Abstract - Both, on- and off-line measurements have beenperformed through the last years on several large powertransformers. This paper describes the most recent devel-opments of sensor technology, such as an electronicBuchholz Relay, an OLTC monitoring based on the powerconsumption of the motor drive and on-line PD-measurement. The presentation of results gained from thecombination of on- and off-line methods shows, that theyare powerful tools to take decisions on the operation of thetransformers. Even more it allows to define concepts forefficient assessment of the transformers condition.

Keywords: Power Transformer, Monitoring, Lifetime As-

sessment, Buchholzgas sensor, OLTC, PD-Measurement, Fre-

quency Response Analysis, Thermal control

1. INTRODUCTION

Transformer outages have a considerable economic im-pact on the operation of an electrical network. Thereforeit is the aim to ensure an accurate assessment of thetransformer condition. Techniques that allow diagnosingthe integrity through non-intrusive tests can be used tooptimise the maintenance effort and to ensure maximumavailability and reliability. With the increasing averageage of the transformer population there is an increasingneed to know the internal condition. For this purposeon- and off-line methods and systems have been devel-oped in recent years. On-line monitoring can be usedcontinuously during the operation of transformers andoffers in that way a possibility to record different rele-vant stresses which can affect the lifetime. The auto-matic evaluation of these data allows the early detectionof an oncoming fault. In comparison to this, off-line

∗ Rheinstr.73, D-41061 Mönchengladbach,Stefan.Tenbohlen@tde.alstom.com

methods require disconnecting the transformer from thepower network and are mainly used during scheduledinspections or when the transformer is already suspi-cious.

2. NEW SENSOR TECHNOLOGIES FOR ON-LINEMONITORING

Transformer outage rate statistics indicate on-load tapchanger, bushings and winding insulation as the mostfrequent causes of long duration outages [1, 2]. There-fore the installation of a comprehensive monitoringsystem to warn in case of an oncoming fault is advisablefor strategically important power transformers.A multitude of different measurable variables can becollected for on-line monitoring. However, it is veryrarely useful to use all the available information. So thesensor technology must be adjusted to the specific re-quirements of a particular transformer or transformerbank, depending on their age and condition [3,4]. Thisdemands a very high degree of modularity and flexibil-ity for the hard- and software of the monitoring system.To fulfil this requirement Alstom designed and devel-oped the on-line monitoring system MS2000 in co-operation with some utilities. Because of its modularity,the system can easily be adapted to the customers needsand the requirements of the monitored transformer. It ispossible to propose a personalised set of sensors andfunctionalities. The customer may prefer to have a widerange of condition indicators or to concentrate on spe-cific ones. Another main advantage is that the integra-tion of all kinds of future sensors is possible without anyproblems. Retrofitting a transformer which is currentlyin operation should be achieved without requiring longwiring, and in the shortest time possible. This is realisedby the use of field bus technology, which reduces sig-nificantly the expenditures for wiring and installation,and allows furthermore the monitoring of several trans-

Enhanced Diagnosis of Power Transformers using On- and Off-line Methods:

Results, Examples and Future Trends

S. Tenbohlen∗, D. Uhde, J. Poittevin H. Borsi, P. WerleALSTOM T&D, Schering-Institute of High-Voltage Technique and France Engineering, University of Hannover,

Germany

U. Sundermann H. MatthesRWE Energie AG, PreussenElektra Netz GmbH u. Co. KG,Germany Germany

formers in a substation by only one monitoring system(Fig. 1). The superiority of the field bus technology forthe retrofitting of on-line monitoring systems is provenby several on-site installations.

2.1. Electronic Buchholz Relay

The Buchholz-Relay, which has been in use for manydecades as a protection tool for oil filled-transformers,is based on a mechanical device consisting of twoswimmers installed in a chamber normally filled withthe oil of the transformer. In case of faults in the trans-former undissolved failure gases come up into thechamber and replace the oil. If the gas amount exceeds adefined quantity for example 100 up to 200 ml one ofthe swimmers moves downwards and an alarm or aswitching signal is activated. In some cases a diagnosisof the failure can be carried out by analysing the gasescollected in Buchholz-Relay and by an additional dis-solved gas–analysis (DGA).One of the main disadvantages of the conventionalBuchholz-Relay is its integral measuring characteristic.It can only show, how much gas had come into thechamber since the last emptying. If an alarm is signal-ised it is difficult to know, if the gases were generatedby a large fault or caused by a small error emergedduring a long time period since the last emptying. Thenthe result is an erroneous alarm and misinterpretation inmany cases. Furthermore the history of the gas-development, which is important for a diagnosis of thefault, is unknown. Beside electrical breakdown there area few reasons for gas generation:• Degassing of saturated oil,• Drawing of air because of underpressure in front of

operating oil pumps,• Behind the oil pumps degassing by cavitation,

• Strong mechanical vibrations can cause bubbling insaturated oil,

• Sudden temperature changes in small oil volumes,• Sudden decrease of ambient pressure,• Blocked air dryers can cause underpressure inside

the tank, which leads to bubbling.

A computer aided monitoring using the Buchholz-Relayis difficult, because of its poor sensitivity. TheBuchholz-Relay in its actual form is thus only a protec-tion device and can not be used as a diagnosis tool. Fail-alarm and fail-interpretation is possible. To reduce thepossibility of erroneous alarms the small amounts ofgases which develop during a long time period and arenot an indication of a large failure should be emptiedout of the chamber. Only in this case a signal from theBuchholz-Relay indicates a large failure and the signalcan be used as protection signal.In the following a new developed sensor is introducedin which the functions of the Buchholz-Relay are ex-tended [5]. An incorrect alarm can be suppressed be-cause the gas-rate can be detected, which may be usedfor diagnostic purposes.The sensor consists of a cylindrical capacitor whichusually can be installed on the top of the Buchholz-Relay above the degassing valve. It has a volume of 10up to 25 ml and is normally filled with the transformeroil. Small amounts of gases entering the chamber of theBuchholz-Relay ascend into the sensor and replace theoil. The capacitance of the sensor changes adequatelyby the gas. The change of the capacitance is thereby ameasure for the gas-amount. After measuring the gasvolume, the gases are transferred into a gas collector,where they can be stored and analysed. The time of thegas-detection and the amount of the measured gases arestored in a memory. Additionally different parameterssuch as temperature, pressure or the load-condition canbe stored. Beside this if the oil level decreases becauseof e.g. a leakage an alarm is activated. If the sensor isconnected to a monitoring system these functionalitiescan be taken over by the central control unit.The sensor was installed for about two years on a200 MVA, 110 kV/220 kV transformer in a substation.On this transformer the Buchholz-Relay generated sig-nals irregularly and therefore the transformer was fail-ure suspicious. The reason for the gas generation wasunknown [6].With the help of the installed sensor it became obvious,that the gases were generated continuously with anamount of about 6-9 ml per month in winter time andabout 1 ml per month in summer time. This finding wasin accordance with the experiences of the utility, that theBuchholz-signals appeared mainly during winter time.Thus it could be concluded, that the gases were notoriginated from a large fault in the transformer. Ananalysis of the collected gases showed, that the maincomponent of the gases was air (containing oxygen andnitrogen). Further investigations allowed the assumptionthat in wintertime during ambient pressure changesgases are dissolved in the oil in the region of the flat ex-

MS 2000

Desktop PC

Modem

Field bus,Fibre Optic

Ethernet,TCP/ IP

Server

Substation Control System

Substation

Control Room

OfficeClients

� � ��

Fig. 1: Architecture of monitoring system MS2000

pansion vessel mounted on the top of the transformer.By temperature changes the gases come into the pipesbetween the expansion vessel and the transformer vesseland can go out of solution. This effect had been sup-ported by vibrations of the pipes thus the generated un-dissolved gases come into the Buchholz-Relay or re-spectively into the new sensor. After repairing the pipesand increasing the oil volume in the expansion vesselthis failure was no longer detected.A further example, which also demonstrates the effi-ciency as well as the diagnosis possibilities of an elec-tronic Buchholz-Relay, is described in the followingwhere the gassing behaviour of a 250 MVA transformerwith horizontal bushings could be clarified. The sensorwas installed on the Buchholz-Relay for the separatedoil volume of the 220 kV bushings. Fig. 2 shows the gasappearance during three weeks in summer after strongincreases of oil or ambient temperature. Each gassingconsists of the small amount of about 4 ml. Cold oil hasa lower gas absorption capability as warm oil, whichleads in the small oil volume inside the bushing to gasgeneration at sudden temperature changes. Because ofthe small rate of gas generation it could be concludedthat this is not a critical condition of the transformer.

The new feature of this sensor to connect temporalresolution of gas generation with operational data suchas temperature and load conditions requires in futureadditional work to evaluate fingerprints of abnormalconditions.

2.2. Mechanical Condition Assessment of OLTC

Due to the fact that serious damage to the transformercan be expected in the event of a failure of the On LoadTap Changer (OLTC), its condition is strategic to thereliability of the transformer. It is therefore important tomonitor this mechanically and electrically highlystressed element on-line.There are a few methods to perform an on-line moni-toring of the OLTC, e.g. the measurement of the tem-perature differential between the main tank and the tap-changer compartment to detect cooking contacts. Toevaluate the mechanical condition between the mainte-

nance intervals of the tap changer, the measurement ofthe active power consumption of the motor drive duringan operation is implemented in the Alstom monitoringsystem MS2000. Compared to other approaches thismethod is more simple and reliable, while maintainingall important information [7].The active power is recorded by means of an Aaronmeasuring circuit with a sample rate of 20 ms. Fig. 3illustrates the characteristic curve for the operation ofdiverter switch, selector and pre-selector contacts. Dur-ing the first 300 ms of the switching process a powerpeak occurs due to the starting current. The intention isto draw conclusions regarding the mechanical state ofthe OLTC from the position and the amplitude of thefollowing power peaks, which form a typical signatureof a specific tap changing. The entire signal consists ofthree parts dependent on the components which have apart in the specific operation. The loading of the springsfor the diverter switch is part of the active power up tothe final load switching at 4500 ms. During this periodthe opening of selector and pre-selector contacts, therevolution and the closing of the contacts take place. Allthese events are represented by typical peaks in the cur-vature of active power.

Additionally an operation with simulated problems, isshown in Fig. 3. These faults were generated by adhe-sive tapes, which were fixed on the metallic surface ofthe selector and pre-selector contacts to simulated me-chanical resistances while moving the contacts. The in-correct operation can be recognised by the higher am-plitude of the corresponding power peaks. For the appli-cation of this method to the on-line monitoring thecomplete switching operation is divided in 8 sections. Inthe event that the maximum peak within one section ex-ceeds an alarm level the monitoring system will triggera warning. In the case of retrofitting the alarm levels areascertained according to the recorded fingerprints foreach tap switching after a few weeks in service.The load current of the transformer is also stored duringa tap changer switching with a sampling rate of 20ms.The difference of load current before and after tapswitching is also a valuable information about the cor-rectness of the switching operation. Furthermore in case

17.6. 20.6. 23.6. 26.6. 29.6. 2.7. 5.7.0

2

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16

ml

20Gas AmountOil TemperatureAmbient Temperature

0

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°C

50

Fig. 2: Gassing behaviour of oil volume of a 220kVbushing

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500

500

550

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650

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Sta

rtin

gcu

rren

t

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switc

hingClosing of contactsof tap selector

Opening of contactsof tap selector

Rev

olut

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ofco

ntac

ts

Ope

ning

ofse

lect

or

Clo

sing

ofpr

e-se

lect

or

Ope

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ofpr

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or

Ope

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ofse

lect

or

Normal operationFaulty operation

Act

ive

pow

er[W

]

Time [ms]

Fig. 3: Active power of motor drive of OLTC

of a failure during tap changing the knowledge of loadcurrents is of a great importance for the identification ofthe causes.

2.3. On-line Partial Discharge Measurement

In the last few years extensive investigations on partialdischarge (PD) measurements at high voltage deviceshave been made, leading to improvement in measure-ment and sensor technology and also to various kinds ofalgorithms for PD-evaluation. On-line PD measure-ments on transformers can be performed using variousmethods, thus beside acoustic measurements electric PDdetection methods are used. Acoustic measurements usespecial sensors often based on the Piezo electric effectfor measuring the compressional waves in a frequencyrange between 50 kHz and 350 kHz. Using this tech-nique in some cases a location of the PD source is pos-sible, but due to the high damping owing to the insula-tion, conductors, magnetic circuit and the vessel of thetransformer the sensitivity of this technique is quitesmall. Nevertheless a PD location is possible within aradius of about 20 cm [8]. Therefore in an unfavorablecase in which a PD source is located at the edge of acoil opposite to the neighbouring coil even the phase ofthe transformer in which the PD appeared can not bedetected. Furthermore usually a lot of sensors are neces-sary thus increasing the efforts concerning the meas-urement equipment as well as the evaluation and theprocessing of the collected data. Also the determinationof the energy of the discharge can not be performed pre-cisely, because calibration measurements are almostimpossible. For these reasons electric PD measurementsare more preferable, because they allow the determina-tion of the apparent charges and in some cases also a PDlocation is possible. Electric PD measurements can bedivided into narrow-band and wide-band measurements.Narrow-band measurements are characterised by a cen-tre frequency and its bandwidth between 9 and 30 kHzwhereas wide-band measurements have a bandwidthbetween 100 and 400 kHz according to the proposal forthe revision of the international standard IEC 60270 [9].Narrow-band techniques enable due to the selection of asuitable centre frequency a noise suppression, but oftenit is necessary to choose a centre frequency of a fewMHz for an adequate noise suppression. This must notbe in accordance to the IEC 60270 where the centre fre-quency is limited to 1 MHz, except the frequency spec-trum of the partial discharge is almost constant up to thechosen centre frequency. Otherwise a measurement inthe high frequency range exclusively allows only astatement about the existence of partial discharges butnot about their apparent charge, thus the comparison ofhigh frequency components only is of less accuracy.Furthermore a PD location as well as a characterisationconcerning the type of the PD based on the evaluationof so called ϕ-q-n patterns or t-q-n patterns is difficultand can in general only be performed with expertknowledge.Therefore a broadband PD detection is preferable foron-line PD measurements, thus investigations have

shown that a bandwidth of about 10 MHz is suitable inorder to enable a location of PD sources using patternrecognition methods [10, 11]. This bandwidth is in ac-cordance with the revision of the IEC 60270 and de-fined as ultra-wide-band measurement, which representsa practical technique especially for the PD location.Using ultra-wide-band techniques the PD signals can bedecoupled with Rogowski coils mounted at the bottomof the bushings, capacitive dividers or measurementtaps, which are usually integrated in the bushings. Themeasurement setup on site is shown in Fig. 4 where thedecoupled signal is low pass filtered and amplified be-fore it is recorded by a digitizer and processed.

During broadband measurements on site various noisesignals influence the measurements, thus it is necessaryto suppress them by different filtering techniques. Firstthe continuous sinusoidal noises are suppressed using anadaptive digital filter. In Fig. 5 a measurement on atransformer in operation is shown before (a) and after(b) the suppression of sinusoidal noises. In this case themeasurements on a 200 MVA transformer have beenrecorded using a frequency range between 20 kHz and10 MHz.

Afterwards it is necessary to suppress periodical pulseshaped noises caused by e.g. thyristor drives which canbe efficiently done with adaptive correlation algorithms

Fig. 4: Measurement setup

Fig. 5: Suppression of sinusoidal noises

or frequency rejection filters. The residual signal onlycontains PD signals and stochastically impulse shapednoises caused by e. g. corona. Because these signals arequite similar it is difficult to separate them. Thereforedirectional coupling techniques are suitable, becausethey determine the energy flux of the signals, thus adistinction between signals from inside and outside thetransformer is possible [12].Fig. 6 shows this technique applied for PD signalsmeasured on a 200 MVA transformer on-line. From acomparison of the PD voltage signal Uc decoupled bythe capacitive divider and the PD current componentURog decoupled by Rogowski coils (see Fig. 6 a and b) aselection between partial discharges and noise pulse ispossible. Because calibration measurements have shownthat pulses with voltage and current component of thesame polarity are coming from outside the transformerthe pulses shown in detail (see Fig. 6 c and d) must benoise pulses.Finally from the recorded signal stream only partial dis-charges are left. This signals can be evaluated usingvarious kinds of pattern recognition methods in order tofind out where they have their PD origin. However, thisstage has to be performed off line because a lot of dif-ferent algorithms are necessary to analyse the data.

Fig. 6: Separation of PD and noise pulses

3. DIAGNOSTIC BY COMBINATION OF ON- AND OFF-LINE METHODS

The experience gained from on-line monitoring ofpower transformers is increasing steadily. There is nev-ertheless still a lack on how to integrate the informationobtained by the on-line monitoring into the actionstaken onto the service of the transformer. The combina-tion of on-line monitoring and off-line diagnosis pro-vides a powerful tool for the complete and economicassessment of the transformer condition. The supple-mentary information obtained by the off-line diagnosticafter the detection of an abnormal condition is a worth-

ful information to be integrated into future on-linemonitoring systems.Off-line diagnosis can use same methods like an on-linesystem, as this is the case for PD-measurements. On-line PD-measurements do allow to detect PD, but it isdifficult due to the difference in the measuring tech-nique to determine the apparent charge of the PD. Off-line diagnosis is carried out under well controlled sys-tem conditions, using enhanced measurement methodslike a multi-channel PD recorder, allowing the detectionand quantification of the PD.Off-line diagnosis provides furthermore powerful toolswhich allow supplementary information on the trans-former condition, which so far cannot be integrated inan on-line monitoring system reliably and cost effec-tively; among these methods are for example FrequencyResponse Analysis (FRA) and detailed Gas Chromato-graph Analysis (DGA).

3.1. Frequency Response AnalysisThe transfer function of a transformer winding is aunique characteristic for each transformer or trans-former winding. A transformer winding behaves as acomplex RLC network at high frequencies and its trans-fer function represents according to the system theorythe characteristic behaviour of a linear shift invariantsystem. Small changes in the geometry of the windinglead to changes of the corresponding capacitances andinductances and consequently to a change in the FRAresult. Different methods exist in order to determine thetransfer function of a transformer winding [14, 15]:• High Voltage Impulse (HVI)

• Low Voltage Impulse (LVI)

• Frequency Sweep Analysis (commonly calledFRA)

The HVI and LVI methods are based on the same prin-ciple, a steep impulse voltage is applied to the trans-former terminal and simultaneously the current in thedifferent terminals is measured. From these two signalsthe transfer function can be calculated.The transfer function between the input and output leadsas well as the transfer function between different wind-ings on the same limb can be calculated.The HVI method uses a High Voltage impulse, as dur-ing the lightning surge factory test or can be used as anon-line measurement during switching or lightning inthe network [16]. The main inconvenient of the HVImethod is the poor frequency spectrum of the input sig-nal and the sensitivity of this technique is not sufficientto detect minor changes in the winding.For the LVI method a low voltage impulse (some hun-dred Volts) is applied to the winding. The steepness ofthe applied impulse can be adjusted in order to obtain awide frequency band. The sample rate must be chosento allow measurements at the highest wished frequency.The main problem using LVI is the repeatability of thetest results, as they are depending on environmentalnoise conditions, thus it is in some cases difficult to

carry out comparisons between original signatures andrepeated measurements.During discrete frequency measurements the impedanceof the transformer winding is measured in function ofthe frequency by applying a low-voltage sinusoidal testsignal with variable frequency. The signals are meas-ured at discrete frequencies to determine amplitude andphase of the transfer function for the full frequencyrange. The disadvantage using the FRA is the relativelong duration for each measurement compared to theLVI method. The main advantage of the FRA is thegood repeatability of the test results, because this tech-nique is less influenced by superimposed noises.The measurements presented in this paper are carriedout in the frequency range between 10 Hz and 2 MHz,with a total of 2000 discrete sample points adjusted overthe frequency range.The assessment of a transformer winding through FRAcan be done by comparison to an earlier recording, bycomparison of the signatures of different phases in caseof a multi-phase transformer or two signatures of identi-cal transformers.Fig. 7 shows the amplitude of an on-site recording of a16.5 MVA, 90kV/27.5 kV single phase transformer be-fore and after a series of three short circuits on site. Thefirst resonance frequency changed from 325 Hz to 285Hz. With a slight amplitude increase. At low frequen-cies the main inductance is determined by the iron of themagnetic circuit. Differences in the residual flux densi-ties during the different measurements lead to changesof the magnetic permeability of the iron core materialand subsequently a change in the main inductance. Thisdoes influence the frequency response for low frequen-cies, but does not change the HF behaviour of thewinding.The HF results before and after the short circuit do

match very well. The HF response is determined by theinterdisc and interturn capacitances as well as thewinding inductances. The transformer did successfullypass the short circuit tests. Short circuit impedancemeasurements before and after the short circuit con-firmed the results obtained by the FRA.

3.2. Gas-in-oil detectionThe on-line monitoring of dissolved gases in oil iswidely used within installed systems. A number of dif-ferent sensors have been developed for this purpose.The information of the rise of dissolved hydrocarbongases in oil does normally not allow a “detailed” state-ment on the condition of the transformer. But it givesthe trigger for an off-line diagnosis in time in order toestablish a reliable diagnostic of the transformer to pre-vent a severe failure.Fig. 8 illustrates a detected increase of dissolved gasesusing a Hydran sensor. The obtained information by theon-line monitoring must be analysed using DGA analy-sis in order to establish a first diagnosis of the trans-former.The increase of the on-line detection of dissolved gasestriggered in this case a PD measurement. PD-measurements and their ultrasonic location allow a de-tailed diagnosis of the transformer insulation.

Fig. 8: Evolution of dissolved gases using a Hydransensor

For this purpose a multi-channel Automatic Partial Dis-charge Recorder (APDR) has been developed [13, 17].The APDR allows to record the signals simultaneouslyon up to 7 channels with a dynamic of 120 dB. The peakvalue, the polarity and the phase of each discharge isrecorded. The obtained record can be analysed cycle bycycle for the whole duration of the test. Methods ofsorting the PD’s into clusters and their characterisationcan be applied. By applying this tool to the presentedcase the supposition could be proved that the rise of thedissolved gases was due to inception of PD in oil.

4. TEMPERATURE MONITORING AND CONTROLINGOF COOLING UNIT

The thermal behaviour of a transformer can be repre-sented by a one-body system [18, 19]. In stationarycondition, all losses (P) are transferred to the environ-ment via the thermal resistance (Rth) of the coolingequipment. For the oil temperature rise, the followingapplies:

thnkairoil RPkP ⋅+⋅=− )( 02

,ϑϑ (1)

-70

-60

-50

-40

-30

-20

-10

0

1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06frequency [Hz]

amp

litu

de

[dB

]

Before short circuit

After short circuit

Fig. 7: FRA response before and after short circuit

The variable k is the ratio of the actual load to ratedload. In the case of strong fluctuations in ambient tem-perature or load, the thermal capacity (Cth) of the trans-former has also to be taken into consideration. This, to-gether with the thermal resistance, results in the thermaltime constant of the transformer. The dynamic charac-teristic of the oil temperature (ϑoil) can be calculated,on-line and iteratively, by means of the monitoring sys-tem in accordance with equation 2. The initial and finaloil temperatures are denoted by ϑoil,act and ϑoil,∞ respec-tively.

( ) actoilCR

t

actoiloiloilththet ,,, 1)( ϑϑϑϑ +���

�

�

���

�

�−⋅−≈ ⋅

−

∞ (2)

By resolving equation 1 to k, the permissible continuousload according to IEC 60354 can be calculated. Thespecific constants (P0, Pk,n, Rth) of the transformer aredetermined by the specific design. The ambient tem-perature can be measured and the top oil temperaturshould be limited to 105°C for OD-cooled transformersaccording to IEC 60354. In this way also the hot-spottemperature has to be controlled, because it is limitedfor normal cyclic loading to 120°C. It is made up ofambient temperature (ϑair), the calculated top oil tem-perature, and the load-dependent temperature differencebetween oil and winding temperature, weighted with thehot-spot factor (h). The demand to compensate periodsof high load and accelerated ageing by periods of lowload and slower ageing can be met by the on-line cal-culation of the ageing rate and calculating the 100-daymean-value.This thermal model was applied to a 250 MVA gridcoupling transformer. The maximum possible continu-ous load of that transformer, with the cooling plant op-erating at rated load, was determined as a function ofthe ambient temperature and is shown in Fig. 9. Due tothe low ambient temperature, maximum continuous loadfactors of up to 1.3 were reached.

The top oil temperature, calculated under considerationof the thermal time constant, corresponded with themeasured top oil temperature. Minor deviation of up to2 K will occur with strong fluctuations in ambient tem-perature and load. These deviations can be establishedby the unprecise detection of top oil temperature due to

the measurement of the top oil mixture. Thus, the reli-ability of the model is proved so that it is possible todetect failures of the cooling system, such as failures ofpumps or fans, by comparison of measured and calcu-lated top oil temperature.For emergency operation of the transformer the durationat a maximum load factor of 1.5 can be pre-calculated.Because of the strong variations of the oil temperatureduring such high load phases, it has to be borne in mind,that for the calculation of the hot-spot temperature atwo-body system with a much smaller time constant asin equ.2 has to be applied [19].This thermal model is also used for a load-dependentregulation of the cooling plant. For this purpose, the de-sired oil temperature is entered in the software of themonitoring system MS2000 as a control value. Thethermal resistance of the cooling plant required fortransferring the losses to the environment can be calcu-lated as a function of the ambient temperature and theactual load. The monitoring system determines thenumber of fans to be activated corresponding to the re-quired thermal resistance. In this way, a largely constantoil temperature is obtained; as this reduces „breathing“of the transformer, less moisture is absorbed by thetransformer oil. Compared with a conventional fan con-trol, the use of this intelligent, load-dependent fan con-trol offers a number of additional advantages:• Use of life can be reduced by optimising the hot-

spot temperature (life management),• Service consumption of the transformer can be re-

duced, as a smaller number of fans will operate atlower ambient temperatures,

• The overload capacity of the transformer can beraised by pre-cooling of the oil before a load peakoccurs,

• The selective control of fans will reduce the overallnoise level of the transformer.

5. CONCLUSIONS

The condition assessment of power transformers by thecombination of on- and off-line methods strongly re-duces the risk of severe failures. So it provides a reliableelectrical power supply in connection with an optimumexploitation of the active part. The most recent devel-opments of sensor technology, such as an electronicBuchholzgas relay, an OLTC monitoring based on thepower consumption of the motor drive and on-line PD-measurement have been described in this contribution.The results gained from the field application of thesenew sensor technologies show the early warning capa-bility.A comprehensive diagnosis however requires that infuture results from on- and off-line measurements has tobe put together by better proven evaluation methods,which have to be achieved in close collaboration be-tween transformer manufacturer and utility.

11.11.99 12.11.99 13.11.99 14.11.99 15.11.99 16.11.99-10

0

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Tem

pera

ture

[°C

]

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)

Fig 9: Calculation of oil temperature and overloadcapacity

6. REFERENCES

[1] "An International Survey on Failures of LargePower Transformers in Service" (CIGRE WorkingGroup 12.05, Electra, No. 88, January 1983).

[2] D.F. Peelo, et al.: "A Value Based Methodol-ogy for Selecting On-line Condition Monitoring of Sub-station Power Equipment" (EPRI Substation EquipmentDiagnostic Conference V, New Orleans, Louisiana, Feb.17, 1997).

[3] S. Tenbohlen, F. Figel: “On-line ConditionMonitoring for Power Transformers” (IEEE Power En-gineering Society Winter Meeting, Singapore,Jan. 2000)

[4] U. Sundermann, S. Tenbohlen, "Der intelli-gente Transformator - Zustandserfassung und Diagnosevon Leistungstransformatoren" (Elektrizitätswirtschaft,Jg. 97 (1998), Heft 10).

[5] H. Borsi; M. Urich, T. Leibfried "Das neueelektronische Buchholzrelais" (Elektrizitätswirtschaft,Jg. 97 (1998), Heft 13)

[6] P. Werle, V. Wasserberg, H. Borsi, E. Gocken-bach "Kombinierte Verfahren zur Zustandserkennungder Isolierung von Leistungstransformatoren" ETGTage, München, November 1999

[7] A. Krämer, et. al.: “Monitoring Methods forOn-load Tap-changers. An Overview and Future Per-spectives” (CIGRE Session 1996, paper12-108).

[8] H. M. Shertukde, J. G. Lackey: "Interpretationof Results obtained from Testing of Oil filled ElectricalPower Transformers in Field for several Utilities in theWorld" (IEEE International Symposium on ElectricalInsulation, Vol. 1, pp. 49-52, June 1998, WashingtonDC, USA)

[9] IEC 60270: "Partial Discharge Measurements"

[10] P. Werle, H. Borsi, E. Gockenbach: "Hierar-chical Cluster Analysis of Broadband Measured PartialDischarges as Part of a Modular Structured MonitoringSystem for Transformers" (11th International Sympo-

sium on High Voltage Engineering, Vol.5, pp. 29-32,August 1999, London, UK)

[11] D. Wenzel: "Teilentladungsmessungen anTransformatoren im Netz mit Verfahren der digitalenSignalverarbeitung und Mustererkennung" (Disserta-tion, Universität Hannover, 1998)

[12] M. Hartje: " Erfassung von Teilentladungen anLeistungstransformatoren im Netzbetrieb" (Dissertation,Universität Hannover, 1990)

[13] C. Moreau, P. Tantin, B. Mansuy, J. Poittevin,P. Despiney: “Monitoring and operational behaviour ofHV and EHV Instrument Transformers” (Cigre 1998,12-101)

[14] P.T.M. Vaessen, E. Hanique: “A New Fre-quency Response Analysis Method for Power Trans-formers” (IEEE Transactions on Power Delivery; Vol .7N° 1, January 1992, pp. 384 – 391)

[15] J. Bak-Jensen, B. Bak-Jensen, S.D. Mikkelsen;“Detection of Faults and Ageing Phenomena in Trans-formers by Transfer Function” (IEEE Transactions onPower Delivery; Vol. 10, N° 1, January 1995, pp. 308-314)

[16] T. Leibfried, K. Feser: “Off-line and On-lineMonitoring of Power Transformers using the TransferFunction Method” (International Symposium on Elec-trical Insulation, Montreal, 1996)

[17] J. Poittevin, D. Uhde, N. Foulon, J.-P. Lucas,G. Barré: “Partial Discharge Measurements on Trans-formers: Fingerprint Analysis” (INSUCON/ISOTEC1998 pp; 278 – 290)

[18] IEC 60354: “Loading guide for oil immersedpower Transformers” (IEC, 1991)

[19] S. Tenbohlen, M. Schäfer, H. Matthes: “Beur-teilung der Überlastbarkeit von Transformatoren mitonline Monitoringsystemen” (Elektrizitätswirtschaft,Jg. 99 (2000), Heft 1-2)