Draft - to be published and presented with "2012 IEEE International Symposium on Electrical Insulation", 10-13 June 2012, San Juan, Puerto Rico
On-site Transformer Partial Discharge Diagnosis
Detlev W. Gross, Markus Soeller
Power Diagnostix Systems GmbH
Aachen, Germany
Abstract—In the lab, partial discharge diagnosis has widely
replaced the traditional RIV measurements. Additionally, partial
discharge acceptance levels are being reduced due to the
increasing use of composite material and a growing awareness of
partial discharge phenomenon and their consequences. Adequate
filtering of the supply voltage for induced voltage testing and the
use of sensitive acoustic measurement has greatly improved the
detection and location of partial discharge activity in power
transformers.
An increasing population of service aged substation equipment
having reached their projected service life demands on-site repair
and, hence, on-site testing to factory standards.
Using inverter based three-phase mobile test sets allows on-site
application of tests previously limited to a mere test room
environment. Besides the unmatched portability of inverterbased sources, especially the less critical generation of reactive
power simplifies on-site testing, if compared with motorgenerator sets. Additionally, the lack of high short circuit
currents limits the damage in case of breakdown. However,
adequately removing the switching noise spectrum becomes a
demanding task in order to reach the required sensitivity of the
partial discharge measurements.
Thus, acceptance testing as commonly applied in the lab,
will be increasingly used on-site to extend the service life of
old transformers, to validate the success of on-site repair, and
to ensure successful commissioning of new units.
Likewise, applying on-line monitoring of various operation
parameters including partial discharge can assist to extend the
lifetime of service aged substation equipment.
II.
PARTIAL DISCHARGE TESTING
Detecting high frequency signals using narrow-band
receivers based on heterodyne principles has been used already
very early in the history of high voltage insulation systems [1].
With early meter-type instruments diagnosis was mostly
limited to the observation of magnitude and inception vs.
extinction voltage, while later oscilloscope-base instruments
added the phase position of the discharge activity. An in-depth
understanding of the gas discharge physics and the statistics of
partial discharge was supported with the introduction of
instruments using the phase resolved partial discharge (PRPD)
pattern, or ϕ-q-n pattern [3, 4, 5].
Keywords: partial discharge; on-site; diagnosis; inverter-based;
acoustic location
I.
INTRODUCTION
The deregulation and privatization of the energy sector,
which started in the 1970ies in many areas of the world, has
had a further impact on transformers and their life. Before
deregulation with often government-owned or government
controlled utilities, availability was the most prominent design
and sourcing criterion. A side effect of this policy was “overengineering”. Deregulation shifted the emphasis to
profitability. As a consequence, re-investment into the grid
dropped significantly. However, in light of the typical service
life of a large power transformer of about 40 years, the
consequences of this change took decades to materialize, while
profits went up immediately.
Nowadays, we do have in many parts of the world serviceaged populations of sub-station equipment, of which a large
portion has reached or already exceeded its projected service
life. In Europe, for instance the majority of the 400kVtransmission-system was commissioned in the 1970ies and
1980ies. Moreover, the ongoing change from fossil fuels to
renewable sources further increase the required transmission
capacity of a service-aged grid.
Figure 1. ϕ-q-n pattern of multiple cavities (voids)
As an example, fig. 1 shows such a ϕ-q-n pattern of several
voids in epoxy resin. Here, each individual sine-shaped trace
belongs to an individual gas inclusion. Moreover, the welldistributed pattern is caused by a low availability of the starting
electron for the discharge avalanche, as it is typical for bubbles
in polymeric material such as fresh epoxy resin [6].
Fig. 2, instead, shows the activity of several gas inclusions
with the casein glue of barriers and spacers on top of the static
shield of large distribution transformer coils. Here, increasing
the field strength due to customer demands reached the
limitations of the factory's production methods.
With a better understanding of the deterioration processes
of materials used in power transformer, partial discharge
measurements gained a more prominent role in the acceptance
testing of large power transformers. The relevant standards for
transformer testing, such as the IEEE C57.113 [9] have shifted
the emphasis to partial discharge detection with the more recent
revisions. A partial discharge acceptance level of 500pC was
commonly used. However, the increasing use of composite
materials and their defect mechanism led to a reduction of the
partial discharge acceptance test levels during the past decade.
III.
MOBILE TEST SET
Generally, on-site partial discharge testing of large
transformers is a demanding task. In order to allow tests at
elevated voltage, as it is part of the standard short duration or
the long duration test [10], the transformer needs to be
energized at a higher frequency. Heavy motor-generator-sets
are commonly used in the test room of a transformer factory. A
motor-generator-set, the step-up transformer and the control
circuits cover several freight container loaded up to their
permissible weight, when testing transformers of 500MVA or
more.
Figure 2. ϕ-q-n pattern of gas inclusions trapped in casein glue
Finally, fig. 3 shows a pattern that is caused by a
delamination in transformer pressboard. Here, the Lichtenberg
figure of the surface discharge does cause the steep increase of
the partial discharge magnitude vs. phase.
In order to overcome the operational and logistic limitations
of such a conventional solution, an inverter-based three-phase
source was developed [11]. The unit is built into a modified
40ft high-cube container (Fig.4) and stays within the load and
size limitations of a conventional road-worthy trailer truck.
Figure 4. Mobile transformer test set built into a 40ft container
Figure 3. ϕ-q-n pattern of pressboard delamination
Power transformer acceptance testing initially focussed on
"radio interference voltage" (RIV). The original intention of
this test, however, was to avoid hampering AM radio reception
due to partial discharge activity. Therefore, narrow-band
circuits and a weighting circuit were used with the RIV meters
[2]. However, the used bandwidth does not allow to process
high-repetition partial discharge, while the weighting circuit
confuses the detection of low-repetition discharge with high
magnitudes.
The mobile test set requires a 400V three-phase supply
feeding three individual 450kVA inverter units covering an
output frequency range of 20-200Hz. The 2MVA step-up
transformer consists of three single-phase transformers in a
common tank. This allows running the unit in single-phase
mode at full power on all three inverters. Both ends of the HV
winding as well as two taps are accessible via four bushings in
line for each of the single-phase units (fig. 5). Additionally,
each low voltage coil has a tap as well. Thus, by selecting the
LV and by applying jumpers equipped with multi-contact
connectors, a large variation of output voltages ranging from
8.5kV to 90kV full-scale can be chosen by interconnecting the
different taps in star or delta configuration (Table 1) to have a
close match to the load requirements.
Figure 6. Noise pattern due to (unfiltered) inverter switching action
Figure 5. Step-up transformer with taps and HV filters
Running the inverters with 120° phase shift offers threephase induced voltage testing, while 0° phase shift allows
single phase induced voltage testing at full power. Although
intended mainly for transformer testing, this also offers testing
a cable of 5µF at 36kV and 50Hz, for instance.
TABLE I.
Finally, the mobile test set comes with a 500kV reactor for
resonant applied voltage testing. The reactor sits on a frame
that can be moved out of the container to provide the required
spacing (fig. 7). The reactor has an inductance of 400H, which
together with the coupling capacitor of 2nF results in a resonant
frequency of about 178Hz. With the minimum frequency of the
inverter of 15Hz, a load capacitance of up to 200nF can be
covered for applied voltage testing. However, with increasing
capacitance, the current limitation of the reactor limits the
maximum voltage. Given the current limit of 4A, the 500kV
can be reached up to about 25nF, whereas the resonance
frequency is close to 50Hz, then.
MOBILE TEST SET, 2MVA STEP-UP TRANSFORMER
Configuration
Three-Phase Output, Voltage and Current
LV Input 1
LV Input 2
HV Output 1, Delta
11.8kV
97.9A
8.5kV
135.2A
HV Output 1, Star
20.4kV
56.5A
14.8kV
78.1A
HV Output 2, Delta
26.1kV
44.3A
18.9kV
61.2A
HV Output 2, Star
45.1kV
25.6A
32.7kV
35.3A
HV Output 3, Delta
52.0kV
22.2A
37.7kV
30.7A
HV Output 3, Star
90.1kV
12.8A
65.2kV
17.7A
The inverters offer full four-quadrant operation and, hence,
can supply reactive power up to their output current limit.
Additionally, the mobile test set comes with switchable
inductive (3 x 180kVA) and capacitive (3 x 603kVA)
compensation to minimize the inverter current.
Generally, of course, the inverters produce switching noise,
which strongly hamper partial discharge measurements, if not
sufficiently filtered. Depending on the load situation, the
inverters produce various impulse noise patterns. Fig. 6 gives
an idea of such noise pattern. One set of filters is fitted on the
LV side between inverter and transformer. Another set of filters
is placed on the high voltage side (fig. 5), which additionally
carry the fiber-optically-isolated load current measurement.
Figure 7. 500kV reactor for applied voltage testing moved out
IV.
ON-SITE TESTING
On-site acceptance testing of transformers including partial
discharge tests is either triggered by abnormal behavior of the
transformer in service detected by on-line dissolved gas
analysis (DGA) or in more extreme cases by tripping a
Buchholz relay. Moreover, such on-site acceptance testing
greatly simplifies assessing the current status in preparation for
an on-site repair or validation of the results after such an on-site
repair [11, 14, 16, 17].
Of course, the transformer needs to be disconnected from
the grid and prepared for the test. Setting up a mobile test set as
such requires few hours, only. Typically, a rented 400V diesel
generator is used to supply the mobile test set. Hence,
decommissioning, measurement, and commissioning the
transformer again can be done within two days, if needed.
However, subsequent attempts to acoustically locate a found
partial discharge source may be more time consuming.
The IEC60270-2000 [7] limits the frequency range to an
upper corner frequency of 400kHz, whereas the highest
permissible lower corner frequency is kept to 100kHz. Often,
this frequency range is (partly) occupied by noise signals, when
doing field tests. Thus, a tunable detection circuit is very
helpful to optimize the signal-to-noise ratio (SNR). Currently,
an addendum to the IEC60270-2000 is under preparation. Redefining frequency ranges is part of the discussion.
Besides the three-phase induced voltage test at elevated
frequency, the unit support as well single-phase induced and
applied voltage tests. The available 1.3MVA active power does
not pose any limitation for no-load tests on even very large
transmission transformers. However, accurately measuring the
load losses is limited by the available output current and by the
limited capacitive compensation.
Generally, the lower cut-off frequency shall exclude
residual noise signals from the three-phase-supply for induced
voltage testing regardless, whether it is IGBT switching noise
from an inverter-based source or thyristor noise of the
excitation system of a conventional motor-generator set. The
upper corner frequency shall not be chosen too high to still
cover larger parts of the winding. However, for proper pulse
processing a bandwidth of several hundred kHz is mandatory
[8].
As an example, table 2 shows the power requirements for
two different transformers tested. In both cases a diesel
generator well below the maximum power was used.
TABLE II.
MOBILE TEST SET, POWER REQUIREMENTS
Given a well de-noised source and the comments above,
on-site partial discharge testing of large power transformers
offers similar sensitivity levels as found in an average, nonoptimized transformer test room.
Power Requirements
Transformer I
Transformer II
112/22kV
235/20kV
Nominal Power
63MVA
660MVA
Apparent Power
38kVA (@1.0UN)
240kVA (@1.5UN)
Power (Diesel Gen.)
24kW (@1.0UN)
190kW (@1.5UN)
Nominal Voltage
V.
PARTIAL DISCHARGE TESTING
As in the lab, typically, the test tap or potential tap of the
bushings is used for coupling to the partial discharge signal.
Only for smaller LV or tertiary bushings coupling capacitors
are used instead. Quadrupoles and preamplifiers (fig. 8) are
fitted to make the signals available to an eight-channel partial
discharge detector. Besides covering the frequency range as
defined with the IEC60270, this unit offers additionally a wider
frequency range using a tunable heterodyne circuit covering
frequencies up to 10MHz [12, 13].
VI.
ACOUSTIC LOCATION
Besides electromagnetic signals and light emission, partial
discharge cause also acoustical signals. Hence, its acoustic
emissions and the respective travel time in different materials
can be used to locate partial discharge.
However, the classical triangulation approach with three
sensors on three faces of a cubic tank fails in most cases of
real-life transformers, as two essential conditions of this
method aren’t met. Firstly, the internal medium of the tank
must be homogeneous and, secondly, the tank wall must have
the properties of a thin membrane [15, 17].
Instead, a transformer is filled with different materials
having different density and, hence, causing different travel
speeds of the sound wave. Thus, for a partial discharge source
deeply buried within the insulation system, the different
possible travel paths, their delays and attenuation strongly
hamper the location of the source. Additionally, the tank wall,
instead of being a thin membrane, has its own transmission
properties and does add lateral transmission within the steel as
an additional path.
Thus, understanding the transmission paths and their
properties of a remote partial discharge source in a winding is
as complex as trying to visually locate a light source within a
complex structure of glass of different refraction indices, for
example.
Figure 8. Quadrupoles and preamplifiers fitted to the test tap
Typically, before attempting an acoustic location, an indepth partial discharge diagnosis is performed in order to
understand the rough location of the source in terms of
dominant phase, phase-to-phase, or phase-to-ground discharge,
for instance. Based on these results, the suspected area of the
transformer is scanned placing several acoustic sensors on the
tank wall (fig. 9) to find acoustic signals correlated to the
electrical partial discharge signal.
Figure 9. Acoustic sensors fitted to the tank wall using magnets
Therefore, an oscilloscope or equivalent equipment is
triggered to the dominant electrical signal, while displaying
the averaged acoustic signals of the piezo sensors placed. Fig.
10 shows such typical signals with different travel time for the
sensor positions.
The ideal situation is having a clear oil path. Here, the
typical transmission speed of about 1.4m/ms can be used for
calculating the distance. Of course, knowing the internal
structure of the transformer under test and the impact of
different materials is essential and detailed drawings should be
at hand. Practical experience of acoustic location of about one
hundred large power transformers led to a comparably simple
method. After having found a correlating acoustical signal,
three sensors are placed in a row, whereas the positions are
optimized in order to have the center sensor showing the
shortest distance. Placing the sensors in a row reduces the
triangulation to a two-dimensional, i.e. “flat problem”. Having
started with a horizontal line, for instance, the sensors are then
placed in a vertical line at the horizontal position found in the
first step. This process is assisted by a software tool to create
then the three-dimensional position results. Fig. 11 shows the
oscilloscope screen of this software, while the location screen
is found with fig. 12.
Figure 10. Acoustic travel time with respect to the electrical PD signal
Given the complexity of a transformer’s insulation system,
the key strategy, when optimizing the sensor positions, is to
have a signal path as simple as possible.
Figure 12. Viewing the resulting position based on the triangulation
The two-dimensional system is intentionally over
determined in order to address the effects of signals traveling in
the tank wall. The transmission in steel is about four times
faster than in transformer oil and, hence, can produce confusing
results in case of a lateral location of the discharge source.
Especially with mounting turrets of high voltage bushings, the
high speed in steel can produce misleading results, when
signals via the epoxy resin of the bushing and the steel of the
turret appear earlier at the sensor than the signals of the direct
oil path. With a clear oil path, the acoustic signal has a sharp
inception and placing a cursor either manually or automatically
on the graph is easy. In such cases an accuracy of about +/-2cm
can be achieved. If several independent transmission paths
contribute to the signal placing the cursor and, hence,
determining the distance becomes more and more difficult.
However, even here in most cases a location precision of +/20cm is possible. Often, the precision can then be improved
with the analysis of the partial discharge pattern, its properties,
and the cross-coupling matrix with respect to phases and coils.
Figure 11. Analyzing the vertical and horizontal results
Based on this analysis, areas are identified, where the
observed partial discharge activity is physically possible and
compared with location result of poorer precision.
VII. MONITORING
Generally, partial discharge monitoring suffers from
electromagnetic noise interference as found in a substation
environment. Thus, the local noise situation needs to be
understood before setting up a monitoring system.
Figure 13. Bushing coupling unit and adapter connected to a test tap
The smoothest application can be expected, if the
transformer is connected to cables and/or gas insulated
switchgear (GIS). Here, almost lab-type frequencies in the
range of the IEC60270 can be used due to the low interference
level. Partial discharge coupling is made using the
measurement taps of the bushings. Care must be taken to have
this connection made durable in order to maintain a safe
operation of the bushing. Fig. 13 shows such coupling to a
bushing tap – the measurement impedance that separates the
power frequency synchronization and the high frequency signal
follows a mere mechanical adapter with surge protection.
Figure 14. Fitting an UHF sensor to a transformer's oil drain valve
Alternatively, higher frequencies can be covered with builtin UHF antennas mounted on a spare flange or fitted to a drain
valve of the transformer under test (fig. 14). Sadly, often
transformer design does not provide such UHF access via the
drain valve. In more than 50% of the cases the drain valve does
not offer a fully open cross-section, continues internally with
an elbow, faces a 45° stiffener plate, or runs via pipe directly to
the conservator.
Fig. 15 shows an example of such a partial discharge
monitoring instrument (upper left corner) as part of an overall
transformer monitoring system. Here, the instrument talks to
the local monitoring system covering temperatures, voltages,
and other parameters, which then reports to the SCADA
system.
Alternatively, the PD monitoring device itself is already
equipped with a TCP/IP interface acting according to the
IEC61850. Besides the values for partial discharge monitoring,
which are already included with the relevant section of this
standard, further parameters can be added into the model
description, including the ϕ-q-n pattern or more in-depth
trending information, for instance.
Figure 15. PD acquisition system as part of an overall monitoring system
VIII. SUMMARY
Besides wanted effects, deregulation has caused a reduced
investment in the grid and, hence, has caused a larger
population of service-aged substation equipment.
[4]
[5]
Mobile, inverter-based transformer test sets for three-phase
induced voltage tests allow assessing the condition of
transformers on site with partial discharge acceptance tests to
factory standards.
[6]
Lower weight, non-critical supply of reactive power, and
smooth breakdown handling makes inverter-based sources by
far superior to conventional motor-generator sets. Additionally,
a large reactor offers applied voltage testing in resonance
mode.
[8]
Combining the partial discharge measurements with
acoustic location of the partial discharge activity assist to
prepare for an on-site repair or to decide for factory repair of a
faulty unit.
During the past decade, the described method of acoustic
location has been successfully applied to about one hundred
large power transformers having had critical partial discharge
activity.
Continuous on-line partial discharge monitoring allows
keeping transformers in service, which have already reached
their projected service life.
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