whole substation has an
isodynamic mesh. The resistance of the earthing must
be less than 1ø and generally the substation must be
constructed in accordance with the existing
instructions and regulations of the local authorities
(e.g. construction authority, electrical utility).
The unloading and transportation of the transformer
should be done in such a way that the transformer
does not deviate by more than 15° from its horizontal
position. When the transformer is installed at its
position, no deviation is allowed from its horizontal
position. If the transformer is equipped with a
Buchholz relay, the Buchholz connection instructions
must be followed.
Before the connection of the transformer to the
network, the transformer must be optically checked,
in order to ensure that it has no damage during its
transportation or it has no oil leakage. In case of
scratches in its painting, the transformer must be
repainted immediately in order to avoid future rust.
B.3 Instructions for transformer Maintenance
The transformer is a very reliable electrical machine and it will practically need no maintenance during the many
years of its operation. However, this presupposes that the transformer remains clean and it is not overloaded more
than the permissible levels of duration and loading. Moreover, it also assumes that the network that the
transformer serves is not affected by short-circuits, overvoltages, thunders, and the coupling apparatus of the high
and low voltage as well as the transformer’s protective devices operate normally. In practice, it is not possible to
guarantee all these conditions, that’s why the following are recommended:
1. Optical inspection (every three months)
Check if the transformer is clean, especially on the
surface of insulators (dust and moisture can cause
flashover).
Check for oil leakage.
Check for damage in the transformer painting. In
case of scratches, they should be repainted in order
to prevent the tank oxidation.
Check of the oil level of the oil indicator, when the
transformer is out of operation. For example, if the
ambient temperature is +20°C and the reading of
the oil indicator is below the reading of +20°C, then
oil filling is required.
Check of the condition of the air dehumidifier. If the
color of the silica gel becomes yellow, then it is in
good condition, while if it has a soft blue-white
color, then it must be dried or it must be replaced.
2. Oil check (every year)
Check of the oil dielectric strength. This is based
on the sample that is taken, by opening the draining
and sampling oil valve of the transformer. 10 lt of
oil are initially taken out and next, a sample of 1 lt is
taken. The cans, bottles and funnels that are going
to be used for sampling, must be absolutely clean
and dry.
The bottle, which is going to be used for the
shipment of the oil, must be hermetically sealed. If
the check results in an oil dielectric strength less
than 40 kV, then the oil must be replaced or must be
reprocessed with a special cleaning apparatus.
Check of the operation of the Buchholz relay, the
thermometer and the condition of their contacts.
Before each action it is necessary:
to turn off the medium and low voltage switches,
to ground the transformer in order to remove any
capacity loads.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 37
B.4 Instructions for thermometer connection
The transformer thermometer is used to follow the
variations of the oil temperature.
The thermometer has two normally open contacts,
which change status when the transformer reaches
the predetermined limits.
The first contact (alarm contact) is used for warning
and the second (trip-off contact) is used for tripping
off the circuit breaker at the low voltage switchboard.
The warning could be:
Activation of alarm,
Load rejection,
Optical indication (warning lamp).
The suggested activation adjustments of the
transformer contacts are:
90°C for warning (left movable pointer with
red end),
100°C for trip-off (right movable pointer with
red end).
A general arrangement of the transformer and
Buchholz relay is shown in Figure 4.
A typical wiring of thermometer and Buchholz
connected to an alarm panel is shown in Figure 5.
Figure 4: General arrangment of thermometer and Buchholz.
Figure 5: Typical wiring of thermometer and Buchholz connected to an alarm panel.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 38
B.5 Instructions for the connection of the Buchholz relay
If the transformer has a Buchholz relay, the following connection instructions are suggested:
Initially, the protective cylindrical cover of the testing button of the Buchholz relay must be unscrewed, and
then the cylindrical piece of wood (which blocks the floats during transportation) must be removed.
Next, it is necessary to check (from the inspection door) if the Buchholz relay is full with oil. In case that it is not
full, the hexagonal cover must be removed and the ventilation valve must be opened so that the Buchholz relay
to be filled with oil. The filling and the free movement of the floats with the contacts are checked through the
inspection door. As soon as the Buchholz relay is filled with oil, the ventilation valve must be closed again.
A general arrangement of the Buchholz relay (and the transformer) is shown in Figure 4.
A typical wiring of thermometer and Buchholz relay connected to an alarm panel is shown in Figure 5.
(“Electrical contacts” and “check of the operation
of the protection circuits”, page 40)
Dry type contacts for signaling (alarm and trip)
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 39
B.5 Instructions for the connection of the Buchholz relay (continue)
Electrical contacts
The electrical contacts consist of two pairs of normally open contacts.
The one pair of contacts is used for alarm and the other pair of contacts for tripping off the circuit breaker.
The required voltage is 24-230V alternative or direct current.
Check of the operation of the protection circuits
The check of the operation of the protection circuits is
implemented through the following steps:
The cylindrical cover of the testing button is
removed and the button is gradually pressed so
that to get down the floats. Next, it is checked if the
alarm contacts and the trip-off contacts are closed.
As soon as the testing button is left free, the floats
are moved to the normal position and the contacts
open.
The alarm contacts (3 and 4) are connected to an
alarm horn, which will operate when gases are
collected to the Buchholz relay or the oil level gets
down.
The trip-off contacts (1 and 2) act on the trip-off
coil of the medium voltage circuit breaker and open
the circuit breaker, when the oil level gets down or
the oil pressure in the transformer tank increases
suddenly.
B.6 Instruction for the connection of the air dehumidifier
If the transformer has an air
dehumidifier, the following
connection instructions are
suggested:
VE.11/TV75 type
;;;
;
;;
;;;
;
;;;
;;
;;
;;
;;
;;
;;
;
;;
;
;;
;;
;;
;;
Waterproof packing is used
for the transportation of the
dehumidifier, in order to avoid
the absorption of moisture by
the silica gel.
During the placement of the
dehumidifier to the transformer,
the oil glass (which is under the
dehumidifier) is removed.
Then the oil glass is filled with
mineral oil until the end of the
tube (which goes out of the
dehumidifier) to be sink in the oil.
In case of transformer
transportation, the dehumidifier
must be removed, its tap must
be sealed and a cap must
be put to the tube of the oil
conservator.
The air dehumidifier is shown
in Figure 6.
1. Top cover
2. Stainless tank
3. Transparent silica
gel tank
4. Tube
5. Transparent oil tank
6. Oil indicator - air input
7. Base
8. Rating plate
9. Draining tube
11. Air input
;;;
;;
;;
;;;
;;
;;;
;;
;;
;;;
;;
;;;
Type
Oil
weight
VE.1
1500 kg
H
D
F
250 mm 100 mm 1/2” GF
silica gel
weight
0,35 kg 0,465 dm3
Figure 6: Air dehumidifier
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 40
Tank
volume
Schneider Electric Servises
Schneider Electric Services cover:
Engineering and technical support
Startup
Training
Preventive maintenance and maintenance after failure
Adaptation works
Spare parts
Maintenance contracts
For questions please contact
the Services department of Schneider Electric AE:
tel.: +301 0 6295243, +301 0 6295247.
Use and Maintenance of ELVIM Oil-immersed Distribution Transformers / page 41
Schneider 85-10AD/05.02
Schneider Electric SA
Athens:
14th km Athens-Lamia N.R.
GR-145 64 Kifissia
Tel. + 301 0 62.95.200
Fax + 301 0 62.95.210
Thessaloniki: 7, Adrianoupoleos str.
GR 551 33 Kalamaria
Tel. + 3031 0 423.224
Fax + 3031 0 423.225
http://www.schneider-electric.com.gr
Customers service line: 0800-11- 62900