AREVA Site Tests Manual

AREVA T&D Enerji Endstrisi A . .

Bar Mah. E 5 Alt, 1801 Sok. No 10441410 Gebze- Kocaeli / TRKYE

Tel. : + 90 262 648 33 00 Fax : + 90 262 641 20 36

SITE TESTS MANUALOF THE

POWER TRANSFORMERS

Site Tests ManualOf The Power Transformers

T&D Enerji Endstrisi A..

INTRODUCTION:

Site Test procedures and test equipment of The Power Transformers are dealt in thescope of this manual.

CONTENTS:Pages

1. Measurement of voltage ratio 2-32. Measurement of capacitance and power factor

2.1 Bushing capacitance and power factor measurements 3-42.2 Transformer capacitance and power factor measurements 5-8

3. Transformer exciting current measurements 8-94. Measurement of winding DC resistance 9-115. Measurement of insulation resistance and polarization index 11-136. Quality analysis of insulation oil 13-147. Total combustible gas test 14

Page : 1



1. MEASUREMENT OF VOLTAGE RATIOThe voltage ratio (i.e. turn ratio) of the transformer is the ratio of voltages at no-load.Purpose of the measurement: verification of no-load voltage ratios specified by thespecification and detection of any problem within the coils or tapping connections.Measurements are carried out on all taps and on all phases.Measuring CircuitTurn ratio measurement is performed by measurement of voltage ratios

windingLV of voltage the windingHV of voltage the ratio turn lTheoretica =

( ) ( ) %100 Deviation ratio turn ltheoretica ratio turn ltheoretica ratio turn measured

=

Voltage ratio measurements are generally performed by the digital instruments producedfor this purpose. In addition to voltage ratio measurement, the determination of the vectorgroup (connection group) and current measurements can be performed with theseinstruments, it is necessary that the instrument must have 3-phase system for vector groupdetermination. The method of comparing the voltages of dual vectors enables themeasurement of phase shifting between the vectors.The deviation in the turn ratio shall be < 0,5 %.

The characteristics of measurement devices used in AREVA Gebze Laboratory indicatedas follows:Ratio measuring instrument :

Type : 2791 and 2793 Accuracy : 0,1% Measuring range : 0,18...1999,9

Measuring voltage :2791 150 V ac2793 100 V ac

Manufacturer : TETTEX

Page : 2



2. MEASUREMENT OF CAPACITANCE AND POWER FACTOR2.1 Bushing capacitance and power factor measurementPrior to taking a condenser bushing into service, and on a suspected fault, the capacitanceand dissipation factor can be measured and compared with the values given on the ratingplate or in the routine test report.Measuring procedureWhen tetsing a bushing having a capacitive test tap, i.e. practically all our bushings, it isnot necessary to disconnect the top of the bushing. It is enough to open the disconnectingswitch.For the sake of safety and for reduction of the influence from the winding inductance, alltransformer windings shall be short-circuited. Windings not connected to the tested bushingshall be grounded. See Figure 2.1.1The bridge shall be placed on a vibration-free base. If the reference capacitor is separate, itshall be placed on a dry, insulating base.Depending on which insulation is to be tested, the voltage source (test voltage) shall beconnected via separate leads to the bushing top or the capacitive test tap. Leads for testvoltage or grounding must not be common with leads for measuring. Measuring lead shallbe as short as possible and must not touch grounded objects. Bands or strings used forspacing must be dry and clean. This also applies to the test object. If the bushings is in itstransport case, it must not be surrounded by wet filling material. The test tap shall be cleanand dry.

Figure 1.1

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At humid conditions drying of the test tap may be required in order to get representativereadings for tan over C2. An air dryer may be used for drying. Cleaning of the air sideinsulator housing is indispensable for correct measuremnet of tan over C1.

Power factor testIt is assumed that the bushing to be tested is equipped with a capacitive test tap. It is furtherassumed that the bridge in use can measure ungrounded, according to the UST-metod(Ungrounded Specimen Test). This way, the influence from the transformer on the test result(power factor) is minimized.To make it possible to compare the test resultwith the value on the rating plate or with theroutine test report supplied with each bushing, the power factor shall be measured at 10kV.The measurement procedure shall begin with low sensitivity on the bridge. The sensitivity isthen increased gradually to the highest possible. In rare cases, external interferences canmake it difficult to set the detector to zero. If the interference cannot be eliminated, thesensitivity must be lowered. Table 2.1.1 shows the connections to be made, whenmeasuring the dissipation factor of the different insulations. It is to be noted that mostbridges give capacitance and dissipation factor in the same measuring operation.

Table 2.1.1Test

SequenceTestLevel

Voltageto

HV testlead to

LV testlead to

Switchposition

Measure powerfactor and

capacitance1 10 kV CL CL Tap UST C12 Note A Tap Tap CL Guard C2

CL = Center conductorTap = Capacitive test tapC1 = Main insulationC2 = Tap insulationUST = Ungrounded measurement (Ungrounded Specimen Test)Guard = In this position C1 by-passed and only C2 is measured. This method is not

available on all bridges.

Figure 2.1.1

Page : 4



Note A = Test 2 shall be carried out at no higher voltage than 1kV if the test tap insulation isdelivery tested with 2 kV. If the test tap is tested with 20 kV, 2.5 or 5 kV may be used.

2.2 Transformers capacitance and power factor measurementAll insulating materials used in practice have slightly small dielectric losses at rated voltageand at rated frequency. These losses are fairly low for good insulating materials. This losschanges proportionally with the square of the applied voltage. Insulation, in terms of basiccircuits elements, is shown in Figure 2.2.1

a) Insulation b) Equivalent circuit c) Vectordiagram

Figure 2.2.1

As it can be seen from Figure 2.2.1 the angle between the total current I andcapacitive current Ic is a basic value.Insulation angle is dependent on the thickness, the surface and the properties of theinsulation material ( the pores, impurities and humidity which cause the ionization in theinsulation material ).Generally, the conditions and the reasons will cause a decrease in the dielectric withstandof the insulation. For this reason, the power factor measurement of the insulation at certainfrequency gives a basic idea about the insulation. The measurements to be made duringthe service are one of the most important indications, showing the ageing of the insulationand the contimination of the oil.The active losses of the circuit is as follows;

P= UICos = U 2. C. . tan

Capacitance, tan , active losses and Cos can be measured by bridge methods orpower factor ( Cos ) measuring instrument at definite voltages.Measurement is performed between the windings and the tank, and the test temperature isrecorded; then according to desired reference value the necessary corrections are done.

CU R G=

1

I

a) b) c)

V

I

I

Ic

IR

V1 = U

V2 = 0IC IR

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2.2.1 Two-winding transformers:The insulations involved in two-winding transformers are shown schematically in Figure2.2.1.1 The insulations indicated as CH, CL and CHL are, respectively, the insulation betweenthe high side and ground, the insulation between the low side and ground, and theinsulation between the high and low windings. These insulations, although actuallydistributed along the windings, are shown as single capacitors in the drawing for the sakeof simplicity. These insulations are not made up of a single dielectric: for instance, CH, theinsulation between the high side and ground, includes the bushings, the insulation betweenthe high winding and ground, and the oil between the winding and ground.

Readings are made on CH and CL directly. When the high winding is energized and the lowwinding is guarded, the insulation CH is measured. When the low winding is energized andthe high winding is guarded, the insulation CL is measured. By making additional tests(shown in the tabulation below) and a simple computation, the insulation CHL is obtained.In general, transformer windings connected to the guard circuit will have no appreciableeffect on the accuracy of the test results.The test procedure is as follows: Isolate transformer from the live bus. Disconnect all external leads from the bushing terminals. Disconnect the neutrals of each winding from ground. Short circuit each winding at its bushing terminals. Ground the tank.Make the series of tests as outlined below:

Fig.2.2.1.1

TransformerTank

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2.2.2 Three-winding transformers:The insulations involved in three-winding transformers are shown schematically in Figure2.2.2.1. The insulations indicated as CH, CL and CT are respectively the insulation betweenthe high side and ground, the insulation between the low side and ground and theinsulation between the tertiary and ground.These insulations indicated, CHL, CHT and CLT are respectively the insulation between thehigh and low windings, the insulation between the high and tertiary windings and theinsulation between the low and tertiary windings. These insulations although actuallydistributed along the windings, are shown as single capacitors in the drawing for the sakeof simplicity. These insulations may not not be made up of a single dielectric; for instanceCH, the insulation between the high side and ground, includes the bushings, the insulationbetween the high winding and ground and the oil between the high winding and the tank.

Readings are made directly on CH, CL and CT. When the high winding is energized and thelow and tertiary windings are guarded, the insulation CH is measured. When the lowwinding is energized and the high and tertiary windings are guarded, the insulation CL ismeasured. When the tertiary winding is energized and the low and high windings areguarded, the insulation CT is measured. By making additional tests volving both guardedand guarded windings the interwinding insulations CHL , CHT and CLT can be determined bysimple computation.In general, transformer windings connected to the guard circuit will have no appreciableeffect on the accuracy of the test results.The test procedure is as follows:

Isolate transformer from the live bus. Disconnect all external leads from the bushing terminals. Disconnect the neutrals of each winding from ground. Short circuit each winding at its bushing terminals. Ground the tank.

Figure 2.2.2.1

TransformerTank

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Make the series of tests as outlined below:

In the case of new oil-filled transformers and reactors, the power factor shoulnot exceed 0,5% (20C); power factor between 0,5% and 1,0%(20C) may beacceptable; however, power factor >1,0% (20C) shall be investigated.The characteristics of measurement devices used in AREVA Gebze Laboratory indicated asfollows:

Test equipmentMeasuring bridge:

Manufacturer : Doble Engineering CompanyModel : M2HSerial Nr. : 905Meas. range : 0 12 kV, ac

3.TRANSFORMER EXCITING CURRENT MEASUREMENTTEST CONNECTIONS:Figure 3.1 though 3.3 illustrate test procedures for measurement of exciting currents intransformer windings.

Page : 8



The characteristics of measurement devices used in AREVA Gebze Laboratory indicated asfollows:

Test equipmentMeasuring bridge:

Manufacturer : Doble Engineering CompanyModel : M2HSerial Nr. : 905Meas. range : 0 12 kV, ac

4. MEASUREThe resistancewindings are cThe winding resistances of

R R2 1= aluminium )where; R2

R1Therefore, whmeasurement

Figure 3.1 Figure 3.2

Fig.7

Figure 3.3

Page : 9MENT OF WINDING DC RESISTANCE measurement will show whether the winding joints are appropriate and theorrectly connected.resistances that vary with the temperature strongly, are the ohmic/d.c. a winding and the resistance is computed as follows:

tt

2

1

235235

+

+( for copper ) R R tt2 1

2

1

225225=

+

+( for

= winding resistance at t2 temperature = winding resistance at t1 temperatureerever the winding resistances are stated, the temperatures during themust be given.



The resistances between all pairs of phase terminals at all tapping connections aremeasured.Direct current obtained from a constant-current supply unit. The value of the directcurrent should h enough to ensure correct measurement and should be low enoughto prevent anythan1,2 x I0 and lethe ratio of L/When the testwhich is series

i UR= (1

If the measuto saturatiocondition inAfter switchwaited untilwill be occurMeasuremeWinding resissensitivity canmeasurementIn the currenreference resivoltage drop value of unknmeasuring insIt is necessarythe circuit.

Figure 4.1

S

Page : 10 can be be hig

effects on the winding temperature. In practise this value should be greater

ss than 0,1x In. The time constant of the measurement circuit will depend onR. object is assumed to be composed of a R resistance and L inductance connected to it, U voltage applied to this circuit will be;

eRL t ) where time coefficient depends on L/R ratio.

ring current increases, it leads to the reduction of inductance duen of the core, these enables the current to reach the steady state a short time.ing on the supply voltage to the measurement circuit, it should be the current becomes stationary, otherwise measurement errorsred.

nt Circuittances can be measured by the following current-voltage method. Measuring be increased by using the digital measuring instruments. The circuit of the by current-voltage method is given in Figure 4.1.t-voltage measuring method, by applying the winding current through thestance in the system, the voltage drops occurred in both resistances. Thisvalues of reference and winding resistances are compared to determine theown resistance (winding resistance) which can be read directly from the digitaltrument. to care, in order avoid of very high voltages during the switching on and off

: Winding resistance measurement by Current-Voltage method

C

BA

a

b

c

A

V N n

Powersupplyunit



Comparison of readings with other phases or previous measurements underfield conditions are recommended. Variations under field conditions shouldnot exceed 5%.

The characteristics of measurement devices used in AREVA Gebze Laboratory indicated asfollows:

Test equipmentsMeasuring instruments:

Manufacturer : TETTEXModel : 2292 and 2285c/100/3Meas. current : 50A and 100A

5.MEASUREMENT OF INSULATION RESISTANCEThe purpose of the measurement is to determine the leakage current of the insulationresistance. This current is changing with the moisture, impurity contents and temperature ofthe insulation.Beside the result of the measurements, the comparison of the periodical measurement givethe information about the condition of the insulation. In order to compare they must be atthe same temperature ( for example at 20C reference temperature ).Time resistance method in the insulation resistance measurement is one of the bestmethods, that is simple and gives correct results.The insulation resistance is measured by means of an insulation resistance meter whichapply a voltage between 1kV dc and 5kV dc. Each winding is measured separately byconnecting the voltage between the winding to be tested and earth. While the otherwindings are connected to the guard circuit of the test instrument. The temperature andhumidity are recorded during the test.The Resistance values R15 , R30, R45, R60 and R600 are taken at 15 sec , 30 sec, 45 sec, 60 secand 600 sec after apply the voltage. Furthermore, the ratio of the insulation resistance R600to the insulation resistance R60 is stated as polarization index (PI) in the test report.

Readings are referred to 20 C by multiplying the value at transformer oil temperature by correction factor given in table below:

C

Correctionfactor

C

Correctionfactor

-10 0,13 35 2,80-5 0,18 40 3,950 0,25 45 5,605 0,36 50 7,8510 0,50 55 11,2015 0,75 60 15,8520 1,0 65 22,4025 1,40 70 31,7530 1,98 75 44,70

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5.1 Two-windings measurement

5.2 Three-windings measurement

Figure 5.1.1

Figure 5.2.1

TransformerTank

TransformerTank

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For small transformers the polarization index will be equal to 1 or slightlyhigher. Larger transformers may exhibit a polarization index of 1,1 1,3. Ingeneral, a high value of polarization index indicates that the insulationsystem is good condition. A polarization index of



6.2 Acidity,Neutralization numberIn the case of new oil as received from refinery the acid number should notecxeed 0,03 mg KOH/g.

6.3 Interfacial tension

6.4 Power factor measurementThe maximum suggested power factor for new oil after filling andstanding, prior to energizing, should not exceed 0,10% (at 25C).

6.5 Water contentMoisture content in oil for new transformer should not exceed 10 ppm.

7. TOTAL COMBUSTIBLE GAS TEST OF INSULATION OIL TAKENFROM TOP VALVE

Total dissolved gas for new transformers shall not exceed 0,5% (if units areequipped with nitrogen blankets, total dissolved gas should not exceed1,0%).

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Bar Mah. E 5 Alt, 1801 Sok. No 10441410 Gebze- Kocaeli / TRKYE

INTRODUCTION:

AREVA Site Tests Manual

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