dirana anp 10017 enu

© OMICRON of 29

Application Note

Measuring and Analyzing the Dielectric Response of Bushings

Author Maik Koch | [email protected]

Date April 2010

Related OMICRON Product DIRANA

Application Area Bushings

Version v2.0

Document ID ANP_10017_ENU

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

mailto:[email protected]

© OMICRON 2010 of 29

Content

1 Using this document ....................................................................................................................... 4

1.1 Operator Qualifications and Safety Standards ........................................................................... 4 1.2 Safety measures ....................................................................................................................... 4 1.3 Related Documents .................................................................................................................. 4

2 Types of Bushings .......................................................................................................................... 5

3 Preparing the bushings................................................................................................................... 7

4 Connecting DIRANA to the bushing ............................................................................................... 8 4.1 Basic Measurement Circuit – The Guarding Principle ................................................................ 8 4.2 General Procedure ................................................................................................................... 9 4.3 Wiring diagram for bushing measurements ..............................................................................11

5 Setting up the Software..................................................................................................................12

6 Setting up the Software..................................................................................................................13 6.1 Pre Measurement Check with the Monitor Device ....................................................................13 6.2 Development of the dissipation factor curve .............................................................................15 6.3 Determination of the Capacitance ............................................................................................16 6.4 Creating a Measurement Report ..............................................................................................17 6.5 Measurement Errors ................................................................................................................18

6.5.1 Voltage Source Overload ........................................................................................................... 18 6.5.2 Input Overflow ........................................................................................................................... 18 6.5.3 Negative Dissipation Factor ........................................................................................................ 18 6.5.4 Dip at the Transition from Time to Frequency Domain ................................................................. 19 6.5.5 Disturbances during Time Domain Measurement ........................................................................ 20

7 Interpretation of Measurement Data ..............................................................................................21 7.1 Limits at Power Frequency .......................................................................................................21 7.2 General interpretation ..............................................................................................................22 7.3 Interpreting the Dielectric Response of OIP Bushings...............................................................23

7.3.1 Superposition of Dielectric Properties ......................................................................................... 23

8 Moisture Analysis for OIP Bushings Using DIRANA ....................................................................25 8.1 Step by step guide for moisture analysis ..................................................................................25

8.1.1 Select the Measurement ............................................................................................................ 25 8.1.2 Enter Variables .......................................................................................................................... 25 8.1.3 Automatic Assessment ............................................................................................................... 26 8.1.4 Optimizing the Moisture Analysis by Hand .................................................................................. 26

8.2 Comparison to Other Moisture Measurement Techniques ........................................................27 8.2.1 Oil Sampling with Equilibrium Diagrams ...................................................................................... 27 8.2.2 Dielectric Response Methods ..................................................................................................... 27 8.2.3 Paper Samples and Karl Fischer Titration ................................................................................... 27

9 Contact Technical Support ............................................................................................................28

10 Literature ........................................................................................................................................28


Please use this note only in combination with the related product manual which contains several important safety instructions. 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, optical character 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 technical 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 is useful, accurate and entirely reliable. 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. Any translation 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.


1 Using this document

This application guide provides detailed information on how to measure and to analyze the dielectric response of bushings using the OMICRON DIRANA. Please refer to national and international 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 the country and at the site of operation has to be fulfilled.

1.1 Operator Qualifications and Safety Standards

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 establish the responsibilities. Personnel receiving training, instructions, directions, or education on 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 listed 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 measures

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 yourself from high-voltage hazards.

1.3 Related Documents

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


2 Types of Bushings

OIP Bushings

OIP (oil impregnated paper) bushings are the predominant bushing type. The dielectric is made of a paper insulation impregnated with transformer grade mineral oil. It is surrounded by an insulating envelope of oil and porcelain or composite material. The paper of OIP bushings ages especially at high temperatures, what results in conductive agingbyproducts and an increasing dissipation factor. Furthermore the ageing byproducts itself, like water and acids, are acting as a catalyst. Therefore the moisture analysis, which can be done with DIRANA only for this bushing type is of special interest for risk assessment.

RIP Bushings

RIP (resin impregnated paper) bushings are made of a resin impregnated condenser core (Figure 1). The core is enclosed in a porcelain or silicon insulator for a completely dry construction. Partial discharges rarely occur for RIP bushings, but they can cause an increment of the capacitance.

Figure 1: Design of a RIP bushing (left) and a application example of a RIP bushing in storage (right)

RBP Bushings

RBP (resin bonded paper) bushings have a condenser core with a resin pre-impregnated paper, which cures upon heating (Figure 2). Nowadays most bushing suppliers have gone to more stable resin impregnated technology. A failure of this bushing type is mainly caused by inclusions or cracks which may result in partial discharges, high dielectric losses which may cause thermal instability or cracks in the laminated paper filled with oil what leads to an increased capacity. Another reason for a failure may be oil leakage after long storage time, causing an increased partial discharge activity and decreased capacitance.


Figure 2: RBP bushing - a) Cross Selection, b) after failure, c) bushing with porcelain insulator


3 Preparing the bushings

In order to determine the dielectric properties of a bushing using a dielectric response measurement, the device needs to be deenergized and then disconnected from the network. All connections to the bushings should be removed in a manner as to conventional dissipation factor tests. If a complete disconnection is impossible a measurement still can be performed. While measuring the capacitance of a bushing the guarding technique prevents disturbing influences by still-connected devices. However, the following requirements must be fulfilled:

Disconnect voltage transformers and neutral point impedances as they cause a short circuit

to ground.

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. 18).

Avoid electromagnetic field coupling since the remaining devices might act as antennas. If these requirements are fulfilled, the instrument can attain the same accuracy as that of a complete disconnection.


4 Connecting DIRANA to the bushing

4.1 Basic Measurement Circuit – The Guarding Principle

A dielectric response measurement is a three terminal measurement that includes the output voltage, the measured current and a guard. Generally, the output voltage should be connected to the bushing conductor, which is exposed to electromagnetic disturbances. Guarding is required to prevent disturbances due to unwanted current paths, caused by dirty bushings and unwanted electromagnetic fields. Figure 3 illustrates the principle of guarding. Without guarding, the ammeter measures the current through the insulation volume Ivol and the unwanted current over the insulation surface Isur. After applying a guard wire, the unwanted current Isur will bypass the ammeter and flow directly to the voltage source.

Figure 3: A dielectric response measurement without guarding (left) and with guarding (right)

Figure 4 illustrates the guarding principle for a bushing. A conductive belt has to be placed on the bushing and connected to the tank, so that the surface current over dirty bushings is not measured by the instrument. Additionally, the tank and the shielded measurement cables will prevent electromagnetic field coupling.

Figure 4: Guarding principle applied to a bushing


4.2 General Procedure

This section gives illustrated introductions how to connect the DIRANA to a bushing.

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

2. Next, connect all bushings of the same voltage level as the desired bushing to each other.

3. Connect the bushing conductor to the output channel (yellow) of the DIRANA (below left).

4. Connect the cable for the input channel (red) to the measurement tap on the bushing (above right). Note that the connector type on the bushing is manufacturer dependent. Use the split connectors delivered with the DIRANA to connect the tri-axial cable to the connector using alligator clips or wires.


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

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

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


4.3 Wiring diagram for bushing measurements

Figure 5 displays a connection diagram of the DIRANA for the insulation properties measurement of two bushings. All bushings of the same voltage level are connected together. The output is connected to the conductor of bushing A. The input channels CH1 and CH2 are connected to the measurement terminals of the bushings A and B. The guard is connected to the tank. Note that the guard connection to the bushings´ surface is not shown.

Figure 5: DIRANA connection diagram for bushings


5 Setting up the Software

1. 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 Transformer Nameplate data, like serial number and bushing type. If a moisture analysis should be done (only possible for OIP bushings), the temperature of the bushing is absolutely necessary and should be noted as well.

3. Press the button "Configure Measurement".

4. By clicking the drop-down-list, choose "Bushing". You may also refer to the corresponding wiring diagram in order to connect DIRANA to the bushing.

5. Click the "Settings" tab and then enter 1°kHz as start frequency and 1 mHz as stop frequency. This is sufficient for most bushing types.

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


6 Setting up the Software

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. Before starting the polarization, the incoupling noise causes a current, which should be considerably lower (at least 1:10th) than the current after the polarization is started (Figure 6). Recommendations are given in the information box, how to improve the measurement.

Figure 6: Pre Measurement check with the PDC Monitor

The dependence of capacitance, tangent delta, power factor or impedance depending on frequency and voltage can be checked using the "FDS Monitor" (Figure 7). The frequency range and the voltage amplitude 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.


Figure 7: Pre Measurement check using the FDS Monitor


6.2 Development of the dissipation factor curve

After setting up the software and checking the measurement cables, press the "Send Configuration to

Device and Start Measurement " button . 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 high frequencies, and developing toward the low frequencies.

Figure 8: Dissipation factor curve starting at the high frequencies

Figure 9: Dissipation factor curve after transition from time to frequency domain at 0.1 Hz

Figure 10: Complete Dissipation factor curve from 1kHz to 1mHz


During the measurement, the DIRANA unit can be disconnected from the computer and the measurement will continue. After reconnection to the computer, the measurement results are loaded into the DIRANA software and displayed in the graphical view pane.

6.3 Determination of the Capacitance

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 curve pane to the desired frequency, the corresponding absolute capacitance will be displayed in the data view on top.

Figure 11: c'/c" display mode

It is critical to measure the capacitance between the measurement tap and the top of the bushing, since these measurements are strongly dependent on external influences like air humidity and dirt between the tap terminal and the flange. 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. Furthermore, it is shorted during service, so the insulation is not important for the normal case.


6.4 Creating a Measurement Report

A measurement report is provided by the software, containing

dissipation factor curve

main measurement information

relevant measurement data like e.g. the dissipation factor and capacitance at power frequency

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.

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


6.5 Measurement Errors

6.5.1 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 bushing and the DIRANA have the same 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. 8).

6.5.3 Negative Dissipation Factor

The dissipation factor curve may turn negative at high frequencies, see Figure 12. Reasons for this problem 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).

Figure 12: Dielectric measurement with negative dissipation factor

To solve the problem:

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

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

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


6.5.4 Dip at the Transition from Time to Frequency Domain

At the transition from time domain (PDC) to frequency domain (FDS), a dip may appear. Two reasons for this are possible: first a remaining polarization of the dielectric and second disturbances at in the time domain measurement.

Figure 13: Dip at the transition from frequency domain (FDS) to time domain (PDC)

Figure 13 illustrates a dip caused by a remaining polarization. For this example, the resistance of the dielectric was tested with 5 kV DC prior to the dielectric response measurement using DIRANA. The remaining polarization shifts the time domain current and, consequently, the dissipation factor as displayed in frequency domain. To solve this problem:

Depolarize the dielectric by connecting the terminals of the bushings to the tank. The

depolarization time should be at least as long as the polarization time (duration, for which the voltage was applied), however this also depends on the applied voltage. After this, the

DIRANA measurement can be repeated.

Measure the dielectric response using DIRANA at first prior to a resistance test of the dielectric.


6.5.5 Disturbances during Time Domain Measurement

Disturbances in the time domain current are transformed into the frequency domain and affect the results displayed in frequency domain (e.g. dissipation factor). Figure 14 shows disturbances on the time domain 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 on their frequency spectrum.

Figure 14: Time domain current with disturbances at around 1000s (left) and its transformation in frequency domain with disturbances at the low frequencies (right). The reason for the disturbances was that guarding was not applicable for this measurement.

To solve this problem:

Use a guarded measurement set-up

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 Measurement Sequence" to "FDS only". Note that this increases the time duration for the measurement substantially.


7 Interpretation of Measurement Data

7.1 Limits at Power Frequency

The following table lists limits of tangent delta, power factor and partial discharge levels at power frequency (50/60 Hz) according to different standards for OIP, RIP and RBP bushings:


7.2 General interpretation

The measurement results indicate different phenomena occurring in a bushing. The table below lists typical phenomena for OIP, RIP and RBP bushings:


7.3 Interpreting the Dielectric Response of OIP Bushings

The dielectric response of oil paper insulation consists of three components: the dielectric response of the cellulose insulation (paper, pressboard), the dielectric response of the oil, and the interfacial polarization effect. The superposition of these three components follows in the dielectric response. Moisture, temperature, insulation geometry, oil conductivity and conductive aging by-products influence the dielectric response. The discrimination of moisture from other effects is a key quality feature for the analysis of dielectric measurements.

7.3.1 Superposition of Dielectric Properties

Figure 15 displays the dissipation factor of non-impregnated pressboard with a moisture content of 1, 2 and 3 % measured at 20°C.

Figure 15: Dissipation factor of non-impregnated pressboard having moisture content of 1, 2 and 3 %

Figure 16 shows the dissipation factor of oil only with a conductivity of 1 pS/m measured at 20°C. Note, that the losses are much higher as for pressboard and that the dissipation factor is just a line with a slope of – 20 dB / decade.

Figure 16: Dissipation factor of oil only having a conductivity of 1pS/m at 20°C


The dielectric properties of pressboard and oil are superimposed together with interfacial polarization. Interfacial polarization is typical for non-homogeneous dielectrics with different permittivity or conductivity. Here charge carriers such as ions accumulate at the interfaces, forming clouds with a dipole-like behavior. This kind of polarization is effective only somewhere below 10 Hz.

Figure 17; Dissipation factor of pressboard and oil together with the interfacial polarization effect (insulation geometry)

Figure 17 displays the dissipation factor of pressboard having 1 % moisture content and oil together with the interfacial polarization effect (insulation geometry). The insulation geometry (ratio of oil to pressboard) determines the interfacial polarization effect. The frequency range of approximately 1000-10 Hz is dominated by the pressboard. Oil conductivity causes the steep slope at about 1-0.01 Hz. The interfacial polarization (insulation geometry) determines the local maximum or "hump". Finally, the properties of pressboard appear again at very low frequencies. The frequency limits will vary in a wide range with moisture, oil conductivity, temperature and amount of conductive aging by-products. Since pressboard also dominates the high frequency area above 10 Hz in Figure 17, it might appear that it is sufficient to measure this frequency range. However, moisture especially affects the low frequency branch of the dissipation factor curve. Figure 15 illustrates, that the high frequency part of the dissipation factor curve is very similar for different moisture contents, but the low frequency part differs. Consequently, if the measurement range is restricted to the high frequencies, the accuracy of water determination will be very low allowing only for a rough discrimination between wet and dry.


8 Moisture Analysis for OIP Bushings Using DIRANA

The interpretation of the dielectric response in frequency domain for OIP bushings is similar to that 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 bushing geometries, a ratio of 70°% barriers to 30°% oil is suitable. Therefore, it is not necessary to measure down to very low frequencies to reach the "hump". As mentioned, a frequency range of 1°kHz to 1°mHz is sufficient.

8.1 Step by step guide for moisture analysis

8.1.1 Select the Measurement

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

8.1.2 Enter Variables

Select the "Settings" button for entering the geometry data range. Set the values for barriers (X - ratio of barriers to oil) to 70% - 100% and the values for spacers (Y - ratio of spacers to oil) to 30% - 100%. Click the "OK" button to close the "Settings" window. The exact geometry data can be calculated automatically. For temperature compensation, type the insulation temperature into the corresponding field. For this measurement, it was 50°C. The oil conductivity will also be calculated automatically. If the oil conductivity is known, it can be entered taking into account the measurement temperature. Using the "Enter Conductivity at Different Temp." button, the conductivity can be recalculated to the insulation temperature. All parameters with a check mark will be calculated automatically by the DIRANA software. The only parameter that is absolutely necessary is the insulation temperature. If you know the exact values, note the values and enable the check box.


8.1.3 Automatic Assessment

Press the "Start Assessment" button. The fitting algorithm arranges the parameters of the model (barriers X, spacers Y, oil conductivity, moisture content) in order to obtain the best fit between the model 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, moisture saturation and bubbling inception temperature can be found here (Figure 18). Also, the measurement results and the fitted model curve are shown here.

Figure 18: Advanced assessment screen after automatic curve fitting

8.1.4 Optimizing the Moisture Analysis by Hand

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 the curve fitting needs to be optimized by hand, what can be easily done by using for the geometry data. After changing the values for barriers and spacers, it is necessary to adjust the values for conductivity and moisture.


8.2 Comparison to Other Moisture Measurement Techniques

8.2.1 Oil Sampling with Equilibrium Diagrams

By applying the water content in oil (ppm) and the sampling temperature (°C) to a moisture equilibrium diagram only a very rough estimation of moisture content in paper can be made. Since aging of oil and paper shifts the equilibrium curves, this method essentially overestimates moisture content in paper. This especially applies if the acidity and / or oil conductivity are high.

8.2.2 Dielectric Response Methods

For the recovery voltage method RVM, the CIGRÈ task force 15.01.09 stated: "For the RVM technique, the old interpretation based only on simple relationship between the dominant time constant of the polarization spectrum and the water content in cellulose is not correct" [7]. The newer methods of polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS) are based on a comparison of the measured dielectric response to a modeled dielectric response. As the data base of the modeled dielectric response was scaled with different Karl Fischer titration techniques, the moisture contents as analyzed by these methods may differ as well.

8.2.3 Paper Samples and Karl Fischer Titration

Taking paper samples offers a good opportunity to validate dielectric response methods. On the other hand, three restrictions apply:

Sampling procedure

During paper sampling and transportation to the laboratory, moisture from the atmosphere easily increases the moisture content of the sample. A few minutes of exposure to air makes the test useless. Therefore the sampling conditions may lead to an overestimation of moisture content.

Comparability of Karl Fischer titration

Karl Fischer titration suffers from a poor comparability between different instruments and laboratories [8]. The laboratory measuring the water content of paper samples may use a different instrument and procedure as the one used to scale the DIRANA data base. Consequently, the indicated moisture content might differ to a certain extent.


9 Contact Technical Support

In case of further questions, please contact OMICRON's technical support: Europe/Middle East/Africa [email protected] Phone: +43 5523-507-333 Fax: +43 5523-507-7333 North and South America [email protected] Phone: +1 713 830-4660 or 1 800-OMICRON Fax: +1 713 830-4661 Asia/Pacific [email protected] Phone: +852 2634 0377 Fax: +852 2634 0390

10 Literature

[1] M. 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 Voltage Engineering, ISH, Cape Town, South Africa, 2009 [2] M. Koch, M. Krüger, S. Tenbohlen: " Comparing Various Moisture Determination Methods for Power Transformers" CIGRE Southern Africa Regional Conference, 2009 [3] M. 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 Diagnosis CMD, Peking, China, 2008 [4] T. V. Oommen: “Moisture Equilibrium Charts for Transformer Insulation Drying Practice” IEEE Transaction on Power Apparatus and Systems, Vol. PAS-103, No. 10, Oct. 1984, pp. 3063-3067. [5] M. Koch, S. Tenbohlen, D. Giselbrecht, C. Homagk, T. Leibfried: “Onsite, Online and Post Mortem Insulation Diagnostics at Power Transformers”, Cigré SC A2 & D1 Colloquium, Brugge, Belgium 2007 [6] M. Koch, M. Krüger: “Moisture Determination by Improved On-Site Diagnostics”, TechCon

Asia Pacific, Sydney 2008, download at www.omicron.at

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dirana anp 10017 enu

Documents

types of bushings

omicron page

measurement report

measurement errors

dissipation factor curve

negative dissipation

application note

application guide