1_PDTestingTransformers (2)
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Transcript of 1_PDTestingTransformers (2)
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The Doble ExchangeAugust 2013 Doble Engineering Company. All Rights Reserved.
Partial Discharge Testing of Power TransformersBy: Falk Werner, Senior Field Engineer, Doble Engineering CompanyPartial Discharges (PD) are small localized breakdowns of an insulation system within high voltageequipment. PD occurs if the electric field strength applied to a dielectric exceeds the dielectric withstandstrength of an insulation system locally. This may be caused by either increased field strengths, e.g. dueto design problems, sharp edges on electrodes, small clearances, floating conductive particlesorincreased voltage levels that exceed the initial design specifications (e.g. switching transients on alreadyweak insulation systems). Other reasons for the inception of PD are weaknesses of the insulation system.In oil-filled power transformers examples of such weaknesses are aged paper, gas bubbles (gassesgenerated by other faults), accumulation of gas at barriers or in paper, non-homogeneous fielddistributions on surfaces of insulating materials e.g. due to contaminations. Other examples aretangentially stressed boundary layers between different insulation materials and poor or oxidized contacts
or connections.
Knowledge about the kind of PD, the insulation system involved, the strength and the location of PD in atransformer provides crucial information about the health of an individual asset. This article describesapproaches of PD assessment on power transformers that can be applied in the field and online withoutservice interruption. These include electrical, high frequency and acoustic PD measurements.
Bushing Tap Measurements
Partial discharge measurements on transformers can be performed using decoupling circuits connected
to the bushing taps of a transformer. The bushing tap capacitance acts as a coupling capacitor similar to
the coupling capacitor in an IEC 60270 compliant standard measurement [1] [2] [3]. Figure 1 shows anexample installation of a bushing tap sensor. The PD detector is connected to a terminal box at the
control cabinet of the transformer.
Figure 1: Bushing Tap PD Sensor
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There is a range of problems connected to this kind of measurement method when applied outside a test
lab, under on-site and on-line conditions:
1. Outage might be required: if no bushing tap sensors are previously installed, a service interruption of
the transformer is required in order to install bushing tap measurement impedances.
2. Noise levels: a transformer in service creates significant noise levels within the IEC frequency range
of 100 kHz to 500 kHz. Reasonably sensitive PD measurements can only be performed if the PD
detector used measures at a frequency range above approximately 1 MHz. This results in the
measurement no longer being in compliance with IEC and the charge magnitude can no longer be
quantified in terms of apparent charge QIEC.
3. Internal and external PD measured: The measurement sensor decouples through the bushing. This
means that PD from within the transformer, as well as from connected equipment (bus work, lightning
arrestors etc.) is picked up using this detection method.
Even though there are certain disadvantages to the bushing tap measurement being made under on-site
conditions, it still provides a lot of useful information, if performed and analyzed correctly. A thorough
pattern analysis can be performed to differentiate various PD phenomena (internal vs. external, type of
defect etc.), as well as the phase association of particular PD sources.
Where bushing tap measurements are not possible, other PD detection and analysis methods such as
UHF and HFCT sensing methods can be applied. These are discussed in the subsequent sections.
UHF PD Measurement
Ultra High Frequency (UHF) measurement methods detect partial discharge pulses in a frequency range
between 100 MHz and 1 GHz. With this method, electromagnetic waves, not current pulses, originating
from PD within the transformer are detected and analyzed. The sensors used for UHF PD measurements
are simple monopole antennas that are available in different versions such as custom made top hatch
sensors or drain valve antennas (Figure 1).
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Figure 1: UHF Drain Valve Antenna
Top hatch sensors are permanently installed and require an outage of the transformer whereas drain
valve antennas can be installed while the transformer is online without service interruption. The PD
measurement itself is performed with a PD detector capable of measuring in a frequency range that suits
the UHF antenna arrangement. UHF measurements can only be used to detect PD, identify the type of
PD and monitor the trend of the PD. Conclusions about the strength of partial discharge are not possible.
However, it is a very sensitive method of PD detection in power transformers, as the antenna represents
a sensor that is placed inside the tank which acts as a Faradays cage, electrically shielding the
measurement setup from external noise sources.
However, a problem associated with UHF drain valve antennas is the requirement for a straight valve with
at least 2 inches in inner diameter, in order for the antenna to pass through the valve into the transformer
tank. This can only be achieved with gate or ball valves. Butterfly and globe valves will not allow the
application of this sensing method. In North America the latter valve types are common, and different
methods of non-invasive PD detection and analysis have to be applied. One of these methods is the
HFCT PD measurement which will be discussed in the next section.
HFCT Partial Discharge Measurements
High Frequency Current Transformers (HFCT) are useful tools when it comes to the assessment of PD in
high voltage equipment. Split core HFCTs such as shown inFigure 2 can be clamped around ground
connections of HV equipment and in conjunction with a broad band PD detector used to determine PD
activity inside the equipment.
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Figure 2: Split Core HFCT
When a PD occurs in the insulation system of HV equipment, equalization current pulses propagatethrough the terminals of the test objects. These current pulses can be detected on both high voltage and
ground connections of the test object.Figure 3 shows an example of an HFCT clamped around the
neutral and/or ground connection of a transformer.
Figure 3: HFCT on Neutral Ground Connection
If measured on the neutral of a transformer, a measurement frequency range of 2 MHz to 20 MHz is
recommended. The lower cut off frequency defines the distance of how far the measurement looks into
the ground grid. Other equipment with PD might emit into the ground grid. Higher frequency content in a
signal is attenuated over distance and thus choosing a lower frequency that is not too low allows
decreasing sensitivity for distant PD sources. If PD is found on a transformers neutral and/or ground
connection, measurements on ground connections of nearby equipment have to be performed.
By comparing PD patterns and magnitudes, conclusions can be drawn on which equipment is the actual
source of Partial Discharge. The upper frequency is basically limited by the transfer function of the
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transformer winding. Close to and above 20 MHz, no signal content of PD pulses that propagated through
the winding will be detectable, as the windings attenuate high frequency content.
Figure 4 shows an example of an HFCT measurement on a transformer without PD. The PRPPD pattern
shows background noise only.
Figure 4: Transformer without PD
The measurement inFigure 5 was performed on the sister unit of the transformer results shown inFigure
4.It exhibits a characteristic phase resolved PD pattern. This measurement was used to identify PD in the
test object. It furthermore allowed to identify on which phase the PD was located based on the phase shift
of the PD pattern in relation to the reference voltage (station supply).
Figure 5: Transformer with Characteristic PD Pattern
Acoustic PD Localization
While electrical, UHF and HFCT Partial Discharge measurements allow for very sensitive detection and
analysis of PD, they do not provide any information on the geometric location of the PD source within the
transformer. Acoustics on the other hand can be extremely insensitive for PD detection if the PD is
located deep inside the transformer, but can be very useful in identifying the geometric location of PD
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inside a transformer. Thus acoustic measurements are not the first choice when it comes to PD detection,
but the only choice when it comes to localization.
There is a range of different localization approaches that can be applied on power transformers [4] [5].
The most commonly used localization method is based on a time of flight model that is similar to the
location calculation of modern GPS devices. Instead of satellites around the planet there are acoustic
sensors placed around the transformer tank.Figure 6 illustrates this arrangement. The geometric
coordinates of these sensors are known. GPS uses differences in the runtime of signals from the GPS
satellites to the GPS receiver. For PD localization this principle is inverted, and the acoustic signal
runtime differences from the PD source (GPS receiver) to the acoustic sensors on the tank wall
(satellites) are used to calculate the location of the PD source. Sensors used for acoustic PD detection
and location in oil filled transformers are contact sensors with a resonance frequency of 150 kHz, which is
the predominant frequency of acoustic emissions from PD in oil.
Figure 6: Acoustic Sensor Arrangement
The equation system used to solve this problem and derive the location of the PD is the same as it is
used in GPS. A minimum count of 4 sensors getting a clear acoustic signal is required to solve this
equation system. The acoustic arrival times need to be determined at each individual sensor. Figure 7
shows a signal that was acquired on a transformer tank. The blue marker indicates the arrival time during
the acquisition.
Figure 7: Acoustic PD Signal
The sensitivity of the acoustic measurement depends on the actual location of the partial discharge. If the
PD source is located close to the tank wall or in an area where the acoustic waves can propagate freely,
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an acoustic measurement can be fairly sensitive. However, if the PD source is surrounded by materials
that attenuate the acoustic emissions (such as paper, pressboard etc.), or if such materials are in the
propagation path of the acoustic waves from the source to the sensor, these acoustic emissions can be
subject to strong attenuation up to the point where detection becomes impossible by acoustic measures
only.
Knowledge about the location of PD within the transformer can be crucial for the condition assessment.
Depending on whether discharge activity is located in the winding, on a poor connection or a bad
crimping, a bushing connection or the DETC, different decisions can be made on future maintenance and
or monitoring measures. If the acoustic sensitivity is not sufficient, combination with other measurement
methods is required. This application is discussed in the next section.
Combined Electro-Acoustic PD Location
Every method has its advantages and disadvantages. While UHF and HFCT methods are very sensitive
at detecting PD, they do not provide any information on the geometric location of the PD source within the
insulation system (apart from phase association). Acoustic PD measurements can be used to
successfully locate PD, but can be (extremely) insensitive on oil-paper insulation systems. Combining
these two methods for PD assessment of transformer insulation systems makes use of their individual
advantages while compensating for the disadvantages.
Acoustic and UHF or HFCT PD measurements combined make the PD assessment more reliable and
increases the chances of successfully localizing PD. Combining means to use both simultaneously while
looking for correlations in both measurements. The sensitive UHF or HFCT measurement is used to
trigger acoustic measurements. By doing so,the acoustic measurement can be focused on known
electric discharge events. One advantage is that the acoustic sensitivity can be increased by using the
trigger for averaging purposes, very similar to the averaging function of an oscilloscope, allowing
identification of very weak signals. In addition, if there is a correlation between electrical and acoustical
signals, meaning that in the wake of a detected electrical PD pulse there is an acoustic signature on the
acoustic sensors, it is certain that the detected PD is coming from inside the transformer and is not
external.Figure 8 illustrates such a correlation.
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Figure 8: Correlation between Electrical and Acoustic PD Signals
Conclusion
It is obvious that for PD related condition assessment of power transformers there is a broad range of
tools available. As with any tool, best use of it can be made if it is part of a well-rounded toolbox. The
combinationof different assessment methods, DGA, electrical/UHF/HFCT and acoustic PD
measurements is of vital importance for a thorough assessment of PD in power transformers. Identifying
whatit is, whereit is and how strongit is, is the goal of this combined approach.
References
[1] International Electrotechnical Commission, High-voltage test techniquesPartial discharge measurements, Third
Edition, Geneva: International Electrotechnical Commission, 2000-12.
[2] IEEE, C57.113: Recommended Practice for Partial Discharge Measurement in Liquid-Filled Power Transformers
and Shunt Reactors, IEEE, 2010.
[3] I. E. Commission, IEC 60076-3:2000 - Power Transformers - Part 3: Insulation levels, dielectric tests and external
clearances in air, Geneva, 2000.
[4] F. W. S. T. S. M. Sebastian Coenen, "Localization of PD Sources inside Transformers by Acoustic Sensor Array
and UHF Measurements," in CMD, Tokyo, 2010.
[5] S. C. S. K. Falk Werner, "New Methods for Multisource UHF-Acoustic PD Location on Power Transformers," in
International Symposium of High Voltage Engineering (ISH), Hannover, 2011.
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