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TAP-CHANGER TESTING USING DRM
Matz Ohlen1*, Nils Wäcklen2, Rene Yvrard2
1Megger Sweden AB, Stockholm, Sweden
2TSV, Lyon, France
*Email: <[email protected]>
Abstract: Dynamic Resistance Measurement (DRM) has been used for circuit-breaker
diagnostics for over 20 years. It is an interesting technique that can be used also to verify
the switching operation of load tap changers (LTC). Existing methods and techniques for
dynamic measurements on load tap-changers are based on measuring current and/or
voltage on the primary side of the transformer and short-circuiting the secondary side to
minimize the inductance in the circuit. Static resistance measurements per tap are
performed in a separate test sequence with the secondary side open. A new technique
(patent pending) is to combine current measurement with voltage measurements on both
primary and secondary side of the transformer and then use the transformer parameters
to calculate inductive and resistive voltages to be able to calculate the dynamic
resistance during a tap change. Measurements have been performed on non-mounted
tap-changers (no-oil condition) and after being mounted inside a transformer (with oil).
Different test setups have also been evaluated including the new technique where current
measurement is combined with voltage measurements on both primary and secondary
side of the transformer.
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INTRODUCTION
The power transformer is an integral and
expensive part of all electric power networks at all
levels from generation and transmission down to
distribution. The on-load tap changer is the only
moving part connected to the transformer
windings. The importance of its reliability cannot be
overemphasised. Taking a transformer off the
system to investigate an internal problem with a
tap changer is an expensive exercise; therefore it
is in every utility’s interest to carry out condition
assessments of their tap changers to help detect
developing faults at an early stage. The tap
changer is the only moving part of a transformer,
and as such is the most susceptible to failure.
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TAP CHANGER TYPES
Tap changers can be divided into two main types;
On-load (OLTC, On-load Tap Changer) and Offload/de-energized (DETC, De-Energized Tap
Changer). The OLTC allows selection of voltage
change while the transformer is in service. This
type of tap changer allows voltage output of a
transformer to be changed while power (current) is
still passing through it. In Europe and
internationally, the most common configuration is
to have the tap-changer on the HV side of the
transformer. Figure 1 depicts a typical on-load tap
changer with tap selector and diverter switch with
transition resistors. Figure 2 depicts a linear-type
OLTC with diverter resistors.
Figure 1. A typical diverter switch type OLTC
showing both tap selector switch and diverter
switch (MR)
The resistors in the diverter switch are typically a
few ohms. Total operation time of an LTC is
between 3 and 10 sec pending design. Contact
switching time is usually in the order of 30-100 ms
(resistor types)
Reactance type LTC’s, common in US and mostly
mounted on the LV side of the transformer, use a
preventive auto transformer instead of the two
resistors in the standard diverter switch which
means that the additional resistance in the diverter
device is very low. Measurements on reactance
type load tap-changers are not covered in this
paper.
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DYNAMIC MEASUREMENTS ON TAPCHANGERS
There are several methods developed for dynamic
testing of tap changers but common for all is that a
current is injected in the tap changer and during
the operation of the tap changer, the current and/or
the voltage is measured as a function of time. Test
currents vary from about 0.1 A to standard test
current for winding resistance measurements
(typically 1% of rated current for the transformer
winding). The measurements are sometimes
performed at the same time as measuring winding
resistance and sometimes as a separate test. The
most common tests are;


Figure 2. A typical OLTC with selector switch (17
taps linear) (ABB)
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STATIC MEASUREMENTS ON TAPCHANGERS
The most common diagnostics test for tapchangers is to perform winding resistance
measurements [1]. WRM is normally performed for
each tap in the same way as an individual winding
without taps. A suitable test current is injected
through the winding and the resistances for each
tap are measured sequentially as the tap changer
is stepped through its positions. Results are
typically presented as a graph or table with
resistance values for each tap. Resistance
changes between taps should be consistent with
only small deviations between different tap position
changes. An example showing deviations between
taps is shown in figure 3. Red phase – OK, Blue
phase – “Questionable”
Figure 3. WRM/tap, diverter design – Old/aged
condition
4.1
Continuity/break-before-make testing
o Shut-off if current path is
interrupted
Dynamic measurements
o Dynamic current/”ripple”
o Dynamic voltage
o Dynamic resistance, DRM
Continuity testing
This test should detect if there is a break-beforemake condition in the tap changer by monitoring
current change. The measurement is typically
performed at the same time as winding resistance
measurements and the instrument detects if the
contact switching is continuous or if there is an
interruption in the current path.
Open contact detection can be made by a variety
of methods/detection principles but common for all
of them is that if the current change (dI/dt) exceeds
a certain value, the instrument identifies this state
as “open circuit” and sets an alarm or stops test
current injection.
4.2
Dynamic current measurements
Dynamic current measurements are pending test
current. If the test is performed at a current level
below saturation level, the inductance in the
transformer winding is high and smoothes the
current change. If the test is performed at a current
level at or above saturation level, the inductance is
low and current level change will be higher [1].
A method to reduce transformer inductance when
performing TC measurements is to short-circuit the
not tested corresponding LV (or HV) windings. This
action is principally “replacing” the inductance of
the winding with the short-circuit impedance.
Inductance is greatly reduced and changes in
current can be measured more precisely. Figures
4-7 below show dynamic current measurements on
a 30 MVA, 130/46 kV, YNyn0 transformer using
different test currents and with LV windings open
and shorted [2]
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As seen in the figures, dynamic current
measurements are pending the inductance of the
circuit. If the test is performed at high current or
with LV shorted (low inductance), timing of the tap
switch can be identified and estimated. Resistor
values cannot be estimated.
4.3
Figure 4. 10 A test current, shorted LV
Dynamic Resistance Measurement
with shorted LV
Dynamic resistance measurement (DRM) is a
standard method for circuit-breaker testing and can
also be applied to tap changer contacts [3]. A
relatively small test current (0.1 to 1A) is injected
through the tap changer using a high-impedance
current source and the LV windings of the
transformer are short-circuited. The voltage over
the test circuit is measured and resistance versus
time can be calculated. Contact timing as well as
diverter resistor values can be measured. The test
setup is described in figure 8 and an example from
a 20MVA, 22/10 kV transformer is shown in figure
9 and 10.
Figure 5. 1 A test current, shorted LV
Figure 8. Dynamic Resistance Measurements on a
load tap-changer (direct method)
Figure 6. 10 A test current, open LV
Figure 9. DRM on OLTC, 0.1 A test current,
constant current source, shorted LV. Red; Current,
Blue; Resistance
As seen in the figure, contact timing and resistor
values are easily recognized. Figure 8 provides a
summary of measured switch times for this linear
type tap-changer with 17 taps. No malfunctions
detected.
Figure 7. 1 A test current, open LV
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OLTC MEASUREMENTS AT TSV, LYON,
FRANCE
A mutual project between Megger and TSV
(Transformer Service Venissieux) to study different
test methods for DRM on tap-changers is ongoing
at TSV, Lyon, France. Several tap-changers are
measured with and without simulated faults, as
separate units as well as mounted in transformers
[4].
5.1
Figure 10. H1 OLTC transition times (ms). 1-2 to
16-17 tap change
4.4
Dynamic resistance measurement
with open LV
A new method (patent pending) is to measure
dynamic resistance in the tap-changer by
simultaneously measuring the test current together
with voltages on both HV and LV windings and
combine the results with transformer modeling.
The test setup is the same as in figure 6 but with
the difference that the LV winding is left open. An
example of a measurement is shown in figure 11.
The source impedance in this example is about 10
ohm and we can see a small current change during
tap change. Due to the inductance in the circuit,
the voltage change on the primary (HV) side is
rather high (open LV). This voltage is a sum of
inductive and resistive voltage and cannot be used
for directly calculating the resistance in the circuit.
However the voltage on the secondary (LV) side is
purely inductive and if we use transformer model
parameters to calculate the inductive voltage on
the primary, we can deduct this value from the
measured primary (HV) winding voltage and
calculate the resistance in the circuit. The result is
shown in figure 11.
Summary of tests
In the initial tests, a separate (not mounted in a
transformer) unit V-type OLTC was used as a test
object for verifying the methodology. Test
equipment was a constant current supply and a
data recording device, test setup as in figure 8.
The initial measurement results for verifying the
methodology on a tap-changer without any known
defects are summarized in Figure 12 and 13.
Figure 12. Contact timing for the load tap-changer
Figure 13. Resistance values during switching of
the tap-changer
Figure 11. DRM on OLTC, 5 A test current, source
impedance 10 Ω, Green; Test current, Red;
Primary voltage, Blue; Secondary voltage, Black;
Resistance
As seen in Fig. 12, contact timing is rather stable
except for the very first two operations 17-16 and
16-15. The reason was that the unit had not been
operated in a quite long time and this is an
example of the importance to “exercise” a
switching device before stable timing results can
be achieved. Measured resistance values (figure
13) are as expected almost constant with R1
slightly higher than R2.
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5.2
Measurement details
Switching times in the tap-changers are critical for
correct operation. Figure 14 describes the
operation of a typical design (MR)
4. R1 released
5. R2 released
6. Load contact makes
Contact bouncing can be seen, especially related
to R2 making.
Figure 15 below presents results for a simulated
fault. Spring damage, one of the two springs
removed.
Figure 14. Switching sequence in a typical OLTC
with diverter switch [5]
Figure 16. DRM on V-type OLTC, test current 5A,
simulated spring defect (one spring removed)
As seen in the figure, total switching time is about
60 ms and contains a number of switching
sequences (+ static positions before and after the
tap change).
With one spring removed, switching time is about
twice as long as when two springs are engaged.
Note that contact bouncing almost disappeared
due to the lower contact speed.
A DRM measurement on a similar design is shown
in Figure 15.
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Several failures/defects have been simulated in
attempts to characterize faults based on DRM
results and determine the viability of this diagnostic
method. The investigated tap-changer is an OLTC
type VIII 350A 60kV 17/19 3W.
Two examples of individual DRM results are
presented in Figure 14 and 15. Nominal load
resistor value is 1.9 Ω
Figure 15. DRM on V-type OLTC, test current 5A,
normal condition.
Note that this tap-changer type has 6 distinctive
states for each tap switch. Each of them is
recognized in the DRM test.
1. Load contact releases
2. R1 inserted
3. R2 inserted in parallel with R1
CONCLUSION
A CB analyzer/signal recorder with analog inputs
together with suitable power supply is a good tool
for performing various dynamic measurements e.g.
on load tap-changers.
To measure dynamic properties in a tap changer
(in this paper assumed to be mounted on the HV
side of the transformer), several methods are
possible:


Measure dynamic current/”ripple” on
primary with a constant voltage source and
secondary open
o Current drop value is strongly
pending test current/winding
inductance and can only be used
as fingerprint
o Contact timing may be measured
(difficult)
o Diverter resistor values cannot be
measured
Measure dynamic current/”ripple” on
primary with a constant voltage source and
secondary short-circuited
o Current drop value is slightly
pending test current/winding
inductance
o Contact timing can be measured
o Diverter resistor values cannot be
measured
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

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Measure dynamic resistance on primary
with a constant current source and
secondary short-circuited
o Contact timing (including loadcontacts) can be measured
o Actual diverter resistor values can
be measured (high impedance
source)
Measure dynamic voltage on secondary
winding (secondary open), dynamic
voltage on primary and test current
o Contact timing (including loadcontacts) can be measured
o Resistance can be calculated
REFERENCES
[1] D. Chajer and Matz Ohlen, “Preventive
Maintenance of Transformer Accessories Bushings & Tap Changers”, 2011 Weidman
conference, USA
[2] J. J. Erbrink et al, “Reproducibility of Dynamic
Resistance Measurement Results of On-Load
Tap Changers – Effect of Test Parameters”,
Proceedings of the 2010 International
Conference on Condition Monitoring and
Diagnosis, September 6-11, 2010, Tokyo,
Japan
[3] N. Wacklen, “Timing of Tap Changers”,
Programma Electric Application Guide, 1996
[4] P. Illuzzi, “Abstract about measures on V-type
OLTC Model”, TSV Lyon-Megger Sweden
internal report, 2012
[5] “Switching Sequence of OLTC Type OILTAP
M
Diverter
Switch”,
Presentation
by
Maschinenfabrik Reinhausen GmbH, 2002
(used with permission)