Dirana Anp 11002 Enu

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 Application Note

 Application Note

Measuring and Analyzing the Dielectric Response of

Measuring and Analyzing the Dielectric Response of

Current Transformers

Current Transformers

 Author 

 Author 

Stephanie Raetzke |

Stephanie Raetzke | stephanie.raetzke@omicron.atstephanie.raetzke@omicron.at

 Alan McGu

 Alan McGuigan|igan| alan.mcguigan@omicron.atalan.mcguigan@omicron.at

Date

Date

 August 20  August 201111

Related OMICRON Product

Related OMICRON Product

DIRANA DIRANA

 Application Area

 Application Area

Current T

Current Transformerransformer Version Version v1.0 v1.0 Document ID Document ID  ANP_1100  ANP_11002_ENU2_ENU  Abstract  Abstract

This application guide informs how to measure and analyze the dielectric response of current transformers in This application guide informs how to measure and analyze the dielectric response of current transformers in order to assess the capacitance, dissipation factor and moisture.

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Content

Content

1

1 Using Using this this docdocumenument ...t ... ... 55 1.1

1.1 Operator Qualifications and Safety Standards ... 5Operator Qualifications and Safety Standards ... 5 1.2

1.2 Safety Safety measmeasures ...ures ... ... 55 1.3

1.3 Related Related DocuDocuments ments ... ... 55 2

2 Preparing the Current Transformer ... 5Preparing the Current Transformer ... 5 3

3 Access and Connections - General Procedure ... 6Access and Connections - General Procedure ... 6 4

4 MeaMeasuresurement ment ConfConfiguiguratiorations ns ... .... 88 4.1

4.1 Measurement of HV terminal to bushing capacitor tap ... 8Measurement of HV terminal to bushing capacitor tap ... 8 4.2

4.2 Measurement of buMeasurement of bushing cashing capacitor pacitor tap to hotap to housing using ... 8... 8 4.3

4.3 Measurement of HV to secondaMeasurement of HV to secondary or housing with capacitor tap earthery or housing with capacitor tap earthed d ... 9... 9 5

5 SettSetting ing up up the the SoftSoftwareware...11...11 6

6 MeaMeasuresurementments s ...1...122 6.1

6.1 Pre Measurement Check with the Monitor Device ...12Pre Measurement Check with the Monitor Device ...12 6.2

6.2 Development of the dissipation factor curve ...13Development of the dissipation factor curve ...13 6.3

6.3 Determination of the Capacitance ...14Determination of the Capacitance ...14 6.4

6.4 Creating Creating a a MeaMeasuresurement ment RepReport ort ...1...144 6.5

6.5 MeasureMeasurement ment Errors Errors ...15...15

6.5.1

6.5.1 Voltage Source Voltage Source OverOverload ... 15load ... 15 6.5.2

6.5.2 Input Input OverOverflow flow ... 1... 155 6.5.3

6.5.3 NegaNegative tive DissDissipation ipation FactFactor ... 15or ... 15 6.5.4

6.5.4 Disturbances Disturbances during Tduring Time Doime Domain main MeasureMeasurement ment ... ... 1616

7

7 Interpretation of Measurement Data ...17Interpretation of Measurement Data ...17 7.1

7.1 DissipatioDissipation n factofactor r curve ...curve ...17...17 7.2

7.2 Frequency dependent capacitance...18Frequency dependent capacitance...18 7.3

7.3 Moisture Analysis for Instrument TransMoisture Analysis for Instrument Transformers Using DIRANA formers Using DIRANA ...19...19 8

8 Step by step guide for the measurement on current trStep by step guide for the measurement on current transformeransformers s ...20..20 9

9 ContaContact ct TechTechnical nical SuppSupport ort ...23...23 10

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Please use this note only

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

instructions. The user is responsible for every application that makes use of an OMICRON productan OMICRON product..

OMICRON electronics GmbH including all international

OMICRON electronics GmbH including all international branch offices is henceforth rbranch offices is henceforth r eferred to as OMICRON.eferred to as OMICRON. © OMICRON 2010. All rights reserved. This application note is

© OMICRON 2010. All rights reserved. This application note is a publication of OMICRON.a publication of OMICRON.  All rights including translation res

 All rights including translation reserved. Reproduction of any kind, for exerved. Reproduction of any kind, for example, photocopying, micample, photocopying, microfilming, opticalrofilming, optical character recognition and/or storage i

character recognition and/or storage i n electronic data processing systems, requires tn electronic data processing systems, requires t he explicit consent of he explicit consent of OMICRON.OMICRON. Reprinting, wholly or in part, is not permitted.

Reprinting, wholly or in part, is not permitted.

The product information, specifications, and technical data embodied in t

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

status 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 i

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1

1

Using this document

Using this document

This application guide provides detailed information on how to measure and to analyze the dielectric This application guide provides detailed information on how to measure and to analyze the dielectric respons

response of e of current transformers using the OMICRON DIRANA. current transformers using the OMICRON DIRANA. Please refer to Please refer to nationanational l and internationaand internationall safety regulations relevant to working with the DIRANA. The regulation EN 50191 "The Erection and safety regulations relevant to working with the DIRANA. The regulation EN 50191 "The Erection and Operation of Electrical Test Equipment" as well as all the applicable regulations for accident prevention in Operation of Electrical Test Equipment" as well as all the applicable regulations for accident prevention in the country and at the site of operation has to be fulfilled.

the country and at the site of operation has to be fulfilled.

1.1

1.1

Operator Qualifications and Safety Standards

Operator Qualifications and Safety Standards

Working on HV devices is extremely dangerous. The measurements described in this Application Guide Working on HV devices is extremely dangerous. The measurements described in this Application Guide must be carried out only by qualified, skilled and authorized personnel. Before starting to work, clearly must be carried out only by qualified, skilled and authorized personnel. Before starting to work, clearly establish the responsibilities. Personne

establish the responsibilities. Personnel l receiving training, instructions, directions, or education on treceiving training, instructions, directions, or education on t hehe measurement setup must be under constant supervision of an experienced operator while working with the measurement setup must be under constant supervision of an experienced operator while working with the equipment. The measurement must comply with the relevant national and international safety standards equipment. The measurement must comply with the relevant national and international safety standards listed below:

listed below:

 EN EN 50191 50191 (VDE (VDE 0104) 0104) "Erection "Erection and and Operation Operation of of ElectricElectrical al Equipment"Equipment" 

 EN EN 50110-1 50110-1 (VDE (VDE 0105 0105 Part Part 100) 100) "Operation "Operation of of Electrical Installations"Electrical Installations" 

 IEEE 510 IEEE 510 "Recommen"Recommended ded Practices Practices for Safor Safety in fety in High-Voltage High-Voltage and and High-PowHigh-Power Teser Testing"ting" 

 1910.261910.269(a)(1)(i)(C) "O9(a)(1)(i)(C) "Occupationaccupational Sal Safety anfety and d Health Health StandaStandards - rds - Electric PoElectric Powerwer

Generation, Transmission, and Distribution" Appendix C Generation, Transmission, and Distribution" Appendix C

 LAPG LAPG 1710.6 1710.6 NASA NASA "Electric"Electrical al Safety"Safety"

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

1.2

1.2

Safety measures

Safety measures

Before starting a measurement, read the safety rules in the DIRANA User Manual and observe the Before 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 protect application specific safety instructions in this Application Note when performing measurements to protect yourself from

yourself from high-voltage hazardshigh-voltage hazards..

1.3

1.3

Related Documents

Related Documents

DIRANA User Manual

DIRANA User Manual – – Contains information on how to use the DIRANA test system and relevant safety Contains information on how to use the DIRANA test system and relevant safety instructions.

instructions.

2

2

Preparing the Current Transformer

Preparing the Current Transformer

In order to determine the dielectric properties of a current transformer using a dielectric response In order to determine the dielectric properties of a current transformer using a dielectric response measurement, the device needs to be de-energized and then disconnected from the network. All measurement, the device needs to be de-energized and then disconnected from the network. All

connections to the current transformer should be removed in a manner as to conventional dissipation factor connections to the current transformer should be removed in a manner as to conventional dissipation factor tests. If

tests. If a complete disconnea complete disconnection is impossible a mction is impossible a measureeasurement still can ment still can be performed. While measbe performed. While measuring theuring the capacitance of a CT the Guarding technique prevents disturbing influences by still-connected devices.

capacitance of a CT the Guarding technique prevents disturbing influences by still-connected devices. However, the f

However, the followollowing requirements must be fing requirements must be f ulfilled:ulfilled:

 Avoid oAvoid overloadiverloading ng of of the the instrument due instrument due to to high high currents, e.g. currents, e.g. long long cables.cables. 

 The The remaining deremaining devices should vices should have have low low capacitcapacitances ances and and losses losses compared to compared to the the measuredmeasured

insulation; otherwise high guard currents may cause a negative dissipation factor (p. insulation; otherwise high guard currents may cause a negative dissipation factor (p.  15) 15)..

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3

3

Access and Connections - General Procedure

Access and Connections - General Procedure

This section gives illustrated introductions how to connect the DIRANA to a current transformer. Please also This section gives illustrated introductions how to connect the DIRANA to a current transformer. Please also refer to t

refer to the user manual.he user manual. 1.

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

the rear panel of the DIRANA, and clamp its other end to the tank.

2.

2. After this, conAfter this, connect the HV conect the HV conductor to nductor to the output chthe output channeannel (yellow) ol (yellow) of the DIRANA.f the DIRANA.

3.

3. When connecting the When connecting the measurement cable of measurement cable of the input cthe input channel (red) to hannel (red) to the measurement tap the measurement tap use theuse the split connectors delivered with the DIRANA to connect the tri-axial cable to the connector using split connectors delivered with the DIRANA to connect the tri-axial cable to the connector using alligator clips or wires.

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4.

4. Connect Connect the the guard guard ofof bothboth measurement cables to the tank. Confirm a good connection, avoidmeasurement cables to the tank. Confirm a good connection, avoid lacquered surfaces or corroded metal. Clean the surfaces, if

lacquered surfaces or corroded metal. Clean the surfaces, if necessnecessary.ary.

5.

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

6.

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4

4

Measurement Configurations

Measurement Configurations

For a CT with a capacitor tap available on the bushing there are three options for dielectric response For a CT with a capacitor tap available on the bushing there are three options for dielectric response measurements:

measurements:

 HV HV to to bushing bushing capacitor capacitor taptap 

 Bushing Bushing capacitor capacitor tap tap to to housinghousing 

 HV HV to to housing housing with with capacitor capacitor tap tap earthedearthed

4.1

4.1

Measurement of

Measurement of

HV terminal to bushing capacitor tap

HV terminal to bushing capacitor tap

For measuring between the HV connections and the capacitor tap on the bushing section the output is For measuring between the HV connections and the capacitor tap on the bushing section the output is connected to the HV conductor of current transformer. Both HV terminations are connected together without connected to the HV conductor of current transformer. Both HV terminations are connected together without the lead touching the insulator. The earth link is removed from the bushing capacitor tap and the DIRANA the lead touching the insulator. The earth link is removed from the bushing capacitor tap and the DIRANA output is connected to the HV termination. The input channel CH1 is contacted to the capacitor tap. In the output 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 bridge "Configuration" window select "Bushing" test and deselect the measurement at bushing B. Ignore the bridge to the other bushings in the diagram

to the other bushings in the diagram (Figure 1)(Figure 1)..

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

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

4.2

4.2

Measurem

Measurement of

ent of bushing capacitor tap to housing

bushing capacitor tap to housing

The second measurement is between the capacitor tap of the bushing and the metal housing. In the The second measurement is between the capacitor tap of the bushing and the metal housing. In the

configuration window select transformer and tick the CL measurement between the secondary winding and configuration window select transformer and tick the CL measurement between the secondary winding and the tank

the tank (Figure 2)(Figure 2). The DIRANA output is connected to the tank and the red measuring lead connected to. The DIRANA output is connected to the tank and the red measuring lead connected to the capacitor tap with the earth bridge disconnected. Both guards are connected to the HV terminals which the capacitor tap with the earth bridge disconnected. Both guards are connected to the HV terminals which are shorted together.

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

4.3

4.3

Measurem

Measurement of

ent of HV to secondary or housing with capacitor tap

HV to secondary or housing with capacitor tap earthed

earthed

For measurements between the HV terminations and secondary winding or

For measurements between the HV terminations and secondary winding or tank select either “Currenttank select either “Current Transformer” or “Current Transformer with Screen Electrode”.

Transformer” or “Current Transformer with Screen Electrode”. Wiring diagram "Current Transformer"

Wiring diagram "Current Transformer"

With the “Current Transformer”

With the “Current Transformer” configurationconfiguration (Figure 3)(Figure 3) the DIRANA  the DIRANA output is connected to the HV toutput is connected to the HV t erminals,erminals, the input CH1 is connected to the CT secondary with all secondary terminations shorted together. No earth the input CH1 is connected to the CT secondary with all secondary terminations shorted together. No earth is to be connected to the CT secondary terminations. The guards from the output and measuring leads are is to be connected to the CT secondary terminations. The guards from the output and measuring leads are connected to the tank. The capacitor tap earth link

connected to the tank. The capacitor tap earth link is closed.is closed.

Figure 3: DIRANA connection diagram "Current Transformer" Figure 3: DIRANA connection diagram "Current Transformer"

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Wiring diagram "

Wiring diagram " Current Transformer with Screen Electrode "Current Transformer with Screen Electrode "

If the CT has a screen electrode the DIRANA output is connected to the tank and the measuring channel If the CT has a screen electrode the DIRANA output is connected to the tank and the measuring channel CH1 is connected to the HV terminations

CH1 is connected to the HV terminations (Figure 4)(Figure 4). The secondary terminals are shorted together and. The secondary terminals are shorted together and connected to the tank. No guards are connected. The capacitor tap earth link is closed.

connected to the tank. No guards are connected. The capacitor tap earth link is closed.

Figure 4: DIRANA connection diagram "Current Transformer with Screen El

Figure 4: DIRANA connection diagram "Current Transformer with Screen El ectrode"ectrode"

Please note:

Please note: If  If the CT is the CT is sand filled the "Moisture Assessment" calculation is not valid for this msand filled the "Moisture Assessment" calculation is not valid for this m easureeasurement.ment. For such cases comparing to a curve with a known moisture content or condition is the best indication of For such cases comparing to a curve with a known moisture content or condition is the best indication of moisture content levels and the general condition of the CT.

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5

5

Setting up the Software

Setting up the Software

1.

1. Connect DIRANA to a Connect DIRANA to a USB port USB port of your of your laptop and laptop and start the DIRANA sostart the DIRANA softftware. The ware. The status field in status field in thethe lower right corner of t

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

2.

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

and should be noted as well. Also record ambient weather conditions. 3.

3. Press Press the the button button "Confi"Configure gure Measurement".Measurement".

4.

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

5.

5. Click the "Settings" tab and Click the "Settings" tab and then enter 1°kHz as then enter 1°kHz as start frequency and 10 mHz start frequency and 10 mHz or 1 or 1 mHz as stopmHz as stop frequency. This is

frequency. This is sufficient for most sufficient for most current transformerscurrent transformers. Furthermore the . Furthermore the measuremenmeasurement modet mode should be switched to "FDS only" which can be found in "Show Advanced Settings".

should be switched to "FDS only" which can be found in "Show Advanced Settings".

6.

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6

6

Measurements

Measurements

6.1

6.1

Pre Measuremen

Pre Measurement Check with t

t Check with the Monitor Device

he Monitor Device

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

noise ratio and noise current for ensuring a successful measurement press the button:

The "PDC Monitor" can

The "PDC Monitor" can be used to estimate be used to estimate the signal-to-noise ratio at the signal-to-noise ratio at different polarization voltagesdifferent polarization voltages. Before. Before starting the polarization, the input-coupling noise causes a current, which should be considerably lower (at starting the polarization, the input-coupling noise causes a current, which should be considerably lower (at least 1:10th) than the current after the polarization is started

least 1:10th) than the current after the polarization is started (Figure 5)(Figure 5). Recommendations are given in the. Recommendations are given in the information box, indicating how to improve t

information box, indicating how to improve t he measurementhe measurement..

Figure 5: Pre Measurement check with the PDC Monitor  Figure 5: Pre Measurement check with the PDC Monitor 

The dependence of capacitance, tangent delta, power factor or impedance depending on frequency and The dependence of capacitance, tangent delta, power factor or impedance depending on frequency and voltage can be checked using the "FDS Monitor"

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

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6.2

6.2

Development of the dissipation factor curve

Development of the dissipation factor curve

 After setting up th

 After setting up the software ae software and checking the nd checking the measuremenmeasurement cables, prest cables, press the "Send Cos the "Send Configuration nfiguration toto Device and Start Measurement” button

Device and Start Measurement” button . During the running measurement do not move the cables since. During the running measurement do not move the cables since the piezoelectric effect may cause disturbing charges. The dissipation factor curve will appear, starting at the piezoelectric effect may cause disturbing charges. The dissipation factor curve will appear, starting at thethe high frequencies, and developing toward the low frequencies.

high frequencies, and developing toward the low frequencies.

Figure 7: Dissipation factor curve starting at the high frequencies Figure 7: Dissipation factor curve starting at the high frequencies

Figure 8: Dissipation factor curve after transition from time to fr

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6.3

6.3

Determination of t

Determination of the Capacitance

he Capacitance

Switch to the " c'/c'' Display Mode" to display the real and imaginary part of the capacitance between Switch to the " c'/c'' Display Mode" to display the real and imaginary part of the capacitance between conductor and measurement tap over frequency as shown in Figure 11. By placing the cursor above the conductor and measurement tap over frequency as shown in Figure 11. By placing the cursor above the curve pane to the desired frequency, the corresponding absolute capacitance will be displayed in the data curve pane to the desired frequency, the corresponding absolute capacitance will be displayed in the data view on top.

view on top.

Figure 10: c'/c" display mode Figure 10: c'/c" display mode

It is critical to measure the capacitance between the measurement tap and the top of the bushing, since the It is critical to measure the capacitance between the measurement tap and the top of the bushing, since the measurements between the tap terminal and the flange are strongly dependent on external influences like measurements between the tap terminal and the flange are strongly dependent on external influences like air humidity and dirt. Also, materials like adhesives with higher tan delta are normally used to fix the active air humidity and dirt. Also, materials like adhesives with higher tan delta are normally used to fix the active part of the bushing against the flange, which are influencing the tan delta.

part of the bushing against the flange, which are influencing the tan delta.

6.4

6.4

Creating a Measurement Report

Creating a Measurement Report

 A measure

 A measurement report is proment report is provided by the softwvided by the software, containingare, containing

 dissipation dissipation factor factor curvecurve 

 main main measurement measurement informationinformation 

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For creating a measurement report, select the desired measurements which should be included in the For creating a measurement report, select the desired measurements which should be included in the report using the check boxes. The "Print Preview" will now show the measurement report.

report using the check boxes. The "Print Preview" will now show the measurement report. Use theUse the "Save as/Export" button to save the measurement report as an Excel or PDF f

"Save as/Export" button to save the measurement report as an Excel or PDF f ile.ile.

6.5

6.5

Measurem

Measurement

ent Errors

Errors

6.5.1

6.5.1 Voltage Source OverloadVoltage Source Overload If the instrument is

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

To solve the problem:

 Check wCheck whether the hether the measurement setup measurement setup has has resulted in resulted in a a short-cshort-circuitircuit.. 

 If capacitive currents cause If capacitive currents cause an oan oververload (typical load (typical for long for long cables), decrease the cables), decrease the output voltage oroutput voltage or

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

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

6.5.2

6.5.2 Input OverflowInput Overflow

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

the ground terminal on the rear panel of the DIRANA (p.  6) 6)..

6.5.3

6.5.3 Negative Dissipation FactorNegative Dissipation Factor

The dissipation factor curve may turn negative at high frequencies, see

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

capacitance and high guard currents (dirty bushings).

Figure 11: Dielectric measurement with negative dissipation factor Figure 11: Dielectric measurement with negative dissipation factor

To solve the problem: To solve the problem:

 Connect all guards Connect all guards of measurement cables of measurement cables and if and if possible an possible an additional wire from additional wire from the triaxialthe triaxial

connectors at the DIRANA front

connectors at the DIRANA front plate to the tank.plate to the tank.

 Try to Try to decrease the decrease the guard currents (clean guard currents (clean bushings, disconnect all bushings, disconnect all devices whdevices which are ich are possibly stillpossibly still

connected). connected).

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

CHL CHL f/Hz f/Hz 0 0.0.0001 1 00.0.0110 0 00.1.1000 0 11.0.0000 0 1100.0.00000 DF DF 0.005 0.005 0.0100 0.0100 0.050 0.050 0.100 0.100 0.500 0.500 1.000 1.000

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6.5.4

6.5.4 Disturbances during Time Domain MeasurementDisturbances during Time Domain Measurement

Disturbances in the time domain current are transformed into the frequency domain and affect the results Disturbances in the time domain current are transformed into the frequency domain and affect the results displayed in frequency domain (e.g. dissipation factor).

displayed in frequency domain (e.g. dissipation factor).  Figure 12 Figure 12 shows disturbances on the time domainshows disturbances on the time domain current for 600-1100 s measurement time as an example. They cause disturbances in dissipation factor for current for 600-1100 s measurement time as an example. They cause disturbances in dissipation factor for the low frequencies. Generally, the disturbances in time domain will appear in frequency domain depending the low frequencies. Generally, the disturbances in time domain will appear in frequency domain depending on their frequency spectrum.

on their frequency spectrum.

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

measurement.

To solve this problem: To solve this problem:

 Use Use a a guarded guarded measurement set-up measurement set-up if if possiblepossible 

 Apply aApply all ll guards guards of of the the measurement cablesmeasurement cables 

 Increase Increase measurement measurement voltagevoltage 

 Try to Try to minimize dminimize disturbances isturbances by e.g. usby e.g. using an eing an electrostatic slectrostatic shieldhield 

 Perform the Perform the measuremenmeasurement in frequet in frequency doncy domain only. In thmain only. In the dialog e dialog field "Confield "Configurefigure

Measurement", click on the "Show Advanced Settings" button. Set the "Type of Measurement Measurement", click on the "Show Advanced Settings" button. Set the "Type of Measurement Sequence" to "FDS only". Please note that this increases the time duration for the measurement Sequence" to "FDS only". Please note that this increases the time duration for the measurement substantially. substantially. f/Hz f/Hz 0. 0.000010 10 0.0.010100 00 0.0.101000 00 1.1.000000 00 1010.0.0000000 DF DF 0.020 0.020 0.050 0.050 0.100 0.100 0.200 0.200 0.500 0.500 1.000 1.000 2.000 2.000 t/s t/s 2 2 5 5 110 0 220 0 550 10 1000 0 22000 0 55000 0 11000000 I/A I/A 0.0000005 0.0000005 0.0000007 0.0000007 0.0000010 0.0000010 0.0000020 0.0000020 0.0000030 0.0000030 HV+LV to tankHV+LV to tank HV+LV HV+LV to to tanktank

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7

7

Interpretation of Measurement Data

Interpretation of Measurement Data

7.1

7.1

Dissipation factor curve

Dissipation factor curve

The dielectric response of instrument transformers has a very

The dielectric response of instrument transformers has a very specific shape, which is similar to specific shape, which is similar to the dielectricthe dielectric respons

response of e of cellulosic material itselfcellulosic material itself (Figure 13)(Figure 13). The curve has a nearly linear part at the lower frequencies. The curve has a nearly linear part at the lower frequencies and is rather flat at high frequencies with a minimum around power frequency.

and is rather flat at high frequencies with a minimum around power frequency.

Figure 13: Dielectric response of cellulosic material Figure 13: Dielectric response of cellulosic material at 20°C with 1% (2%

at 20°C with 1% (2% and 3%) water contentand 3%) water content

Figure 14: Dielectric response of four current transformers of the Figure 14: Dielectric response of four current transformers of the same type

same type

Instrument transformers of the same type, which are aged similar and also having a similar water content will Instrument transformers of the same type, which are aged similar and also having a similar water content will have nearly identical curves, like shown in

have nearly identical curves, like shown in Figure 14. Figure 14. Ageing as well as moisture in the solid insulation will Ageing as well as moisture in the solid insulation will increase the dissipation factor

increase the dissipation factor especialespecially at ly at low frequencies, whereas the dissipation factor low frequencies, whereas the dissipation factor at powerat power frequencies is relatively stable. Only for high water contents or strongly aged insulations the dissipation frequencies is relatively stable. Only for high water contents or strongly aged insulations the dissipation factor will significantly increase at power frequencies. Therefore the 50/60 Hz v

factor will significantly increase at power frequencies. Therefore the 50/60 Hz v alue often only significantalue often only significantlyly changes, when the ageing or moisture ingress has already led to a bad insulation condition

changes, when the ageing or moisture ingress has already led to a bad insulation condition (Figure 15)(Figure 15).. Since the low fr

Since the low fr equenequencies are much more sensitive to cies are much more sensitive to ageageing and moisture, it miing and moisture, it might be helpful to compareght be helpful to compare the values e.g. at 10 mHz.

the values e.g. at 10 mHz.

The height of the dissipation factor is

The height of the dissipation factor is specific for the voltage class and the type specific for the voltage class and the type the instrument transformer.the instrument transformer. Therefore the limits should be defined only within such a group.

Therefore the limits should be defined only within such a group.

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

f/Hz f/Hz 0 0..00001 1 00..001 1 00..1 1 11..0 0 1100..0 0 110000 DF DF 0.005 0.005 0.010 0.010 0.020 0.020 0.050 0.050 0.100 0.100 0.200 0.200 0.500 0.500 1.000 1.000

1%@20°C

1%@20°C

3 3 2 2 1 1 0,001 0,001 0,01 0,01 0,1 0,1 1 1 0 0,,0011 00,,11 11 1100 110000 11000000    d    d    i    i  s  s   s   s    i    i  p  p   a   a    t    t    i    i  o  o

  n   n    f    f  a  a   c   c    t    t  o  o   r   r frequency in Hz frequency in Hz phase A phase A phase B phase B phase C phase C phase A-2 phase A-2 0,001 0,001 0,01 0,01 0,1 0,1 1 1 10 10 0 0,,001 1 00,,1 1 1 1 110 0 11000 0 11000000    d    d    i    i  s  s  s  s    i    i  p  p  a  a    t    t    i    i  o  o  n  n    f    f  a  a  c  c    t    t  o  o  r  r frequency in Hz frequency in Hz old; 4

old; 4% % water contenwater content, t, 9 pS/m oil 9 pS/m oil condconductivityuctivity old; 2

old; 2% % water content, 2 pS/water content, 2 pS/ m oil conductivitym oil conductivity new; 1,2% water con

new; 1,2% water content, tent, 3 pS/3 pS/ m oil condm oil conductivityuctivity new; 0,5% water con

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7.2

7.2

Frequency dependent capacitance

Frequency dependent capacitance

Figure 16: Frequency dependent capacitances of various

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

The assessment of the frequency dependent capacitance is useful to gain knowledge about the insulation The assessment of the frequency dependent capacitance is useful to gain knowledge about the insulation condition. An ideal insulation has a frequency independent capacitance. However, the capacitance of real condition. An ideal insulation has a frequency independent capacitance. However, the capacitance of real insulations is increasing at l

insulations is increasing at low frequenciesow frequencies (Figure 16)(Figure 16). For new and dry oil-paper-insulations the increase is. For new and dry oil-paper-insulations the increase is very small. For aged and wet insulations the capacitance at low frequencies will increase stronger. This can very small. For aged and wet insulations the capacitance at low frequencies will increase stronger. This can be visualized by the ratio of the capacitances at very low frequencies, e.g. 10 mHz and power frequency be visualized by the ratio of the capacitances at very low frequencies, e.g. 10 mHz and power frequency 50/60 Hz. For

50/60 Hz. For new and dry insulations this ratio is about 1.05. During tnew and dry insulations this ratio is about 1.05. During t he lifetime the ratio will increase. Mosthe lifetime the ratio will increase. Most instrument transformers in service with acceptable insulation condition have a ratio of 1.3 or below

instrument transformers in service with acceptable insulation condition have a ratio of 1.3 or below (Figure(Figure 17)

17). This value is rather independen. This value is rather independent of t of CT type and size.CT type and size.

Figure 17: Ratio of capacitance values between 10 mHz and 50 Hz for vari

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

100 100 1.000 1.000 0 0,,001 1 00,,1 1 1 1 110 0 11000 0 11000000   c   c   a   a   p   p   a   a   c   c    i    i    t    t  a  a  n  n   c   c   e   e    i    i  n  n   p   p    F    F frequency in Hz frequency in Hz old; 4

old; 4% w% water content, 9 pS/m oil conductivityater content, 9 pS/m oil conductivity old; 2

old; 2% w% water contenater content, t, 2 p2 pS/S/m oil conductivitym oil conductivity new; 1,2% water content, 3

new; 1,2% water content, 3 pS/m oil conductivitypS/m oil conductivity new; 0,5% water content, 6

new; 0,5% water content, 6 pS/m oil conductivitypS/m oil conductivity

1,0 1,0 1,2 1,2 1,4 1,4 1,6 1,6 1,8 1,8 0 1 2 3 4 5 0 1 2 3 4 5    C    C   1   1    0    0  m  m    H    H  z  z    /    /    C    C   5   5    0    0    H    H  z  z water content in % water content in %

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7.3

7.3

Moisture Analysis for

Moisture Analysis for Instrument Transformers Using DIRANA

Instrument Transformers Using DIRANA

The interpretation of the dielectric response in frequency domain for

The interpretation of the dielectric response in frequency domain for instrument transformers is similar to thatinstrument transformers is similar to that of power transformers. Both systems have oil impregnated paper insulations. The difference to the analysis of power transformers. Both systems have oil impregnated paper insulations. The difference to the analysis for power transformers lies mainly in the settings for the geometry data. For most instrument transformer for power transformers lies mainly in the settings for the geometry data. For most instrument transformer geometries, a ratio of 70°% barriers to 30°% oil is suitable.

geometries, a ratio of 70°% barriers to 30°% oil is suitable. 1.

1. Select Select the the MeasurementMeasurement

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

window by clicking on the "Assessment" button. 2.

2. Automatic Automatic AssessmentAssessment

Press the "Start Assessment" button. The fitting algorithm arranges the parameters of the model 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 the (barriers X, spacers Y, oil conductivity, water content) in order to obtain the best fit between the model curve and the measurement curve. If more i

model curve and the measurement curve. If more i nformation is needenformation is needed, press the "Advanced..."d, press the "Advanced..." button. Beside moisture content and oil

button. Beside moisture content and oil conducticonductivity, the vvity, the v alues for insulation geometry, moisturealues for insulation geometry, moisture saturation and bubbling inception temperature can be found here

saturation and bubbling inception temperature can be found here (Figure 18)(Figure 18). Also, the. Also, the measurement results and the fitted model curve are shown here.

measurement results and the fitted model curve are shown here.

Figure 18: Advanced assessment screen after automatic curve fitting Figure 18: Advanced assessment screen after automatic curve fitting

3.

3. Optimizing the Optimizing the Moisture Moisture Analysis Analysis by by HandHand

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

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8

8

Step by step guide for the measurement on current transformers

Step by step guide for the measurement on current transformers

Preparation

Preparation

1.

1. Use Use normal operating normal operating and and safety procedures safety procedures to to access access the the CT.CT. 2.

2. Disconnect Disconnect all all external external apparatus.apparatus. 3.

3. Examine Examine and and clean clean the the bushing bushing if if needed.needed. 4.

4. Short both Short both HV HV terminatiterminations ons and and short all short all secondary terminations secondary terminations and and connect to connect to earth.earth. 5.

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

the CT during a measurement. 6.

6. Clean Clean HV HV connecticonnection on and and earth earth bar bar connection connection locations.locations. 7.

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

Measurement configuration

Measurement configuration

8.

8. If C1 If C1 (HV to (HV to cap tacap tap) p) measuremenmeasurement is t is requiredrequired apply DIRANA output (yellow) lead to HV

apply DIRANA output (yellow) lead to HV termination.termination. Connect CH1 measuring lead (red) to cap t

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

Connect guards to each clamp. 9.

9. Select “Monitor” button and check PDC and FDS noise levels.Select “Monitor” button and check PDC and FDS noise levels. 10.

10. In ConIn Configuration sefiguration select "Bushinglect "Bushing" measure" measurement inment in “Connections” tab and deselect b

“Connections” tab and deselect bushing B measurement.ushing B measurement. 11.

11. In Setting tab select “FDS” as type of measurementIn Setting tab select “FDS” as type of measurement .. 12.

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Performing the measurement

Performing the measurement

14. Confirm

14. Confirm CT CT is is clear clear and and start start measurement measurement by by selecting selecting the the in in top top tool tool bar.bar. 15. Confirm that red led on front panel is flashing.

15. Confirm that red led on front panel is flashing. 16.

16. Observe the noticObserve the notices in bottom left hand corner of PC screen and the development of the curve plot.es in bottom left hand corner of PC screen and the development of the curve plot. 17.

17. If the curve is not regular (please have a look at the 50 Hz values) or error messages are generatedIf the curve is not regular (please have a look at the 50 Hz values) or error messages are generated use detailed notes to locate the problem.

use detailed notes to locate the problem. 18.

18. When measuWhen measurement is completed save the file.rement is completed save the file. Interpretation of measurement data

Interpretation of measurement data

19.

19. Check the dissipation factor at power frequeCheck the dissipation factor at power frequency and at 10 mHz.ncy and at 10 mHz. 20.

20. Check the increase of capacitance at low frequeCheck the increase of capacitance at low frequencies.ncies. 21.

21. Do an Do an advanced automatic assessment to confirm suitabilitadvanced automatic assessment to confirm suitability oy of results and curve f results and curve matching.matching. Additional measurements

Additional measurements

22.

22. If C2 (cap tap to tank) and current transIf C2 (cap tap to tank) and current transformer (overall) measuformer (overall) measurements are required add adrements are required add additionalditional templates.

templates. In measurement tab of top toolbar select “Add New Template..In measurement tab of top toolbar select “Add New Template.. ..””. Repeat for second. Repeat for second additional measurement.

additional measurement.

23.

23. For each measurement in turn select the appropriate Test Type in the “Measurement ConfigurationFor each measurement in turn select the appropriate Test Type in the “Measurement Configuration > Connections“ tab.

> Connections“ tab. Connect output, measuring lead and guards as per diagram for eachConnect output, measuring lead and guards as per diagram for each appropriate selection.

appropriate selection. Use “Power Tr 

Use “Power Tr ansformeransformer – – 2 Winding” for C2 and select CL measurement only and take HV winding2 Winding” for C2 and select CL measurement only and take HV winding in diagram as CT HV terminations.

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Select either “Current Transformer” or “Current Transformer with Screen Electrode”

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

"Current

"Current Transformer" Transformer" "Current "Current Transformer Transformer with with Screen Screen Electrode"Electrode"

24. Repeat steps from

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9

9

Contact Technical Support

Contact Technical Support

In case of

In case of further questions, please contact OMICRON's technicafurther questions, please contact OMICRON's technical support:l support: Europe/Middle East/Africa

Europe/Middle East/Africa support@omicron.atsupport@omicron.at Phone: +43 5523-507-333

Phone: +43 5523-507-333 Fax: +43 5523-507-7333 Fax: +43 5523-507-7333 North and South America

North and South America techsupport@omicronusa.comtechsupport@omicronusa.com Phone: +1 713 830-4660 or 1 800-OMICRON

Phone: +1 713 830-4660 or 1 800-OMICRON Fax: +1 713 830-4661

Fax: +1 713 830-4661  Asia/Pacific

 Asia/Pacific support@asia.omicron.atsupport@asia.omicron.at Phone: +852 2634 0377 Phone: +852 2634 0377 Fax: +852 2634 0390 Fax: +852 2634 0390

10

10

Literature

Literature

[1]

[1] S. Raetzke, M. KS. Raetzke, M. Koch, M. Krueger, A. och, M. Krueger, A. Schroecker: "The Assessment of Instrument transformersSchroecker: "The Assessment of Instrument transformers by Dielectric

by Dielectric ResponResponse Analysis" TechCon Asia se Analysis" TechCon Asia Pacific,Pacific, Sydney 2011Sydney 2011 [2]

[2] M. Koch, M. Krüger: “The Negative Dissipation Factor and The Interpretation of the DielectricM. Koch, M. Krüger: “The Negative Dissipation Factor and The Interpretation of the Dielectric Response of Power Transformers" Proceedings of the XVIth International Symposium on High Response of Power Transformers" Proceedings of the XVIth International Symposium on High Voltage Engineering, ISH, Cape Town, South Africa, 2009

Voltage Engineering, ISH, Cape Town, South Africa, 2009 [3]

[3] M. Koch, M. Koch, M. Krüger, S. M. Krüger, S. Tenbohlen: " Comparing Various Tenbohlen: " Comparing Various Moisture DetermiMoisture Determination Methods fornation Methods for Power Transformers" CIGRE Southern Africa Regional Conference, 2009

Power Transformers" CIGRE Southern Africa Regional Conference, 2009 [4]

[4] M. Koch, M. Krüger: “A Fast and Reliable Dielectric Diagnostic Method to Determine Moisture inM. Koch, M. Krüger: “A Fast and Reliable Dielectric Diagnostic Method to Determine Moisture in Power Transformers" Proceedings of the International Conference on Condition Monitoring and Power Transformers" Proceedings of the International Conference on Condition Monitoring and Diagnosis CMD, Peking, China, 2008

Diagnosis CMD, Peking, China, 2008 [5]

[5] T. V. T. V. Oommen: “Moisture EquilibriuOommen: “Moisture Equilibrium Charts fm Charts f or Transformer Insulatioor Transformer Insulation Drying Practice”n Drying Practice” IEEEIEEE Transaction on Power Apparatus and Systems,

Transaction on Power Apparatus and Systems, Vol. PAS-103, No. 10, Oct. 1984, pp. 3063-Vol. PAS-103, No. 10, Oct. 1984, pp. 3063-3067.

3067. [6]

[6] M. Koch, SM. Koch, S. Tenbohlen. Tenbohlen, D. Giselbrecht, C. , D. Giselbrecht, C. Homagk, T. LeibfrieHomagk, T. Leibfriedd: “Onsite, Online and Post: “Onsite, Online and Post Mortem Insulation Diagnostics at Power

Mortem Insulation Diagnostics at Power TransforTransformers”,mers”, Cigré SC A2 & D1 Colloquium,Cigré SC A2 & D1 Colloquium, Brugge,Brugge, Belgium 2007

Belgium 2007

[7]

[7] M. Koch, M. Krüger: “Moisture Determination by Improved OnM. Koch, M. Krüger: “Moisture Determination by Improved On--Site Diagnostics”,Site Diagnostics”, TechCon AsiaTechCon Asia Pacific,

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OMICRON

OMICRON is an international company serving the electrical power is an international company serving the electrical power

industry with innovative testing and diagnostic solutions. The application of industry with innovative testing and diagnostic solutions. The application of OMICRON products provides users with the highest level of confidence in OMICRON products provides users with the highest level of confidence in the condition assessment of primary and secondary equipment on their the condition assessment of primary and secondary equipment on their systems. Services offered i

systems. Services offered in the area n the area of consulting, commissioninof consulting, commissioning,g, testing, diagnosis, and training make the product range complete. testing, diagnosis, and training make the product range complete. Customers in more than 140 countries rely on the company's ability to Customers in more than 140 countries rely on the company's ability to supply leading edge technology of excellent quality. Broad application supply leading edge technology of excellent quality. Broad application knowledge and extraordinary customer support provided by offices in knowledge and extraordinary customer support provided by offices in North America, Europe, South and East Asia, and the Middle East, North America, Europe, South and East Asia, and the Middle East, together with a worldwide network of distributors and representatives, together with a worldwide network of distributors and representatives, make the company a market leader in its

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