Testing Manual 3

7/29/2019 Testing Manual 3

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Power Transformer Testing

MTEKPRO Technologies Pvt. Ltd, New Delhi.

2

Testing of

Power Transformers

1. Verification of voltage Ratio and vector

Group or phase displacement and polarity.


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31. Turn Ratio Measurement

1.1 Purpose of measurementThe no-load voltage ratio between two windings of a transformer is called turn ratio. The aim of measurement

is; confirming the no-load voltage ratio given in the customer order Specifications, determining the conditions

of both the windings and the connections and examining the problems (if any).

The measurements are made at all tap positions and all phases.

1.2 Turn Ratio Measurement

Bridge methodMeasurement of turn ratio is based on, applying a phase voltage to one of the windings using a bridge

(equipment) and measuring the ratio of the induced voltage at the bridge. The measurements are repeated in

all phases and at all tap positions, sequentially. During measurement, only turn ratio between the winding

couples which have the same magnetic flux can be measured, which means the turn ratio between the

winding couples which have the parallel vectors in the vector diagram can be measured.

Theoretical turn ratio = HV winding voltage / LV winding voltage

The theoretical no-load turn ratio of the transformer is adjusted on the equipment by an adjustable

transformer; it is changed until a balance occurs on the % error indicator. The value read on this error

indicator shows the deviation of the transformer from real turn ratio as %.

(Measured turn ratio) - (Designed turn ratio)

% Deviation = * 100

(Designed turn ratio)

The % deviation of the turn ratios should be 0.5 %.


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4

1.2 Determining the Vector Group

Depending on the type of the transformer, the input and output windings of a multi-phase transformer are

connected either as star ( Y) or delta( D ) or zigzag(Z). The phase angle between the high voltage and the low

voltage windings varies between 0 and 360.

Representing as vectors, the HV winding is represented as 12 (0) hour and the other windings of the

connection group are represented by other numbers of the clock in reference to the real or virtual point. For

example, in Dyn 11 connection group the HV winding is delta and the LV winding is star and there is a phase

difference of 330 (11x30) between two windings. While the HV end shows 12 (0), the LV end shows 11

oclock (after 330).

Determining the connection group is valid only in three phase transformers. The high voltage winding is shown

first (as reference) and the other windings follow it.

If the vector directions of the connection are correct, the bridge can be balanced.

Also, checking the connection group or polarity is possible by using a voltmeter. Direct current or alternating

current can be used for this check.

The connections about the alternating current method are detailed in standards. An example of this method is

shown on a vector diagram below.

Fig: - Connection grouprepresentation and

measuring

The order of the measurements:

1) - 3 phase voltage is applied to ABC phases

2) - voltage between phases (e.g. AC) is measured

3) - A short circuit is made between C and n

4) - voltage between B and B is measured

5) - voltage between A and c is measured

As seen from the vector diagram, in order to be Dyn 11 group, A.c > AB > B.b correlation has to realized.

Taking the other phases as reference for starting, same principles can be used and also for determining the

other connection groups, same principles will be helpful.


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5


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6

1.3 Polarity test

1.3.1 Polarity test using voltmeters

Single-phase transformersFor single-phase transformers the polarity can be either additive or subtractive. The low voltage winding is

connected in series with the high voltage winding, either in phase or in opposite phase. For additive

determination of polarity, if the phase displacement is correct, see figure 1.3.1.

Figure 1.3.1: Connection for additive polarity test

And for subtractive determination of polarity, see figure 1.3.2

Figure 1.3.2: Connection for subtractive polarity test.

Polyphase Transformers.

The vector group must be checked for three-phase transformers. This is done by connecting a terminal from

the low voltage side to a terminal on the high voltage side, see figure 1.3.3. When a three-phase supply is

connected to the high voltage winding, potential differences appear between the open terminals and are used

to determine the vector group.

Figure 1.3.3: Polarity test and connection test on three-phase transformer

Using one voltmeter


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7

1.3.2 Polarity check using DC current.

This method establishes the polarity of single and three-phase transformers by briefly switching on a DC

current source at the high voltage winding, see figure 1.3.4. The polarity is shown on a polarized voltmeter

connected to the low voltage side.

Figure 1.3.4: Basic analog ratio bridge circuit


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8

Testing of

Power Transformers

2. Winding Resistance

Measurement.


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9

2. Winding Resistance Measurement.

2.1 Purpose of the test

Winding resistance serves a number of important functions like:

Providing a base value to establish load loss.

Providing a basis for an indirect method to establish winding temperature and Temperature rise

within a winding.

Inclusion as part of an in-house quality assurance program, like verifying electric continuity within a

winding.

2.2 General.Winding resistance is always defined as the DC-resistance (active or actual resistance) of a winding in Ohms

[].

Temperature dependenceIt should be noted that the resistivity of the conductor material in a winding copper or aluminum is

strongly dependent on temperature. For temperatures within the normal operating range of a transformer the

following relationship between resistance and temperature is sufficiently accurate:

C+2

R2 = R1

C+1Where:

R1 = resistance at temperature 2R2 = resistance at temperature 1

= temperature in C

C= constant which is a function of material type

IEC [1] specifies: C= 235 for copper= 225 for aluminum

Principle and methods for resistance measurement:-There are basically two different methods for resistance measurement: namely, the so-called voltmeter-

ammeter method and the bridge method.

Voltmeter-ammeter MethodThe measurement is carried out using DC current. Simultaneous readings of current and voltage are taken. The

resistance is calculated from the readings in accordance with Ohms Law. This measurement may be

performed using conventional analog (rarely used nowadays) or digital meters; however, today digital devices

such as Data Acquisition Systems (DAS) with direct resistance display are being used more and more.

Measurement with voltmeter and ammeterThe measuring circuit is shown in figure 2.1.

Resistance RXis calculated according to Ohms Law:

RX=U/I


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10The advantage of this method is the simplicity of the test-circuit. On the other hand, this method is ratherinaccurate and requires simultaneous reading of the two instruments.

Resistance measurement using a Kelvin (Thomson) Bridge

This measurement is based on the comparison of two voltage drops: namely, the voltage drop across the

unknown winding resistance RX, compared to a voltage drop across a known resistance RN (standard resistor),

figure 2.2.

DC-current is made to flow through RX and RN and the corresponding voltage drops are measured and

compared.

The bridge is balanced by varying the two resistors Rdec and RV, which have relatively high resistance values.

A balanced condition is indicated when the galvanometer deflection is zero, at which time the following

relationship holds:

Rdec

RX = RN

RvThe influence of contact resistances and the connection cable resistances (even of the connection between RX

and RN) can be neglected.

Figure 2.1: Voltmeter - ammeter method measuring circuit

Figure 2.2: Kelvin (Thomson) Bridge method


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11Value of the DC-current of measurementMaximum value:

To avoid an inadmissible winding temperature rise during the measurement, the DC-current should be

limited to a maximum 10% of the rated current of the corresponding winding.

Minimum value:

The lower limit of the DC-current is given by the following considerations:

The measuring circuit for all resistance measuring methods consists of a DC-source and a transformer

winding fixed around an iron core as represented by the following equivalent circuit, Winding inductance

is strongly dependent on current and displays the following characteristic for transformers, see figure 3.4.

As the measuring circuit time-constant is given by the relation L/R, the current-time characteristic differs quite

significantly when switching on the DC-source, depending on the measuring current value (magnetizing

current).

Therefore, the DC measuring current should be at least 1.2 times higher than the crest value of the

magnetizing current to be sure to saturate the iron core

Figure 3.3: Equivalent circuit of a Figure 3.4: Inductance of transformer winding

Winding as a function of the current.


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12

Testing of

Power Transformers

3. Magnetic Balance and

Magnetising Current

Measurement.


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13

Magnetic Balance Test on 3-phase Transformers

This test is conducted only in three-phase transformers to check the imbalance in the magnetic circuit. In this

test, no winding terminal should be grounded; otherwise results would be erratic and confusing. The test shall

be performed before winding resistance measurement. The test voltage shall be limited to maximum power

supply voltage available at site.

Evaluation Criteria

The voltage induced in the center phase is generally 50% to 90% of the applied voltage on the outer phases.

However, when the center phase is excited then the voltage induced in the outer phases is generally 30 to

70% of the applied voltage.

Zero voltage or very negligible voltage with higher excitation current induced in the other two windings should

be investigated. The voltage induced in different phases of transformer in respect to neutral terminals given in

the table below.

Left side phase Central phase Right side phase

AN BN CN

Voltage applied at left side phase 230 V 180 V 50 V

Voltage applied at central phase 115 V 230 V 115 V

Voltage applied at right side phase 50 V 180 V 230 V


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14

EXCITING/ MAGNETISING CURRENT MEASUREMENT

This test should be done before DC measurements of winding resistance to reduce the effect of residual

magnetism. Magnetising current readings may be effected by residual magnetism in the core. Therefore,

transformer under test may be demagnetised before commencement of magnetizing current test.

Three-phase transformers are tested by applying Single-phase 10 kV voltage to one phase (HV terminals) and

keeping other winding open circuited and measuring the current at normal, minimum and max. tap positions.

Keep the tap position in normal position and keep HV and LV terminals open. Apply 1phase 10kV supply on IV

terminals.

Measure phase to phase voltage between the IV terminals and current on each of the IV terminals. The set of

reading for current measurement in each of the tap position should be equal. Unequal currents shall indicate

possible short circuits in winding. Results between similar single-phase units should not vary more than 10 %

.The test values on the outside legs should be within 15 % of each other, and values for the centre leg should

not be more than either outside for a three-phase transformers. Results compared to previous tests made

under the same conditions should not vary more than 25%. If the measuredexciting current value is 50 times

higher than the value measured during precommissioningchecks, then there is likelihood of a fault in the

winding which needs further analysis. The identical results confirm no damage due to transportation. The

availability of test data of normal condition and faulty condition results help us to analyze the problem in

future.

Measurement of Magnetization Current at Low Voltage

For 3-phase transformers, the test shall be conducted either with 415 V, 3-phase (neutral grounded) or 230 V,

1 phase (preferred). For single phase transformers, the test shall be conducted with 230 V.

This test is performed to locate defect in magnetic core structure, shifting of windings, failures in turn

insulation or problem in tap changers.

The acceptance criteria for the results of exciting current measurement should be based on the comparison

with the previous site test results or factory test results. The general pattern is two similar high readings on

the outer phases and one lower reading on the center phase, in case of three phase transformers. Anagreement to within 25% of the measured exciting current with the previous test is usually considered

satisfactory. If the measured exciting current value is 50% higher than the value measured during pre-

commissioning checks, then the winding needs further analysis.


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15

Testing of

Power Transformers

4. MEASUREMENT OF SHORTCIRCUIT IMPEDANCE


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16MEASUREMENT OF SHORT CIRCUIT IMPEDANCE

This test is used to detect winding movement that usually occurs due to heavy fault current or mechanical

damage during transportation or installation since dispatch fro the factory.

Ensure the isolation of Transformer from High Voltage & Low voltage side with physical inspection of open

condition of the concerned isolators/disconnectors. In case tertiary is also connected, ensure the isolation of

the same prior to commencement of testing

The measurement is performed in single phase mode. This test is performed for the combination of two

winding. The one of the winding is short circuited and voltage is applied to other winding. The voltage and

current reading are noted.

The test shall be conducted with variac of 0-280 V, 10 A, precision RMS voltmeter and ammeter. The

conductors used for short-circuiting one of the transformer windings should have low impedance (less than

1m-ohm) and short length. The contacts should be clean and tight.

The acceptable criteria should be the measured impedance voltage having agreement to within 3 percent of

impedance specified in rating and diagram nameplate of the transformer. Variation in impedance voltage of

more than 3% should be considered significant and further investigated.

The formula for calculating the percentage impedance with current and frequency correction is:

Where:

Vtest = Test voltage

Vrated=Rated voltageItest= Test current

Irated = Rated current

ft= Test frequency

fr = Rated frequency


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17

Testing of

Power Transformers

5. Measurement of dissipationfactor (tan) of the insulation

system capacitances.


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18Measurement of dissipation factor (tan) of the insulation system capacitances

The purpose of the measurementThe insulation power-factor test, similar to the insulation resistance test, allows certain conclusions to be

drawn concerning the condition of the transformer insulation.

The significance of the power factor figure is still a matter of opinion. Experience has shown, however, that

the power-factor is helpful in assessing the probable condition of the insulation when good judgment is used

[51].

GeneralIEC defines the power factor as the ratio between the absorbed active power to the absolute value of the

reactive power. This corresponds to tan. IEEE [51], on the other hand defines the insulation power-factor as

the ratio of the power dissipated in the insulation in watts, to the product of the effective voltage and current

in volt-amperes (corresponding to the apparent power) when tested using a sinusoidal voltage. Insulation

power-factor is usually expressed in percent [51].Measurement of power-factor values in the factory is useful for comparison with field power-factor

measurements and assessing the probable condition of the insulation. It has not been feasible to establish

standard power-factor values for the following reasons:

There is little or no relationship between power-factor and the ability of the Transformer to withstand the

prescribed dielectric tests.

The variation of power-factor with temperature is substantial and erratic.

The various liquids and insulation materials used in transformers result in Large variations in insulation

power factors [51].

The measuring circuit / the measuring procedure [51]

Measurement using a bridgeThe method is based on comparing the capacitance CX(transformer under test) with a well-known

capacitance CN (standard capacitor).

Conventional Schering-BridgeFigure shows the measuring circuit for the insulation power-factor measurement of a two-winding

transformer using a conventional Schering-bridge.

InstrumentationThe Schering-Bridge test circuit consists of three main parts:

The unknown capacitance CX, which represents the transformer under test whose power-factor (or tan)

and capacitance are to be measured.

The standard capacitor CN, which must be a HV capacitor with very low dielectric losses. Normally its

capacitance is between 100 pF and 10 nF.

The Schering-Bridge casing contains resistors R3, R4 and r, adjustable capacitor C4 and galvanometer G.

In order to reduce the influence of external disturbances, coaxial cables must be used for the connection

between CX(the transformer under test) to the bridge and also between standard capacitance CN and thebridge.


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19

Figure: Measuring circuit for the measurement of power factor and winding

Capacitances.

When the bridge is balanced, the unknown capacitance CX and tan can be calculated using the following

equations:

In most bridges the following resistance values are used for R4, to simplify the calculation: 100/, 1000/ or

10000/ etc. in ohms.

For a 50 Hz measurement, with R4 = 1000/ and C4 in nF, the insulation power factor tan will be:


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20A modern tanbridge with current comparator and microprocessor

This bridge uses basically the same measuring principle as described above. Figure 18.1 b shows the measuring

circuit for dissipation factor and capacitance measurement with a modern tan _ measuring bridge withincorporated microprocessor.

The currents are balanced in a comparator (more-winding differential transformer) and quadrature current is

injected to balance the losses.

For the unknown capacitance Cx, the standard capacitor CN and the connections between transformer and

bridge are the same as mentioned above for the conventional Schering Bridge.

MEASURING METHODS:

CAPACITANCE AND TAN MEASUREMENT OF BUSHINGS

C & Tan measurement of bushings shall be done at 10kV with fully automatic test kit so as to have reliable

test result.

For 3-Ph auto-transformer, short together all 400kV, 220kV and Neutral (isolated from earth) Bushings. Also

short all 33kV Bushings and earth the same.

Measurement of C1 Capacitance and Tan: Connect the crocodile clip of the HV cable to the top terminal of

the shorted HV/IV bushings. Unscrew the test tap cover, Insert a pin in the hole of the central test tap stud by

pressing the surrounding contact plug in case of 245 kV OIP Bushing and remove the earthing strip from the

flange by unscrewing the screw (holding earth strip to the flange body) in case of 420 kV OIP Bushing. Connect

the LV cable to the test tap (strip/central stud) of the bushing under test to the C & TAN KIT through a

screened cable and earth the flange body. Repeat the test for all Bushings by changing only LV lead connection

of the kit to test tap of the Bushing which is to be tested.

Measurement of C2 Capacitance and Tan: HV lead to be connected to the test tap of the bushing under test

(if required additional crocodile type clip may be used) and LV of the kit to be connected to the ground. HV of

the bushing is to be connected to the Guard terminal of the test kit. Test to be carried out in GSTg mode at

1.0kV.

For measurement of 33kV Bushing Tan Delta, earth HV/IV Bushings (already shorted). Apply HV lead of theTest kit to shorted 33kV Bushings and connect LV lead of the test kit to Test tap of the Bushing under test.

Measurements shall be made at similar conditions as that of a previous measurement. The oil-paper

insulation combination of bushings exhibit fairly constant tan delta over a wide range of operating

temperature. Hence, effort is to be made for testing attemperature near to previous test and Correction factor

need not be applied.

Do not test a bushing (new or spare) while it is in its wood shipping crate, or while it is lying on wood. Wood

is not as good an insulator as porcelain and will cause the readings to be inaccurate. Keep the test results as a

baseline record to compare with future tests.


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21 It is to be ensured that C& Tan measurement of bushings and testing of turrets carried out beforeinstallation. This will prevent installation of bushings having C& Tan values beyond permissible limits.

It is to be ensured that Test Tap points are earthed immediately after carrying out the measurements for

that particular Bushing and earthing of test tap to be ensured by carrying out continuity test.

CAPACITANCE AND TAN MEASUREMENT OF WINDINGS

The combination for C & tan measurement of winding is same as that of measurement of IR value. The

summery of probable combination is given below:

Auto-Transformer (Two

winding)

Test Mode Shunt

Reactor

Test Mode 3 winding

Transformer

Test Mode

HV + IV to LV UST V HV to E GST HV to LV1 US

HV + IV to E GSTg HV to LV2 US

LV to E GSTg LV1 to LV2 US

HV to Ground GSTg

LV1 to Ground GSTg

LV2 to Ground GSTg

Table: Combination for C & tan measurement of winding for various

Transformers/ Shunt Reactor.

Ensure that test specimen is isolated from other equipments. Removal of Jumpers from Bushings is Pre-

Requisite for C & Tan Measurement of Windings.

For ICTs (Auto-Transformers): Shorting of all three phase Bushings (400kV&220kV) and neutral to be done.

In case of single phase, 400kV, 220kV and neutral Bushings to be shorted Capacitance and Tan measurement

of windings should be done in following combinations:

Test

No.Winding Combination

Test

modeCap Symbol Test lead Connection Remarks


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221. HV-IV/LV UST CHL HV lead of test kit to

HV/IV bushings of

transformer

LV lead of test kit to LV

bushing of transformer

2. HV-IV/ LV+G GST CHL +CHG -do-

3. HV-IV / LV

with Guard

GSTg C HG - do- LV to be

Guarded

4 HV-IV/LV UST CHL LV lead of test kit to

HV/IV bushings of

transformer

HV lead of test kit to LV

bushing of transformer

5 LV/ HV-IV +G GST CHL +CLG LG -do-

6. LV/ HV-IV

with Guard

GSTg C LG -do- HV to be

Guarded

Table :.Winding combination for C & tan measurement for auto transformer.

Measurement inter-check can be done by calculating C1= C2-C3 & C4= C5- C6 & DF1=C2DF2-C3DF3 / C2-C3=

C4DF4-C5DF5 / C4-C5 Where C stands for capacitance and DF for dissipation factor or tan and attached

suffix (16) denotes the sr. no. oftest in above table.

For Reactors: All 400kV and neutral Bushings to be shorted. HV of the test kit to be connected to shorted

Bushings and LV of the test kit to be connected to Earth connection. Measure the Capacitance and tan Delta in

GST mode. Neutral connection with earth/ NGR to be isolated before the test.


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23

Testing of

Power Transformers

6. Measurement

Insulation Resistance.


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24Measurement of insulation resistance

Insulation resistance tests - Megger tests - are performed to determine the insulation resistance from

individual windings to earth or between individual windings. Knowledge of the insulation resistance is of value

when evaluating the condition of the transformer insulation.

Insulation resistance is commonly measured in megohms, (M).

It should be stated, that variations in insulation resistance can be caused by numerous factors including:

design, temperature, dryness, and cleanliness of parts, especially of bushings. When insulation resistance falls

below specified value, it can often be brought back to the required value by cleaning and drying.

Insulation resistance varies with the applied voltage. Any measurement comparisons should always be carried

out at the same voltage.

Figure: Principal measuring circuit for the insulation resistance measurement

IEEE Std. C57.12.00 [50] also specifies the insulation resistance measurement between core and earth. It shall

be measured after complete assembly of the transformer at a level of at least 0.5 Kv DC for a duration of 1

minute.

The test is conducted with the help of mega-ohmmeter. IR is proportional to the leakage current through/over

the insulation after capacitive charging and absorption currents become negligible on application of DC

voltage. Insulation resistance shall be measured after the intervals of 15 sec, 60 sec and 600 sec. The

polarization index (PI) is defined as the ratio of IR values measured at the intervals of 600 and 60 secondsrespectively. Whereas, the dielectric absorption is the ratio of IR values measured after 60 sec and 15 sec. IR is

normally measured at 5 kV DC or lower test voltage, but the test voltage should not exceed half the rated

power-frequency test voltage of transformer windings.

Polarization index (PI) is useful parameters for logistic interpretation of IR test results. This ratio is

independent of temperature and gives more reliable data for large power transformers. A PI of more than 1.3

and dielectric absorption factor of more than 1.25 are considered satisfactory for a transformer when the

results of other low voltage tests are found in order. PI of less than 1 calls for immediate corrective action. For

bushings, an IR value of above 10000 M-ohms is considered satisfactory.

The IR value of transformer is dependent on various factors such as configuration of winding insulation

structures, transformer oil, atmosphere condition etc. therefore, present trend is to monitor oil characteristics


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25

for judging the condition of dryness of the transformer and not to rely solely on absolute values of IR. It may

be note that no national/international standards specify minimum insulation resistance values of

transformers. The value of IR may be very low under heavy fog or humid conditions.

During IR measurement, we must ensure following conditions: Transformer is disconnected from other associated equipment Bushings are cleaned and free of moisture Transformer tank and core are properly grounded Both ends of winding under test are short-circuited.

Measuring Methods:-

IR measurements shall be taken between the windings collectively (i.e. with all the Windings being connected

together) and the earthed tank (earth) and between each winding and the tank, the rest of the windings being

earthed. Before taking measurements the neutral should be disconnected from earth. Following table givescombinations of IR measurements for auto-transformer, three -winding transformer & Shunt Reactor.

For Auto-transformer For 3 winding

transformer

For Shunt

Reactor

HV + IV to LV HV + IV to LV HV to E

HV + IV to E HV + LV to IV

LV to E HV + IV +LV to E

Where HV-High voltage, IV-Intermediate voltage, LV-Low voltage/Tertiary voltage windings, E- Earth

Unless otherwise recommended by the manufacturer the following IR values as a thumb rule may be

considered as the minimum satisfactory values at 30C (one minute measurements) at the time of

commissioning.

Insulation resistance varies inversely with temperature and is generally corrected to a standard temperature

(usually 20 C) using table (Source: BHEL instruction Manual) as given below.


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26

PI= R10 / R1 (dimensionless),Where PI is Polarisation Index and R is resistance

The following are guidelines for evaluating transformer insulation using polarization index values:

A PI of more than 1.25 and DAI of more than 1.3 are generally considered satisfactory for a transformer when

the results of other low voltage tests are found in order. PI less than 1 calls for immediate corrective action.

For bushings, an IR value of above 10,000 M is considered satisfactory.


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27

Testing of

Power Transformers

7. CHECK LIST FOR ENERGISATION

OF TRANSFORMER.


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28CHECK LIST FOR ENERGISATION OF TRANSFORMER/ REACTOR

PRELIMINARY CHECKS

1. Release air at the high points, like oil communicating bushings, buchholz petcock, tank cover and the cooling

devices including headers, radiators, pumps, expansion joints etc. of the transformer. Air release should be

resorted from low points to high points.

2. Check the whole assembly for tightness and rectify where necessary.

3. Check the general appearance and retouch the paint work if need be.

4. Check that the valves are in the correct position:

Tank: valves closed and blanked

Cooling circuit: valves open

Conservator connection: valves open

By-pass: valves open or closed as the case may be.

On-load tap changer: valves open

5. Check that the silica gel in the breather is blue and that there is oil in the breather cup (oil seal)

6. Check that CC-CL-G are shorted

7. Check the oil level in the main conservator and the conservator of on-load tap changer, bushing caps,

flanges, turrets, expansion bellows as per manufacturers recommendation. Level should correspond to 35 C

mark on oil level gauges for BHEL transformers

8. Check the bushings:

Oil level (bushings fitted with sight-glasses)

Adjustment of spark-gaps /arcing horn gaps, if provided

Conformity of connection to the lines (no tensile stress on the terminal heads)

Bushing CT secondary terminals must be shorted and earthed, if not in use.

Neutral bushing effectively earthed

9. Check the on-load tap changer:

Conformity of the positions between the tap changer control cubicle and the tap Changer head

Adjustment of the tap-changer control cubicle coupling

Electric and mechanical limit switches and protective relays

Step by step operation- local and remote electrical operation as well as manual Operation and parallel

operation, if any

Signaling of positions

10. Check the quality of the oil:

Draw off a sample from the bottom of the tank

Carry out DGA and oil parameters test (i.e. BDV, Moisture content, resistivity & tan at 90C and IFT )


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before energisation.

11. Check the oil of OLTC chamber, if not good, drain and fill with filtered oil upto desired level.

12 Check that equalising link between OLTC tank and Main tank is removed

13 Extraneous materials like tools, earthing rods, pieces of clothes, waste etc. should be removed before

energisation.

CHECKING OF AUXILIARY AND PROTECTIVE CIRCUITS

1. Check temperature indicator readings and their calibrations

2. Check the setting and working of the mercury switches of winding and oil temperature indicators and

presence of oil in the thermometer pockets

Follow the same procedure for the thermal replicas

3. Check the direction of installation of buchholtz relay.

4. Check the operation of the buchholz relay and the surge protective relay of the tap-changer :

Alarm and tripping

Protections and signals interlocked with these relays

5. Check the insulation of the auxiliary circuits in relation to the ground by 2 kV megger for 1 min.

6. Check the earthing of the tank and auxiliaries like cooler banks at two places.

7. Measure the supply voltages of the auxiliary circuits

8. Check the cooling system :

Check the direction of installation of oil pumps

Check the direction of rotation of the pumps and fans

Check the working of the oil flow indicators

Check the setting of the thermal overload relays

Go through the starting up sequences, control and adjust, if necessary, the relay time delays

9. Check that there is efficient protection on the electric circuit supplying the accessories and tightness of all

electrical connections

10. Check the heating and lighting in the cubicles

11. Check the differential protection, over-current protection, restricted earth fault protection, over-fluxing

protection etc. are in service and settings are as per CC/Engg recommendations. After the inspection / tests

are completed, the transformer may be energised from the incoming side on NO LOAD. The initial magnetising

current at the time of switching will be very high, depending upon the particular moment in the cycle. The

transformer should always be soaked for few hours under constant care i.e. keep it energised for twelve

hours. Excessive vibrations of radiator parts etc. should be located and corrected. The transformer hum should

be observed for any abnormality. After that it may be checked for gas collection. Should the gas prove to beinflammable, try to detect the cause which may probably be an internal fault? If the breaker trips on

Testing Manual 3

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