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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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