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Use and Maintenance of Oil-immersed Distribution Transformers - Elvim range / page 1

Use and Maintenance of

Oil-immersedDistribution TransformersElvim range


Elvim, Merlin Gerin and Telemecanique become Schneider ElectricMoving our brands under the flagship brand Schneider Electric is an ambitious project that will make things simpler for all our partners and customers.Selection becomes simpler, the guarantee of equipment compatibility is reinforced and the integration of recently acquired technologies is optimized. The products themselves remain the same. With a single brand, Schneider Electric confirms its presence as a global specialist in energy management. With a network close to you and fast-responding teams, we commit to help you make the most of your energy, improve your efficiency and ensure the sustainable development of your business.

New product markingGradually, all major product families will be marked “Schneider Electric”. Only the product marking visual design changes. All the other specifications will remain the same: Technical features, function, weight, quality, commercial reference, technical information,… From the 2nd trimester 2009, oil-immersed distribution transformers will be branded Schneider Electric and the name Elvim will become a range name.

From now on you will see


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

Oil-immersedDistribution TransformersElvim range


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

A.1.4 Classification of transformers according to the construction of the magnetic circuit page 6

A.2 Elvim Distribution A.2.1 General characteristics page 7

Transformers A.2.2 Advantages of Elvim distribution transformers page 7

A.3 Transformer A.3.1 Magnetic circuit page 8

Manufacturing A.3.2 Windings page 8 Features

A.3.3 Metallic parts page 9

A.3.4 Assembly page 9

A.3.5 Cooling medium page 9

A.4 Transformer A.4.1 Tank page 10

Components 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 Transformer tests A.5.1 Type tests page 14

A.5.2 Routine tests page 14

A.5.3 Special tests page 15

A.6 Transformer A.6.1 Rated power page 16 electrical A.6.2 Temperature rise page 16

characteristics

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 Vector group page 16

A.6.7 No-load losses page 17

Contents


A.6 Transformer A.6.8 Load losses page 17

electrical A.6.9 Rated voltage page 17

characteristics 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 A.9.1 Overloading page 20

operation A.9.2 Parallel operation page 21

A.9.3 Load distribution of transformers in parallel operation page 21

A.10 Transformer order form page 22

A.11 Transformer A.11.1 Electrical utilities page 23

selection A.11.2 Industrial users page 23

A.12 Elvim A.12.1 Single-phase transformers, transformers from 5 to 50 kVA, 20/0.231 kV page 24

series A.12.2 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV page 26

A.12.3 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV, with low losses page 28

A.12.4 Three-phase transformers, from 250 to 1600 kVA, 20-15/0.4 kV page 30

A.12.5 Three-phase sealed type transformers, from 25 to 1600 kVA, 20/0.4 kV page 32

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

Schneider Electric Services page 41


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 Transformer types

Section A:

Use of Transformers

A.1.1 Classification of transformers according to the use

According to their use, the transformers are classified into the following categories:

(a) Distribution transformersThey 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 2500 kVA.

(b) Power transformersThey 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 2,5 MVA.

(c) AutotransformersThey 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.

(d) Test transformers They are used for the execution of performance tests with high or ultra-high voltage.

(e) Special power transformersThey are used for special applications, e.g. in furnaces and in welding.

(f) Instrument transformersThey are used for the accurate measurement of voltage or current.

(g) Telecommunication transformersThey 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 transformersThe insulating medium is mineral oil or synthetic (silicon) oil.

(b) Dry type transformersThe 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 transformersThe resin type transformer is a dry type transformer insulated with epoxy resin cast under vacuum.


(a) With three 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.

(b) With five legs (vertical limbs)Free return of the flux through the external legs.

The construction of the magnetic circuit of the three-phase transformers can be done, alternatively, as follows:

(a) Stack coreThe layers of the sheets of the magnetic material are placed one over the other and the vertical and the horizontal layers are over lapped.

(b) Wound coreThe magnetic circuit is of shell type and the sheets are wound.

There are two different technologies for stacking the sheets of the magnetic material of the core:

(a) Silicon steel sheetThe 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).

(b) Amorphous metal sheetThe 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.

Two different materials are used for core construction:

A.1.4 Classification of transformers according to the construction of the magnetic circuit


A.2 Elvim Distribution Trasformers

At the industrial site of Schneider Electric Greece, 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 2500 kVA.

A.2.1 General characteristics

More than 35 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 application of the OHSAS 18001 Health & Safety system assures that each employee enjoys the maximum level of health and safety in the working environment.

The use of the best materials for the transformer construction. The reliability of the suppliers of the transformer materials is systematically monitored and checked.

The high automation of the industrial process has dramatically decreased the delivery time. For example, special transformers can be delivered within 3 weeks.

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.

A potential transformer user has a lot of reasons to choose Elvim distribution transformers that are manufactured by the industrial site of Schneider Electric Greece. Some of the most important reasons are the following:

A.2.2 Advantages of Elvim distribution transformers


A.3 Transformer Manufacturing Features

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.

A.3.1 Magnetic circuit

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.

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.

A.3.2 Windings

Figure 1: Wound core.

Figure 2: Transformer active part.

CORES

COILS


CNC machines for cutting, punching and bending of steel sheets.

Different types of welding machines (i.e. MIG-MAG, TIG, and electrode) for the welding of the metallic parts.

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.

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 μm. This painting procedure results in a durable corrosion protection and therefore lengthy life expectancy.

A.3.3 Metallic parts

For the transformer assembly, the following basic equipment is used:

For the construction of transformer metallic parts, the following basic mechanical equipment is used:

various cranes from 1 to 35 tons,

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,

two vacuum chambers, in which the transformers are filled with oil,

machines for the processing of transformer oil, so that the oil obtains the appropriate characteristics, according to the international standards.

A.3.4 Assembly

Transformer oil according to IEC 60296 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.

A.3.5 Cooling medium


This link ensures the neutral earthing of the three-phase winding with the transformer tank.

A.4.6 Neutral earthing link

A.4 Transformer Components

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.4.1 Tank

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.4.2 Cover

The lifting lugs are used for lifting and carrying the transformer.

A.4.3 Lifting lugs

The transformers up to 160 kVA are usually manufactured as pole-mounted. The transformers above 160 kVA are equipped with bi-directional rollers.

A.4.4 Rollers

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.4.5 Draining and sampling oil valve


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.4.7 High voltage bushings

Low voltage bushings of 1 kV series, according to DIN 42530, are used in the low voltage.

A.4.8 Low voltage bushings

Low voltage connectors, according to DIN 43675, are used.

A.4.9 Low voltage connectors

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.4.10 Tap changer

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.4.11 Voltage selector

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.

A.4.12 Transformer thermometer


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.4.13 Oil conservator

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.4.14 Buchholz relay

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 should have the following colors: Yellow (silica gel is fully dry).

Soft blue-white (silica gel is full of 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.

A.4.15 Air dehumidifier

The transformers are equipped with a filling valve, in order to have the ability to fill the transformers with mineral oil.

A.4.16 Filling valve

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 transformers

with oil conservator, an oil level indicator of tube type (glass transparent tube) or magnetic type is placed on the oil conservator.

A.4.17 Oil level indicator


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.4.18 Rating plate

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.4.19 Tank earthing point

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.

A.4.20 Accessories of sealed type transformers

Oil conservator

Buchholz relay

High voltage bushings

Tap changer

Low voltage bushings

Lifting lug

Thermometer

Oil level indicator

Air dehumidifier

Rating plate

Tank

Rollers

Elvim Oil-immersed Distribution Transformer


A.5 Transformer tests

The transformer tests are classified, in accordance with the specification IEC 60076, as follows:

Type tests.

Routine tests.

Special tests.

(a) Temperature rise testThe procedure of the temperature rise test is performed according to IEC 60076-2. 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.

(b) Lighting impulse testThe procedure of the lightning impulse test is performed according to IEC 60076-3. 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.

The type tests, which are performed on one transformer from every transformer type, are the following:

A.5.1 Type tests

(a) Measurement of winding resistance The procedure of the measurement of windings resistance is performed according to IEC 60076-1. 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.

(b) Measurement of the voltage ratio and check of phase displacementThe measurement of the voltage ratio is performed according to IEC 60076-1. 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:

(c) Measurement of short-circuit impedanceThe measurement of short-circuit impedance is performed according to IEC 60076-1. 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.

(d) Measurement of load lossesThe 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 lossesThe measurement is performed according to IEC 60076-1. 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.

The routine tests are performed on every transformer separately.The routine tests include:

A.5.2 Routine tests

U1 N1

U2 =

N2


(a) Dielectric special tests

(b) Determination of capacitances of windings-to-earth and between windings

(c) Short-circuit withstand testAccording 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.

(d) Determination of sound levelsThe 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.

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:

(e) Measurement of the harmonics of the no-load current

( f) Measurement of insulation resistance and/or measurement of dissipation factor (tanδ) of the insulation system capacitances

(g) Radio interference voltage

(h) Measurement of zero-sequence impedance

A.5.3 Special tests

(f) Dielectric routine tests

The dielectric routine tests are the following:

Applied voltage dielectric test

The duration of the test, according to IEC 60076-3, 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.

(a) MV windingsThe 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 windingsThe 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.

Induced voltage dielectric test

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.5.2 Routine tests (continue)


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.6.6 Vector group

A.6 Trasformer Electrical Characteristics

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.6.1 Rated Power

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.6.2 Temperature rise

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.6.3 Ambient Temperature

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 voltage drop. The lower the short-circuit impedance, the higher the short-circuit current, in case of short-circuit. 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.6.5 Short-circuit impedance

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.6.4 Altitude of installation


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 5 lists (E0, D0, C0, B0, A0) of no-load losses for transformers from 50 to 2500 kVA, according to EN 50464-1:2007.

A.6.7 No-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 4 lists (Dk, Ck, Bk, Ak) of load losses for transformers from 50 to 2500 kVA, according to EN 50464-1:2007.

For example, a transformer has a combination of losses of Dk-A0, if its load losses belong to list Dk, and its no-load losses belong to list A0.

A.6.8 Load losses

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 60076). 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.

A.6.9 Rated voltage

Rated power

kVA

Dk

W

Ck

W

Bk

W

Ak

W

Short circuit impedance

%

50 1350 1100 875 750

4

100 2150 1750 1475 1250

160 3100 2350 2000 1700

250 4200 3250 2750 2350

315 5000 3900 3250 2800

400 6000 4600 3850 3250

500 7200 5500 4600 3900

630 8400 6500 5400 4600

630 8700 6750 5600 4800

6

800 10500 8400 7000 6000

1000 13000 10500 9000 7600

1250 16000 13500 11000 9500

1600 20000 17000 14000 12000

2000 26000 21000 18000 15000

2500 32000 26500 22000 18500

Rated power E0 D0 C0 B0 A0

Short circuit impedance

kVAP0

W

LwA

dB(A)

P0

W

LwA

dB(A)

P0

W

LwA

dB(A)

P0

W

LwA

dB(A)

P0

W

LwA

dB(A)%

50 190 55 145 50 125 47 110 42 90 39

4

100 320 59 260 54 210 49 180 44 145 41

160 460 62 375 57 300 52 260 47 210 44

250 650 65 530 60 425 55 360 50 300 47

315 770 67 630 61 520 57 440 52 360 49

400 930 68 750 63 610 58 520 53 430 50

500 1100 69 880 64 720 59 610 54 510 51

630 1300 70 1030 65 860 60 730 55 600 52

630 1200 70 940 65 800 60 680 55 560 52

6

800 1400 71 1150 66 930 61 800 56 650 53

1000 1700 73 1400 68 1100 63 940 58 770 55

1250 2100 74 1750 69 1350 64 1150 59 950 56

1600 2600 76 2200 71 1700 66 1450 61 1200 58

2000 3100 78 2700 73 2100 68 1800 63 1450 60

2500 3500 81 3200 76 2500 71 2150 66 1750 63

NOTE: P0 = no load losses LwA = sound power level

Table 2: Load losses Pk (W) at 75ºC for Um 24kV

Table 1: No load losses P0 (W) and sound power level (Lw(A)) for Um 24kV


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:

n =

where S is the transformer load in VA, losses are the losses in W and cos φ 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 =

where NLL are the no-load losses, LL are the load losses and SB is the rated power of the transformer in VA.

A.6.12 Efficiency

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.

A.6.14 No-load 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 κ√2 times the value of the symmetrical short-circuit current. The factor κ√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 κ√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:

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 with-stand short-circuit currents according to IEC 60076-5.

Ux / Ur κ√2 1 1.51 1.5 1.63 2 1.75 3 1.95 4 2.09 5 2.19 6 2.28 8 2.38 10 2.46 15 2.56 25 2.66 50 2.77

A.6.13 Short-circuit current

S cos φ

S cos φ + losses

S cos φ

S cos φ + NLL + LL(S/SB)2

IK 100

In =

UK

Table 3: Values of the factor κ√2 versus the ratio Ux/Ur

The frequency at which the transformer is designed to operate is 50 Hz or 60 Hz in accordance with the network frequency.

A.6.10 Frequency

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.6.11 Noise


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 Description1 IEC 60076-1 Power transformers: general2 IEC 60076-2 Power transformers: temperature rise3 IEC 60076-3 Power transformers: insulation levels and dielectric tests4 IEC 60076-5 Power transformers: ability to withstand short circuit5 IEC 60137 Bushings for alternating voltages above 1000 V6 IEC 60354 Loading guide for oil-immersed power transformers7 IEC 60726 Dry-type power transformers8 IEC 60905 Loading guide for dry-type power transformers

Table 4: Transformer standards according to IEC.

The above standards are related with the electrical characteristics and the accessories of transformers. The IEC 60076 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 EN 50464-1 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 60076-1.

Ιtem ToleranceVoltage 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 tappingShort-circuit impedance ±10% of the guaranteed short-circuit impedanceNo-load losses +15% of the guaranteed no-load lossesLoad losses +15% of the guaranteed load lossesTotal losses (load and no-load) +10% of the guaranteed total losses (load and no-load)No-load current +30% of the guaranteed no-load current

Table 5: Tolerances on certain transformer items according to IEC 60076-1.


A.9 Transformer Operation

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.

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.

A.9.1 Overloading

Previous continuous Oil temperature Duration (min) of overloadingloading (°C) for specific levels of overloading(% of rated power) (% of rated power)

10% 20% 30% 40% 50% min min min min min

50 55 180 90 60 30 15

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. 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.


The parallel operation of two or more transformers is feasible, when the following requirements are met:

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 60076-1, Table 5, § A.8).

Their short-circuit impedance should be the same (the permitted tolerance is according to IEC 60076-1, Table 5, § A.8).

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.

A.9.2 Parallel operation

Transformer group Group of existing Connection between phasesfor parallel operation transformer High Voltage Low Voltage R S T r s t

5 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 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.

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.

Consequently, the load distribution is given by the following equation:

Pi = Pni ,

A.9.3 Load distribution of transformers in parallel operation

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 short-circuit impedance of the n parallel operated transformers.

Finally, the total power of the n parallel operated transformers is:

(Pi ) < Pi .

An arithmetic example of the load distribution of transformers in parallel operation is given in § A.13.3.

UK min

UKi

n

Σi=1

n

Σi=1

UK min

UKi


A.10 Transformer order form

Customer

Sales engineer

Transformer type three-phase single-phase

Rated power (kVA)

Rated primary/secondary voltage (kV)

Frequency (Hz) 50 60

Installation indoor outdoor

Altitude ≤1000 m >1000 m

Cooling ONAN other

Transformer type oil dry type

Oil conservator yes no

Transformer dimensions (mm) length width height

Taps ± 2x2.5% 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

The transformer order form includes the following data:


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.

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.11.1 Electrical Utilities

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.

A.11.2 Industrial Users


Electrical Characteristics

(*) Transformers with different losses and short-circuit impedance can be manufactured, on request.

A.12.1 Single-phase transformers, from 5 to 50 kVA, 20/0.231 kV

In this paragraph, five indicative Elvim transformers’ series are presented.

A.12 Elvim Transformers Series

Rated Power (kVA) 5 10 15 20 25 30 50No-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 cosφ =1 3.04 3.23 3.26 3.28 2.94 2.71 2.74

at full load (%) cosφ = 0.8 3.99 4.00 4.00 4.00 3.97 3.93 3.93

Rated short-circuit impedance (%)(*) 4 4 4 4 4 4 4

Load cosφ =1 96.06 96.39 96.43 96.46 96.79 97.02 97.03

Efficiency (%) 100% cosφ =0.8 95.12 95.52 95.58 95.61 96.02 96.31 96.32

Load cosφ =1 96.42 96.96 97.04 97.08 97.34 97.53 97.56

75% cosφ = 0.8 95.56 96.23 96.33 96.38 96.69 96.93 96.96

General Description Single-phase distribution transformers, 50 Hz. IEC 60076 standard is followed. The cooling is implemented with natural circulation (ONAN) of mineral oil according to IEC 60296. 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 60076. 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.

Order Details

Rated power

Short-circuit impedance

Rated voltages

No-load losses

Rated frequency

Load losses

Altitude of installation (if the altitude exceeds 1000 m)

Primary tappings

Special accessories

Ambient temperature


Dimensions (mm)

Rated power (kVA) 5 10 15 20 25 30 50

A (mm) 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.

General Arrangement of Single-phase Transformers, from 5 to 50 kVA, 20/0.231 kV

1. Filling plug DIN 42553 form "D"2. Drain and sampling valve DIN 42551 NW223. Lifting lugs4. H.V. bushings5. L.V. bushings6. Rating plate7. Off-load tap changer8. Transformer base9. Pole mounted elements


A.12.2 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV


Order Details

Rated power


Rated voltages

No-load losses

Rated frequency

Load losses

Vector group


Primary tappings

Special accessories

Ambient temperature

Details of cable boxes (on request)


Rated power (kVA) 250 400 500 630 800 1000 1250 1600

No-load losses (W)(*) 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 cosφ=1 1.94 1.78 1.73 1.65 1.55 1.52 1.48 1.41

at full load (%) cosφ=0.8 4.92 4.82 4.79 4.74 4.68 4.66 4.64 4.59

Rated short-circuit impedance (%)(*) 6 6 6 6 6 6 6 6

Load cosφ=1 98.02 98.21 98.27 98.36 98.47 98.50 98.54 98.62

Efficiency (%)) 100% cosφ=0.8 97.53 97.77 97.85 97.96 98.09 98.13 98.18 98.28

Load cosφ=1 98.37 98.53 98.58 98.66 98.74 98.77 98.81 98.87

75% cosφ=0.8 97.97 98.17 98.24 98.33 98.43 98.47 98.51 98.59

General Description Three-phase disrtibution transformers, 50 Hz. IEC 60076 standard is followed. Transformers with oil conservator. The cooling is implemented with natural circulation of mineral oil according to IEC 60296. 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 60076. 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.


General Arrangement of Three-phase Transformers, from 250 up to 1600 kVA, 20/0.4 kV

Dimensions (mm)

Rated power (kVA) 250 400 500 630 800 1000 1250 1600

A (mm) 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

D (mm) 520 670 670 670 670 820 820 820

Total weight (Kg) 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.

1. Transformer tank2. Tank cover3. Lifting lugs4. Roller DIN 425615. Draining and sampling valve DIN 425516. Neutral earthing link7. High voltage bushing DIN 425318. Low voltage bushing DIN 425309. Low voltage connector DIN 4367510. Tap changer11. Thermometer with two electrical contacts12. Oil conservator13. Buchholz relay14. Air dehumidifier15. Filling valve DIN 4255316. Oil level indicator17. Rating plate18. Tank earthing point

A

B

C

D D


A.12.3 Three-phase transformers, from 250 to 1600 kVA, 20/0.4 kV, with low losses



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


at full load (%) cosφ=0.8 3.33 3.25 3.22 3.17 4.49 4.47 4.48 4.48


Load cosφ=1 98.55 98.71 98.77 98.85 98.81 98.85 98.84 98.84

Efficiency (%) 100% cosφ=0.8 98.20 98.40 98.46 98.56 98.52 98.57 98.56 98.56

Load cosφ=1 98.81 98.95 98.99 99.05 99.04 99.07 99.07 99.07

75% cosφ=0.8 98.52 98.69 98.73 98.82 98.80 98.85 98.84 98.84

General Description Three-phase distribution transformers, 50 Hz. Combination of losses Dk-A0, EN 50464-1:2007. IEC 60076 standard is followed. The cooling is implemented with natural circulation of mineral oil according to IEC 60296. 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 60076. 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.

Order Details

Rated power


Rated voltages

No-load losses

Rated frequency

Load losses

Vector group


Primary tappings

Special accessories

Ambient temperature



General Arrangement of Three-phase Transformers, from 250 to 1600kVA, 20/0.4 kV, with low losses


1. Transformer tank2. Tank cover3. Lifting lugs4. Roller DIN 425615. Draining and sampling valve DIN 425516. Neutral earthing link7. High voltage bushing DIN 425318. Low voltage bushing DIN 425309. Low voltage connector DIN 4367510. Tap changer11. Thermometer with two electrical contacts12. Oil conservator13. Buchholz relay14. Air dehumidifier15. Filling valve DIN 4255316. Oil level indicator17. Rating plate18. Tank earthing point

Dimensions (mm)

Rated power (kVA) 250 400 500 630 800 1000 1250 1600

A (mm) 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) 520 670 670 670 670 820 820 820

Total weight (Kg) 1150 1500 1750 2100 2400 2800 3200 4050

D D

A

C

B


A.12.4 Three-phase transformers, from 250 to 1600 kVA, 20-15/0.4 kV



Rated power (kVA) 250 400 500 630 800 1000 1250 1600

No-load losses (W)(*) 575 810 930 1000 1180 1360 1720 1950

Load losses as 20 kV (W)(*) 4000 6350 7500 9300 10800 12800 13500 17400


at full load (%) cosφ=0.8 4.82 4.81 4.76 4.74 4.66 4.62 4.49 4.49


Load cosφ=1 98.20 98.24 98.34 98.39 98.52 98.60 98.80 98.81

Efficiency (%) 100% cosφ=0.8 97.76 97.81 97.94 98.00 98.16 98.26 98.50 98.51

Load cosφ=1 98.52 98.56 98.65 98.70 98.81 98.87 99.02 99.03

75% cosφ=0.8 98.15 98.21 98.31 98.38 98.51 98.59 98.77 98.79

General Description Three-phase distribution transformers, 50 Hz. IEC 60076 standard is followed. The cooling is implemented with natural circulation of mineral oil according to IEC 60296. 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 60076. 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.

Order Details

Rated power


Rated voltages

No-load losses

Rated frequency

Load losses

Vector group


Primary tappings

Special accessories

Ambient temperature



General Arrangement of Tree-phase Transformers, from 250 to 1600 kVA, 20-15/0.4 kV

Dimensions (mm)

Rated power (kVA) 250 400 500 630 800 1000 1250 1600

A (mm) 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

D (mm) 520 670 670 670 670 820 820 820

Total weight (Kg) 1100 1560 1800 2100 2550 2800 3200 3760


1. Transformer tank2. Tank cover3. Lifting lugs4. Roller DIN 425615. Draining and sampling valve DIN 425516. Neutral earthing link7. High voltage bushing DIN 425318. Low voltage bushing DIN 425309. Low voltage connector DIN 4367510. Tap changer11. Voltage selector12. Thermometer with two electrical contacts13. Oil conservator14. Buchholz relay15. Air dehumidifier16. Filling valve DIN 4255317. Oil level indicator18. Rating plate19. Tank earthing point

D

C

D

B

A


A.12.5 Three-phase sealed type transformers, from 25 to 1600 kVA, 20/0.4 kV


Rated power (kVA) 25 40 50 63 100 125 160 250 400 630 800 1000 1600

No-load losses (W)(*) 110 170 180 230 320 380 460 650 930 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 cosφ=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

at full load (%) cosφ=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

Rated short-circuit impedance (%)(*) 4 4 4 4 4 4 4 6 6 6 6 6 6

Load cosφ=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

Efficiency (%) 100% cosφ=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 cosφ=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% cosφ=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

General Description Three-phase distribution transformers, 50 Hz. IEC 60076 standard is followed. The cooling is implemented with natural circulation of mineral oil according to IEC 60296. 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 60076. 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.


Order Details

Rated power


Rated voltages

No-load losses

Rated frequency

Load losses

Vector group


Primary tappings

Special accessories

Ambient temperature



General Arrangement of Three-phase Sealed Type Transformers, from 25 to 1600 kVA, 20/0.4 kV

Dimensions (mm)

Rated power (kVA) 25 40 50 63 100 125 160 250 400 630 800 1000 1600

A (mm) 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.

1. Corrugated panels2. Tank earthing point3. Filling valve DIN 425534. Draining and sampling valve5. Lifting lugs6. High voltage bushings7. Low voltage bushings DIN 425308. Rating plate9. Thermometer with two electrical contacts10. Tap changer11. Pressure relief device12. Rollers13. Neutral earthing link14. Oil level indicator

D

A

C

D

B

Alternative position of rating plate


Case 1: full loadFor cos φ = 1 , sin φ=0.

For cos φ = 0.8, sin φ = 1 - (cos φ)2 = 0.6.

Case 2: load 75%For cos φ = 1, the voltage drop is calculated as follows:

For cos φ = 0.8, the voltage drop is:

Case 1: full load The efficiency at full load and for power factor equal to 1.0 (cos φ=1.0) is calculated as follows:

The efficiency at full load and cos φ=0.8 is:

Case 2: load 75%The efficiency at load 75% and cos φ=1.0 is:

The efficiency at load 75% and cos φ=0.8 is:

η1 = = S cos φS cos φ + NLL + LL(S/SB)2

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.

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:

A.13.1 Calculation of Transformer Efficiency

A.13.2 Calculation of Voltage Drop

A.13 Examples

Udrop = (er cos φ + ex sin φ) + ( )2 (er sin φ - ex cos φ)2 , whereSSB

SSB

12

1100

Udrop = (er cos φ + ex sin φ) + SSB

Udrop = (1.0) * (1.4762 * 0.8 + 5.816 * 0.6) +

+ ( )2 (er sin φ - ex cos φ)2 =

= 1.0*(1.4762*1 + 5.816*0) +

SSB

12

1100

+ (1.0)2 (1.4762*0 - 5.816*1)2 = 1.645 %12

1100

+ (1.0)2 (1.4762 * 0.6 - 5.816 * 0.8)2 = 4.741 %12

1100

Udrop = (0.75) * (1.4762 * 1 + 5.816 * 0) +

+ (0.75)2 (1.4762 * 0 - 5.816 * 1)2 = 1.202 %12

1100

Udrop = (0.75) * (1.4762 * 0.8 + 5.816 * 0.6) +

+ (0.75)2 (1.4762 * 0.6 - 5.816 * 0.8)2 = 3.543 %12

1100

er = = = 0.014762 = 1.4762% and ex = U2k - e2

r = 0.062 - 0.0147622 = 0.05816 = 5.816 % LLSB

9300630000

= 0.9836 = 98.36 %= 630000*1.0630000*1.0 + 1200 + 9300*(1.0)2

= 97.96 %η2 = 630000*0.8630000*0.8 + 1200 + 9300*(1)2

= 98.66 %η3 = 472500*1.0472500*1.0 + 1200 + 9300*(0.75)2

= 98.33 %η4 = 472500*0.8472500*0.8 + 1200 + 9300*(0.75)2


Among the three transformers, the third transformer has the minimum short-circuit impedance, i.e Uk, min = 4.0 %.

The load of transformer 1 is:

The load of transformer 2 is:

The load of the transformer 3 is:

The maximum total load of the three transformers is:

Ptot = Pn,1 + Pn,2 + Pn,3 = 728 + 417 + 315 = 1460 kVA.

The three transformers have total installed power:

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.

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.

A.13.3 Parallel Operation of Transformers

Manufacturer A

The annual buying cost (€) is:

The annual charge (€) for the no-load losses is:

The annual charge (€) for the load losses is:

The annual total owning cost (€) is:

TOC1 = OC1 + NLLC1 + LLC1 = 2338.24 €

Manufacturer B

The annual buying cost (€) is:

The annual charge (€) for the no-load losses is:

The annual charge (€) for the load losses is:

The annual total owning cost (€) is:

TOC2 = OC2 + NLLC2 + LLC2 = 2318.18 €

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).

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).

A.13.4 Transformer Selection

Pn,1 = P1 = 800 = 728 kVA.Uk, minUk,1

44.4

OC1 = = 1174 €5870 €

5

NLLC1 = 8,760 h * 1.2 kW * 0.075 = 788.4 €€

kWh

LLC1 = (200 * 8 h) * 0,62 * 8.7 kW * 0.075 = 375.84 €€

kWh

OC2 = = 1409 €7045 €

5

NLLC2 = 8,760 h * 0.94 kW * 0.075 = 617.58 €€

kWh

LLC2 = (200 * 8 h) * 0,62 * 6.75 kW * 0.075 = 291.6 €€

kWh

Pn,2 = P2 = 500 = 417 kVA.Uk, minUk,2

44.8

Pn,3 = P3 = 315 = 315 kVA.Uk, minUk,3

44


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 mesh, the value WM =1.5 is added to the value of Wmin .

Calculation of air resistance

Calculation of cross-section area of the input and output openings

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.

The cross-section area of the opening for the input of air, F1 (m2), is calculated by the following formula:

F1 = . V . ,

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).

Figure 3: Dimensions of transformer installation room.

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.

B.1 Dimensions of transformer installation area

Section B:

Transformer Installation and Maintenance

Gravel

Pit for oil

4.25100

104WH . t3


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 1Ω 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.2 Instructions for transformer installation

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.

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.

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:

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.

B.3 Instructions for transformer Maintenance


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.


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)


Figure 6: Air dehumidifier

B.6 Instruction for the connection of the air dehumidifier

If the transformer has an air dehumidifier, the following connection instructions are suggested:

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.

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.

Electrical contacts

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.

Check of the operation of the protection circuits

1. Top cover2. Stainless tank3. Transparent silica gel tank4. Tube5. Transparent oil tank6. Oil indicator - air input7. Base8. Rating plate9. Draining tube11. Air input

Type Oil H D F silica gel Tank weight weight volume

VE.1 1500 kg 250 mm 100 mm 1/2” GF 0,35 kg 0,465 dm3

VE.11/TV75 type

B.5 Instructions for the connection of the Buchholz relay (continue)


Schneider Electric Services offer:

• Engineering and technical support

• Startup

• Training

• Preventive maintenance and maintenance after failure

• Adaptation works

• Spare parts

• Maintenance contracts

For questions please contact our Customer Care Center:

+30 800 11 62900 (toll free) or

+30 210 62 95 325

Schneider Electric Services


Make the most of your energy

Sch

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00 -

00A

D/0

2.09

Inofyta Plant55th km Athens-Lamia N.R., GR-32011 Inofyta, Greece

Customer care center: +30 800 11 62900 (toll free)

http://www.schneider-electric.gr

Documents

TR Looses (A0Ak0.pdf