AF01414

AF01414

Presentation of the France Transfo factories

France Transfo's internationally recognised and highly esteemed expertise istoday exported to more than 80 countries throughout the world.Over the past 15 years, France Transfo has produced and sold 350,000transformers or the equivalent of the entire installed power of a majorEuropean country.

Production sites

France Transfo currently has three production sites in the Lorraine region tothe North of Metz, including one of Europe's largest production facilities forcast resin transformers.The factories are located close to one another in order to benefit from thesynergy of resources and expertise right through from the project designstage to the despatch of finished goods.These purpose-built, modern production units are regularly refitted and up-dated in the constant pursuit of optimum quality and safety.

n the Maizires-Ls-Metz factory (1970)Site of France Transfo's head office.Area : 25,000 m2

Production : oil-immersed distribution transformers up to 5,000 kVA and36 kV.

n the Ennery factory (1985) Area : 15,000 m2

Production : - oil-immersed power transformers of between 5 and 60 MVA up to 123 kV- TRIHAL cast resin transformers of between 100 kVA and 15 MVA up to

36 kV.

n the Marange-Silvange factory (1980)Area : 11,000 m2

Production : tanks, heat exchangers and all metal accessories required fortransformers produced on the other 2 sites.

Location of the 3 factories

Engineering and design facilities are computer-linked between the technical,sales and production departments at each of the factories to ensure manu-facturing reliability.

The "TRIHAL cast resin transformers" activity of the Ennery factory com-prises the following main manufacturing sections :

n electrical steel sectionThe magnetic cores are cut from very wide sheets of electrical steel using aseries of slitters and core cutting machines.

n windings sectionThis section comprises a set of coil winding machines, each using varioustechnologies according to the winding type being produced.- Medium Voltage windings (MV) are wound single strand using a method

developed and patented by France Transfo : continuous linear gradientwinding without interlayer (vertical winding machine).

- Low Voltage windings (LV) are wound in bands whose turns are insulatedusing F class film.

n Medium Voltage encapsulating systemThis system is used for encapsulation using the vacuum casting technique,with a filled, fire-proof resin. This technique has been developed and paten-ted by France Transfo.

Product range

France Transfo's product range comprises :

4 oil-immersed distribution transformers for an insulation voltage level 36 kV :

- pole mounted for powers of 50 to 160 kVA- standard pad-mounted type for powers of 100 to 3,150 kVA- low-noise pad-mounted type of 250 to 1,000 kVA- standing substation type of 100 to 160 kVA

4 "TRIHAL" cast resin transformers up to powers of 15 MVA andvoltages 36 kV.

4 power transformers up to 60 MVA and 123 kV

4 special transformers : neutral point reactor coils, zero sequence genera-tors, short-circuit limiting reactors, motor start autotransformers, HV rec-tifier sets for power supply systems of electrostatic precipitators, etc.

Standards

Transformers are designed and tested in accordance with the proceduresdescribed in the international standard IEC 76 and 726, as well as in theeuropean harmonization document HD 464-S1.In the appendix are listed most of the national standards corresponding tothese specifications.

Quality system

All France Transfo transformers are manufactured in accordance with a pro-cedural quality system certified by AFAQ (the French Association for QualityAssurance) to be in conformity with international standard ISO 9001.

The international standard ISO 9001 defines the supplier's obligations to thecustomer in terms of quality, service, design and adaptation of its equipmentin line with technology.

For example, it defines the following requirements for tests; measurements,calibration procedures, etc.

This international quality assurance standard includes, amongst otherthings : internal audits on products and processes by the manufacturer, self-checks and analyses, corrective actions, the quality of suppliers, etc.

At France Transfo, quality is managed by the Quality Assurance departmentand the operators on the production lines and by the AFAQ which performsan annual audit of the factory to award or withdraw its ISO 9001 certification.

The transformer

Definition

A static electrical energy transducer intended to transform an alternatingcurrent system into one or several alternating current systems of the samefrequency, but generally of different current and voltage.

The transformer receives a primary current I1 at a primary voltage U1, andtransforms it to a secondary current I2, different from the primary current, ata secondary voltage U2.

n operating principle"A winding subject to a variable flux, generated by a variable voltage via across section of a given magnetic core, induces an electromotive forceacross its terminals proportional to the number of turns in the winding".

This electromotive force determines the voltage across the terminals of thetransformer according to the Boucherot equation :

U = 4,44 Bmax x N x S x fwith U = voltage across the terminals of the primary or secondary winding

Bmax = the maximum value of the magnetic field in the magnetic coreN = the number of turns in the primary or secondary windingS = magnetic core cross-sectional areaf = supply frequency of the transformer expressed in Hertz

n diagramatical representation of a transformerAny multi-phase circuit is a combination of single phase circuits and conse-quently, we can diagramatically represent the transformer in single phase asfollows :

N.B.: The impedance of the secondary winding can be referred to the pri-mary winding by multiplying it by the square of the transformation ratio "k".We therefore have :

The magnetic core

This comprises grain-oriented electrical steel on which are laid 2 windings.One of these has current I1 running through it, the other has current I2 run-ning through it.It is characterised by the no-load losses (Po) : these combine losses due tohysteresis, Eddy currents and Joule effects (insignificant) as well as dielec-tric losses (insignificant).The choice of the quality of the steel sheeting as well as the cutting andassembly method used determine the performance level of the magneticcore.

(supply)

(supply) (output)

(output)

The windings

The windings are characterised by :

n the transformation ratio, "k", corresponding to the ratio of the primaryand secondary voltages

U1 N1 I2k = = =

U2 N2 I1

n the connection arrangement :

The connection of the highest voltage (MV) is in upper case and that of thelowest voltage in lower case (LV). If there is a neutral terminal, we have :YN or ynZN or znThe clock-hour index allows us to express the phase displacement, as amultiple of 30, between the MV winding and the LV winding.E.g. : Dyn 11 = Delta connected MV (D) and star connected LV (y) with a neu-tral output. The clock-hour index shows a phase displacement of 30 bet-ween the MV and LV voltage vectors in an anti-clockwise direction.

n the load losses (Pcc) :Also called copper losses (even for aluminium transformers !). They compri-se Joule or ohmic losses (RI2) and Eddy current losses.These losses are expressed at the standard reference temperature in accor-dance with the IEC standard and are proportional to the square of the load.

n France Transfo windings :The LV winding is made using aluminium or copper foils which ensure zeroaxial stresses in the instance of a short circuit. Once the LV coils are posi-tioned on the magnetic core, the unit is encapsulated without casting in aclass F alkyd resin, then polymerised.This process guarantees excellent resistance to external attack in industrialatmospheres and excellent dielectric withstand.

The Medium Voltage winding is wound in aluminium or copper foils using thecontinuous linear gradient winding technique without interlayer. This tech-nique enables a very low inter-turn voltage gradient to be achieved togetherwith greater series capacitance in the coil. This favours the linearity of impul-se wave dispersion and reduced stresses between turns.This winding is then encapsulated and vacuum cast in a class F resin, whichstrengthens the dielectric characteristics and ensures a low partial dischar-ge level.

delta (D,d) star (Y,y) zigzag (Z,z)

The test

Transformers are tested to ensure that their electrical and thermal characte-ristics are in conformity with those specified on the order.

Three different types of test exist :- routine tests- type tests- special tests

This guide is intended to give you an insight into these different types oftests and into the test methods used at France Transfo for cast resin trans-formers produced at its ENNERY factory.

Test equipment at the ENNERY unit

n 2 production line-integrated test rigs for routine tests and lightning impul-se tests.

n 2 test rigs for customer-requested acceptance tests. These are used forroutine tests, type tests and certain special tests.

n 1 dedicated equipment testing laboratory used for the testing of materialsreceived and for R&D and type-approval of new products.

Lastly, each manufacturing section has its own specific testing resourcesand procedures to ensure the quality of components and sub-assemblies.

All the tests listed in the index are performed at France Transfo's Ennery fac-tory with the exception of the short circuit test, which is performed at anapproved laboratory.

The transformers are only tested again, in your presence, (routine checktests, type tests and special tests) on special request at the time of orderand at additional cost.

Safety recommendations in testing areas

Transformer test areas are clearly marked. Once within these zones, allequipment can be live, with exposed parts. There is therefore a risk thatcontact with this equipment will cause fatal electrocution.

Access to these areas is strictly forbidden to any unauthorised personfrom outside the "Testing" department.

Visitors may only enter these areas under the supervision of authorisedtesting department personnel, after having first been informed of the inhe-rent dangers in the installation, when no test sequence is taking place.

The following pages give for each test :- on the left hand page : a summary of the main standards requirements- on the right hand page : the method used by France Transfo.

Glossary

Reference Symbols Units

1) Rating plate

Rated power Sn VoltAmpere (VA) orkiloVoltAmpere (kVA) orMegaVoltAmpere (MVA)

Rated frequency Fn Hertz or kiloHertz (Hz or kHz)

High voltage MV kiloVolt (kV)

Low voltage LV Volt or kiloVolt (V or kV)

Temperature C degrees Celsius (C)

Temperature rise K Kelvin

No-load losses Po Watt or kiloWatt (W or kW)

Load losses Pcc Watt or kiloWatt (W or kW)

No-load current Io Ampere (A)

Rated current IN Ampere (A)

Short circuit current Icc Ampere (A)

Impedance voltage Ucc Volt (V)

Power factor cos percentage (%)

Magnetic field B Tesla (T)

Number of turns N

2) Routine tests

l Measurement of no-load losses

Corrected no-load losses Po Watt (W)

First wattmeter reading dW1 Watt (W)

Second wattmeter reading dW2 Watt (W)

Constant Cte

Power correction k Watt (W)

Approximation D percentage (%)

l Measurement of load losses

Currents in the 3 windings l1, l2, l3 Ampere (A)

Measured power Pmeas Watt (W)

Special losses = additional losses Ps Watt (W)

l Resistance of windings

Resistance of MV and LV RMV, RLV ohm ( W )

Joules losses RI2 Watt (W)

l Partial discharge level measure m e n t

Transformer voltage class Um KiloVolt (kV)

Apparent charge q picoCoulomb (pC)

3) Type tests

l Temperature rise tests

Total losses Pt Watt (W)

Average current Iave Ampere (A)

Ambient temperature Tamb Celsius (C)

l Lightning impulse tests

Sweep time microsecond (*s)

4) Special tests

l Noise level measurement

Background noise decibel (dB)

Weighted (A) acoustic pressure level Lp(A) decibel (A)

Weighted (A) acoustic power level Lw (A) decibel (A)

The standard

This standard is taken from IEC standards 76-3 (1980) and 726 (1982).

Definitions

- uniform insulation of a transformer winding : this is the insulation of a windingfor which all the terminals connected to a transformer's terminals, have the samepower frequency withstand voltage.

- Um voltage : this is the highest voltage of a system to which the winding can beconnected, in view of it's insulation level. In practice, it refers to the voltage of thetransformer's class of insulation.

Test description

The type of induced voltage test used, depends on the type of transformer winding.

The induced voltage test described in the standard, specifies that an alternatingvoltage with a near-sinusoidal waveform must be applied to the terminals of thetransformer.The test voltage must be twice the rated voltage and must not exceed the minimumapplied overvoltage at the power frequency (see the table below), between the lineterminals of a three-phase winding.

The test frequency must be greater than the rated frequency in order to avoid satu-ration of the magnetic core (excessive magnetising current).The test lasts 60 seconds at full voltage for all test frequencies less than or equalto twice the rated frequency, unless otherwise specified by the customer.

When the test frequency exceeds twice the rated frequency, the test durationshould be :

[120 x (rated frequency / test frequency)] seconds, with a minimum of 15 s.

Three phase windings are preferably tested with balanced voltages induced in thethree phases of the winding. If the winding has a neutral output, this may be ear-thed during the test.The induced voltage test is considered satisfactory if the test voltage does not col-lapse in any way.

system highest voltage for the power frequency transformer Um, in kV applied overvoltage in kV

1,1 33,6 107,2 20

12,0 2817,5 3824,0 5036,0 70

Induced voltage dielectrictest

Test objective

This test highlights whether any fault exists between theturns of the windings (e.g. : fault between and ` , and ) or if there is a fault between phases.

Test procedure

With the MV winding open circuited, a voltage of 2 x Urated is applied to the LV winding ; to avoid satura-tion of the transformer's magnetic core, the frequencyused for the test is 200 Hz or 150 Hz depending on thetest rig used.

The maximum test duration is :

l (120 x 50) / 200 = 30 s. for a test frequency of 200 Hz.l (120 x 50) / 150 = 40 s. for a test frequency of 150 Hz.

Test rig diagram

The transformer is considered to have passed when thereis no voltage collapse or variation in current.

The standard

This summary of the test standard is taken from the IEC publication on dielectrictestings IEC 76-3 (1988) and 726 (1982).

Definition

Applied voltage : this is the single phase voltage that is applied to one of a trans-former's windings, whilst the others are earthed.

Description of the test

The applied voltage test must be performed using a single phase alternating volta-ge with a sinusoidal waveform at an appropriate frequency equal to at least 80% ofthe rated frequency.

The test voltage must be in conformity with the table below, taking account of thespecified insulating class for the transformer, if no value has been otherwise agreedbetween the manufacturer and the buyer.

The full test voltage must be applied for 60 seconds between the winding undertest and all the terminals of the other windings, the magnetic core, the frame andthe enclosure of the transformer, these last four being connected together to earth.

The test is considered as satisfactory if the test voltage remains constant.

system highest voltage for the minimum power frequency applied transformer Um, in kV overvoltage in kV

1,1 33,6 107,2 20

12,0 2817,5 3824,0 5036,0 70

Test objective

Testing the dielectric strength of each of the transformerwindings at the power frequency (50 Hz), with regard toboth earth and to the other windings.The transformer's schematic diagram is modified as fol-lows :

Test procedure

The applied voltage test is performed using a singlephase 50 Hz power supply at a voltage specified in thestandard, according to the transformer's class of insula-tion.The voltage is applied successively to each winding for60 s. whilst all the terminals of the other windings and themetal parts of the transformer are earthed.

The voltage measurement is taken directly using a volta-ge divider or a voltage transformer.

Test rig diagram

Characteristics of the measuringapparatus

Voltage transformer : transformation ratio = 1,000

Applied voltage dielectrictest

The standard

The IEC standard 726 (1982) refers to the measurement procedure described instandard IEC 76-1 (1993)All the values measured during this test are expressed for the principal tappingunless otherwise specified by the customer, with the transformer initially at ambienttemperature.

Definitions

n no load losses : this is the active power absorbed by the transformer when therated voltage at the rated frequency is applied to the terminals of one of the win-dings with the other windings open-circuited

n no load current : this is the effective value of current required to magnetise themagnetic core.

Test description

The no load losses and current must be measured on one of the windings (with theother winding(s) open-circuited) :

- at the rated frequency and rated voltage, if the test is performed on the principaltapping,

- at the rated frequency and at a voltage equal to the appropriate tapping voltage,if the test is performed on another tapping.

Tolerances

Certain values are subject to manufacturing variations and to errors of measu-rement and cannot be determined with absolute accuracy.

This is why, in order to guarantee the performance of transformers, the"International Electrotechnical Committee" has set tolerances to be complied within its standard 76-1 9, according to the measurement carried out.

Values Tolerances

1. No load losses +15% of the declared values on conditionthat tolerances on total losses are compliedwith

2. No load current +30% of the value declared by the manu-facturer

3. Total losses +10% of the declared values(Po + Pcc)

Test objective

- characterising the no load losses and the no load cur-rent of the transformer

- checking that the characteristics are in conformity withthe current standard

In real terms, no load losses are generated by the areamarked within the dotted line below :

Test procedure

The low voltage (LV) winding is supplied with the ratedvoltage at the rated frequency, with the medium voltage(MV) winding open-circuited.A three-phase precision wattmeter is used for measu-rements and gives directly :- the applied voltage (true r.m.s.)- the 3 currents (true r.m.s.)- the average current- the no load losses

NB : the values measured on transformers with severalLV windings are obtained on the winding with thehighest voltage, in other words the one offering thebest accuracy.

Test rig diagram

Voltmeters and ammeters integrated on the test rigenable the values to which the transformer is subjectedto be measured at any point in time.

Tolerances

The application of small tolerances on the no load lossesis negotiable at the time of the request for tender.

Measurement of no loadlosses and no load current

The standard

The IEC standard 726 (1982) refers to indications given in IEC standard 76-1 (1993).

Definition

MV and LV resistances are internal transformer resistances from the MV and LVsides that generate Joule losses proportionally to the square of the current.

Test description

The resistance of each winding is measured and the temperature is recorded ; thetest is performed with direct current.The effects of self-induction must be reduced to a minimum in all resistance mea-surements.

For dry type transformers, the test starts after a rest period of at least 3 hours forthe transformer at a stabilised ambient temperature. The resistance and the tem-perature must be measured simultaneously using sensors placed in significantpositions, preferably in the windings to measure the temperature of the windings.

Measurement of the resistanceof MV and LV windings

Test objective

To measure the resistance in ohms of each of the win-dings of the transformer.These resistances are shown within the dotted lines in thefollowing diagram :

Test procedure

During this test, the ambient temperature is taken in thechannel between the MV and LV windings.

Several methods are possible :Measuring the resistance of the MV and LV windings isgenerally performed using a digital micro-ohmmeter. Thisdevice is connected between each phase : it injects acontinuous current (which varies according to the ratingfrom 0.1 to 10 A), and measures the voltage across theterminals, which enables it to calculate the required valueof resistance.

We can also use the voltamperimetric method for highresistance windings, or a Thompson double bridge forlow resistance windings.

Test rig diagram

Measurement of MV and LV resistances using the voltamperemethod

Measurement of the LV resistance using a Thompson doublebridge

with

RT = (2/3) x rfor delta connected windings

RT = 2 x rfor star connected windings.

RT being the equivalent resistance between phases and rthe resistance of one of the transformer's windings.

The calculations

R = (U / I)

For the Thompson bridge, we have :

R3 R1 = RT R2

if R1 = R4 and R2 = R5.

The standard

This measurement procedure is described in IEC standards 76-1 (1993) and 726(1982).

Definitions

n the impedance voltage : is the voltage that must be applied to the line terminalsof a winding in order to give the rated current when the terminals of the otherwindings are short-circuited. It is given in percentage of the rated voltage.

n load losses : these correspond to the absorbed active power (at the rated fre-quency and reference temperature), when the tapping's rated current flowsthrough the line terminals of one of the windings whilst the terminals of the otherwinding are short-circuited and the other windings, if there are any are open-cir-cuited. These losses are also called short-circuit losses (= Joule losses + speciallosses)

Test description

Impedance voltages and load losses must be measured with a supply current of atleast 50% of the tapping's rated current.The measured value of these losses must be multiplied by the square of the ratiobetween the rated current of the tapping and the current used for the test.

The measurement must be made quickly to avoid temperature rises introducing anysignificant errors.

For transformers with a tapped winding whose tapping range is greater than +/-5%,the impedance voltage must be measured across the principal tapping and the twoend tappings.

The reference temperature must be equal to the maximum temperature rise for thewindings, as given in the table below, increased by 20 K.

Part Insulating system Maximum temperaturetemperature (C) rise (K)

Windings(temperature rise measured using the variation in resistance)

Magnetic core, metal parts The temperature mustand adjacent materials in no instance reach

a value high enough todamage the magnetic core itself, or other parts of adjacent materials.

105 (A)120 (E)130 (B)155 (F)180 (F)

200220

607580

100125135150

Measurement of the impedancevoltage and load losses

Test objective

- Measurement of the transformer's load losses (Pcc)- Measurement of the impedance voltage (Ucc)

Equivalent circuit :

Test procedure

The Medium Voltage winding is supplied at the rated fre-quency of 50 Hz and at a voltage that gives a current asnear as possible to the rated current, whilst the LowVoltage winding is short-circuited.A three-phase precision wattmeter is used for measu-rements and gives directly :- the applied voltage (true r.m.s.)- the 3 currents (true r.m.s.)- the average current- the no load lossesThe impedance voltage is expressed as a percentage ofthe rated voltage.

Ucc (%) = (Ucc measured/Urated) x 100

Test rig diagram

The test procedure and the apparatus used are identicalto those used for measurement of no-load losses.

Calculation of load losses

- at ambient temperature (20C on average)

Pcc = PjoulesMV + PjoulesLV + Pspecial

= [(3/2) x RMVx I2] + [(3/2) x RLV x I2 ]+ Pspecialwith RMV, RLV = resistances between phases

In the case of Delta connected windings :

= 3 x r x (I/ 3)2Pjoules

= (3/2) x RT x I2 with RT = (2/3) x r

In the cas of star connected windings :

= 3 x r x I2Pjoules

= (3/2) x RT x I2 with RT = 2 x r

The special losses (or stray losses) are mainly composedof Eddy current losses.

- at the reference temperature (maximum temperaturerise +20 K)

The Joules losses (or resistive losses) vary according tothe temperature whereas the special losses are inverse-ly proportional to the temperature.

Pcc T rf = (K x Pj Tamb.) + (1/K x Ps Tamb.)

K = temperature correction constant for copper : K = (235+Trf) / (235+T amb.). for aluminium : K = (225+Trf) / (225+T amb.).

The standard (cont.)

Tolerances

Tolerances are taken from the standard, unless otherwise specified by the custo-mer.

Values Tolerances

1.b) Partial losses : + 15 % of each of the partial losses, on (No-load or load losses) condition that the tolerance on the total losses is

complied with (+10 % of the declared values)

3. Short circuit voltagea) on the principal tapping 7.5 % of the value declared by the manufactu-

rer if the impedance voltage value (Ucc) is 10 %. 10 % of the value declared by the manufactu-rer if Ucc is < 10 %

b) on the other tappings 10 % of the value declared by the manufactu-rer if Ucc is 10 %

15 % of the value declared by the manufacturer if Ucc is < 10 %

Standard values for impedance voltage

For example : we can give the values recommended by the standards applicableas a function of the reference geographic region :

n in Europe, according to harmonization document HD 538.1 S1.

Insulation level Rated power (Sn) Impedance voltage expressedin kV in kVA as a percentage of the

applied rated voltage forthe winding

7,2 and 12 100 Sn 630 4 or 6630 Sn 2500 6

17,5 and 24 160 Sn 2500 6

n for other countries, in accordance with IEC publication 76-5 (1982).

Rated power (Sn) Impedance voltage expressed as a in kVA percentage of the applied rated voltage for

the winding

Sn 630 4630 < Sn 1250 51250 < Sn 3150 6,253150 < Sn 6300 7,156300 < Sn 12 500 8,3512500 < Sn 25 000 10

The impedance voltage Ucc

The impedance voltage comprises a reactive component,Ux, and Ur, which unlike Ux is dependent on temperatu-re.

This can be expressed in terms of the rated values as fol-lows :

(Ucc%)2 = (Ux%)2 + (Ur%)2

with Ur% = (Pcc x 100) / SnUcc% = (Ucc x 100) / Un

E.g. : at ambient temperature of 20C, and at a referencetemperature of 120C, we have, always given in percen-tage :

(Ucc120C)2 = (Ux)2 + (Ur 120C)

2

(Ucc120C)2 - (Ur120C)

2 = (Ux)2

(Ucc120C)2 - (Ur 120C)

2

= (Ucc20C)2 - (Ur20C)

2

Ucc at 120 C can therefore be expressed in the followingmanner :

Ucc 2 0 C 2 Pcc 2 0 C 2 Pcc 1 2 0 C 2Ucc 1 2 0 C = 100 x ( ) ( ) + ( )Un Sn Sn

Measurement of the impedancevoltage and load losses (cont.)

The standardThe following information is taken from IEC standards 76-1 (1993) and 726 (1982).

Definitions

n phase displacement in a three phase winding : this is the phase angle betweenthe vectors representing the MV and LV voltages across the corresponding termi-nals of a pair of windings.The vectors are considered to turn counter-clockwise.The vector of the MV winding, which 1st phase is oriented at 12 o'clock on the clockface, is used as a reference, and the phase displacement between all the other win-dings is usually expressed relative to this using a clock-hour figure.

n the vector group : this is the conventional notation indicating the connectionarrangement of the MV and LV windings and their relative phase displacements,expressed as a combination of letters and clock-hour figures.

Description of measurements

The transformation ratio is measured on each transformer tapping. The vectorgroup of three phase transformers, as well as the polarity of single phase transfor-mers must be checked.

Tolerances

Values Tolerances

2. No-load transformation ratio :

for the principal tapping The lowest of the following two values :a) 0.5 % of the ratio specified by the

manufacturer

b) 10 % of the real percentage of theimpedance voltage

on other tappings Must be subject to an agreement betweenthe customer and the supplier, but the ratiomust be greater than whichever is smallestof a) or b).

Measurement of the transformationratio and verification of the vector group

Test objectives

- To check the conformity of the transformer connectionarrangement with that stated

- To check the conformity of the transformation ratio, k,on each tapping compared with the guaranteed values.

The part of the transformer studied during this test lieswithin the dotted line on the following diagram :

Test procedure

The vector group is checked and the transformer ratio ismeasured on each tapping with a voltage of 110 or 220 Vapplied on the MV side.

The measurement involves comparing the phase displa-cement of the MV voltage with the LV voltage for eachphase.This operation is performed using a Vettiner bridge, thatopposes the voltages in the phases in order to comparetheir moduli. The bridge is balanced by adjustment of thedials to give null deflection on the galvanometer. The ratiois then read off the dials. The connection is correct whenthe ratio is identical on each phase.

Example : Dyn11 connection

- the MV side of the transformer is delta connected- the LV side is star connected with a neutral terminal

Measurement of the transformation ratio

The vector group and the transformation ratio are identi-cal to those defined by the manufacturer when :

[modulus of the MV I IA BI I I IB CI I I IC AI I voltage on the same leg] = = =

I Ia nI I I Ib nI I I Ic nI I [modulus of the LV voltage on the same leg]

The calculations

Calculation of the rated transformation ratio for each tap-ping : This ratio is only valid for the same reference voltage.

Example :Simple or complex ratios of MV/LV voltages

To obtain the rated transformation ratio for Yd or Dyconnections, the values must be multiplied or divided by 3 depending on whether the voltages measured aresimple or complex.For Yy or Dd connections, the rated transformation ratiois not modified.

Usual connections

The most common connections in Europe are :

The capital letters denote to the highest voltage winding.

The standard

Most of the test procedure is described in IEC stan-dards 270 (1981) and 726 (1982).For the purposes of these standards, partialdischarges are considered as electrical dischargesoccurring in an insulating medium, generally occur-ring as individual impulses that cause the dielectricproperties to deteriorate.Partial discharges originate from sharp edges andpoints on metal surfaces and electrically stressedcavities within solid insulation.

Definitions

- Partial discharges : electrical discharges that onlypartially bridge the insulation between conductors.

- the apparent charge : expressed in pico-Coulombs (pC), this is the value chosen to charac-terise the intensity of a partial discharge.

Method of applying the test voltage

The partial discharge test voltage applied to thetransformer must be as near sinusoidal as possibleand be of greater frequency than the rated frequen-cy, in order to avoid an excessive magnetising cur-rent during the test.

This voltage depends on the standard according towhich :

1 the windings are intended to be connected tonetworks that are bolted to earth, or earthedthrough a low value impedance : in this case, a pre-load voltage between earth and line of 1.5 Um/ 3is induced for 30 seconds followed without pauseby a voltage of 1.1 Um/ 3 for 3 mn.

2 - the windings are intended to be connected toinsulated networks or those that are earthedthrough a high value impedance: in this case, a pre-load voltage between earth and line of 1.5 Um/ 3is induced for 30 seconds followed without pauseby a voltage of 1.1 Um/ 3 for 3 mn. This test isrepeated with another line terminal earthed.

Test circuit and measuringdevices

Whatever type of test circuit and measuring deviceare used, they must first be calibrated.Unless otherwise specified by the customer, thetransformer will be tested at ambient temperatureand the insulator surfaces will be clean and dry.

The standard gives a number of test circuits ofwhich France Transfo has decided to use the follo-wing :

Test circuit for self-actuated devices

This diagram represents a test circuit in which thevoltage is induced in the power transformer.The circuit comprises :l a source of alternating voltagel an impedance or Filter Z : blocking out the

discharge impulses produced by the test equip-ment and reducing disturbances from the source.

l a capacitance, Ca : corresponding to the capaci-tive impedance of the transformer

l a connecting capacitor, Ck.l a measuring circuit, connected to the terminals of

the tested transformer through a suitable capaci-tor and comprising :- the measuring impedance Zm, including a resis-

tor or a resistor and a capacitor or some othermore complex filtering device

- coaxial connecting cables- the detecting device and display unit : this sets

the pass band of the measuring circuit and dis-plays the partial discharges.

The partial discharges produced by the transformercause transfer of charge within the test circuit andcurrent impulses across the impedance Zm.This impedance, together with the transformer andthe connecting capacitor, determine the durationand shape of the measured impulses.These impulses are then smoothed and amplified toprovide the discharge detector with the transfor-mer's quadratic flow value

Measurement of partial discharge level

Test objectives

The objective of this test is to determine the overall die-lectric condition of the transformer

Test procedure

The transformer is energised with no load at a frequencyof 270 Hertz.The voltage applied to the MV varies according to the twocases provided for in the standard.

Case 1 : the line to earth test voltage must follow the fol-lowing cycle :

Case 2 : the phase to phase test voltage must follow thefollowing cycle :

Partial discharge levels are measured using capacitancesconnected between each MV phase and earth.

Test rig diagram

Case 1 :

Case 2 :

- the low pass filter lets the 270 Hz pass and blocks par-tial discharges coming from the power supply.

- the high pass filter, connected between the capacitan-ce outputs and earth lets all frequencies apart from270 Hz past, in other words all those corresponding topartial discharges.The capacitors are rated for high voltages, 50 kV, whilstthemselves being free of any partial discharge.

- the discharge detector, connected to a 3-way switchenables measurements of the peak partial dischargevalues on each phase to be taken in accordance withthe standard.

The measurement range is usually in the pass band bet-ween 40 and 400 kHz.

The standard (cont.)

Calibration

Before taking any partial discharge measurements a calibration procedure must beperformed. This is because the measured pulses are attenuated both in the win-dings and in the measurement circuit.To do this, simulated pulses are injected using a calibrated discharge generatorconnected across the transformer's terminals.

Measurement sensitivity and permissible partialdischarge levels :

Partial discharge measurements are subject to greater errors than other types ofmeasurements performed during High Voltage tests.The measurements are affected on one hand by many unquantifiable parametersand on the other by the raw background value that must not exceed 50 % of thespecified acceptable partial discharge level. However, when the intensity of thespecified partial discharges is less than or equal to 10 picoCoulombs (pC), a back-ground noise level of up to 100 % of the specified value is acceptable.The limit of the partial discharge levels measured must be agreed by the manufac-turer and buyer. However, unless otherwise specified, the value of this level is setat 20 pC for cast resin transformers.

Partial discharge levels(cont.)

Calibrating

A discharge calibrator connected across the MediumVoltage terminal and earth, sends out calibratedimpulses.When the observed value on the detector is identical tothat generated by the reference calibrator, this meansthat the signal is not being attenuated.The partial discharge value is then calculated directlyfrom the reading measurement.Otherwise, i.e. when the reading from the detector is notthe same as that sent out by the impulse calibrator, thevoltage attenuation must be measured and added toeach measurement value.

The admissible partial dischargelevel

France Transfo guarantees the partial discharge levelsat : 10 pC at 1,1 Um 10 pC at 1,375 Um if Um >1,25 Un

n Interpreting the partial discharge measurement onthe detectorPartial discharges are displayed over 2 half cycles of asinusoidal curve :

Detector screen Description

Impulses over both half cycles betweenz e ro and towards the peak voltagevalue. The average amplitude of theimpulses is the same in both half cycles

Impulses over both half cycles betweenz e ro and towards the peak voltagevalue. The average amplitude of theimpulses is greater for one of the halfcycles

Impulses symmetrical over all halfcycles relative to zero

Cavities within the insulation. Insulatedconductors in contact.C o rona type discharges on surfaceswithout conductive metal contacts

Cavities within the insulation around thee l e c t rodes. Corona type discharg e saround the electrodes. Maximum ampli-tude for the positive half cycle : voltageside discharge. Maximum amplitude forthe negative half cycle : earthing sidedischarge

Poor contact between metal parts orbetween semi-conductor screen.

Discharge origin

The standardThis test is detailed in IEC standard 726 (1982),completed by the document HD 464-S1 (1988).

Test objective

Determination of the temperature rise of the oil andthe transformer's MV and LV windings.

Test procedure

A transformer is specified according to its coolingmode (ONAN, ONAF, ODWF) which is intended tolimit its temperature rise.

Its specification and rating plate must therefore giveinformation on the power levels for which the trans-former complies with the temperature rise limits.

The standard defines the temperature rise values,characteristics of transformers that are guaranteedand tested at the specified conditions.

Normal temperature rise limits apply, unless therequest for tender and the order specify "specialoperating conditions".This is notably the case for the temperature rise testof a transformer with one winding having a tappingsrange exceeding 5 %, for which the test is to beperformed on the tapping with the highest currentvalue.The temperature rise limits therefore apply for thistapping and power, at the appropriate voltage andcurrent levels.

If the transformer is to be installed in a location thatdoes not correspond to the normal operating condi-tions (altitude < 1000 m, -25C < T ambient < 40C,sinusoidal supply voltages, low pollution environ-ment), the temperature rise limits of the transformermust be modified to take account of this.

Test description

The transformer must be equipped with its protec-tion devices.

The standard proposes several methods : that cho-sen by France Transfo is the simulated loadingmethod.

It involves carrying out two tests separately to mea-sure the total losses due to temperature rise :- the no load test : the individual temperature risesof the windings D o are measured (no load losses),at the rated voltage and up to steady state.- the short circuit test is performed immediatelyafter the no-load test : one winding has the ratedcurrent passed through it and the other is short-cir-cuited. The temperature rises D Ucc of each windingare measured (short circuit losses) until steady stateis reached.

The overall temperature rise of each winding is thencalculated D using the formula :

D = D cc . [I + ( D o/D cc)1,25] 0,8

Temperature rise corrections inthe case of reduced current

If 90 % In test current It < In, the temperature risesD t of the windings are measured using the resis-tance variation method when steady state is rea-ched ; they are then corrected to obtain the tempe-rature rises D n at the rated load conditions usingthe formula :

D n = D t [In/It]q

with q=1.6 for AN transformer (natural air cooled transformers)

with q=1.8 for AF transformer (forced air cooled transformers)

Temperature rise test

Test procedure

Temperature rise tests are performed with the transfor-mer positioned on its main tapping link. 3 major stagescan be distinguished in chronological order :

a) taking of reference measurementsb) no-load testc) short circuit test

At least 24 hours before the reference measurements aremade, the transformer is placed in a suitable room inorder to attain a uniform temperature throughout its win-dings.

a) Taking of reference values:The values of the MV and LV resistances are taken toge-ther with the initial temperature of the windings, with thetransformer not live and at ambient temperature.The measured values are used as references and deter-mine the calculation of the winding temperature rises:

n Measurement of the transformer temperature :A thermocouple probe is used which is placed betweenthe MV and LV windings to deduce an average tempera-ture.

n Measurement of the initial MV resistance betweenphases to determine the cooling curve :This measurement is performed using the voltamperime-tric method.

n Measurement of the initial LV resistance betweenphases to determine the cooling curve :Since resistance values are of the order of a milliohm,

this measurement is performed using the Thompsondouble bridge method.

n Measurement of the initial MV resistance betweenphases in the case of superposition :A "direct current superposition" arrangement is used.Once the current has stabilised, the current and voltageare measured, to determine the initial MV resistance bet-ween phases.

b) No-load test :The transformer's LV windings are supplied at the trans-former's rated voltage.The ambient temperature is measured using the readingsfrom 3 thermometers situated 1 metre distant at half thetransformer's height. They are immersed in pots of oil toavoid any fluctuations due to drafts affecting the measu-rements.The thermocouples measuring the transformer's tempe-rature rise are placed at the top of the magnetic core andon the LV winding, in accordance with the standard.

These measurements are displayed in real time on a plot-ter showing the temperature rise over time.

Once steady state has been reached (constant tempera-ture rise), the circuit is broken by cutting off the transfor-mer's power supply. The transformer then cools from thismoment on : the resistances of the MV and LV windingsare measured every 15 seconds using the voltamperime-tric method for the MV winding and the Thompsondouble bridge method for the LV winding.

The resistance measurements at regular time intervalsare used to produce the cooling curve : this is the resis-tance variation method.

N.B.: in the case of a no-load test, the thermal inertia ofthe magnetic circuit is high and generally the value of theresistance varies little : the average value of resistancecan therefore be taken.

The standard (cont.)

Determining conditions for thermal

The final temperature rise is the point at which the temperature rise becomesconstant, in other words at which it varies by no more than 2% of the permissibletemperature rise or 2 K (whichever of these 2 values is smallest).

For this we use thermocouples or thermometers installed at the centre of the uppercasing, as near as possible to the inner conductors of the LV winding, at its sum-mit. The internal measurement is made on the central column.

Temperature rise test (cont.)

By extrapolation, the value of the MV and LV windings'resistances (RMV and RLV) are determined at the momentof switching power off (at t = 0) ; we can then deduce thetemperature rise D T for each of the MV and LV windingsusing the formula :

D T = [(Ro Rr f) / Rr f] x [(1/ ) + r f] + r f o

Ro = winding resistance at t = 0

o = ambient temperature at t = 0Rref = initial winding resistance

ref = initial ambient temperature

1/ = temperature coefficient

c) Short circuit test :This test is performed immediately after the no-load test.The LV winding is short circuited and the rated current Inis injected to the MV winding.

Here again we wait until the transformer's temperaturerise reaches steady state. Once reached, we take off theshort circuit to the LV winding and the resistance variationmethod is applied with a Thompson double bridgemethod. This will enable us to estimate the LV windingresistance in short circuit at t=0.

For the MV winding, the direct current superpositionmethod is generally best suited. It enables the change inresistance to be monitored over time in short circuit untilsteady state is reached at which time the required mea-surement is obtained.

Once this stage is finished, we use the above mentionedformula to calculate the temperature rise D T of the win-dings in short circuit.

All that remains is then to calculate the global temperatu-re rise for each LV and MV winding using the formula :

D = D cc . [1 + ( D o/D cc) 1,25] 0,8

D = global temperature rise for the windingD o = no-load temperature rise of the windingD cc = short circuit temperature rise of the winding

Layout diagram

- resistance variation method : same layout at that for thevoltamperimetric method cf. p19.

- direct current superposition method.

The standardThe test methods and tolerances that follow aretaken from IEC standard 726 (1982) and 76-3 (1980and 1987).

Test objective

The full or chopped-wave lightning impulse test,applied to a winding's line terminals aims to checkthe impulse withstand at each line end relative toearth and to the other windings as well as that alongthe tested winding.

Description of the full-wavelightning impulse test

n GeneralLightning impulse tests must be performed with thetransformer approximately at ambient temperature.The impulse test voltage is normally of negativepolarity and is applied to the transformer insulation(see table of values on p36).By agreement between the manufacturer and thebuyer, drawn up at the time of offering, the tests canbe performed using positive polarity ; this will enableabrupt changes of polarity to be avoided.The shape of the applied wave has two features : itsfront and tail :Front (voltage rising to peak) : 1.2 s, with a toleran-ce of 30 %Tail (voltage dropping by 1/2) : 50 s, with a toleran-ce of 20 %

However, in the case of this shape of impulse notbeing reasonable to attain, due to the low inductan-ce of the windings or a high capacitance to earth,larger tolerances may be allowed subject to agree-ment between the manufacturer and the customer.

n Test sequenceThe standard sets out the test sequence composedof a calibration impulse of between 50 and 75 % ofthe full voltage followed by 3 impulses at full volta-ge.This test impulse sequence is successively appliedacross the line terminals of the tested winding.For three phase transformers, the frame and theother line terminals of the winding must be connec-ted to earth either directly or indirectly via a lowimpedance, e.g. a current measuring shunt.If the winding has a neutral output, then it must beconnected to earth or to a low impedance and theframe connected to earth.

n Recording measurementsThe oscillographic data recorded during the testsmust clearly show the shape of the applied impulsevoltage (time taken for voltage to rise to peak, timetaken to drop by half), the scanning time and theselected attenuator.The recording must include an oscillogram of thecurrent in the winding to earth and be as sensitive aspossible to enable any faults to be detected.

n Test criteriaIf there is no noticeable difference in the voltage andcurrent curve recordings of the reduced and full vol-tage tests, then it is considered that the insulationhas withstood the test without any damage.However, should there be any doubt concerning theinterpretation of any differences between the oscil-lograms, three full voltage impulses must again beapplied, or the whole impulse test procedure repea-ted.For dry type transformers, the lightning impulse testcan give rise to capacitive partial discharges in airwhich present no hazard to the insulation. Thesepartial discharges cause variations in the currentwaveforms, whilst voltage waveforms vary very littleif at all. These slight variations in current waveformsare not a reason for rejection.

Lightning impulse test (B.I.L)

Test objective

Checking the lightning impulse withstand of each win-ding relative to earth, relative to the other windings andalong the tested winding.This test may be diagramatically represented by :

N.B. : At high frequency, the test voltage is spread expo-nentially along the length of the tested winding ;however, at low frequency (e.g. 50 to 60 Hz, etc.)the voltage is spread evenly along the winding.

Description of the full-wavelightning impulse test

The impulse wave is applied to each of the MV windingswhilst short circuiting the others to earth through a shunt,with the LV and the frame being earthed.

In other words, any terminal to which the impulse is notapplied is earthed either directly or indirectly through ameasuring shunt.Impulse waves of negative polarity are applied (in order toreduce the risk of a random external breakdown in thetest circuit), that are characterised by the standard 1.2 /50 wave.

Initially, the transformer is subjected to a calibratingimpulse at 50% of the full impulse voltage.Each winding tested is then subjected to 3 calibratingimpulses followed by 3 more impulses at full impulse vol-tage magnitude.

Standard sphere gaps or calibrated voltage divider areused to calibrate the impulse voltage to be applied.

During each test, two values are simultaneously recordedusing an oscilloscope, at the appropriate sweep speeds :- the applied voltage- the primary current resulting from the propagation of theimpulse wave along the windings.

For practical reasons, a set of attenuators are used toadjust the oscillograph peak deflection at full voltage andat the calibrating voltage in order to make the obtainedtraces easy to compare.

The impulse test report includes a print out of the oscillo-grams recorded during the test.The traces are examined to determine whether the trans-former has passed or failed the test.In the instance of a fault, this may be seen as an increa-se in the magnitude of current and / or a deformation inthe voltage caused by : either an earthing fault or short-circuit between turns.

The standard (cont.)

Description of the chopped-wave impulse withstand test

This chopped-wave lightning impulse test is a specific test applied to the line ter-minals of a winding.It is recommended to combine this test, when required, with the full-wave lightningimpulse test, in the following sequence :- one full-wave impulse at reduced voltage ;- one full-wave impulse at 100 % ;- one chopped-wave impulse at reduced voltage ;- 2 chopped-wave impulses at 100 % ;- 2 full-wave impulses at 100 %.

The peak value of the chopped-wave impulse defined by the standard, for eachtype of MV transformer voltage, must be the same as for the full-wave impulse test.

In practice, the manufacturers apply the same rules concerning the impulse gene-rator and measurement devices as for the full-wave lightening impulse test, simplyadding a spark-gap chopping device, and adapting the current attentuators as app-ropriate.The voltage must be chopped at between 2 and 6 s.

As for the full-wave lightning impulse test, the detection of faults during the chop-ped wave impulse test is performed by comparing the oscillograms taken beforeand after the chopped impulse wave.

Impulse voltage values defined in the standards

Rated insulation level in kV 3,6 7,2 12 17,5 24 36

rated voltage at the power 10 20 28 38 50 70frequency in kV at 50 Hz, 1 mn

impulse voltage in kV 4 0 6 0 7 5 9 5 1 2 5 1 7 0(list 2 of table 1 in IEC standard 76-3-1 (1987)

Lightning impulse test(cont.)

Description of the chopped-wavelightning impulse test

With the exception of the measurement that is performedwith a spark-gap, the test method is the same as for thefull wave impulse test.

Test rig layout

- the full-wave impulse wave

- the chopped-wave impulse wave

Impulse voltage values

rated insulation level in kV 3,6 7,2 12 17,5 24 36

rated voltage at the power 10 20 28 38 50 70frequency in kV, at 50 Hz, 1 mn

impulse voltage in kV 40 60 75 95 125 170

The standard

The test procedure is described in IEC standards 726 (1982) and 551 (1987). Whenit is planned to operate transformers in an enclosure supplied by the customer, themagnetic core and winding noise measurements can be performed in the manu-facturers premises without the enclosure.

Definitions

- the sound pressure level, Lp : this is the value expressed in decibels (dB), equalto 20 times the decimal logarithm of the ratio of the given acoustic pressure andthe re f e rence acoustic pre s s u re ; the re f e rence acoustic pre s s u re being 20 Pascals.

- the sound power level, Lw : this value is expressed in dB, equal to 10 times thedecimal logarithm of the ratio of the given acoustic power and the referenceacoustic power ; the reference acoustic power being 1 picoWatt.

- the ambient noise level : this is the weighted level (A) of the acoustic pressurewhen the transformer is de-energised.

Measuring devices

Measurements must be performed using a class 1 sound level meter. In addition,the background noise level must be measured immediately before and after themeasurement is performed on the transformer.

If the difference between the Lp of the background noise and the sum of the levelsresulting from the background noise and the transformer is 10 dB, the backgroundnoise levels may only be measured from one measurement position without it beingn e c e s s a ry to apply any correction to the noise level measured for the device.

If the difference is between 3 and 10 dB, the corrections in the table below mustbe applied.

Moreover, if the difference is less than 3 dB the test will not be accepted unless thelevel resulting from the background noise and the noise of the transformer are lessthan the guaranteed values.Should this be the case, a lower value will be taken for this difference and a totallevel reduced by 3 dB could be considered as the upper limit of the acoustic pres-sure level in this position.This condition must be included on the official test report.

Correction for the effect of background noise

Depending on the above mentioned acoustic pressure levels that are taken fromeach of the measurement positions, a correction may be performed for the effectof background noise, in accordance with the following table :

Difference between the measured Lp Correction to be subtracted from theof the device in service and the Lp measured Lp of the device in service to of the background noise alone in dB obtain the Lp due to the device in dB

3 34 5 26 8 19 10 0,5

Measurement of sound level

Test objective

Comparing the noise generated by the transformer withthat set in the standard.

Test procedure

The noise is caused by magnetostriction of the core, thereactors and their associated cooling devices.

Once the background noise has been measured, poweris supplied to the transformer under no load, at the ratedvoltage and frequency, with the tapping selector on theprincipal tapping.The sound pressure level is then measured at variouspoints around the transformer.

The noise level can be expressed in two ways :

l in terms of the (A) weighted sound pressure level Lpmeasured using a sound level meter at a defined dis-tance from the transformer.The value obtained is the quadratic average of themeasured values :

- at 1/3 and 2/3 of the active part (or of the enclosure ifthere is one) when this is greater than 2.5 m : otherwi-se, the measurement is performed at half the equipmentheight.

- at 0.3 m from the main surface for transformers withoutcasing, unless for reasons of security, it is not increasedto 1m. When equipped with a casing, the measurementdistance is 0.3 m from the latter.

N.B. : At France Transfo, this test is performed systema-tically 1 m from the main surface for transformers withoutenclosure. Measuring points should be spaced at an interval of atmost 1 meter and at least such that the test has a mini-mum of 6 measuring points.

Measuring without enclosure

Measuring with enclosure

l in terms of the device's (A) weighted sound powerlevel, Lw, calculated from the sound pressure using thefollowing equation :

Lw (A) = Lp (A) + 10 log S - X

Lw (A) = the weighted acoustic power level in dB (A)Lp (A) = the sound pressure in dB (A)X = correction for background noise (see previous page)S = the equivalent surface area in m2, defined by the equa-

tion : S = 1.25 x H x Pwith H = the transformer height in metres

P = the length of the measuring perimeter at a dis-tance of max. 30 cm, in meters

1.25 = empirical factor designed to take account ofthe acoustic energy radiated by the top of thetransformer or the coolers.

The sound power takes account of the geometry of thetransformer and enables a noise level to be expressedindependently of the distance from the transformer atwhich it is measured ; it is therefore possible to compareit between transformers.

The standard

This test is summarised in IEC standard 76-5 (1982and amendment 2 dated 1994). It specifies thattransformers must be designed and built to withs-tand the thermal and mechanical consequences ofexternal short-circuiting without suffering any dama-ge.

General

The transformer must have previously been subjectto the routine tests and accessories need notnecessarily be mounted for this test.If the windings are fitted with tappings, the reactan-ce, X and the resistance, R must be measured forpositions corresponding to the tappings that will besubjected to the short-circuit test.At the start of the short-circuit tests, the averagewinding temperature must be between 0 and 40C.

n Short circuit current :The short-circuit current is characterised by :- an initial asymmetric value equal to k. 2 x Icc sym-

metric where k. 2 is dependent on the ratio of thereactance and the resistances X/R (see the tablebelow)This peak current, lasting for a duration that is lessthan one cycle, is the source of electromagneticforces generated in the transformer.

- a symmetric value of short circuit current, that willmainly cause thermal effects in the transformer.This current is dependent among other things onthe short-circuit voltage.

(e.g. : Icc symmetric for Ucc 4 % = 25 x In).

The symmetric short circuit current is calculatedtaking account of the short circuit impedance andthe network impedance, for transformer with a ratedpower of > 3150 kVA, and also for those 3150 kVA,if the network impedance is greater than 5 % of theshort circuit impedance (otherwise, the networkimpedance is ignored).

The accepted tolerance between the specifiedvalues and the measured values is 10 % for thesymmetric short-circuit current, and 5 % for thepeak current value.

Values of the factor k. 2

X / R 1 1 , 5 2 3 4 5 6 8 1 0 1 4

k . 2 1 . 5 1 1 . 6 4 1 . 7 6 1 . 9 5 2 . 0 9 2 . 1 9 2 . 2 7 2 . 3 8 2 . 4 6 2 . 5 5

n apparent power :The apparent short circuit power of the networkwhere the transformer is installed, may be specifiedby the buyer in his request for tender, in order togive the value of Icc symmetric that is to be used inthe calculation and in the tests.If the short circuit power level is not specified, thevalues given in the table below may be used :

The highest network The apparent shortvoltage in kV circuit power in MVA

7.2 - 12 - 17.5 and 24 50036 1000

n Tapping selector switchThis device must be able to withstand the sameshort circuiting overcurrents as the windings.

n The neutral terminalsThe neutral terminal of zigzag or star connectedwindings must be designed to withstand the highestovercurrent that could pass through it.

n temperatureThe maximum winding temperature after short cir-cuit must be in conformity with the following tableaccording to the transformer insulation's temperatu-re rating.

Insulation system's

ratedtemperature

105 (A)120 (E)130 (B)155 (F)180 (H)

220

180C250C350C350C350C350C

180C200C200C200C200C200C

Maximum permissible

temperature forcopper windings

Maximum permissible

temperature foraluminium windings

Short circuit test in an independentlaboratory

Test objective

To confirm that the transformer can withstand the shortcircuit currents defined in the standard.The test rig layout is identical to the test for short circuitlosses, but the transformer is subjected to Ucc = Un, orin other words to a high short circuit current :

e.g. : for a transformer with Ucc = 4 % (when measuringPcc), the short circuit withstand current can reach :II I symmetric II = 25 InBy choosing the making time at U = 0 : I asymmetric = k. 2 x 25 InThe transformer is then subjected to :1) high electrodynamic forces, since the

Force = constant x I 2

2) then high temperature in the windings

Test procedure

To obtain the short circuit current, one of the transfor-mer's windings is supplied a voltage that must notexceed 1.15 times its rated voltage, with the other win-ding short circuited.

Moreover, the making time of the peak initial value in thewinding of the phase being tested, must be set using asynchronous making breaker;The values of these currents as well as the voltageapplied across the line terminals are recorded using anoscillograph.

For three phase transformers, a total of 9 tests must beperformed, 3 on each column, with a test duration of 0.5seconds and with a tolerance of 10 %.

Unless otherwise specified by the customer, the tests oneach of the phases for transformers with tappings areperformed on various tapping positions :- 3 tests on the position corresponding to the highest

transformation ratio on one of the end phases.- 3 tests on the principal tapping on the middle phase- 3 tests on the position corresponding to the lowest

transformation ratio on the other end phase.

After each of the tests, the impedance values are measu-red and compared with the original values.

Fault detection and test sanctions

After the test, the transformer, must be inspected.The short circuit reactance measurement and the oscillo-grams taken at different stages of the tests must be exa-mined to find any anomalies.The transformer is again subjected to routine tests andnotably to dielectric tests which are performed at 75 % oftheir original value, after which the tank is removed toinspect the active part of the transformer in order to findany visible faults.

(e.g. a fault may be seen in the connection position, theremay be deformation of the windings, etc.)

The transformer is deemed to have passed the short cir-cuit test if :- the routine tests are repeated with success- measurements during the short circuit tests and ins-

pection after tank removal show no sign of a fault.- the short circuit reactance measured after the tests dif-

fers from that measured initially by no more than 2 %for concentrically wound transformers. However, whenthe winding conductor is made from foil, a higher limitof 4 % can be used for transformers with a Ucc of atleast 3 % after agreement between the manufacturerand the buyer.

Testing 60 Hz transformers on a 50 Hz test rig

The test rigs have a 50 Hz supply, and testing 60 Hztransformers requires a few adaptations to be made.

Measurement of the impedancevoltage and load losses :

l The impedance voltage as a percentage (Ucc%) hastwo components : active and reactive

Active voltage :PccUr % (60 Hz et 120C) = 100 Sn

Reactive voltage :

60Ux 60 Hz = Ucc 2 (ambient, 50 Hz) - Ur2 (ambient)50

and

Ucc% (60Hz 120C) = Ur2 (60Hz, 120C) + Ux2 (60 Hz)

Pcc = load losses at 120C and 60 HzSn = rated power

l Joule effect losses (resistive) are independent of thefrequency.Special losses (stray) Ps are mainly losses due to Eddycurrents, that are inversely proportional to the resistivi-ty and directly proportional to the square of the fre-quency.Therefore the special losses Ps2, at temperature 2and frequency fb, and losses Ps1 at temperature 1and frequency fa are in a ratio of :

Ps2 X + 1 fb2

= Ps1 X + 2 fa

2

with X = 235 for copper and 225 for aluminium.

Measurement of no-load losses

The no-load losses are virtually the same as the losses inthe magnetic core. Losses in the core are due to hystere-sis and to Eddy currents.At constant induction the hysteresis losses vary propor-tionally with the frequency and those due to Eddy cur-rents vary proportionally to the square of the frequency.

To remain at constant induction, at 50 Hz we will apply avoltage of U = Un x (50/60).

Losses at 60 Hz (Pv60) are calculated using losses mea-sured at 50 Hz (Pv50) by :

Pv60 = 1,32 x Pv50

In the same way, no-load current at 60 Hz is deducedfrom the one at 50 Hz by :

Io60 = 1,1 x Io50

Dielectric tests

- the induced voltage test must last :l 60 seconds for any test frequency (fe) 2 x the

rated frequency (fn), i.e. fe 120 Hzfn

l [ 120 x ]sec. with a minimum of 15 sec. fe

for fe 2 fn, i.e. fe 120 Hz. - the applied voltage tests has to be performed at any

appropriate frequency at least equal to 80 % fn.Consequently, for fn = 60 Hz, the test frequency must be :fe 0,8 x 60 or in other words fe 48 Hz.Therefore testing at 50 Hz is valid without increasing thetest duration.

Temperature rise test

- no-load testing : the transformer's LV windings are at avoltage corresponding to the no-load losses at 60 Hz(see below)

- the short circuit test : the LV winding is short circuitedwhilst applying the current corresponding to the shortcircuit losses at 60 Hz to the transformer.

These losses at 60 Hz are deduced from those at 50 Hzusing the formula :

X + 2 X + 1 602

Pcc60 = Pj + Ps50 X + 1 X + 2 502

To obtain the same losses at 50 Hz as at In and 60 Hz, itis necessary to inject a current of :

Pcc60I = In

Pcc50

Measurement of the noise level

Since the sound power is proportional to the square ofthe frequency, the increase in noise level is of approxi-mately 10 log (60/50)2 = 1.6 dBIt is therefore possible to estimate the noise level of atransformer at 60 Hz through a 50 Hz test at the sameinduction but there will be a small increase in the measu-rement uncertainty.

Test certificate n 627822-01tl. : 03 87 70 57 57 adresse postale :fax : 03 87 51 10 16 BP 10140tlex : 860 418 FRAFO F 57281 Maizires-ls-Metz cedex

Product : Cast resin transformer Rated power : 2500 kVAType : Step down - Indoor - Three-phase Rated frequency : 50 HzStandard : IEC 726 Total mass : 8600 kgDielectric : TrihalType of cooling : AN classes (HD464 S1) : C2 E2 F1

Year : 1997

Maximum ambient according to IEC. 76 : 40 CAltitude service : X < 1000 mPartial discharges : 10pC at 1.10 Um

Rated voltage : HV 20000 V Curents : 72.17 ATappings : HV 21000 V - 20500 V - 19500 V - 19000 V

Insulation : 24 kV (125/50) Applied voltage : 50 kV Duration : 60 s

Rated voltage : LV 400 V Currents : 3608.4 A

Insulation : 1.1 kV (0/10) Applied voltage : 10 kV Duration : 60 s

Connection : Dyn11 Induced voltage : 40000 V Duration : 22 s Frequency : 270 Hz

Remarks: Thermic class FTest procedures N MOD/IQ/ESS007 and MOD/IQ/ESS008. Measurement of partial discharge, acceptance criterion, except special agreement between FT and the buyer : 10 pC at 1.10 Um/ 10 pC guaranted at 1.375 Um if Um > 1.25 UnThis transformer has been subjected to the partial discharges test. We therefore guarantee that the partial discharges levelobtained is less than 10 pC.

Po IV/IN PCC at 120C UCC at 120C P0+PCC at 120C Transformation ratioPrincipal tapping Other

Guarantee 8000 W 1.20 % 28000 W 7.00 % 36000 W 0.5 % 1 %

Rated voltage ratio

HV/LV1- 52.622- 51.293- 49.984 - 48.665 - 47.34

No-load losses Results

Hz U(V) I1(A) I2(A) I3(A) IV(A) dW1dW2 cte k Po D Po IV/IN D IV/IN

50 400 25.22 19.09 23.13 22.48 7386 1 7386 W -7.68 % 0.623 % -48.08 %

Load losses at 20.0 C (HV/LV) Results at 120 C

U(V) I1(A) I2(A) I3(A) cte dW1dW2 cte k Pmes PCC D PCC UCC D UCC

1297 72.17 72.17 72.17 1 13550 1 13550

1297 IN = 72.17 A 13550 15714 W -43,88 % 6.50 % -7.18 %

Issued on Po+PCC 23100 W D Po+PCC -35.83 %

Efficiency Voltage regulation

cos 0.8 : 98.858 % cos 0.8 : 4.498 %Test manager M. WALTER

cos 1.0 : 99.084 % cos 1.0 : 0.838 %

sige social : Voie Romaine - Pont de Semcourt S.A. au capital de 10 280 000 FRF - APE 311 B

Resistances at 20.0 C

HV : 0.540 W LV : 0.000210 W0.540 W 0.000250 W0.550 W 0.000240 W

Average : 0.543 W 0.000233 W RI2 : 4244 W 4557 W R12 120 C : 5942 W 6380 W

Bibliography and correspondenceof standards

cast resin transformer TRIHAL

IEC

HD

FranceNFC 52-726NFC 52-100

NFC 52-115

NBN C 52 101NBN C 52 102

NBN C 52 103

NBN C 52 104NBN C 52 105

United KingdomBS 171

GermanyVDE 0532 Teil 1VDE 0532 Teil 2VDE 0532 Teil 3VDE 0532 Teil 4VDE 0532 Teil 5VDE 0532 Teil 6VDE 0532 Teil 7DIN 42 523BelgiumNBN C 52 726NBN C 52 223

NBN HD 538.1-S1

NBN C 52-001

International publications(International Electrical Commission)

European harmonization document(European electrical standards commission)

Dry-type power transformersPower transformers : GeneralPower transformers : Temperature risesPower transformers : Insulation levels and testsPower transformers : Tappings and connectionsPower transformers : Short circuit withstandLoading guide for transformers Measurement of partial discharge levelsDetermination of the noise levels of transformers

Dry-type power transformersPower transformersThree-phase dry-type distribution transformers 50 Hz, from100 to 2500 kVA, with highest voltage not exceeding 36 kV.

AllgemeinesbertemperaturenIsolationspegel und SpannungsprfungenAnzapfungen und SchaltungenKurzschlufestigkeitTransformatoren und DrosselspulenBestimmung der GeruschpegelTrockentransformatoren 50 Hz, 100 bis 2500 kVA

Dry-type power transformersThree phase distribution transformersThree-phase dry-type distribution transformers 50 Hz, from100 to 2500 kVA, with highest voltage for equipment not exceeding 36 kVDetermination of the noise levels of transformers and inductance coilPower transformers ; part 1 : GeneralPower transformers ; part 2 : Temperature risesPower transformers ; part 3 : Insulation levels and testsPower transformers ; part 4 : Tappings and connectionsPower transformers ; part 5 : Short circuit withstand

Power transformers

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Bibliography and correspondenceof standards (cont.)

IEC

HD

SpainUNE 20-178UNE 20-101UNE 21-315ItalyCEI 14-8CEI 14-4

SwedenSS IEC 726

CEI 14-12

SS 427 01 01

SS IEC 551HollandNEN 2761NEN 2762

NEN 2763

NEN 2764NEN 2765NorwayNEN 05.71AustriaVE-M 20, TEIL 1VE-M 20, TEIL 2VE-M 20, TEIL 3VE-M 20, TEIL 4VE-M 20, TEIL 5PortugalFinland

International publications(International Electrical Commission)

European harmonization document(European electrical standards commission)

Transformadores de potencia tipo seccoTransformadores de potenciaMedida de los nivele de ruido de los transformadores

Trasformatori di potenza a seccoTrasformatori di potenzaTrasformatori trifase di distribuzione di tipo a secco 50 Hz, da100 a 2500 kVA, con una tensione massima per il componentenon superiore a 36 kV

Dry-power transformersPower transformers (sauf divergence aux clauses 2 de IEC 76-1 et 5 de IEC 76-3)Measurement of transformers sound levels

Energietransformatoren. AlgemeenEnergietransformatoren. TemperatuurverhogingEnergietransformatoren. Isolatieniveaus en dilektrische proe-ven (in voorbereiding)Energietransformatoren. Aftakkingen en schakelingenEnergietransformatoren. Kortsluitskerte

Norske normer for krafttransformatorer : power transformers

Allgemeinesber temperaturenIsolationspegel und SpannungsprfungenAnzapfungen und SchaltungenKurzschlufestigkeitNo particular standards referencesNo particular standards references

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