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This article is limited i
This article is limited in scope to standard testing procedures for transformers prior to installation, n scope to standard testing procedures for transformers prior to installation, withwith their general purpose and methodology. Testing of th
their general purpose and methodology. Testing of the transformers is done to determine their electrical,e transformers is done to determine their electrical, thermal and mechanical suitability for the s
thermal and mechanical suitability for the system where they will be applied ystem where they will be applied or used. Most of the testsor used. Most of the tests performed on power transformers are defined in national standards created b
performed on power transformers are defined in national standards created by IEEE, NEMA and ANSI,y IEEE, NEMA and ANSI, whose purpose is to define a unifor
whose purpose is to define a uniformm set of tests set of tests recognized by both the manufacturrecognized by both the manufacturer and the user.er and the user.
Categories of Field Testing
Categories of Field TestingIn general, field testing can be dIn general, field testing can be divided into three categoriesivided into three categories
1.
1. AcAcceceptptanance ce teteststss 2.
2. PePeririododic ic ttesesttss 3.
3. TeTeststs afs afteter fr faiailulurere
Acceptance tests should be performed immediately after the product arrives at the
Acceptance tests should be performed immediately after the product arrives at the destination. A fewdestination. A few tests can be carried out which
tests can be carried out which are stated below:are stated below:
Turns ratioTurns ratio
Insulation resistance(Winding and core)Insulation resistance(Winding and core)
Power factorPower factor
Resistance (winding)Resistance (winding)
Polarity and phase relationPolarity and phase relation
Oil tests (DGA, moisture, dielectrics, etc.)Oil tests (DGA, moisture, dielectrics, etc.)
Visual inspectionVisual inspection
Periodic tests are done after the product is installed in its permanent location. The main purpose of this Periodic tests are done after the product is installed in its permanent location. The main purpose of this test is to monitor the condition of the
test is to monitor the condition of the unit so that any unit so that any potential trouble may be spotted early before apotential trouble may be spotted early before a failure
failureoccurs. Some occurs. Some of these aof these are listed re listed below:below:
Turns ratioTurns ratio
Insulation resistanceInsulation resistance
Power factorPower factor
Resistance (winding)Resistance (winding)
Oil tests (DGA, moisture, dielectrics, etc.)Oil tests (DGA, moisture, dielectrics, etc.)
Excitation current testExcitation current test
Visual inspectionVisual inspection
An unscheduled outage and the potential of outright failure can be prevented by following a periodic test An unscheduled outage and the potential of outright failure can be prevented by following a periodic test schedule.
Failure tests
Failure tests, which can be p, which can be perfoerformed,rmed,are:are:
Turns ratioTurns ratio
InsInsulaulatiotionn resresististanceance
Power factorPower factor
ResistanceResistance
Oil testsOil tests
Excitation current testExcitation current test
Combustible gas/ gas-in-oil analysisCombustible gas/ gas-in-oil analysis
Visual inspection (internal)Visual inspection (internal)
When a transformer fails, the time of failure tests
When a transformer fails, the time of failure tests will decide whether the unit can will decide whether the unit can be repaired at the sitebe repaired at the site or whether it needs to be r
or whether it needs to be returned to the manufacturer, or a specialized center for repair. eturned to the manufacturer, or a specialized center for repair. By comparingBy comparing the results of the tests with the establishe
the results of the tests with the established norms, a 'history' of the transformer can be comd norms, a 'history' of the transformer can be compiled, and thepiled, and the reasons for failure can
reasons for failure can be diagnosed. Herebe diagnosed. Hereis a quick overview of the is a quick overview of the above mentioned tests:above mentioned tests:
Turns Ratio Test (Common to all categories)Turns Ratio Test (Common to all categories) TheTheTransformer TurTransformer Turns Ratio tesns Ratio test (TTR) is t (TTR) is usedused
to make sure that the Turns Ratio bet
to make sure that the Turns Ratio between the windings of ween the windings of the transformer is correct. This ratiothe transformer is correct. This ratio decides what the output voltage of the
decides what the output voltage of the transformer will be with respect to the transformer will be with respect to the input voltage. Theinput voltage. The ratio is calculated under no-load
ratio is calculated under no-load conditions, with ratios calculated at the tap conditions, with ratios calculated at the tap positions for eachpositions for each winding and for the winding as a whole.
winding and for the winding as a whole.
A voltage is applied to one winding and the voltmeters connected to both low voltage and high A voltage is applied to one winding and the voltmeters connected to both low voltage and high voltage windings are read s
voltage windings are read simultaneously. The transformer ratio is the ratio of the HV imultaneously. The transformer ratio is the ratio of the HV voltmetervoltmeter and the LV voltmeter readings. When
and the LV voltmeter readings. When ratio tests are being made on three-phase transformers, theratio tests are being made on three-phase transformers, the ratio is taken on one phase at a time, and the measured ratio should be compared with the ratio ratio is taken on one phase at a time, and the measured ratio should be compared with the ratio calculated using nameplate vo
calculated using nameplate voltages. Any variation ltages. Any variation should be withinshould be within0.5%.0.5%.
Insulation Resistance Test (Common to all categories)Insulation Resistance Test (Common to all categories) thethewindinwinding insulation resg insulation resistanistance testce test
(also known as the Meggar
(also known as the Meggar test) is a measure of quality test) is a measure of quality of insulation within the transformer. It canof insulation within the transformer. It can vary due to moisture content, cleanliness and
vary due to moisture content, cleanliness and the temperature of the insulation parts. Allthe temperature of the insulation parts. All measurements are corrected to 20'C for comparison purposes. It is recommended that tank measurements are corrected to 20'C for comparison purposes. It is recommended that tank andand core are always grounded when this
core are always grounded when this test is performed. Each windingtest is performed. Each windingshould be short-circuited atshould be short-circuited at the bushin
the bushing terminals. Resig terminals. Resistancstanceses are then measurare then measured between each winding and all othered between each winding and all other windings and ground (for 2 winding tra
windings and ground (for 2 winding transformer -nsformer -H-LG, L-HG and HL-G H-LG, L-HG and HL-G and three windingand three winding transformer H-LTG,
transformer H-LTG, L-HTG, T-HLG, HL-TG, HT-LG, L-HTG, T-HLG, HL-TG, HT-LG, LT-LT-HG and HLT-G ).HG and HLT-G ).
vPower Factor (Common to all categories)vPower Factor (Common to all categories)This test is made to mThis test is made to monitor the dryness ofonitor the dryness of
transformer insulation. Power factor is defined as the r
input volt-ampere multiplied by 100. The
input volt-ampere multiplied by 100. The measurement of power factor is made with ameasurement of power factor is made with a capacitance bridge and the connections are
capacitance bridge and the connections are the same as for the insulation the same as for the insulation resistance tests.resistance tests.
Resistance (Common to all categories)Resistance (Common to all categories)thetheresistresistance of a transformeance of a transformer winding can ber winding can be
measured after current has not passed through the transformer for several hours, allowing measured after current has not passed through the transformer for several hours, allowing it toit to reach the same temperature as its surroundings. Winding
reach the same temperature as its surroundings. Winding resistance is calculated by measuringresistance is calculated by measuring the voltage and current simultaneously, with the
the voltage and current simultaneously, with the current as close to the rated current as current as close to the rated current as possible.possible. Calculating the winding resistance can be helpful as it lets you calculate and compensate for I2R Calculating the winding resistance can be helpful as it lets you calculate and compensate for I2R losses, a major component of load losses as a whole.
losses, a major component of load losses as a whole.
Winding resistance measurements can be made to determi
Winding resistance measurements can be made to determine if any changes have occurred inne if any changes have occurred in the current carrying path. The winding resi
the current carrying path. The winding resistance measurements should be made with astance measurements should be made with a Wheatstone bridge, Kelvin bridge or similar bridge capable of measuring fractional ohms Wheatstone bridge, Kelvin bridge or similar bridge capable of measuring fractional ohms accurately.
accurately. For Wye cFor Wye connected values, onnected values, measurements should measurements should be made be made between each between each pair ofpair of bushings, then summed and multiplied
bushings, then summed and multiplied by three-halves to get the comparison value.by three-halves to get the comparison value.
Oil Test (Common to all categories)Oil Test (Common to all categories) AAsample of insulating sample of insulating oil from a transforoil from a transformer in service mer in service cancan
reveal much information about what is taki
reveal much information about what is taking place inside the transformer. There are three basicng place inside the transformer. There are three basic enemies to insulating
enemies to insulating oil -oil -oxidation, contamination and oxidation, contamination and excessive temperature. excessive temperature. The followingThe following tests can be done:
tests can be done:
o
o Acid numberAcid number o
o Dielectric breakdownDielectric breakdown o
o Power factorPower factor o
o Moisture contentMoisture content o
o Interfacial tensionInterfacial tension
After performing the tests the oil can classified as
After performing the tests the oil can classified as reusable; reusable with mireusable; reusable with minor reconditioning; ornor reconditioning; or disposable.
disposable.
Polarity (Acceptance test)Polarity (Acceptance test) thethepolarity of a transformepolarity of a transformer is either additive or subtractive. r is either additive or subtractive. In orderIn order
to find out
to find out the polarity of a the polarity of a transformer, a transformer, a voltage is applied betvoltage is applied between the primary busween the primary bushings.hings. If theIf the resultant voltage between the secondary bushings is
resultant voltage between the secondary bushings is greater than the applied voltage greater than the applied voltage that meansthat means that the t
that the transformer has ransformer has additive polarity.additive polarity. If it is If it is lower, the transformer lower, the transformer has subtractive has subtractive polarity.polarity. Polarity is not important for a single
Polarity is not important for a single connected distribution transformer, but it is a vital connected distribution transformer, but it is a vital concern ifconcern if transformers are to be paralleled or bank
transformers are to be paralleled or bank connected. Three phase transformers are also checkedconnected. Three phase transformers are also checked for polarity by the same means.
for polarity by the same means.
Phase Relation (Acceptance test)Phase Relation (Acceptance test) aaphase rphase relation telation test is caest is carried out rried out for polypfor polyphasehase(for(for
instance, three-phase) transformers to make sure that they have been connected in
instance, three-phase) transformers to make sure that they have been connected in such a waysuch a way that their
and polarity tests.
and polarity tests. The voltages of The voltages of the phase of the phase of primary and secondary can primary and secondary can be recorded be recorded andand comparisons made to get the phase relation.
comparisons made to get the phase relation.
Visual Inspection (Periodic and Failure tests)Visual Inspection (Periodic and Failure tests) thisthismay remay reveal eitveal eitherherprespresent or poent or potenttentialial
problems that may not be picked up
problems that may not be picked up by diagnostic testing. For example, by diagnostic testing. For example, deteriorating gaskets, lowdeteriorating gaskets, low oil level or chipped bushing skirts. A standard list of check points should be established for each oil level or chipped bushing skirts. A standard list of check points should be established for each unit and then a record of each inspection maintained.
unit and then a record of each inspection maintained.
Gas/ Gas-in-Oil Test (Failure test):Gas/ Gas-in-Oil Test (Failure test): A study of gases either dissolved in the oil or from the gasA study of gases either dissolved in the oil or from the gas
above the oil can also show abnormal conditions within the transformers, such as incipient faults. above the oil can also show abnormal conditions within the transformers, such as incipient faults.
Three considerations are very important: Three considerations are very important:
o
o The total percentage of combustible gasThe total percentage of combustible gas o
o The percentage of each gas componentThe percentage of each gas component o
o The rate of change in combustible gas contentThe rate of change in combustible gas content o
o If the percentage of combustible gases is above 5%, If the percentage of combustible gases is above 5%, then immediate action is requiredthen immediate action is required
ExcitaExcitationtionCurreCurrentntTest (PTest (Perioderiodic and Failuric and Failure tests)e tests)thetheexcitexcitation curration current is the minimument is the minimum
amount of current needed to maintain the
amount of current needed to maintain the core in a state of mcore in a state of magnetic excitation. It is measured atagnetic excitation. It is measured at the rated voltage, and usually given
the rated voltage, and usually given as a percentage of the as a percentage of the rated current.rated current.
The test is performed with a
The test is performed with a single phase supply with,single phase supply with,preferably, a voltagpreferably, a voltage rated ate rated at
approximately 10% of the phase voltage of the winding to which the supply is to be connected, approximately 10% of the phase voltage of the winding to which the supply is to be connected, although lower voltages can be used.
although lower voltages can be used. There are 2 methods that caThere are 2 methods that cann be used: the first is tbe used: the first is too connect a single-phase supply to any available winding with an ammeter in the circuit to monitor connect a single-phase supply to any available winding with an ammeter in the circuit to monitor the exciting
the exciting current.current. Three such Three such single-phase testsingle-phase tests are necessary s are necessary for a for a three-phase three-phase transformer.transformer. The relationship between
The relationship between the singlethe singlephase readings is phase readings is important; it should important; it should be as follows:be as follows:
The readings taken on phase A and C should be within 5% of each other.The readings taken on phase A and C should be within 5% of each other.
The reading on phase B should be between 65 and 90% of the readings on phase A and C.The reading on phase B should be between 65 and 90% of the readings on phase A and C.
Readings that fall outside of the r
Readings that fall outside of the relationships given above may be indicators elationships given above may be indicators of a winding fault. Inof a winding fault. Inthethe other method, the same voltage level and
other method, the same voltage level and ammeter requirements apply except the following connectionsammeter requirements apply except the following connections should be made:
should be made:
Short one winding on phase C and apply voltage and read the exciting current on phase A.Short one winding on phase C and apply voltage and read the exciting current on phase A.
Short one winding on phase A and apply voltage and read the exciting current on phase C.Short one winding on phase A and apply voltage and read the exciting current on phase C.
Short one winding on phase B and apply voltage and read the exciting current on phase A orShort one winding on phase B and apply voltage and read the exciting current on phase A or
phase C. phase C.
Other Tests Other Tests
Other tests which can be p
Other tests which can be performed are:erformed are:
Core Loss TestCore Loss Test
Under no-load conditions, a transformer will
Under no-load conditions, a transformer will continue to drain sources of electrical energcontinue to drain sources of electrical energy. They. The chief source of this drain is core loss, which occurs in the magnetic core through a combination of chief source of this drain is core loss, which occurs in the magnetic core through a combination of hyste
hysteresis anresis and eddy currend eddy current loss, amont loss, amonggotheothers.rs. Core-lCore-loss is calculaoss is calculated by applyted by applying the rating the rateded voltage and
voltage and frequency to frequency to a transformer a transformer under no-load under no-load conditions.conditions. The resultant The resultant current is current is thenthen measured, from which the loss of energy
measured, from which the loss of energy can be extrapolated.can be extrapolated.
Load Loss TestLoad Loss Test
Load loss is a combination
Load loss is a combinationof I2R losses, stof I2R losses, stray losses and eddy losses, all of which contribute ray losses and eddy losses, all of which contribute toto the loss of electrical energy
the loss of electrical energy that is seen as current transferred from one winding to that is seen as current transferred from one winding to another. Loadanother. Load loss changes with the magnitude of the load: that is to say, higher loads see higher rates of loss. loss changes with the magnitude of the load: that is to say, higher loads see higher rates of loss. The load loss is t
The load loss is therefore generally calculated for the rated load, whiherefore generally calculated for the rated load, while the transformer is underle the transformer is under full-load conditions. It can be measured by applying a voltage to one winding while the other full-load conditions. It can be measured by applying a voltage to one winding while the other winding is short-circuited. The voltage is adjusted until the current flowing through the circuit is winding is short-circuited. The voltage is adjusted until the current flowing through the circuit is the same as the rated current. The power l
the same as the rated current. The power loss measured at this time is oss measured at this time is the load loss.the load loss.
Impedance TestImpedance Test
Impedance is a measure of the resistance that l
Impedance is a measure of the resistance that leads to the loss of electrical eads to the loss of electrical energy in aenergy in a transformer at full load, causing the ratio of
transformer at full load, causing the ratio of the input and output voltages to diffthe input and output voltages to differ from the Turnser from the Turns Ratio. It can be measured at the sam
Ratio. It can be measured at the same time as load loss. e time as load loss. Impedance is found by measuring theImpedance is found by measuring the voltage required to pass the rated current through one
voltage required to pass the rated current through one winding of the transformer, while the otherwinding of the transformer, while the other winding is short-circuited. This voltage is called the impedance voltage.
winding is short-circuited. This voltage is called the impedance voltage.
Applied Potential TestApplied Potential Test
The applied potential test is used to see how well the transformer's insulation deals with voltages The applied potential test is used to see how well the transformer's insulation deals with voltages higher than the rated voltage, for given periods of time. The applied potential test checks the higher than the rated voltage, for given periods of time. The applied potential test checks the insulation between individual windings; and between windings and ground by appl
insulation between individual windings; and between windings and ground by applying voltages toying voltages to each of these areas.
each of these areas.
Induced Potential TestInduced Potential Test
The induced potential test is
The induced potential test is used to test the quality used to test the quality of the transformer's insulation, as with theof the transformer's insulation, as with the applied potential test above. It tests the insulation of the individual windings of the transformer by applied potential test above. It tests the insulation of the individual windings of the transformer by applying voltages between turns, between layers and between lines.
applying voltages between turns, between layers and between lines.
Quality control impulse testQuality control impulse test
Quality control impulse tests are made
Quality control impulse tests are made on transformers in order to simulate lightning; to see on transformers in order to simulate lightning; to see howhow well they
well they withstand such withstand such high bursts of high bursts of voltage.voltage. The electric The electric impulses applied here impulses applied here can includecan include reduced
reducedfull-wave tests, chopped-wave tests afull-wave tests, chopped-wave tests and front-of-wave testnd front-of-wave tests, to simulate a range ofs, to simulate a range of extreme voltage situations.
Pressure Leak TestPressure Leak Test
A transformer can be checked for pressure leaks
A transformer can be checked for pressure leaks by pressurizing the tank and then lby pressurizing the tank and then leaving iteaving it alone for several hours. If the pressure drops
alone for several hours. If the pressure drops during the intervening time, or if during the intervening time, or if there are signs ofthere are signs of liquid leakage, than
liquid leakage, than a leak is a leak is present.present. Otherwise, the Otherwise, the transformer is transformer is leakage freeleakage free..
While learning about and overseeing
While learning about and overseeing the standard testing procedures of your transformer can be athe standard testing procedures of your transformer can be a laborious task, it definitely helps better your understanding
laborious task, it definitely helps better your understanding of the transformer's operation, minimizesof the transformer's operation, minimizes hazard to life and property, reduces downtim
hazard to life and property, reduces downtime, minimizes the chance of sudden failure and e, minimizes the chance of sudden failure and thus allowsthus allows optimum use of the transformer.
