Dirana Anp 11002 Enu

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OMICRON Page 1 of 24

Application Note

Measuring and Analyzing the Dielectric Response ofCurrent Transformers

Author Stephanie Raetzke | [email protected]

Alan McGuigan| [email protected]

Date August 2011

Related OMICRON ProductDIRANA

Application AreaCurrent Transformer

Versionv1.0

Document ID ANP_11002_ENU

AbstractThis application guide informs how to measure and analyze the dielectric response of current transformers inorder to assess the capacitance, dissipation factor and moisture.
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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OMICRON 2010 Page 2 of 24


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Content

1 Using this document ....................................................................................................................... 5 1.1 Operator Qualifications and Safety Standards ........................................................................... 5 1.2 Safety measures ....................................................................................................................... 5 1.3 Related Documents .................................................................................................................. 5

2 Preparing the Current Transformer ................................................................................................ 5

3 Access and Connections - General Procedure .............................................................................. 6

4 Measurement Configurations ......................................................................................................... 8 4.1 Measurement of HV terminal to bushing capacitor tap ............................................................... 8 4.2 Measurement of bushing capacitor tap to housing ....... ........ ....... ....... ....... ........ ....... ....... ........ ... 8 4.3 Measurement of HV to secondary or housing with capacitor tap earthed ....... ........ ....... ....... ...... 9

5 Setting up the Software..................................................................................................................11

6 Measurements ................................................................................................................................12

6.1 Pre Measurement Check with the Monitor Device ....................................................................12 6.2 Development of the dissipation factor curve .............................................................................13 6.3 Determination of the Capacitance ............................................................................................14 6.4 Creating a Measurement Report ..............................................................................................14 6.5 Measurement Errors ................................................................................................................15

6.5.1 Voltage Source Overload ........................................................................................................... 15 6.5.2 Input Overflow ........................................................................................................................... 15 6.5.3 Negative Dissipation Factor ........................................................................................................ 15 6.5.4 Disturbances during Time Domain Measurement ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... 16

7 Interpretation of Measurement Data ..............................................................................................17 7.1 Dissipation factor curve ............................................................................................................17 7.2 Frequency dependent capacitance...........................................................................................18 7.3 Moisture Analysis for Instrument Transformers Using DIRANA ........ ....... ....... ........ ....... ....... .....19

8 Step by step guide for the measurement on current transformers ....... ........ ....... ....... ....... ........ ..20

9 Contact Technical Support ............................................................................................................23

10 Literature ........................................................................................................................................23


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Please use this note only in combination with the related product manual which contains several important safetyinstructions. The user is responsible for every application that makes use of an OMICRON product.

OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON.

OMICRON 2010. All rights reserved. This application note is a publication of OMICRON. All rights including translation reserved. Reproduction of any kind, for example, photocopying, microfilming, opticalcharacter recognition and/or storage in electronic data processing systems, requires the explicit consent of OMICRON.Reprinting, wholly or in part, is not permitted.

The product information, specifications, and technical data embodied in this application note represent the technicalstatus at the time of writing and are subject to change without prior notice.

We have done our best to ensure that the information given in this application note is useful, accurate and entirelyreliable. However, OMICRON does not assume responsibility for any inaccuracies which may be present.OMICRON translates this application note from the source language English into a number of other languages. Anytranslation of this document is done for local requirements, and in the event of a dispute between the English and a non-English version, the English version of this note shall govern.


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1 Using this documentThis application guide provides detailed information on how to measure and to analyze the dielectricresponse of current transformers using the OMICRON DIRANA. Please refer to national and internationalsafety regulations relevant to working with the DIRANA. The regulation EN 50191 "The Erection andOperation of Electrical Test Equipment" as well as all the applicable regulations for accident prevention inthe country and at the site of operation has to be fulfilled.

1.1 Operator Qualifications and Safety StandardsWorking on HV devices is extremely dangerous. The measurements described in this Application Guidemust be carried out only by qualified, skilled and authorized personnel. Before starting to work, clearlyestablish the responsibilities. Personnel receiving training, instructions, directions, or education on themeasurement setup must be under constant supervision of an experienced operator while working with theequipment. The measurement must comply with the relevant national and international safety standardslisted below:

EN 50191 (VDE 0104) "Erection and Operation of Electrical Equipment" EN 50110-1 (VDE 0105 Part 100) "Operation of Electrical Installations" IEEE 510 "Recommended Practices for Safety in High-Voltage and High-Power Testing" 1910.269(a)(1)(i)(C) "Occupational Safety and Health Standards - Electric Power

Generation, Transmission, and Distribution" Appendix C LAPG 1710.6 NASA "Electrical Safety"

Moreover, additional relevant laws and internal safety standards may have to be followed.

1.2 Safety measuresBefore starting a measurement, read the safety rules in the DIRANA User Manual and observe the

application specific safety instructions in this Application Note when performing measurements to protectyourself from high-voltage hazards.

1.3 Related DocumentsDIRANA User Manual Contains information on how to use the DIRANA test system and relevant safetyinstructions.

2 Preparing the Current Transformer

In order to determine the dielectric properties of a current transformer using a dielectric responsemeasurement, the device needs to be de-energized and then disconnected from the network. Allconnections to the current transformer should be removed in a manner as to conventional dissipation factortests. If a complete disconnection is impossible a measurement still can be performed. While measuring thecapacitance of a CT the Guarding technique prevents disturbing influences by still-connected devices.However, the following requirements must be fulfilled:

Avoid overloading of the instrument due to high currents, e.g. long cables. The remaining devices should have low capacitances and losses compared to the measured

insulation; otherwise high guard currents may cause a negative dissipation factor (p. 15) . Avoid electromagnetic field coupling since the remaining devices might act as antenna


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3 Access and Connections - General ProcedureThis section gives illustrated introductions how to connect the DIRANA to a current transformer. Please alsorefer to the user manual.

1. In order to have the same reference potential, connect the grounding cable to the ground terminal onthe rear panel of the DIRANA, and clamp its other end to the tank.

2. After this, connect the HV conductor to the output channel (yellow) of the DIRANA.

3. When connecting the measurement cable of the input channel (red) to the measurement tap use thesplit connectors delivered with the DIRANA to connect the tri-axial cable to the connector usingalligator clips or wires.


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4. Connect the guard of both measurement cables to the tank. Confirm a good connection, avoidlacquered surfaces or corroded metal. Clean the surfaces, if necessary.

5. If available, wrap a conductive belt around the bushing section, and connect it to the tank.

6. Finally, plug the measurement cables into the DIRANA instrument.


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4 Measurement ConfigurationsFor a CT with a capacitor tap available on the bushing there are three options for dielectric responsemeasurements:

HV to bushing capacitor tap Bushing capacitor tap to housing HV to housing with capacitor tap earthed

4.1 Measurement of HV terminal to bushing capacitor tapFor measuring between the HV connections and the capacitor tap on the bushing section the output isconnected to the HV conductor of current transformer. Both HV terminations are connected together withoutthe lead touching the insulator. The earth link is removed from the bushing capacitor tap and the DIRANAoutput is connected to the HV termination. The input channel CH1 is contacted to the capacitor tap. In the"Configuration" window select "Bushing" test and deselect the measurement at bushing B. Ignore the bridgeto the other bushings in the diagram (Figure 1) .

Figure 1: DIRANA connection diagram for the measurement of HV terminal to bushing capacitor tap

4.2 Measurement of bushing capacitor tap to housingThe second measurement is between the capacitor tap of the bushing and the metal housing. In theconfiguration window select transformer and tick the CL measurement between the secondary winding andthe tank (Figure 2) . The DIRANA output is connected to the tank and the red measuring lead connected tothe capacitor tap with the earth bridge disconnected. Both guards are connected to the HV terminals whichare shorted together.


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Figure 2: DIRANA connection diagram for the measurement of bushing capacitor tap to housing

4.3 Measurement of HV to secondary or housing with capacitor tap earthedFor measurements between the HV terminations and secondary winding or tank select either CurrentTransformer or Current Transformer with Screen Electrode.

Wiring diagram "Current Transformer"With the Current Transformer configuration (Figure 3) the DIRANA output is connected to the HV terminals,the input CH1 is connected to the CT secondary with all secondary terminations shorted together. No earthis to be connected to the CT secondary terminations. The guards from the output and measuring leads areconnected to the tank. The capacitor tap earth link is closed.

Figure 3: DIRANA connection diagram "Current Transformer"


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5 Setting up the Software1. Connect DIRANA to a USB port of your laptop and start the DIRANA software. The status field in the

lower right corner of the main window indicates that the connection is established.

2. Record all relevant current transformer nameplate data, like serial number and bushing type. If amoisture analysis should be done, the temperature of the internal insulation is absolutely necessaryand should be noted as well. Also record ambient weather conditions.

3. Press the button "Configure Measurement".

4. By clicking the drop-down-list, choose the configuration diagram.

5. Click the "Settings" tab and then enter 1kHz as start frequency and 10 mHz or 1 mHz as stopfrequency. This is sufficient for most current transformers. Furthermore the measurement modeshould be switched to "FDS only" which can be found in "Show Advanced Settings".

6. After this, close the dialog field "Configure Measurement" by clicking on "OK".


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6 Measurements

6.1 Pre Measurement Check with the Monitor Device

Often simple connection problems may affect the measurement. To determine the capacitance, signal-to-noise ratio and noise current for ensuring a successful measurement press the button:

The "PDC Monitor" can be used to estimate the signal-to-noise ratio at different polarization voltages. Beforestarting the polarization, the input-coupling noise causes a current, which should be considerably lower (atleast 1:10th) than the current after the polarization is started (Figure 5) . Recommendations are given in theinformation box, indicating how to improve the measurement.

Figure 5: Pre Measurement check with the PDC Monitor

The dependence of capacitance, tangent delta, power factor or impedance depending on frequency andvoltage can be checked using the "FDS Monitor" (Figure 6) . The frequency range and the voltage amplitudecan be changed using the settings. After applying settings the capacitance, tangent delta, power factorand/or impedance are displayed. They should be stable for a good measurement.

Figure 6: Pre Measurement check using the FDS Monitor


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6.2 Development of the dissipation factor curve After setting up the software and checking the measurement cables, press the "Send Configuration toDevice and Start Measurement button . During the running measurement do not move the cables sincethe piezoelectric effect may cause disturbing charges. The dissipation factor curve will appear, starting at thehigh frequencies, and developing toward the low frequencies.

Figure 7: Dissipation factor curve starting at the high frequencies

Figure 8: Dissipation factor curve after transition from time to frequency domain at 0.1 Hz

Figure 9: Complete Dissipation factor curve from 1kHz to 1mHz


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For creating a measurement report, select the desired measurements which should be included in thereport using the check boxes. The "Print Preview" will now show the measurement report. Use the"Save as/Export" button to save the measurement report as an Excel or PDF file.

6.5 Measurement Errors6.5.1 Voltage Source Overload

If the instrument is unable to reach the desired voltage, an error message will indicate instrument overload.

To solve the problem: Check whether the measurement setup has resulted in a short-circuit. If capacitive currents cause an overload (typical for long cables), decrease the output voltage or

start the measurement at lower frequencies than 1000 Hz; i.e. at 100 Hz.

6.5.2 Input Overflow

In case the software displays an input overflow error, check that the CT and the DIRANA have the samereference potential. Usually this error appears when the tank is on a floating potential. Connect the tank tothe ground terminal on the rear panel of the DIRANA (p. 6) .

6.5.3 Negative Dissipation Factor

The dissipation factor curve may turn negative at high frequencies, see Figure 11. Reasons for this problemmay be at first a high guard impedance, a small measured capacitance in conjunction with a large guardcapacitance and high guard currents (dirty bushings).

Figure 11: Dielectric measurement with negative dissipation factor

To solve the problem:

Connect all guards of measurement cables and if possible an additional wire from the triaxialconnectors at the DIRANA front plate to the tank.

Try to decrease the guard currents (clean bushings, disconnect all devices which are possibly stillconnected).

Confirm a proper connection of the DIRANA housing to the reference potential is made.

CHL

f/Hz0.001 0.010 0.100 1.000 10.000

DF

0.0050.0100

0.0500.100

0.5001.000


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6.5.4 Disturbances during Time Domain Measurement

Disturbances in the time domain current are transformed into the frequency domain and affect the resultsdisplayed in frequency domain (e.g. dissipation factor). Figure 12 shows disturbances on the time domaincurrent for 600-1100 s measurement time as an example. They cause disturbances in dissipation factor forthe low frequencies. Generally, the disturbances in time domain will appear in frequency domain dependingon their frequency spectrum.

Figure 12: Time domain current with disturbances at around 1000s (left) and its transformation in frequency domain withdisturbances at the low frequencies (right). The reason for the disturbances was that guarding was not applicable for thismeasurement.

To solve this problem:

Use a guarded measurement set-up if possible Apply all guards of the measurement cables Increase measurement voltage Try to minimize disturbances by e.g. using an electrostatic shield Perform the measurement in frequency domain only. In the dialog field "Configure

Measurement", click on the "Show Advanced Settings" button. Set the "Type of MeasurementSequence" to "FDS only". Please note that this increases the time duration for the measurementsubstantially.

f/Hz0.0010 0.0100 0.1000 1.0000 10.0000

DF

0.020

0.050

0.100

0.200

0.500

1.000

2.000

t/s2 5 10 20 50 100 200 500 1000

I/A

0.0000005

0.0000007

0.0000010

0.0000020

0.0000030HV+LV to tank HV+LV to tank


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7 Interpretation of Measurement Data

7.1 Dissipation factor curve

The dielectric response of instrument transformers has a very specific shape, which is similar to the dielectricresponse of cellulosic material itself (Figure 13) . The curve has a nearly linear part at the lower frequenciesand is rather flat at high frequencies with a minimum around power frequency.

Figure 13: Dielectric response of cellulosic materialat 20C with 1% (2% and 3%) water content

Figure 14: Dielectric response of four current transformers of thesame type

Instrument transformers of the same type, which are aged similar and also having a similar water content willhave nearly identical curves, like shown in Figure 14. Ageing as well as moisture in the solid insulation willincrease the dissipation factor especially at low frequencies, whereas the dissipation factor at powerfrequencies is relatively stable. Only for high water contents or strongly aged insulations the dissipationfactor will significantly increase at power frequencies. Therefore the 50/60 Hz value often only significantlychanges, when the ageing or moisture ingress has already led to a bad insulation condition (Figure 15) .Since the low frequencies are much more sensitive to ageing and moisture, it might be helpful to comparethe values e.g. at 10 mHz.The height of the dissipation factor is specific for the voltage class and the type the instrument transformer.Therefore the limits should be defined only within such a group.

Figure 15: Dielectric response of instrument transformers of different age and condition

f/Hz0.001 0.01 0.1 1.0 10.0 100

DF

0.005

0.010

0.020

0.050

0.100

0.200

0.500

1.000 1%@20C3

2

1

0,001

0,01

0,1

1

0,01 0,1 1 10 100 1000

d i s s

i p a

t i o n

f a c

t o r

frequency in Hz

phase Aphase Bphase Cphase A-2

0,001

0,01

0,1

1

10

0,01 0,1 1 10 100 1000

d i s s i p a t

i o n

f a c

t o r

frequency in Hz

old; 4% water content, 9 pS/m oil conductivity


new; 1,2% water content, 3 pS/m oil conductivity



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7.2 Frequency dependent capacitance

Figure 16: Frequency dependent capacitances of various instrument transformers depending on frequency

The assessment of the frequency dependent capacitance is useful to gain knowledge about the insulationcondition. An ideal insulation has a frequency independent capacitance. However, the capacitance of realinsulations is increasing at low frequencies (Figure 16) . For new and dry oil-paper-insulations the increase isvery small. For aged and wet insulations the capacitance at low frequencies will increase stronger. This canbe visualized by the ratio of the capacitances at very low frequencies, e.g. 10 mHz and power frequency50/60 Hz. For new and dry insulations this ratio is about 1.05. During the lifetime the ratio will increase. Mostinstrument transformers in service with acceptable insulation condition have a ratio of 1.3 or below (Figure17) . This value is rather independent of CT type and size.

Figure 17: Ratio of capacitance values between 10 mHz and 50 Hz for various CTs of different insulation condition

100

1.000

0,01 0,1 1 10 100 1000

c a p a c

i t a n c e

i n p F

frequency in Hz





1,0

1,2

1,4

1,6

1,8

0 1 2 3 4 5

C 1 0 m

H z

/ C

5 0 H z

water content in %


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7.3 Moisture Analysis for Instrument Transformers Using DIRANAThe interpretation of the dielectric response in frequency domain for instrument transformers is similar to thatof power transformers. Both systems have oil impregnated paper insulations. The difference to the analysisfor power transformers lies mainly in the settings for the geometry data. For most instrument transformergeometries, a ratio of 70% barriers to 30% oil is suitable.

1. Select the Measurement

Select the desired measurement in the measurement collection, and open the moisture assessmentwindow by clicking on the "Assessment" button.

2. Automatic Assessment Press the "Start Assessment" button. The fitting algorithm arranges the parameters of the model(barriers X, spacers Y, oil conductivity, water content) in order to obtain the best fit between themodel curve and the measurement curve. If more information is needed, press the "Advanced..."button. Beside moisture content and oil conductivity, the values for insulation geometry, moisturesaturation and bubbling inception temperature can be found here (Figure 18) . Also, themeasurement results and the fitted model curve are shown here.

Figure 18: Advanced assessment screen after automatic curve fitting

3. Optimizing the Moisture Analysis by Hand For excellent moisture analysis, a good fitting should be observed. If the ratios for barriers andspacers are not in the usual range, the curve fitting may be not as good as shown in Figure 18. Thenthe curve fitting needs to be optimized by hand, what can be easily done by using the arrow buttons.


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8 Step by step guide for the measurement on current transformers

Preparation

1. Use normal operating and safety procedures to access the CT.

2. Disconnect all external apparatus.

3. Examine and clean the bushing if needed.

4. Short both HV terminations and short all secondary terminations and connect to earth.

5. Set up DIRANA instrument in proximity to the CT and extend leads where they will have minimumexposure to casual approaches. If possible keep PC a few meters from DIRANA. Do not approachthe CT during a measurement.

6. Clean HV connection and earth bar connection locations.

7. Connect instrument earth and guard leads on earthing points.

Measurement configuration

8. If C1 (HV to cap tap) measurement is requiredapply DIRANA output (yellow) lead to HV termination.

Connect CH1 measuring lead (red) to cap tap.Connect guards to each clamp.

9. Select Monitor button and check PDC and FDS noise levels.

10. In Configuration select "Bushing" measurement inConnections tab and deselect b ushing B measurement.

11. In Setting tab select FDS as type of measurement .

12. Confirm Measurement Frequency Range from 1 kHz to 10 mHz (or 1 mHz for new and dry CTs).

13. Close the Configuration window and enter location and name plate details, CT temperature andambient weather conditions.


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Select either Current Transformer or Current Transformer with Screen Electrode for full CT test.

"Current Transformer" "Current Transformer with Screen Electrode"

24. Repeat steps from 9 to 21.


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9 Contact Technical SupportIn case of further questions, please contact OMICRON's technical support:

Europe/Middle East/Africa [email protected]

Phone: +43 5523-507-333Fax: +43 5523-507-7333

North and South America [email protected]: +1 713 830-4660 or 1 800-OMICRONFax: +1 713 830-4661

Asia/Pacific [email protected]: +852 2634 0377Fax: +852 2634 0390

10 Literature[1] S. Raetzke, M. Koch, M. Krueger, A. Schroecker: "The Assessment of Instrument transformers

by Dielectric Response Analysis" TechCon Asia Pacific, Sydney 2011

[2] M. Koch, M. Krger: The Negative Dissipation Factor and The Interpretation of the DielectricResponse of Power Transformers" Proceedings of the XVIth International Symposium on HighVoltage Engineering, ISH, Cape Town, South Africa, 2009

[3] M. Koch, M. Krger, S. Tenbohlen: " Comparing Various Moisture Determination Methods forPower Transformers" CIGRE Southern Africa Regional Conference, 2009

[4] M. Koch, M. Krger: A Fast and Reliable Dielectric Diagnostic Method to Determine Moisture inPower Transformers" Proceedings of the International Conference on Condition Monitoring andDiagnosis CMD, Peking, China, 2008

[5] T. V. Oommen: Moisture Equilibrium Charts for Transformer Insulation Drying Practice IEEETransaction on Power Apparatus and Systems, Vol. PAS-103, No. 10, Oct. 1984, pp. 3063-3067.

[6] M. Koch, S. Tenbohlen, D. Giselbrecht, C. Homagk, T. Leibfried : Onsite, Online and Post

Mortem Insulation Diagnostics at Power Transformers, Cigr SC A2 & D1 Colloquium, Brugge,Belgium 2007

[7] M. Koch, M. Krger: Moisture Determination by Improved On -Site Diagnostics, TechCon AsiaPacific, Sydney 2008


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OMICRON is an international company serving the electrical powerindustry with innovative testing and diagnostic solutions. The application ofOMICRON products provides users with the highest level of confidence inthe condition assessment of primary and secondary equipment on theirsystems. Services offered in the area of consulting, commissioning,testing, diagnosis, and training make the product range complete.

Customers in more than 140 countries rely on the company's ability tosupply leading edge technology of excellent quality. Broad applicationknowledge and extraordinary customer support provided by offices inNorth America, Europe, South and East Asia, and the Middle East,together with a worldwide network of distributors and representatives,make the company a market leader in its sector.