Schneider Distribution Transformer1

Use and Maintenance of ELVIM Oil-immersed Distribution Transformers

Technical Collection

Building a New Electric World

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

The transformer is an electrical machine that allows the transmission anddistribution of electric energy simply and inexpensively, since its efficiency isgreater than 95%.Through the brief description of the use and maintenance of the oil-immerseddistribution transformers, the present technical leaflet provides usefulinformation 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


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 7Transformers A.2.2 Advantages of ELVIM distribution transformers page 7

A.3 Transformer A.3.1 Magnetic circuit page 8Manufacturing A.3.2 Windings page 8Features

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 10Components 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 16electrical A.6.2 Temperature rise page 16characteristics

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

Contents


A.6 Transformer A.6.8 Rated voltage page 17electrical A.6.9 Vector group page 17characteristics

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 20operation 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 23selection A.11.2 Industrial users page 23

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


(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 theconsumers. Their power is usually ranging from 50 to 1600 kVA.

(b) Power transformersThey are used in the high-power generating stationsfor voltage step up and in the transmission substationsfor voltage step up or step down. Usually their poweris bigger than 2 MVA.

(c) AutotransformersThey are used for voltage transformation withinrelatively small limits, for connection of electric energysystems of various voltages, for starting of AC(alternative current) motors, etc.

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

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.

(d) Test transformersThey are used for the execution of performance testswith high or ultra-high voltage.

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

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

(g) Telecommunication transformersThey are used in telecommunication applicationsaiming at the reliable reproduction of the signal in a wide range of frequency and voltage.

A.1 Transformer types

SECTION A: Use of Transformers

A.1.1 Classification of transformers according to the use


ONAF: Oil Natural Air Forced.

OFAN: Oil Forced Air Natural.

OFAF: Oil Forced Air Forced.

OFWF: Oil Forced Water Forced.

The identification of oil-immersed transformers according to the cooling method is expressed by a four-lettercode. The first letter expresses the internal cooling medium in contact with the windings. The second letteridentifies the circulation mechanism for internal cooling medium. The third letter expresses the external coolingmedium. The fourth letter identifies the circulation mechanism for external cooling medium. For example, if theinternal 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 aircirculation by fans, or both of the above. As a result, the following cooling methods exist:

Combinations like ONAN/ONAF, ONAN/OFAN or ONAN/OFAF are also applicable.

A.1.2 Classification of transformers according to the cooling method

(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 upto 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.

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

A.1.3 Classification of transformers according to the insulating medium


(a) With three legs (vertical limbs)The magnetic flux of one leg must close through theother two legs and the flux also flows through thewindings of the other phases, namely the transformerhas 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 areplaced one over the other and the vertical and thehorizontal layers are over lapped.

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

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 coreconstruction is an alloy consisting of 97% iron and 3%silicon. This material has crystallic structure. Thesilicon steel sheets have thickness from 0.18 up to 0.5mm. There are also silicon steel sheets for operation inhigh magnetic induction (Hi-B).

(b) Amorphous metal sheetThe amorphous metal sheet that is used for the coreconstruction is an alloy consisting of 92% iron, 5%silicon and 3% boron. This material has not crystallicstructure. It has 70% lower no-load loss than the siliconsteel. The thickness of the amorphous metal sheet is0.025 mm, namely it is about 10 times thinner than thetypical 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 oil of Schneider Electric AE, ELVIM distribution transformers are manufactured, with voltagesup 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.2.1 General characteristics

More than 30 years of experience in transformermanufacturing (the manufacturing site is activesince 1969). As a result, the best techniques andmethods are used during transformer design andmanufacturing.

The application of the ISO 9001 quality assurancesystem in combination with the very carefulmonitoring of the whole industrial process lead inthe manufacturing of high quality transformers.

The application of the ISO 14001 environmentalmanagement system assures the protection of theenvironment and the reasonable use of naturalresources during the transformer production.

The use of the best materials for the transformerconstruction. The reliability of the suppliers of thetransformer materials is systematically monitoredand checked.

The high automation of the industrial process hasdramatically decreased the delivery time. Forexample, special transformers can be deliveredwithin 3 weeks.

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

All the transformer offers are treated very carefullyin order to finally give an offer, which fullysatisfies the needs of the transformer user. The offer is technically complete and representsthe optimum technical and economical solution forthe specific transformer application.

The wound core technology that is followed hasthe following advantages, in comparison with thestack 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.2.2 Advantages of ELVIM distribution transformers


A.3 Transformer Manufacturing Features

The wound core technology is followed and magneticmaterials 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 asfollows: the magnetic material is slit into sheets ofstandard 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 copperrectangular wire is mainly used. The high voltage coil is constructed from copper wireor copper rectangular wire. The combination of coppersheet in low voltage with copper wire in high voltageplus coated press paper with epoxy resin as interlayerinsulation, increases the coil’s ability to withstandshort-circuit.

Important points during the production procedure arethe following:

Coil heat treatment at 100ºC so that epoxy resin ispolymerized 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 typetransformer.

A.3.2 Windings

Figure 1: Wound core.

Figure 2: Transformer active part.

CORES

COILS


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. MIG-MAG, TIG, and electrode) for the welding of themetallic parts.

Machines for construction and welding ofcorrugated panels and tanks. Certified techniciansand welders are the operators of these machines.

Stud welding equipment for stud welding on thetransformer cover so that secure insulatorplacement is achieved.

Equipment for oil leak detection of the transformertanks.

Modern painting shop for the painting of themetallic parts. The usual painting procedureincludes the following steps: sandblasting,decreasing-phosphatizing Fe, painting with 4 coats(two primer coats and two final color coats) withtotal thickness of 160 Ìm. This painting procedureresults in a durable corrosion protection andtherefore lengthy life expectancy.

A.3.3 Metallic parts

For the transformer assembly, the following basicequipment is used:

one crane of 35 tons and two cranes of 5 tons,

one drying chamber to dry the active parts in orderto remove the moisture, which is absorbed by thetransformer insulating materials during theproduction procedure,

two vacuum chambers, in which the transformersare filled with oil,

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

A.3.4 Assembly

Transformer oil according to IEC 296 specifications is used as cooling medium. The initial filling oftransformer with oil is done under high vacuum inorder to assure the high penetration of oil everywhereand to remove air bubbles or moisture that could causedielectric failure of coil.

Oil can also be filled later on without vacuum under theprerequisite that the oil level covers the active part andthe oil has been filtered. In agreement with thecustomer, the oil can be supplied from SchneiderElectric or another company provided that the oil isaccording 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 orderto increase the total cooling area.

The tank of sealed type transformers (without oilconservator) 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 temperatureduring transformer’s operation.

The transformer tank has two earthing points. The rolling system or the base skid is welded to thetank 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 thethermometer with two electrical contacts are placedon the cover.

Moreover, a neutral earthing link is also placed on thecover. A pressure relief device is usually placed on thecover 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 areequipped 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 orderto 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, onrequest, 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 windingof transformer is not stable and depends upon thetransformer position in the distribution network.Therefore, taken the primary voltage as granted, thetap changer is used in order to keep the secondaryvoltage 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 onthe cover. The handling of the tap changer must bedone when the transformer is out of voltage, asfollows: initially, the handle of the tap changer is pulledupwards so that the pin is released and entered intothe fixed annulus.

Then we turn the handle right or left so that the pin isplaced to the desirable tap position. If it is desirable toswitch 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 ofthe transformer. For example, when the transformer is designed tooperate in two voltage levels, e.g. 20 kV and 15 kV,then using a 5-position tap changer, the regulation ofthe primary voltage can be ±2x2.5 % for mediumvoltage 20 kV (i.e. voltages 19.0, 19.5, 20.0, 20.5, and21.0 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 forthe change of the transformer operating voltage fromone voltage level to another (e.g. from 15 kV to 20 kVand vice-versa) in proportion with the voltage of thenetwork that the transformer is connected. Thehandling of the voltage selector is the same with the

handling of the tap changer, the only difference is thatthe annulus has two positions (e.g. 15 kV or 20 kV). For example, if we want a 20-15/.4 kV transformer tooperate with primary voltage 19.5 kV, we set thevoltage selector at the 20 kV position and the tapchanger at the -2.5% position.

A.4.11 Voltage selector

The thermocouple of the thermometer is set at thehigher oil layer, in order to measure the maximum oiltemperature. The electrical contacts of thethermometer are regulated to the desirable

temperatures and are connected to the protectioncircuit for alarm and tripping of the circuit, when thecorresponding temperature limits are exceeded.

A.4.12 Transformer thermometer


During the transformer oil temperature variation, andconsequently the oil volume variation, the oilconservator undergoes this oil volume fluctuation. Theoil 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 equippedwith an air dehumidifier and a Buchholz relay.

A.4.13 Oil conservator

The protection of the oil-immersed transformers frominternal faults, which cause the development of gasesor strong oil leakage, is implemented with Buchholzrelay, which is installed between the transformer tankand the oil conservator. In case of gases creation (as aresult of internal fault) or lack of oil, the first float ismoved downwards and the alarm contact is activated.

If the gases are sufficient (i.e. the internal fault issignificant), then the second float is moveddownwards and the trip contact is activated. The tripcontact is also activated in case of strong oil flux to theoil conservator after short-circuit or internal fault.Moreover, the Buchholz relay provides protectionfrom 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 andfrom the oil conservator. The air dehumidifier containsSiO2 crystals (silica gel), which absorb the airmoisture.

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 isyellow. When it is full of moisture and it changes itscolor and becomes soft blue-white, it must be dried or it must be replaced. Drying is achieved by heating itat temperatures between 120ºC and 150ºC until itscolor 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 oilconservator), the oil level indicator is placed on thetank side or on the transformer cover. For the

transformers with oil conservator, an oil level indicatorof tube type (glass transparent tube) or magnetic typeis placed on the oil conservator.

A.4.17 Oil level indicator


According to the international standards, all thetransformer 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 ofmanufacturing, core and windings weight, oil weight,total weight, maximum ambient temperature, winding

temperature rise, oil temperature rise, rated voltage ofthe primary winding, rated voltage of the secondarywinding, rated current of the primary winding, ratedcurrent of the secondary winding, no-load losses, loadlosses, positions of tap changer and positions ofvoltage 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 directionwith 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 withcontact for the trip of the circuit.

A.4.20 Accessories of sealed type transformers

Oil conservator

Buchholz relay

High voltagebushings

Tap changer

Low voltagebushings

Lifting lug

Thermometer

Oil levelindicator

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 76, as follows:

Type tests.

Routine tests.

Special tests.

(a) Temperature rise testThe procedure of the temperature rise test isperformed according to IEC 76-2. With this specifictest, the following tasks are implemented: a) the determination of the temperature rise of the oil,and b) the determination of the average temperature riseof the windings.

(b) Lighting impulse testThe procedure of the lightning impulse test isperformed according to IEC 76-3. With this specifictest, 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 resistanceThe procedure of the measurement of windingsresistance is performed according to IEC 76-1. Duringthis test the resistance of each winding is measuredand the temperature is recorded. The test is performedwith DC (direct current). The measurement of theresistance of the windings is performed using aresistance bridge.

(b) Measurement of the voltage ratio andcheck of phase displacementThe measurement of the voltage ratio is performedaccording to IEC 76-1. The objective of the specific test is to compare themeasured values of the transformer ratio with therespective guaranteed values. For the transformer, the turns ratio is equal to thevoltage ratio of primary and secondary winding,namely:

(c) Measurement of short-circuit impedanceThe measurement of short-circuit impedance isperformed according to IEC 76-1. The short-circuitimpedance, which is expressed as a percentage of therated voltage, represents the transformer’simpedance. The international standards require the short-circuitimpedance to be calculated at the referencetemperature of 75ºC.

(d) Measurement of load lossesThe measurement of load losses is implemented withthe secondary winding short-circuited and byincreasing the voltage of the primary winding till thecurrent of the primary winding reaches its nominalvalue. The load losses are calculated at the referencetemperature of 75ºC.

(e) Measurement of no load current and no-load lossesThe measurement is performed according to IEC 76-1.The no load current represents the real value of currentthat is required to magnetize the magnetic core. The no-load losses represent the active power that isabsorbed by the transformer core when it is appliedrated 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


(f) Dielectric routine tests

The dielectric routine tests are the following:

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 thetank, and c) the insulation between the tested windings and themagnetic circuit.

The procedure of the measurement is as follows.

(a) MV windingsThe LV windings are short-circuited and grounded withthe transformer tank. Then, single-phase voltage isapplied to the MV windings, this voltage is determinedby the voltage of the MV system, in which thetransformer is going to be connected.

(b) LV windingsThe MV windings are short-circuited and groundedwith the transformer tank. Then, single-phase voltageis applied to the LV windings, this voltage isdetermined by the voltage of the LV system, in whichthe transformer is going to be connected.

Induced voltage dielectric test

Three-phase voltage, twice the rated voltage, isinduced to the transformer for 1 min. However, due tothe doubling of the voltage, the magnetic induction isalso doubled, resulting in transformer saturation and,consequently, there is a danger for the transformer tobe destroyed. In order to avoid saturation, thefrequency is doubled, so that the magnetic inductionremains constant. Finally, during this test, the volts perturn and therefore the volts per layer are doubled.

With this test, the dielectric strength between turnsand layers is verified.

(a) Dielectric special tests

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

(c) Short-circuit withstand testAccording to this test, the transformer is subjected tosuccessively short-circuits of 0.5 sec duration and thetransformer must withstand these short-circuits. Sincethis test requires high power, it is executed in specialtest centers. For example, in Greece, the TestsResearch and Standards Center of Public PowerCorporation executes this test.

(d) Determination of sound levelsThe transformer is energized at no-load and at ratedvoltage and rated frequency, so the noise peripherallyto the transformer can be measured. The test isperformed 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 agreementbetween customer and manufacturer. The special tests are the following:

(e) Measurement of the harmonics of theno-load current

( f) Measurement of insulation resistanceand/or measurement of dissipation factor(tan‰) of the insulation system capacitances

(g) Radio interference voltage

(h) Measurement of zero-sequenceimpedance

A.5.3 Special tests

A.5.2 Routine tests (continue)


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 thetransformer 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 typicallycalculated 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 differentambient temperature conditions can be produced.

A.6.3 Ambient Temperature

The short-circuit impedance is the percentage of theprimary rated voltage that has to be applied at thetransformer primary winding, when the secondarywinding is short-circuited, in order to have the ratedcurrent 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, thefollowing are determined: the voltage drop due to thetransformer loading, the distribution of loads in case oftransformers parallel operation, and the short-circuitcurrent.

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 beinstalled 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 corelaminations, 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 toCENELEC HD 428.1 S1/1992.

Table 1: Lists of no-load losses according to CENELEC HD 428.1 S1/1992.

A.6.6 No-load losses

The load losses include losses dueto load currents and eddy currentlosses in conductors due to leakagefields.Table 2 presents the 3 lists (A, B, C)of load losses for transformers from50 to 2500 kVA, according toCENELEC HD 428.1 S1/1992.For example, a transformer has acombination of losses of A-C’, if itsload losses belong to list A, and itsno-load losses belong to list C’.

A.6.7 Load losses

The rated primary voltage (input voltage) is thevoltage at which the transformer is designed tooperate. The rated primary voltage determines thebasic insulation level (BIL) of the transformer,according to international standards (IEC 76). The BILis a basic transformer characteristic, since it indicatesthe transformer ability to withstand the overvoltages

that can appear in the network. The calculation of thewinding insulation is based on the basic insulationlevel.The rated secondary voltage (output voltage) is thevoltage at the terminals of the secondary winding atno-load, under rated primary voltage and ratedfrequency.

A.6.8 Rated voltage

The vector group determines the phase displacementbetween the primary and the secondary winding.The three primary or secondary windings can beconnected with different ways in order to have athree-phase transformer.

These connections are the following:D (d): delta connection for high voltage (low voltage) windingY (y): star connection for high voltage (low voltage)windingZ (z): zigzag connection for high voltage (low voltage) windingN (n): the neutral exists in high voltage (low voltage) winding for connection outside thetransformer.

A.6.9 Vector group

List A’ List B’ List C’

Rated No-load Noise No-load Noise No-load Noise Short-circuitpower (kVA) losses P0 (W) Lw (dB) losses P0 (W) Lw (dB) losses P0 (W) Lw (dB) impedance (%)

50 190 55 145 50 125 47 4100 320 59 260 54 210 49 4160 460 62 375 57 300 52 4250 650 65 530 60 425 55 4400 930 68 750 63 610 58 4630 1300 70 1030 65 860 60 4630 1200 70 940 65 800 60 61000 1700 73 1400 68 1100 63 61600 2600 76 2200 71 1700 66 62500 3800 81 3200 76 2500 71 6

Table 2: Lists of load losses according to CENELEC HD 428.1 S1/1992.

List A List B List C

Rated Load Load Load Short-circuitpower (kVA) losses Pk (W) losses Pk (W) losses Pk (W) impedance (%)

50 1100 1350 875 4100 1750 2150 1475 4160 2350 3100 2000 4250 3250 4200 2750 4400 4600 6000 3850 4630 6500 8400 5400 4630 6750 8700 5600 61000 10500 13000 9500 61600 17000 20000 14000 62500 26500 32000 22000 6


The distribution transformers are very efficientmachines since their efficiency is greater than 95%.The power efficiency of any electrical machine isdefined as the ratio of the useful power output to thetotal power input. The efficiency can be defined bysimultaneously measuring the output and the inputpower. However, this measurement is expensive anddifficult, especially for large machines. Moreover, incase of high efficiency machines (e.g. transformer),higher precision can be achieved, if the efficiency isexpressed through the losses. Consequently, thetransformer efficiency is calculated by the followingformula:

n =

where S is the transformer load in VA, losses are thelosses in W and cos Ê is the power factor.The transformer efficiency is increased with thedecrease of transformer losses.The transformer losses are divided into no-load lossesand load losses. The no-load losses are constant, whilethe load losses are proportional to the transformerload. Consequently, the efficiency of the transformeris calculated by the following formula:

n =

where NLL are the no-load losses, LL are the loadlosses 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 theprimary 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 theasymmetrical and the symmetrical short-circuitcurrent. The amplitude of the first peak of theasymmetrical short-circuit current is equal to Î√2 timesthe 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 ofthe transformer and Ur is the voltage drop in theresistance components of the transformer.

Table 3 presents the values of the factor Î√2 versusthe ratio Ux/Ur.

The symmetrical short-circuit current, IK , is expressedas a function of the rated current In. If the secondarywinding is short-circuited and the nominal current isapplied at the primary winding, the following equationholds:

where UK is the short-circuit impedance.

The asymmetrical short-circuit current stressesmechanically the transformer, while the symmetricalshort-circuit current stresses thermally the transformer.ELVIM transformers are designed and tested towithstand short-circuit currents according to IEC 76-5.

Ux / Ur Î√2 1 1.51

1.5 1.632 1.753 1.954 2.095 2.196 2.288 2.3810 2.4615 2.5625 2.6650 2.77

A.6.13 Short-circuit current

S cos Ê

S cos Ê + losses

S cos Ê

S cos Ê + NLL + LL(S/SB)2

IK 100

In

=UK Table 3:

Values of the factor Î√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 networkfrequency.

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 theinternational standards as well as on specificcustomer needs. From time to time, some of the

Number Standard Description1 IEC 76-1 Power transformers: general

2 IEC 76-2 Power transformers: temperature rise

3 IEC 76-3 Power transformers: insulation levels and dielectric tests

4 IEC 76-5 Power transformers: ability to withstand short circuit

5 IEC 137 Bushings for alternating voltages above 1000 V

6 IEC 354 Loading guide for oil-immersed power transformers

7 IEC 726 Dry-type power transformers

8 IEC 905 Loading guide for dry-type power transformers

Table 4: Transformer standards according to IEC.

standards may be modified and in that case they arerepublished.A list of transformer standards, according to IEC, is shown in Table 4.

The above standards are related with the electricalcharacteristics and the accessories of transformers.The IEC 76 standard describes the electricalcharacteristics and the transformer tests that arerelated with the dynamic, thermal and electrical

withstand of transformers. The DIN standard definesthe transformer accessories and the CENELECstandard defines the lists of transformer losses andshort-circuit impedance.

A.8 Tolerances

Constructional reasons result in deviations betweenthe measured parameters and the values that aredefined by the specification of the transformer user

πtem 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 tapping

Short-circuit impedance ±10% of the guaranteed short-circuit impedance

No-load losses +15% of the guaranteed no-load losses

Load losses +15% of the guaranteed load losses

Total 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 76-1.

(i.e. the guaranteed values).Table 5 presents the tolerances that are applied tocertain items, according to IEC 76-1.


A.9 Transformer Operation

The rated overloading of transformer depends on thetransformer’s previous load or the corresponding oiltemperature at the beginning of the overloading. Examples of the permissible duration and therespective levels of the acceptable overloadings areshown in Table 6.

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.

For example, if the transformer is loaded with 50% ofits rated power continuously, then the transformer canbe overloaded to150% of its rated power for 15minutes or to 120% of its rated power for 90 minutes.

Moreover, it should be noted that the oil temperatureis not a safe measure for the winding temperature,since the time constant of the oil is 2 to 4 hours, whilethe time constant of the winding is 2 to 6 minutes.

Therefore, the determination of the permissibleduration of the overloading must be done verycarefully, since there is a danger for the windingtemperature to exceed the critical temperature of105ºC, without this being visible by the oiltemperature.


The parallel operation of two or more transformers isfeasible, 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 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).

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.

Their vector groups should be the same and theconnection should be implemented with thecorresponding terminals U-u, V-v, W-w. In otherwords, the transformers must have the sameinherent phase angle difference between primaryand secondary terminals, the same polarity and thesame phase sequence. It should be noted that, incase that the vector groups are not the same, theparallel operation of transformers of groups 5 and11 is permitted, if the connection is implementedaccording to Table 7.

If the parallel operated transformers have the samevoltage ratio but different short-circuit impedance,then the load is distributed among them in such a waythat each transformer accepts a specific level of loadfor which the short-circuit impedance becomes thesame for all the parallel operated transformers.

When none of the parallel operated transformers ispermitted to be overloaded, the transformer with theminimum short-circuit impedance must operatemaximum under its rated power.

Consequently, the load distribution is given by thefollowing equation:

Pi = Pni ,

A.9.3 Load distribution of transformers in parallel operation

where Pi is the load that is distributed to the itransformer, Pni is the rated power of the i transformerUKi is the rated short-circuit impedance of the i

transformer and UK min is the minimum rated short-circuit impedance of the n parallel operatedtransformers.

Finally, the total power of the n parallel operatedtransformers is:

(Pi ) < Pi .

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

UK min

UKi

n

™i=1

n

™i=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 detailedspecification. 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 evaluationmethod of the transformers by the electrical utilities and industrial users is presented below.

The electrical utilities evaluate the transformers basedon 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 theno-load losses, B (;/W) is the load losses factor, andLL (W) are the load losses.

A.11.1 Electrical Utilities

Among alternative transformer offers, the economical optimum is the one with the minimumtotal owning cost. The values of the parameters BP, NLL, and LL aredetermined by the transformer manufacturer. The values of the parameters A and B are determinedby the electrical utilities.

The procurement of transformers by the industrial users is based mainly on the transformer sales price andsecondary on the transformer losses. An arithmetic example for the determination of the most economicaltransformer 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 ELVIMtransformers’ 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 cosÊ =1 3.04 3.23 3.26 3.28 2.94 2.71 2.74

at full load (%) cosÊ = 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 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

Load 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

General DescriptionSingle-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 Equipment3-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 ourtransformer 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

BC

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 cosÊ=1 1.94 1.78 1.73 1.65 1.55 1.52 1.48 1.41

at full load (%) cosÊ=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 cosÊ=1 98.02 98.21 98.27 98.36 98.47 98.50 98.54 98.62

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

Load 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

General DescriptionThree-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 Equipment5-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 ourtransformer 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 (%) cosÊ=0.8 3.33 3.25 3.22 3.17 4.49 4.47 4.48 4.48


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

Efficiency (%) 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

General DescriptionThree-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 Equipment5-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 1600 kVA, 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



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 (%) cosÊ=0.8 4.82 4.81 4.76 4.74 4.66 4.62 4.49 4.49


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

Efficiency (%) 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

General DescriptionThree-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 EquipmentVoltage 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


D

C

DB

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 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

at full load (%) 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

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

Load 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

Efficiency (%) 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

General DescriptionThree-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 Equipmemt5-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 designdepartment.

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 load

For cos Ê = 1 , sin Ê=0.

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

Case 2: load 75%

For cos Ê = 1, the voltage drop is calculated as follows:

For cos Ê = 0.8, the voltage drop is:

Case 1: full loadThe efficiency at full load and for power factor equalto 1.0 (cos Ê=1.0) is calculated as follows:

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

Case 2: load 75%

The efficiency at load 75% and cos Ê=1.0 is:

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

Ë1 = = S cos ÊS cos Ê + 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-circuitimpedance. 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 Ê + ex sin Ê) + ( )2 (er sin Ê - ex cos Ê)2 , whereSSB

SSB

12

1100

Udrop = (er cos Ê + ex sin Ê) + SSB

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

+ ( )2 (er sin Ê - ex cos Ê)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 = U2

k - 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 %Ë2 = 630000*0.8630000*0.8 + 1200 + 9300*(1)2

= 98.66 %Ë3 = 472500*1.0472500*1.0 + 1200 + 9300*(0.75)2

= 98.33 %Ë4 = 472500*0.8472500*0.8 + 1200 + 9300*(0.75)2


Among the three transformers, the third transformerhas 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 totalload (1460 kVA) represents the 90.4% of the totalinstalled power (1615 kVA).It should be noted that, in order the maximum totalload to be equal to the total installed power, thetransformers must have the same short-circuitimpedance.

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 500kVA 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 twodifferent offers to the industrial user. The first manufacturer offers a transformer with 1200 W no-load losses and8700 W load losses at a sales price of 5870 :. The second manufacturer offers a transformer with 940 W no-loadlosses and 6750 W load losses at a sales price of 7045 :. Considering that the depreciation of the transformerpurchase investment is going to be done in 5 years and the energy charge is 0.075 :/kWh, calculate theeconomical optimum offer.

The comparison of the two offers will be based on the annual total owning cost, which is the sum of the annualbuying cost and the annual usage cost. An approximation of the annual buying cost can be found by dividing thesales price with the years of depreciation. An approximation of the annual usage cost can be calculated based onthe 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 annualtotal owning cost is 0.9% less (i.e. 20.06 : less). From the above, it is concluded that the cheapesttransformer is not always the most economical. In particular, the difference at the annual total owning costcould be more than 0.9%. This will happen, if we consider more years for the depreciation (instead of thecurrent 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, min

Uk,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, min

Uk,2

44.8

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

Uk,3

44


For the calculation of the dimensions of theopenings for the input and output of air in theelectrical room, the calculation of the airresistance is required.For the air resistance, the symbol W is used inthe sequel. The value of the air resistancedepends on the existence or not of lattices,meshes and venetian blinds.

If there are no lattices, meshes and venetianblinds in the input and output openings of theair, then the minimum air resistance is : Wmin = 4.

For each lattice, the value WL=1 is added to thevalue of Wmin .

For each mesh, the value WM =1.5 is added tothe 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 withtwo meshes (one in the input and one in the output ofair), the minimum air resistance is:

W = Wmin + 2 WM = 4 + 2 x 1.5 = 7.

The lowest possible temperature in the transformerelectrical room is achieved with the following ways:

the opening for the output of the hot air is placed inthe highest possible location, and

the opening for the input of the cold air is placed inthe lowest possible location.

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

F1 = . V . ,

where V is the total transformer losses (kW), W is theair resistance, H is the height (m) of the opening for theoutput of air from the horizontal symmetry axis oftransformer (Figure 3), and t is the temperature rise(°C) of the transformer room.The cross-section area of the opening for the output ofair, F2 (m2), should be 10% to 15% larger than thecross-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 attentionshould be paid to the calculation of the dimensions of the installation area as well as to the ventilation of theinstallation room. The ventilation of the electrical room influences the cooling, and consequently, the transformer’slife. 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 Maint

Gravel

Pit for oil

4.25100

104WH . t3


The transformer is delivered at the industrial site ofSchneider Electric at Inofyta, Greece. The responsibility for the safe transportation,unloading, and connection to the network belongs tothe transformer user. The substation must beconstructed after study and design from a certifiedengineer and the relevant authorities (e.g. electricalutility, etc) must approve the substation design.Under the transformer, there should be an oil collectiontank, which has on its upper part a metallic mesh andgravel. The oil collection tank must have theappropriate volume, so that in case of leakage all thequantity of the transformer oil can be collected withinthe oil collection tank. The whole substation has anisodynamic mesh. The resistance of the earthing mustbe less than 1ø and generally the substation must beconstructed in accordance with the existing

instructions and regulations of the local authorities(e.g. construction authority, electrical utility).The unloading and transportation of the transformershould be done in such a way that the transformerdoes not deviate by more than 15° from its horizontalposition. When the transformer is installed at itsposition, no deviation is allowed from its horizontalposition. If the transformer is equipped with aBuchholz relay, the Buchholz connection instructionsmust be followed.Before the connection of the transformer to thenetwork, the transformer must be optically checked, in order to ensure that it has no damage during itstransportation or it has no oil leakage. In case ofscratches in its painting, the transformer must berepainted 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 thesurface of insulators (dust and moisture can causeflashover).

Check for oil leakage.

Check for damage in the transformer painting. Incase of scratches, they should be repainted in orderto prevent the tank oxidation.

Check of the oil level of the oil indicator, when thetransformer is out of operation. For example, if theambient temperature is +20°C and the reading ofthe oil indicator is below the reading of +20°C, thenoil filling is required.

Check of the condition of the air dehumidifier. If thecolor of the silica gel becomes yellow, then it is ingood condition, while if it has a soft blue-whitecolor, 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 anycapacity loads.

The transformer is a very reliable electrical machine and it will practically need no maintenance during the manyyears of its operation. However, this presupposes that the transformer remains clean and it is not overloaded morethan the permissible levels of duration and loading. Moreover, it also assumes that the network that thetransformer serves is not affected by short-circuits, overvoltages, thunders, and the coupling apparatus of the highand low voltage as well as the transformer’s protective devices operate normally. In practice, it is not possible toguarantee 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 drainingand sampling oil valve of the transformer. 10 lt ofoil are initially taken out and next, a sample of 1 lt istaken. The cans, bottles and funnels that are goingto be used for sampling, must be absolutely cleanand dry.The bottle, which is going to be used for theshipment of the oil, must be hermetically sealed. Ifthe check results in an oil dielectric strength lessthan 40 kV, then the oil must be replaced or must bereprocessed with a special cleaning apparatus.

Check of the operation of the Buchholz relay, thethermometer and the condition of their contacts.

B.3 Instructions for transformer Maintenance

enance


B.4 Instructions for thermometer connection

The transformer thermometer is used to follow thevariations 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 warningand the second (trip-off contact) is used for trippingoff 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 thetransformer 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 andBuchholz relay is shown in Figure 4.

A typical wiring of thermometer and Buchholzconnected 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, andthen 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 notfull, the hexagonal cover must be removed and the ventilation valve must be opened so that the Buchholz relayto be filled with oil. The filling and the free movement of the floats with the contacts are checked through theinspection 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 operationof 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 airdehumidifier, the followingconnection instructions aresuggested:

Waterproof packing is used for the transportation of thedehumidifier, in order to avoidthe absorption of moisture bythe silica gel. During the placement of thedehumidifier to the transformer,the oil glass (which is under thedehumidifier) is removed. Then the oil glass is filled withmineral oil until the end of thetube (which goes out of thedehumidifier) to be sink in the oil.

In case of transformertransportation, the dehumidifiermust be removed, its tap mustbe sealed and a cap must be put to the tube of the oilconservator.

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 isimplemented through the following steps:

The cylindrical cover of the testing button isremoved and the button is gradually pressed sothat to get down the floats. Next, it is checked if thealarm contacts and the trip-off contacts are closed.

As soon as the testing button is left free, the floatsare moved to the normal position and the contactsopen.

The alarm contacts (3 and 4) are connected to analarm horn, which will operate when gases arecollected to the Buchholz relay or the oil level getsdown.

The trip-off contacts (1 and 2) act on the trip-offcoil of the medium voltage circuit breaker and openthe circuit breaker, when the oil level gets down orthe oil pressure in the transformer tank increasessuddenly.

Check of the operation of the protection circuits

��

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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)


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For questions please contact the Services department of Schneider Electric AE:tel.: +301 0 6295243, +301 0 6295247.

Schneider Electric Servises

Schneider Electric SA Athens: 14th km Athens-Lamia N.R. GR-145 64 KifissiaTel. + 301 0 62.95.200Fax + 301 0 62.95.210

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