tech con 2011 kruger new experience with diagn meas on pt

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TechCon® Asia Pacific 2011

Modern Tools for Diagnostic Measurements on Power

Transformers

By

Michael Krüger Maik Koch

Key Rethmeier Alexander Kraetge

Alan McGuigan

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Modern Tools for Diagnostic Measurements on Power Transformers

Michael Krüger, Maik Koch, Kay Rethmeier, Alexander Kraetge, OMICRON electronics (Austria)

Alan Mc Guigan, OMICRON Australia

Abstract

Power transformers are critical, capital-intensive assets for utilities and industry.

Transformers are extremely reliable; however, many of the transformers in use today have

already exceeded their design life. Today transformers are not automatically replaced, if they

have reached the end of their life span, but left in service as long as possible. In contradiction

to the past many power transformers are operated nowadays at or above rated power. With

advancing age of power transformers, a regular check of the operative condition becomes

more and more important. The Dissolved Gas Analysis (DGA) is a proven and meaningful

method such that if increased proportions of H2 and hydrocarbon gases are found in the oil,

the fault must be located as soon as possible. In order to find out the reason for high gas rates,

further tests have to be performed. Common methods are: winding resistance measurement

(static), On-Load Tap Changer (OLTC) test (dynamic resistance test), turns ratio and

excitation current measurement, measurement of the leakage reactance and of the frequency

response of stray losses (FRSL), the measurement of the transfer function (FRA), capacitance

and dissipation factor measurement, the measurement of Partial Discharges (PD) and

dielectric response measurement with PDC and FDS.

DIELECTRIC RESPONSE MEASUREMENT

Increase of water in oil-paper insulations goes hand in hand with transformer aging, it

decreases the dielectric withstand strength, accelerates cellulose decomposition and causes

the emission of bubbles at high temperatures. State of the art for moisture measurements are

equilibrium diagrams where one tries to derive the moisture in the solid insulation (paper,

pressboard) from moisture in oil. This method fails for several reasons [1]. To assess the

insulation's water content some dielectric diagnostic methods were widely discussed and

occasional used during the last decade. The multilayer insulation of common power

transformers consists of oil and paper and therefore shows polarization and conductivity

effects. Dielectric diagnostic methods work in a range dominated by interfacial polarization

at the boarders between cellulose and oil, cellulose conductivity and oil conductivity.

Moisture influences these phenomena. Temperature and the insulation construction have a

strong impact too. In [2] a comparison of the mentioned methods was analysed. FDS and

PDC methods give rather reliable results and reflect also the influence of the temperature and

the geometry by using an X-Y model. The results of the PDC measurement can be

transformed from the time domain into the frequency domain [3], [4] and [5]. Although the

results of PDC and FDS methods are comparable and can be transformed from the time

domain into the frequency domain and vice versa, both methods have advantages and

disadvantages. If the FDS shall be used down to 100uHz, a measuring time of up to twelve

hours is needed for one measurement e.g. the insulation gap between HV and LV winding. If

also other insulation gaps e.g. HV winding to tank or LV to TV winding shall be measured,

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even more time is necessary. The PDC measurement needs much less time but is limited to

frequencies up to about 1Hz. A new approach combines both methods [6]. The FDS

measurement is replaced by the PDC method in the low frequency range and the results are

transformed into the frequency domain, whereas the FDS is used for higher frequencies,

which can be done rather quickly. Two input channels for simultaneous measurement of two

insulation gaps make it even faster. New model curves for aged oil-pressboard insulation, an

outcome of a research project at the University of Stuttgart make the results for aged

transformers much more reliable compared to the standard model curves for new oil-

pressboard insulation which were used up to date.

ROUTINE TEST AND FAULT LOCATION

The most common electrical diagnostic measurement methods are listed in table I. For all

impedance and dissipation factor measurements described in this paper a test system with a

power amplifier, which generates currents and voltages in a frequency range of 15 to 400 Hz

[7] was used. Therefore tests do not have to be made at line frequency only, but can be made

in a wide frequency range. Using frequencies other than 50/60 Hz and their harmonics,

precise results can be obtained even in substations with high electromagnetic interference by

filtering out the 50/60Hz with very effective digital filters.

Table I: electrical measurement methods

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WINDING RESISTANCE MEASUREMENT AND OLTC TEST

Winding resistances are measured in the field to check for loose connections, broken strands

and high contact resistance in tap changers. Additionally, the dynamic resistance

measurement enables an analysis of the transient switching operation of the diverter switch.

In most cases, the tap changer consists in most cases of two units. The first unit is the tap

selector, which is located inside the transformer tank and switches to the next higher or lower

tap without carrying current. The second unit is the diverter switch, which switches without

any interruption from one tap to the next while carrying load current. The commutation

resistances R or inductors L limit the short circuit current between the taps which could

otherwise become very high due to the switching of the diverter contacts without during the

period, where both taps are connected. The switching process between two taps takes

approximately 40–80 milliseconds.

Dynamic behaviour of the diverter switch

In the past only the static behavior of the contact resistances has been taken into account in

maintenance testing. With a dynamic resistance measurement, the dynamic behavior of the

diverter switch can be analysed (Figure 1) [8]. For the dynamic resistance measurement, the

test current should be as low as possible otherwise short interruptions or bouncing of the

diverter switch contacts cannot be detected. In this case, the initiated arc has the effect of

shortening the open contacts internally. Comparison to "fingerprint" results, which were

taken in a known (good) condition and to the other phases, allows for an efficient analysis. A

peak detector measures the peak of the ripple (delta I) and the slope (di/dt) of the measured

current, as these are important criteria for correct switching. If the switching process is

interrupted, even for less than 500us, the ripple and the slope of the current change

dramatically. Figure 2 shows some difference between the phases which are corresponding to

aged contacts.

FREQUENCY RESPONSE OF STRAY LOSSES (FRSL)

The frequency response measurement of stray losses is a tool to determine short circuits of

parallel strands. The resistive part of the short circuit impedance is measured over a

frequency range from 15Hz up to 400Hz. The resistance curves of the three phases are

Fig.1 Dynamic resistance measurement Fig.2 Comparison of the ripple of the three phases

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compared. The 15Hz values are very close to the DC values of the primary winding

resistance plus the resistance of the secondary winding multiplied by the square of the ratio.

If the curve of one phase is more than 2-3% different from the other phases a short circuit

fault between parallel strands can be the reason for this behavior. Local overheating can

cause dangerous breakdowns this behavior. Local overheating can cause dangerous

breakdowns [8].

CAPACITANCE AND DISSIPATION FACTOR (tan ) MEASUREMENT

In the past, the dissipation or power factor was measured at line frequency only. With the

described test system it is now possible to make these insulation measurements in a wide

frequency range. Beside the possibility to apply frequency sweeps, measurements can be

made at frequencies different from the line frequency and their harmonics. With this

principle, measurements are possible also in the presence of high electromagnetic

interference in high voltage substations.

Dissipation factor measurements on high voltage bushings

Bushings with high moisture in the insulation show increased 50/60Hz tan values

particularly at higher temperatures. Figure 3 shows the DF of OIP bushings at 50Hz for

different water contents as f(T) [9], figure 4 the DF as function f(f) of the frequency at

ambient temperature.

Limits for the dissipation factor

In the existing standards limits are given for 50/60Hz only. The measurement of the

dissipation factor at other frequencies should be also included in the standards. Low

frequency results (e.g. 15Hz) allow for a very sensitive moisture assessment, measurements

at high frequencies (e.g. 400Hz) allow a very sensitive detection of contact problems at the

measuring tap or at the layer connections. Also high resistive partial break downs between

grading layers can be detected. The table II shows indicative limits for new and aged

bushings at different frequencies [8]. All tests were done with test voltages of 2kV.

Fig.3 Tan-Delta = f(T) at 50Hz Fig.4 Tan-Delta = f(f) at 30°C

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The values in table II were extracted out of more than 2000 different measurements. They

were calculated as average values plus two times the standard deviation. That means that

95% of the results were below these values [10].

SWEEP FREQUENCY RESPONSE ANALYSIS (SFRA)

Sweep Frequency Response Analysis (SFRA) has turned out to be a powerful, non-

destructive and sensitive method to evaluate the mechanical integrity of core, windings and

clamping structures within power transformers by measuring the electrical transfer functions

over a wide frequency range. This is usually done by injecting a low voltage signal of

variable frequency into one terminal of a transformers winding and measuring the response

signal on another terminal. This is performed on all accessible windings following according

guidelines. The comparison of input and output signals generates a frequency response which

can be compared to reference data, to other phases, or to sister transformers (figure 5). The

core-and-winding-assembly of power transformers can be seen as a complex electrical

network of resistances, self- and mutual inductances, ground capacitances and series

capacitances. The frequency response of such a network is unique and, therefore, it can be

considered as a fingerprint.

Table II Indicative limits for bushings [8],[10]

Fig. 5 Principle operation of SFRA

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Geometrical changes within and between the windings change mainly the capacitor elements

of the network and cause deviations in its frequency response. Differences between an FRA

fingerprint and the result of an actual measurement are an indication of electrical variations of

the components of the equivalent circuit diagram. Different failure modes affect different

parts of the frequency range and can usually be discerned from each other. Practical

experiences as well as scientific investigations show that currently no other diagnostic test

method can deliver such a wide range of reliable information about the mechanical status of a

transformer's active part.

PARTIAL DISCHARGE MEASUEMENT

Partial Discharge (PD) measurement is a worldwide accepted tool for quality control of high

voltage apparatus. Outside screened laboratories PD signals are very often superposed by

noise pulses, a fact that makes a PD data analysis more difficult for both human experts and

software expert systems. Therefore the handling of disturbances is one of the main tasks

when measuring PD.

For DC PD measurements where the expected PD rate might be very low, even single

disturbance pulses can influence the test result significantly.

Modern methods of data evaluation

A new field of evaluation methods is opened by fully synchronous multi-channel PD

acquisition in order to gain more reliable measuring results combined with effective noise

suppression. A technical overview of the system is given in [11].

Being able to perform synchronous multi-channel PD measurements, the Three-Phase-

Amplitude-Ratio-Diagram (3PARD) was introduced as a new powerful analysis tool to

distinguish between different PD sources and noise pulses when measuring high voltage

equipment such as power transformers, rotating machines and cable systems. Responses

of the PD are received simultaneously at three different positions of the measurement

circuit. The amplitudes of three PD channels are geometrically added like vectors in a star

diagram, where the three axes are corresponding to the three channels. A PD source

corresponds normally to an amplitude ratio of the three responses which is defined by the

coupling of the PD signal into the three channels. Noise is normally more or less received

with similar amplitude responses from the three channels. The addition of the three

similar noise signals gives a position close to the star point of the 3PARD.

CASE STUDIES

Overheating of a 123kV transformer

A transformer for 220kV / 50MVA showed strongly increased values of Methane, Ethane,

Ethylene and Acetylene. Carbon Monoxide and Carbon Dioxide were within tolerable limits

(figure 6) which is indicating that no cellulose is involved. First the high voltage winding

resistance was measured on all taps. The star point of the transformer was not accessible, so

the measurement was made phase to phase. The values were first not really stable on some

taps on the U-V measurement. The test was repeated after some movements of the tap

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changer. The repeated test didn't indicate any fault. By moving the tap changer contacts, the

contact surface was cleaned. The problem could be found by measuring the low voltage

windings. Phase 2w showed an increase of the resistance value of 12% compared to the other

phases (figure 7). The inner connection between the bushing and the conductor had a rather

high contact resistance, most probably by a high short circuit current close to the transformer

in the past.

High dielectric losses on a 123kV RBP bushing

One bushing on a 123kV transformer showed extreme high Tan Delta values particularly at

low frequencies (figure 8). The bushing was exchanged and then opened (figure 9). The

reason for the fault was a crack in the metal hood of the bushing. So water could come into

the porcelain and caused creeping discharges on the resin surface.

FRA on a damaged 110kV transformer

A transformer for 110kV/30MVA showed unbalanced voltages on the LV windings after a

short circuit close to the transformer. The FRA measurement results are shown in figure 10.

The phases U and W are very similar whereas Phase V was totally different. The transformer

was transported to a workshop and analysed. The short circuit current on the LV side caused

an interruption of one of the two parallel windings of the phase V. The winding was

Fig.8 Tan Delta curves Fig.9 Surface of the opened active part

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

8.0%

0.0

Hz

50.0

Hz

100.0

Hz

150.0

Hz

200.0

Hz

250.0

Hz

300.0

Hz

350.0

Hz

400.0

Hz

450.0

Hz

A B C

TanDelta

Fig.6 FRA on the 110kV transformer Fig.7 Increased internal contact resistance on 2w

Gases

Normal <

ppm

Abnormal >

ppm

Result

ppm Interpretation

Hydrogen H2 150 1,000 110 PD, Arcing

Methane CH4 25 80 2,250 Sparking

Ethane C2H6 10 35 625 Local overheating

Ethylene C2H4 20 150 1,916 Severe overheating

Acetylene C2H2 15 70 372 Arcing

Carbon monoxide CO 500 1,000 60 Overheating of paper

Carbon dioxide CO2 10,000 15,000 780 Overheating of paper

Nitrogene N2

10000 to

100000 n.a 60,600

Oxygene O2

2000 to

35000 n.a 10,050

Total combustible gasses 300.00 500.00 5,273.00

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interrupted at one end (figure 11). The whole clamping structure with the press rings and the

spacers was broken. It was decided to recycle the transformer.

FRA on a 110kV off-shore transformer

A transformer for 132kV/180MVA on an off-shore wind power plant was exposed to a

short circuit on a current transformer nearby. This caused an inner transformer fault

which was indicated by a Buchholz relay tripping. Figure 12 shows the FRA and figure

13 the winding fault.

Influence of remanent flux on the FRA

A generator step up transformer for 10kV/220kV showed abnormal FRA curves.

Normally the FRA trace of the middle limb shows one single minimum (parallel

resonance) at low frequencies, typically below 1kHz, whereas the outer phases show two

minima. This is caused by the fact, that the outer phases have two different loops for the

flux, through the middle limb and through the other outer limb. The two slightly different

inductivities are in resonance with the winding capacitance. Therefore two resonances,

Fig.12 FRA of the 132kV transformer Fig.13 Winding fault

Fig.10 FRA on the 110kV transformer Fig.11 Interrupted winding

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close to each other are visible. In the described case the middle phase had two resonances

below 1kHz, one of the outer phase had only one (figures 14 and 15).

After demagnetizing of the core FRA curves were measured as expected (figures 16 and 17).

The demagnetizing was made with reduced frequency. This has the advantage that the

voltage which is needed to get the core into saturation is reduced accordingly. If the low

frequency range is used for diagnosis of core problems, the core should always be

demagnetized otherwise FRA could lead to wrong conclusions.

PD measurement on a repaired transformer

Figure 18 shows a PD measurement with four simultaneously measuring channels which

are connected to the three HV bushings and the star point. The transformer was excited

from the low voltage side by a small hydro generator. With a step-up transformer the 6kV

output voltage was transformed to the rated secondary voltage of the test object of 21kV.

It can be seen in figure 19 that the three different clusters in the 3PARD diagram are

generated by three different PD sources: statistical noise, pulse disturbances and inner

partial discharges. The interference noise shown in the right hand picture of figure 15 has

Fig.16 FRA after demagnetising, phase U,V and W Fig.17 FRA of phase U before and after demagnetising

Fig.14 FRA of the 220kV transformer Fig.15 First resonances

before

after

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disappeared in the other pictures. This way PD sources can be separated from each other

and from noise.

Two case studies with 3 Center Frequency Ratio Diagram Filtering

As an enhancement of 3PARD the 3-Center-Frequency-Ratio-Diagram (3CFRD) was

introduced as an additional tool for PD data analysis and PD fault separation in real-time

on single-phase test objects [12]. The synchronous consideration of PD amplitudes at

three different frequencies within the PD spectrum provides information on its discharge

nature and indicates its possible PD fault location due to PD signal propagation and

attenuation. The 3CFRD method requires 3 different PD band-pass filters, measuring each

PD event simultaneously at their predefined center frequencies. Here the proper selection of

these three band-pass positions in the frequency domain is the key to get the optimum benefit

from this method. These three filters have to be set in a way that the spectral differences of

PD pulses and other pulses are at their maximum. Figure 20 shows the spectra of three PD

pulses and the three filters marked as blue bars. The red arrows indicate the absolute charge

values of PD pulse 1 (red curve) at the discrete filter frequencies.

The charge values are drawn into the star diagram as shown in figure 21. The lengths of the

vectors represent the measured charge and the axes indicate the respective filter. By

geometrically adding the vectors one single dot is the final representation of the initial triplet.

The use of this principle is shown in two examples. The first example is the 3CFRD

measurement on a high voltage bushing. Figure 22 shows the phase resolved pattern of

Fig.20 FFT spectra of different PD pulses Fig. 21 3CFRD construction of pulse 1

Fig.18 PD measurement on a 110kV transformer Fig. 19 3PARD filtering

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different PD sources. The overlaying of different patterns doesn't allow a detailed analysis.

Figure 23 shows a separated pattern of one source by 3CFRD filtering.

A second example for CFRD filtering is a measurement on a transformer. Figure 24 shows

the PRPD at app. 40kV. The noise is about 10pC.

An analysis with the 3CFRD filtering is shown in figure 25. The filtered signal shows a clear

pattern of internal void discharges with 5pC, although the PD pulses are below the noise

level.

Summary

With advancing age transformers require regular checks of the operating conditions.

Measurement of the water content in oil-paper insulation is a helpful tool for making an

assessment of the ageing of the cellulose. The analysis of the gas in oil is a well-proven

method of analysis but must be complemented by efforts to locate any faults indicated by

excess hydrocarbon gases in the oil. This way important maintenance can be performed in

time to avoid a sudden total failure. The fault location can be successfully performed using

modern type test equipment for resistance, winding ratio, short circuit impedance, C, tan ,

FRA and PD measurements.

Fig.24 Phase resolved pattern without 3CFRD filtering Fig. 25 PRPD of one PD source with 3CFRD

Fig.22 PRPD pattern without 3CFRD filtering Fig. 23 PRPD of one PD source with 3CFRD filtering

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References

[1] M. Koch “Improved Determination of Moisture in Oil-Paper-Insulations by

Specialised Moisture Equilibrium Charts” Proceedings of the XIVth International

Symposium on High Voltage Engineering, p. 508, Beijing, China, 2005

[2] M. Koch, K. Feser "Reliability and Influences on Dielectric Diagnostic Methods to

Evaluate the Ageing State of Oil-Paper-Insulations" APTADM Wroclaw 2004

[3] D. Giselbrecht, T. Leibfried "Modelling of Oil-Paper Insulation Layers in the

Frequency Domain with Cole-Cole functions" ISEI (IEEE) Toronto 06/2006

[4] V. Der Houhanessian “Measurement and Analysis of Dielectric Response in Oil-

Paper Insulation Systems”. Ph. D. dissertation, ETH No. 12832, Zurich, 1998

[5] A.A. Shayegani, H. Borsi, E. Gockenbach, H. Mohseni "Time Optimization of

Dielectric Response Measurements" Nordic Insulation Symposium, Trondheim,

Norway, June 2005

[6] H. Borsi, E. Gockenbach, M. Krüger "Method and apparatus for measuring a

dielectric response of an electrical insulating system" US2006279292

[7] Hensler, Th., Kaufmann, R., Klapper, U., Krüger, M., Schreiner: S., 2003, "Portable

testing device", US Patent 6608493

[8] "Guide for Transformer Maintenance", Cigre Brochure 445, February 2011,

ISBN: 978-2-85873-134-3

[9] ABB, "Dissipation factor over the main insulation on high voltage bushings", product

information, ABB 2002

[10] M.Krüger, A. Kraetge, M. Koch, K. Rethmeier et al. „New Diagnostic Tools for High

Voltage Bushings“, Cigre VI Workspot, Foz do Iguacu, April 2010

[11] K. Rethmeier, M. Krüger, A. Kraetge, R. Plath, W. Koltunowicz, A. Obralic, W.

Kalkner, Experiences in On-site Partial Discharge Measurements and Prospects for

PD Monitoring, CMD Beijing 2008

[12] K. Rethmeier, A. Obralic, A. Kraetge, M. Krüger, W. Kalkner , R. Plath. "Improved

Noise Suppression by real-time pulse-waveform analysis of PD pulses and pulse-

shaped disturbances", International Symposium on High Voltage on High Voltage

Engineering (ISH), Cape Town, August 2009

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Biographies

Michael Krüger is manager of engineering services for primary testing solutions with

OMICRON electronics GmbH, Austria. He studied electrical engineering at the University of

Aachen (RWTH) and the University of Kaiserslautern (Germany) and graduated in 1976

(Dipl.-Ing.). In 1990 he received the Dr. techn. degree from the University of Vienna.

Michael Krüger has more than 30 years experience in high voltage engineering and insulation

diagnosis. He is member of VDE, Cigre and IEEE. He works in several working groups of

Cigre and IEC.

Maik Koch is manager of the product management for primary testing solutions with

OMICRON electronics GmbH, Austria. He studied Electrical Engineering at the University

of Applied Science “Lausitz” in Germany (1990 to 1995) and afterwards electrical

engineering at the Technical University of Cottbus in Germany (2000 to 2003). He worked as

a research assistant at the Institute of Power Transmission and High Voltage Technology of

the University of Stuttgart in Germany. In 2008 he received the Dr.-Ing. degree from the

University of Stuttgart. His special focus is the field of ageing and moisture measurement in

oil paper insulated power transformers using chemical and dielectric analysis methods.

Kay Rethmeier is application engineer with OMICRON electronics GmbH, Austria. He

studied electrical engineering at the Technical University of Berlin. In 2006 he received the

Dr.-Ing. degree from the Technical University of Berlin in the field of partial discharge

measurement. Before he joined OMICRON he was working as R&D engineer with the focus

on insulation diagnosis on high voltage cables. His special focus is now partial discharge

measurement and monitoring.

Alexander Kraetge is product manager for primary testing equipment with OMICRON

electronics GmbH, Austria. After vocational education as electrician he studied high voltage

and power engineering at the Technical University of Berlin (Germany) and graduated in

2001 (Dipl.-Ing.). Until 2006 he worked as a research engineer for power transformer

condition assessment at the High Voltage Department of TU Berlin. In 2007 he received the

Dr.-Ing. degree from the Technical University of Berlin. He is member of VDE and IEEE.

Alan McGuigan is Field Applications Engineer with OMICRON Australia. He has a long

and diverse career in all aspects of the electrical supply industry. He is a past member of the

Institute of Engineers Australia and his previous experience includes many years as a Test

Engineer in the Transmission department of the SECV, covering construction and

maintenance of medium to large substations and power stations. Alan also has over 10 years

experience in the distribution area in roles of design, system planning and construction. Alan

commenced his present position as Field Applications Engineer with the role to provide

technical support, training and demonstrations for OMICRON primary test equipment and to

organise special focus symposiums and workshops throughout Australia.

tech con 2011 kruger new experience with diagn meas on pt

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