Winding Resistance Measurement: Case Studies and Field Experiences

Winding Resistance Measurement:
Case Studies and Field Experiences
1. What parameters are used to define a stable winding resistance reading?
The stability of winding resistance measurements is driven by the source of power
used and the transformer under test. Important factors for obtaining a stable
reading are the magnitude of test current selected, the inductive nature of the
transformer and the winding configuration. If the test current level is too low, it may
take a long time for the current to settle and even longer to get a stable reading.
On the other hand, for a Delta winding, the resistance reading may seem stable
initially but this is not the final reading. The other two phases in the Delta
connection form a parallel path for test current and these phases must saturate
only after which a final accurate resistance measurement reading is obtained.
Megger winding resistance instruments have two conditions for a stable reading.
The first is the percentage stability where the reading is compared to the previous
value and the change is calculated based on the setting used (99.5% to 99.9%).
The second condition is the time for which the resistance reading does not change.
This setting is configurable from 2 to 100 seconds.
2. How do you calculate the percent difference in winding resistance
measurements?
IEEE C57.152 (Diagnostic Field Testing guide for fluid-filled power transformers,
reactors and regulators) states that the resistance of windings between phases
should be less than 2%. The percentage difference is calculated as below:
(
𝑅𝑚𝑎𝑥 − 𝑅𝑚𝑖𝑛
𝑅𝑎𝑣𝑔
) ∗ 100

Where, Rmax and Rmin are the highest and lowest resistance readings respectively
while Ravg is the average of resistance readings for transformer winding under test.
3. Is it recommended to exercise DETC contacts for distribution transformers?
Which standard should I refer?
IEEE C57.152 Diagnostic Field Testing Guide discusses De-energized Tap
Changers (DETC) and states the following,
“The construction of DETCs is such that they shall be operated only with the
transformer de-energized. Failure to do so will result in severe equipment damage,
personal injury, and possible loss of life. If the DETC has been left in the same
position for a long period of time, extreme caution should be used when changing
taps because the drive mechanism may seize or fail from the contacts becoming
stuck or possibly coked due to non-operation.”
4. Can I use a low resistance meter to perform winding resistance
measurements on a small transformer? Do you recommend a DLRO test
when troubleshooting a connection (Hotspot) issue?
Due to the inductive nature and magnetization requirements of transformers, it is
recommended that specifically designed test instruments be used for winding
resistance measurement. Instruments with the capability to handle the
energy discharged during the test should be used to test small distribution
transformers and motors. Megger instruments such as the MTO, MWA and TRAX
can also demagnetize the core after completing the test – a feature not available
with Low Resistance testers. Winding resistance measurements are sensitive
enough to detect hotspot issues and comparing results with historical data and oil
diagnostic data will help to determine the location and nature of hotspots.

5. When testing for different Open Circuit Voltages, why did the ‘B’ phase take
shorter time to provide a stable reading in both cases?
While measuring winding resistance, it is imperative to saturate the transformer
core to nullify the inductive effect of the winding and the core. To establish
saturation in the core, the flux must overcome the reluctance (analogous to
resistance in electric circuits) of the core. Core reluctance is directly proportional
to the length of the magnetic path that the flux has to travel. Typically, for the
‘middle’ phases of the transformers, the length of the magnetic flux path is smaller
when compared to that of the ‘outer’ phases. As a result, the reluctance of the
magnetic path for the middle or B phase is lesser and it saturates faster than the
outer phases A and C.
6. Most winding resistance instruments only select current while the voltage is
selected by the instrument automatically. How do you select voltage?
Resistance measurement of transformer windings is an application where test
instruments have to be specifically designed to saturate the transformer core in
order to get an accurate measurement. Transformer saturation is dependent on
the magnitude and the stability of test current as well as on the voltage applied to
drive the current. The open circuit voltage of a winding resistance test set is
important to ensure proper core saturation prior to making a resistance
measurement. This open circuit voltage is inversely proportional to the test current
magnitude. Thus, a higher test current magnitude may not have a high enough
voltage to saturate the transformer core. It is recommend that a lower current
range be selected such that the open circuit voltage is higher and can give better
voltage stabilization.

7. Could winding resistance measurements differ when testing On Load Tap
Changers (OLTC) from raise to lower positions and vice versa? Is there any
specific method to use when testing OLTCs?
OLTCs are part of the transformer winding and as such do not require any specific
methods different from winding resistance measurements. It is crucial to maintain
the direction of flux while making resistance measurements and hence the
advancing taps should be followed to maintain flux direction. For OLTCs, it
important to check the make-before-break functionality such that the OLTC
maintains a continuous circuit during normal operation and tap changes. Dynamic
Resistance Measurement (DRM) is an advanced diagnostic tool for OLTCs to
detect problems that cannot be determined by static winding resistance
measurement. In this test, the resistance trace is plotted while the tap changer is
switched from one extreme tap position to the other and back.
8. What is the importance of demagnetization? Can other tests be affected if I
do not demagnetized? What current magnitude to use?
A transformer core with residual magnetism could affect Turns Ratio results,
excitation current results and Sweep Frequency (SFRA) measurements. The
excitation current values measured could be erroneous because of the residual
magnetism. Residual magnetism results in a shift in the low frequency range in
open circuit measurements during a SFRA test. Hence, it is important to
demagnetize the transformer core at the end of the winding resistance test. If the
winding resistance test set is incapable of demagnetizing the core, then the tests
mentioned above need to be done before the winding resistance test. In order to
demagnetize, it is recommended to select the same level of test current that was
used to make resistance measurements. One of the acceptable demagnetization
methods involves reducing the magnitude and changing polarity of the current
throughout the demagnetization process.
9. When should the dual winding injection be used?
The Dual Winding Injection method could be used for transformers that have
winding configurations that would normally not stabilize quickly to provide an
accurate reading. For example, transformers with Delta secondary windings are
good candidates. The untested phases of a Delta winding load the measurement
of the phase under test by drawing current. This affects the time for saturating the
transformer core and could give erroneous resistance measurements if sufficient
time is not given for the reading to stabilize. However, it is important to note that
the Dual Winding Injection technique is found to be most effective when the turns
ratio of the transformer is 10 or less. For transformers that have higher ratios, the
difference between minimum currents required for winding resistance
measurement could be quite large. This could lead to either excessive current flow
through the high voltage winding while testing the low voltage winding or very low
current flow through low voltage windings if the high voltage winding is tested.
10. Do you recommend performing three-phase winding resistance
measurement?
Three-phase testing includes utilizing three current sources to inject current
simultaneously into all three windings and measuring the voltage drop. Threephase
testing provides some timesavings as all three phases are measured
simultaneously and three currents are used to magnetize the core to achieve core
saturation faster. However, single-phase and three-phase testing results may differ
slightly because of the measurement error introduced during three-phase
measurement. For Wye configured windings, performing a three-phase
measurement would result in current flowing through the neutral that is 3 times as
compared to single-phase measurements. The differences between the two
results can be significant, pending WR measurement values (< 5 m ohms) and
may cause variation of more than the 2% limit allowed. Consequently, three-phase
measurement results would generally introduce a positive error. Another limitation
is that three-phase testing can only be performed on Wye windings. A closed loop
delta winding would provide erroneous results when utilizing the three-phase WR
method. For tap changer with Wye configured windings, it may be possible to
perform three-phase measurements but a separate round of tests would be
needed to verify the make-before-breaker functionality that is essential to tap
changer operation.

Here are some interesting things about...