NPTI Transformer New

8/20/2019 NPTI Transformer New

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Transformer testing

By

Nafees Ahamed

Megger India


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Transformer operation:

Power in Power out

Vout * IoutVin * Iin

Power IN = Power OUT

• STEP-UP transformer: Vout > Vin and Iout< Iin

• STEP-DOWN transformer: Vout < Vin and Iout > Iin


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Maintenance of Transformer should include Routine Inspection & repair

Transformer liquid maintenance & testing Transformer winding Insulation maintenance & testing


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Why test transformers?

Transformers are tested to prevent failures

Safeguard against breakdowns by detecting potential causes and eliminating

them and ensure many years of trouble free operation

Unplanned outage – high cost to operations

• Many Utilities believe their wires not only carry electricity, but they also

carry money !

Replacement Transformer - very expensive

Catastrophic failure can cause ancillary damage and even death


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Transformer – Failure Symptoms

Turn-to-Turn short

Open Winding

Poor connection

Insulation damage

Moisture in Insulation

Mechanical damage

Core overheating


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Transformer – Prescribed tests

Failure symptom Preventative and Diagnostic test

Turn-to-turn short Turns Ratio

Winding

Resistance

Excitation

Current

Open Winding Turns Ratio

Winding

Resistance

Poor connection Turns Ratio

Winding

Resistance

Insulation damage Power Factor

Excitation

Current

Insulation

Resistance FRA

Moisture in Insulation Power Factor

Insultation

Resistance

Moisture in

Oil test

Mechanical damage

Excitation

Current Power Factor FRA

Core overheating

Insulation

Resistance

Excitation

Current


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Transformer Acceptance tests

Insulation resistance test

AC or DC hipot test(Optional)

Insulation PF test

Transformer turns ratio test

Polarity test

Excitation current test

Insulating fluid dielectric(BDV) tests

Sweep frequency response analyzer


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Routine field tests on Transformer

Insulation resistance test

AC or DC hipot test(Optional)

Insulation PF test

Transformer turns ratio test

Polarity test

Excitation current test

Induced potential test(Optional)

Transformer core ground test Insulating fluid dielectric(BDV) tests

Dissolved gas analysis(DGA)tests

Sweep frequency response analyzer

Polarization recovery voltage test(Obsolete) now replaced withDielectric frequency response test

DC winding resistance


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Transformer Turns Ratio

Applies voltage to one winding of a Transformer and detects the voltage being

generated on another winding on the same core

Voltage Ratio is directly proportional to the Windings Turns Ratio:

Np VpTTR = ------- = ------

Ns Vs

Where:

NS = Number of turns of the Secondary winding

NP = Number of turns of the Primary winding

VS = Volts on the Secondary

VP = Volts on the Primary


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Three–Phase Transformer

CoreWindings

A B C

Phases:A,B,C


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3-Phase Transformer

Configurations

DELTA

A

B C

A B

C

Possibleconfigurations:

WYE


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TTR tests provides the following information Turns ratio and polarity of single & three phase transformers, one

phase at a time Confirms name plate ratio, polarity and vectors

Identifies trouble in transformer windings, such as open-circuit andshort-circuits of turn to turn sensitivity

The standard deviation as defined by ANSI / IEEE C57.12.00-2006,

states that results should be within 0.5% of name plate markings.


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What is Power Factor ?

What is Dissipation Factor ?


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The PF / DF tests conducted in the field for acceptance(start - up)and routine field(Maintenance) testing of insulation of electricalequipment.

The purpose of these tests is to identify if the equipment has beeninstalled properly, determine if corrective maintenance and repair isneeded, and /or track the gradual deterioration of the equipment overits life.

PF /DF are not go-no-go tests, and can measure dielectric loss,Capacitance, and AC resistance of insulation.

Do not overstress the insulation and can determine if the insulation is

slowly degrading by comparison with previous tests results, or withtest results of similar equipment.


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Single–Phase Transformer

Primary Secondary


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High Low

H L

G

Chassis

Three-terminal devicefor PF/DF testing

Single–Phase Transformer

CHG CLG

CHL


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G

A

B C

A B

C

HL

Three-terminal devicefor PF/DF testing

Three – Phase Transformer


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Three-terminal device

H

L

G


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A “Three-terminal” transformer (with Primary and

Secondary winding(s), has three capacitances:

CHL, CHG, and CLG

A perfect capacitor allows AC current to flow (IC), leading

the voltage by 90º.

In “practice”, however, capacitors allow leakage current

(IR), which is in phase with the voltage (0 º).


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In practice, Capacitors have leakage (loss) current:

V0

IC

IR

I

δ

IR

Tan δ = = DFIC

IR= PF

I

Normally, IR is less than 1/100th of IC (or


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Factor influence PF measurements

Temperature

Humidity

Surface leakage

Dependent upon moisture and cleanliness of the surface of thespecimen under test.

When making PF tests, the effects of surface leakage (due tohumidity and dirt etc.,) should be recognised and addressed

accordingly. The effect of surface leakage current may be minimised by cleaning

and drying external surfaces to reduce the losses, or using guardcollars to divert the surface leakage current from measuring circuit


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Excitation current

Commonly known as no load or magnetizing current of thetransformer.

In this test,voltage is applied to the primary winding one at a time

with all other winding left open. Excitation current is the current the transformer draws when voltage

is applied to its primary terminals with the secondary terminal open.

Excitation current provides means of detection for winding

problems(short-circuited or open turns, poor joints or contacts, coreproblems etc.,

Conducted on each phase winding at a time, that is only one windingis under test with the other winding including the secondary windingare floating.

Measure is in UST mode.


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Evaluation and grading of PF and DF Test Results

The evaluation criteria may be divided into four

catagories.

Good: Insulation condition is good and suitable for continued service

Deteriorated: Insulation condition is satisfactory for service butshould be checked within six months to see if the condition hasfurther degraded

Marginal: Insulation condition is not satisfactory for service-

immediate investigation of the degraded conditions should be begunand if this is not possible then it should begun as soon as possible

Bad: Remove from service and recondition to restore insulation togood condition, if not possible, then replace


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Winding resistance

Measuring a transformer's winding (DC) resistance from one external terminalto another can reveal a great deal of information about the transformer. Inaddition to the obvious faulted winding (i.e., an open winding), more subtleproblems can be detected. The DC current, in addition to flowing through thewinding, must also flow through the off-load ratio adjusting switch (De-Energized Tap Changer or DETC) and the on-load ratio adjusting switch(Load Tap Changer or LTC), as well as numerous welded and mechanicalconnections. Hence, the integrity of all these components can be verified.

Measuring the resistance of the windings assures that the connections arecorrect and the resistance measurements indicate that there are no severemismatches or opens. Many transformers have taps built into them. Thesetaps allow ratio to be increased or decreased by fractions of a percent. Any ofthe ratio changes involve a mechanical movement of a contact from one

position to another. These tap changes should also be checked during awinding resistance test.


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Regardless of the configuration, either star or delta, the measurements arenormally made phase to phase and comparisons are made to determine if thereadings are comparable. If all readings are within one percent of each other,then they are acceptable. Keep in mind that the purpose of the test is tocheck for gross differences between the windings and for opens in theconnections. The tests are not made to duplicate the readings of themanufactured device which was tested in the factory under controlledconditions and perhaps at other temperatures.


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When to measure?

At InstallationRisk of damage is significant whenever a transformer is moved. This is inherent to thetypical transformer design and modes of transportation employed. Damage can alsooccur during unloading and assembly. The damage will often involve a current carryingcomponent such as the LTC, DETC or a connector. Damage to such components mayresult in a change to the DC resistance measured through them. Hence, it isrecommended that the DC resistance be measured on all on-load and off-load taps priorto energizing. If the transformer is new the resistance test also serves as a verification ofthe manufacturers work. Installation measurements should be filed for future reference.

At Routine(scheduled) Transformer maintenance

Routine maintenance is performed to verify operating integrity and to assure reliability.Tests are performed to detect incipient problems

At unscheduled maintenance / troubleshootingUnscheduled maintenance generally occurs following a system event. The objectives ofunscheduled maintenance are:To detect damage to the transformer.

To determine if it is safe to re-energize.To determine if corrective action is necessary.To establish priority of corrective action.


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Interpretation of Measurements

Measurements are evaluated by:

Comparing to original factory measurements Comparing to previous field measurements Comparing one phase to another

The industry standard (factory) permits a maximum difference of 1/2 percentfrom the average of the three phase windings. Field readings may varyslightly more than this due to the many variables. If all readings are within onepercent of each other, then they are acceptable. Variation from one phase toanother or inconsistent measurements can be indicative of many different

problems: Shorted turns

Open turns Defective DETC or LTC

Poor connections (brazed or mechanical)

The winding resistance test is very useful in identifying and isolating thelocation of suspected problems.

NPTI Transformer New

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