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 SoellerPower Diagnostix Systems GmbH

Aachen, Germany

Abstract—In the lab, partial discharge diagnosis has widelyreplaced the traditional RIV measurements. Additionally, partialdischarge acceptance levels are being reduced due to theincreasing use of composite material and a growing awareness ofpartial discharge phenomenon and their consequences. Adequatefiltering of the supply voltage for induced voltage testing and theuse of sensitive acoustic measurement has greatly improved thedetection and location of partial discharge activity in powertransformers.

An increasing population of service aged substation equipmenthaving reached their projected service life demands on-site repairand, hence, on-site testing to factory standards.

Using inverter based three-phase mobile test sets allows on-siteapplication of tests previously limited to a mere test roomenvironment. Besides the unmatched portability of inverter-based sources, especially the less critical generation of reactivepower simplifies on-site testing, if compared with motor-generator sets. Additionally, the lack of high short circuitcurrents limits the damage in case of breakdown. However,adequately removing the switching noise spectrum becomes ademanding task in order to reach the required sensitivity of thepartial discharge measurements.

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, hashad a further impact on transformers and their life. Beforederegulation with often government-owned or governmentcontrolled utilities, availability was the most prominent designand sourcing criterion. A side effect of this policy was “over-engineering”. Deregulation shifted the emphasis toprofitability. As a consequence, re-investment into the griddropped significantly. However, in light of the typical servicelife of a large power transformer of about 40 years, theconsequences of this change took decades to materialize, whileprofits went up immediately.

Nowadays, we do have in many parts of the world service-aged populations of sub-station equipment, of which a largeportion has reached or already exceeded its projected servicelife. In Europe, for instance the majority of the 400kV-transmission-system was commissioned in the 1970ies and1980ies. Moreover, the ongoing change from fossil fuels torenewable sources further increase the required transmissioncapacity of a service-aged grid.

Thus, acceptance testing as commonly applied in the lab,will be increasingly used on-site to extend the service life ofold transformers, to validate the success of on-site repair, andto ensure successful commissioning of new units.

Likewise, applying on-line monitoring of various operationparameters including partial discharge can assist to extend thelifetime of service aged substation equipment.

II. PARTIAL DISCHARGE TESTING

Detecting high frequency signals using narrow-bandreceivers based on heterodyne principles has been used alreadyvery early in the history of high voltage insulation systems [1].With early meter-type instruments diagnosis was mostlylimited to the observation of magnitude and inception vs.extinction voltage, while later oscilloscope-base instrumentsadded the phase position of the discharge activity. An in-depthunderstanding of the gas discharge physics and the statistics ofpartial discharge was supported with the introduction ofinstruments using the phase resolved partial discharge (PRPD)pattern, or ϕ-q-n pattern [3, 4, 5].

Figure 1. ϕ-q-n pattern of multiple cavities (voids)

As an example, fig. 1 shows such a ϕ-q-n pattern of severalvoids in epoxy resin. Here, each individual sine-shaped tracebelongs to an individual gas inclusion. Moreover, the well-distributed pattern is caused by a low availability of the startingelectron for the discharge avalanche, as it is typical for bubblesin polymeric material such as fresh epoxy resin [6].

Fig. 2, instead, shows the activity of several gas inclusionswith the casein glue of barriers and spacers on top of the staticshield of large distribution transformer coils. Here, increasingthe field strength due to customer demands reached thelimitations of the factory's production methods.

Figure 2. ϕ-q-n pattern of gas inclusions trapped in casein glue

Finally, fig. 3 shows a pattern that is caused by adelamination in transformer pressboard. Here, the Lichtenbergfigure of the surface discharge does cause the steep increase ofthe partial discharge magnitude vs. phase.

Figure 3. ϕ-q-n pattern of pressboard delamination

Power transformer acceptance testing initially focussed on"radio interference voltage" (RIV). The original intention ofthis test, however, was to avoid hampering AM radio receptiondue to partial discharge activity. Therefore, narrow-bandcircuits and a weighting circuit were used with the RIV meters[2]. However, the used bandwidth does not allow to processhigh-repetition partial discharge, while the weighting circuitconfuses the detection of low-repetition discharge with highmagnitudes.

With a better understanding of the deterioration processesof materials used in power transformer, partial dischargemeasurements gained a more prominent role in the acceptancetesting of large power transformers. The relevant standards fortransformer testing, such as the IEEE C57.113 [9] have shiftedthe emphasis to partial discharge detection with the more recentrevisions. A partial discharge acceptance level of 500pC wascommonly used. However, the increasing use of compositematerials and their defect mechanism led to a reduction of thepartial discharge acceptance test levels during the past decade.

III. MOBILE TEST SET

Generally, on-site partial discharge testing of largetransformers is a demanding task. In order to allow tests atelevated voltage, as it is part of the standard short duration orthe long duration test [10], the transformer needs to beenergized at a higher frequency. Heavy motor-generator-setsare commonly used in the test room of a transformer factory. Amotor-generator-set, the step-up transformer and the controlcircuits cover several freight container loaded up to theirpermissible weight, when testing transformers of 500MVA ormore.

In order to overcome the operational and logistic limitationsof such a conventional solution, an inverter-based three-phasesource was developed [11]. The unit is built into a modified40ft high-cube container (Fig.4) and stays within the load andsize limitations of a conventional road-worthy trailer truck.

Figure 4. Mobile transformer test set built into a 40ft container

The mobile test set requires a 400V three-phase supplyfeeding three individual 450kVA inverter units covering anoutput frequency range of 20-200Hz. The 2MVA step-uptransformer consists of three single-phase transformers in acommon tank. This allows running the unit in single-phasemode at full power on all three inverters. Both ends of the HVwinding as well as two taps are accessible via four bushings inline for each of the single-phase units (fig. 5). Additionally,each low voltage coil has a tap as well. Thus, by selecting theLV and by applying jumpers equipped with multi-contactconnectors, a large variation of output voltages ranging from8.5kV to 90kV full-scale can be chosen by interconnecting thedifferent taps in star or delta configuration (Table 1) to have aclose match to the load requirements.

Figure 5. Step-up transformer with taps and HV filters

Running the inverters with 120° phase shift offers three-phase induced voltage testing, while 0° phase shift allowssingle phase induced voltage testing at full power. Althoughintended mainly for transformer testing, this also offers testinga cable of 5µF at 36kV and 50Hz, for instance.

TABLE I. MOBILE TEST SET, 2MVA STEP-UP TRANSFORMER

Three-Phase Output, Voltage and CurrentConfiguration

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 switchableinductive (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 notsufficiently filtered. Depending on the load situation, theinverters produce various impulse noise patterns. Fig. 6 givesan idea of such noise pattern. One set of filters is fitted on theLV side between inverter and transformer. Another set of filtersis placed on the high voltage side (fig. 5), which additionallycarry the fiber-optically-isolated load current measurement.

Figure 6. Noise pattern due to (unfiltered) inverter switching action

Finally, the mobile test set comes with a 500kV reactor forresonant applied voltage testing. The reactor sits on a framethat can be moved out of the container to provide the requiredspacing (fig. 7). The reactor has an inductance of 400H, whichtogether with the coupling capacitor of 2nF results in a resonantfrequency of about 178Hz. With the minimum frequency of theinverter of 15Hz, a load capacitance of up to 200nF can becovered for applied voltage testing. However, with increasingcapacitance, the current limitation of the reactor limits themaximum voltage. Given the current limit of 4A, the 500kVcan be reached up to about 25nF, whereas the resonancefrequency is close to 50Hz, then.

Figure 7. 500kV reactor for applied voltage testing moved out

IV. ON-SITE TESTING

On-site acceptance testing of transformers including partialdischarge tests is either triggered by abnormal behavior of thetransformer in service detected by on-line dissolved gasanalysis (DGA) or in more extreme cases by tripping aBuchholz relay. Moreover, such on-site acceptance testinggreatly simplifies assessing the current status in preparation foran on-site repair or validation of the results after such an on-siterepair [11, 14, 16, 17].

Of course, the transformer needs to be disconnected fromthe grid and prepared for the test. Setting up a mobile test set assuch requires few hours, only. Typically, a rented 400V dieselgenerator is used to supply the mobile test set. Hence,decommissioning, measurement, and commissioning thetransformer again can be done within two days, if needed.However, subsequent attempts to acoustically locate a foundpartial discharge source may be more time consuming.

Besides the three-phase induced voltage test at elevatedfrequency, the unit support as well single-phase induced andapplied voltage tests. The available 1.3MVA active power doesnot pose any limitation for no-load tests on even very largetransmission transformers. However, accurately measuring theload losses is limited by the available output current and by thelimited capacitive compensation.

As an example, table 2 shows the power requirements fortwo different transformers tested. In both cases a dieselgenerator well below the maximum power was used.

TABLE II. MOBILE TEST SET, POWER REQUIREMENTS

Power RequirementsTransformer I Transformer II

Nominal Voltage 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)

V. PARTIAL DISCHARGE TESTING

As in the lab, typically, the test tap or potential tap of thebushings is used for coupling to the partial discharge signal.Only for smaller LV or tertiary bushings coupling capacitorsare used instead. Quadrupoles and preamplifiers (fig. 8) arefitted to make the signals available to an eight-channel partialdischarge detector. Besides covering the frequency range asdefined with the IEC60270, this unit offers additionally a widerfrequency range using a tunable heterodyne circuit coveringfrequencies up to 10MHz [12, 13].

Figure 8. Quadrupoles and preamplifiers fitted to the test tap

The IEC60270-2000 [7] limits the frequency range to anupper corner frequency of 400kHz, whereas the highestpermissible lower corner frequency is kept to 100kHz. Often,this frequency range is (partly) occupied by noise signals, whendoing field tests. Thus, a tunable detection circuit is veryhelpful to optimize the signal-to-noise ratio (SNR). Currently,an addendum to the IEC60270-2000 is under preparation. Re-defining frequency ranges is part of the discussion.

Generally, the lower cut-off frequency shall excluderesidual noise signals from the three-phase-supply for inducedvoltage testing regardless, whether it is IGBT switching noisefrom an inverter-based source or thyristor noise of theexcitation system of a conventional motor-generator set. Theupper corner frequency shall not be chosen too high to stillcover larger parts of the winding. However, for proper pulseprocessing a bandwidth of several hundred kHz is mandatory[8].

Given a well de-noised source and the comments above,on-site partial discharge testing of large power transformersoffers similar sensitivity levels as found in an average, non-optimized transformer test room.

VI. ACOUSTIC LOCATION

Besides electromagnetic signals and light emission, partialdischarge cause also acoustical signals. Hence, its acousticemissions and the respective travel time in different materialscan be used to locate partial discharge.

However, the classical triangulation approach with threesensors on three faces of a cubic tank fails in most cases ofreal-life transformers, as two essential conditions of thismethod aren’t met. Firstly, the internal medium of the tankmust be homogeneous and, secondly, the tank wall must havethe properties of a thin membrane [15, 17].

Instead, a transformer is filled with different materialshaving different density and, hence, causing different travelspeeds of the sound wave. Thus, for a partial discharge sourcedeeply buried within the insulation system, the differentpossible travel paths, their delays and attenuation stronglyhamper the location of the source. Additionally, the tank wall,instead of being a thin membrane, has its own transmissionproperties and does add lateral transmission within the steel asan additional path.

Thus, understanding the transmission paths and theirproperties of a remote partial discharge source in a winding isas complex as trying to visually locate a light source within acomplex structure of glass of different refraction indices, forexample.

Typically, before attempting an acoustic location, an in-depth partial discharge diagnosis is performed in order tounderstand the rough location of the source in terms ofdominant phase, phase-to-phase, or phase-to-ground discharge,for instance. Based on these results, the suspected area of thetransformer is scanned placing several acoustic sensors on thetank wall (fig. 9) to find acoustic signals correlated to theelectrical partial discharge signal.

Figure 9. Acoustic sensors fitted to the tank wall using magnets

Therefore, an oscilloscope or equivalent equipment istriggered to the dominant electrical signal, while displayingthe averaged acoustic signals of the piezo sensors placed. Fig.10 shows such typical signals with different travel time for thesensor positions.

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 tohave a signal path as simple as possible.

Figure 11. Analyzing the vertical and horizontal results

The ideal situation is having a clear oil path. Here, thetypical transmission speed of about 1.4m/ms can be used forcalculating the distance. Of course, knowing the internalstructure of the transformer under test and the impact ofdifferent materials is essential and detailed drawings should beat hand. Practical experience of acoustic location of about onehundred large power transformers led to a comparably simplemethod. After having found a correlating acoustical signal,three sensors are placed in a row, whereas the positions areoptimized in order to have the center sensor showing theshortest distance. Placing the sensors in a row reduces thetriangulation to a two-dimensional, i.e. “flat problem”. Havingstarted with a horizontal line, for instance, the sensors are thenplaced in a vertical line at the horizontal position found in thefirst step. This process is assisted by a software tool to createthen the three-dimensional position results. Fig. 11 shows theoscilloscope screen of this software, while the location screenis found with fig. 12.

Figure 12. Viewing the resulting position based on the triangulation

The two-dimensional system is intentionally overdetermined in order to address the effects of signals traveling inthe tank wall. The transmission in steel is about four timesfaster than in transformer oil and, hence, can produce confusingresults in case of a lateral location of the discharge source.Especially with mounting turrets of high voltage bushings, thehigh speed in steel can produce misleading results, whensignals via the epoxy resin of the bushing and the steel of theturret appear earlier at the sensor than the signals of the directoil path. With a clear oil path, the acoustic signal has a sharpinception and placing a cursor either manually or automaticallyon the graph is easy. In such cases an accuracy of about +/-2cmcan be achieved. If several independent transmission pathscontribute 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 improvedwith the analysis of the partial discharge pattern, its properties,and the cross-coupling matrix with respect to phases and coils.

Based on this analysis, areas are identified, where theobserved partial discharge activity is physically possible andcompared with location result of poorer precision.

VII. MONITORING

Generally, partial discharge monitoring suffers fromelectromagnetic noise interference as found in a substationenvironment. Thus, the local noise situation needs to beunderstood 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 thetransformer is connected to cables and/or gas insulatedswitchgear (GIS). Here, almost lab-type frequencies in therange of the IEC60270 can be used due to the low interferencelevel. Partial discharge coupling is made using themeasurement taps of the bushings. Care must be taken to havethis connection made durable in order to maintain a safeoperation of the bushing. Fig. 13 shows such coupling to abushing tap – the measurement impedance that separates thepower frequency synchronization and the high frequency signalfollows a mere mechanical adapter with surge protection.

Alternatively, higher frequencies can be covered with built-in UHF antennas mounted on a spare flange or fitted to a drainvalve of the transformer under test (fig. 14). Sadly, oftentransformer design does not provide such UHF access via thedrain valve. In more than 50% of the cases the drain valve doesnot offer a fully open cross-section, continues internally withan elbow, faces a 45° stiffener plate, or runs via pipe directly tothe conservator.

Fig. 15 shows an example of such a partial dischargemonitoring instrument (upper left corner) as part of an overalltransformer monitoring system. Here, the instrument talks tothe local monitoring system covering temperatures, voltages,and other parameters, which then reports to the SCADAsystem.

Alternatively, the PD monitoring device itself is alreadyequipped with a TCP/IP interface acting according to theIEC61850. Besides the values for partial discharge monitoring,which are already included with the relevant section of thisstandard, further parameters can be added into the modeldescription, including the ϕ-q-n pattern or more in-depthtrending information, for instance.

Figure 14. Fitting an UHF sensor to a transformer's oil drain valve

Figure 15. PD acquisition system as part of an overall monitoring system

VIII. SUMMARY

Besides wanted effects, deregulation has caused a reducedinvestment in the grid and, hence, has caused a largerpopulation of service-aged substation equipment.

Mobile, inverter-based transformer test sets for three-phaseinduced voltage tests allow assessing the condition oftransformers on site with partial discharge acceptance tests tofactory standards.

Lower weight, non-critical supply of reactive power, andsmooth breakdown handling makes inverter-based sources byfar superior to conventional motor-generator sets. Additionally,a large reactor offers applied voltage testing in resonancemode.

Combining the partial discharge measurements withacoustic location of the partial discharge activity assist toprepare for an on-site repair or to decide for factory repair of afaulty unit.

During the past decade, the described method of acousticlocation has been successfully applied to about one hundredlarge power transformers having had critical partial dischargeactivity.

Continuous on-line partial discharge monitoring allowskeeping transformers in service, which have already reachedtheir projected service life.

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