Transformer testing
By
Nafees Ahamed
Megger India
Transformer operation:
Power IN
Power OUT
Power in
Power out
Vin * Iin
Vout * Iout
STEP-UP transformer:
Vout > Vin and Iout< Iin
STEP-DOWN transformer: Vout < Vin and Iout > Iin
Maintenance of Transformer should include
Routine Inspection & repair
Transformer liquid maintenance & testing
Transformer winding Insulation maintenance & testing
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
Transformer Failure Symptoms
Turn-to-Turn short
Open Winding
Poor connection
Insulation damage
Moisture in Insulation
Mechanical damage
Core overheating
Transformer Prescribed tests
Failure symptom
Preventative and Diagnostic test
Turns Ratio
Winding
Resistance
Turns Ratio
Winding
Resistance
Turns Ratio
Winding
Resistance
Power Factor
Excitation
Current
Insulation
Resistance
Moisture in Insulation
Power Factor
Insultation
Resistance
Moisture in
Oil test
Mechanical damage
Excitation
Current
Power Factor
FRA
Core overheating
Insulation
Resistance
Excitation
Current
Turn-to-turn short
Open Winding
Poor connection
Insulation damage
Excitation
Current
FRA
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
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 with
Dielectric frequency response test
DC winding resistance
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
Vp
TTR = ------- = -----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
ThreePhase Transformer
Windings
Phases:
A,B,C
Core
3-Phase Transformer
Configurations
A
A
B
C
C
DELTA
Possible
configurations:
WYE
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 and
short-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.
What is Power Factor ?
What is Dissipation Factor ?
The PF / DF tests conducted in the field for acceptance(start - up)
and routine field(Maintenance) testing of insulation of electrical
equipment.
The purpose of these tests is to identify if the equipment has been
installed properly, determine if corrective maintenance and repair is
needed, and /or track the gradual deterioration of the equipment over
its 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 with
test results of similar equipment.
SinglePhase Transformer
Primary
Secondary
SinglePhase Transformer
CHL
Chassis
High
Low
CHG
LG
Three-terminal
device
for PF/DF testing
Three Phase Transformer
A
B
C
C
Three-terminal device
for PF/DF testing
Three-terminal device
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 90.
In practice, however, capacitors allow leakage current
(IR), which is in phase with the voltage (0 ).
In practice, Capacitors have leakage (loss) current:
IC
Tan =
IR
= DF
IC
IR
= PF
I
0
IR
Normally, IR is less than 1/100th of IC (or <1% DF)
Factor influence PF measurements
Temperature
Humidity
Surface leakage
Dependent upon moisture and cleanliness of the surface of the
specimen under test.
When making PF tests, the effects of surface leakage (due to
humidity 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 guard
collars to divert the surface leakage current from measuring circuit
Excitation current
Commonly known as no load or magnetizing current of the
transformer.
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, core
problems etc.,
Conducted on each phase winding at a time, that is only one winding
is under test with the other winding including the secondary winding
are floating.
Measure is in UST mode.
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 but
should be checked within six months to see if the condition has
further degraded
Marginal: Insulation condition is not satisfactory for serviceimmediate investigation of the degraded conditions should be begun
and if this is not possible then it should begun as soon as possible
Bad: Remove from service and recondition to restore insulation to
good condition, if not possible, then replace
Winding resistance
Measuring a transformer's winding (DC) resistance from one external terminal
to another can reveal a great deal of information about the transformer. In
addition to the obvious faulted winding (i.e., an open winding), more subtle
problems can be detected. The DC current, in addition to flowing through the
winding, must also flow through the off-load ratio adjusting switch (DeEnergized Tap Changer or DETC) and the on-load ratio adjusting switch
(Load Tap Changer or LTC), as well as numerous welded and mechanical
connections. Hence, the integrity of all these components can be verified.
Measuring the resistance of the windings assures that the connections are
correct and the resistance measurements indicate that there are no severe
mismatches or opens. Many transformers have taps built into them. These
taps allow ratio to be increased or decreased by fractions of a percent. Any of
the ratio changes involve a mechanical movement of a contact from one
position to another. These tap changes should also be checked during a
winding resistance test.
Regardless of the configuration, either star or delta, the measurements are
normally made phase to phase and comparisons are made to determine if the
readings 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 to
check for gross differences between the windings and for opens in the
connections. The tests are not made to duplicate the readings of the
manufactured device which was tested in the factory under controlled
conditions and perhaps at other temperatures.
When to measure?
At Installation
Risk of damage is significant whenever a transformer is moved. This is inherent to the
typical transformer design and modes of transportation employed. Damage can also
occur during unloading and assembly. The damage will often involve a current carrying
component such as the LTC, DETC or a connector. Damage to such components may
result in a change to the DC resistance measured through them. Hence, it is
recommended that the DC resistance be measured on all on-load and off-load taps prior
to energizing. If the transformer is new the resistance test also serves as a verification of
the 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 / troubleshooting
Unscheduled maintenance generally occurs following a system event. The objectives of
unscheduled 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.
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 percent
from the average of the three phase windings. Field readings may vary
slightly more than this due to the many variables. If all readings are within one
percent of each other, then they are acceptable. Variation from one phase to
another 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 the
location of suspected problems.
