npti transformer new
TRANSCRIPT
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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.