dirana application guide - measuring and analyzing power transformers

7/25/2019 DIRANA Application Guide - Measuring and Analyzing Power Transformers

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DIRANA

Application Guide

Measuring and Analyzing the Dielectric

Response of a Power Transformer

This application guide informs how to measure and

analyze the dielectric response of power

transformers in order to reliably assess the moisture

content of the paper and pressboard insulation.

June 2008


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DIRANA Application Guide Measuring and Analyzing the Dielectric Response of a Power Transformer

OMICRONelectronics GmbH 2/33

OMICRON electronics 2008. All rights reserved.

This Application Guide is a publication of OMICRON electronics GmbH. All rights including

translation reserved. Reproduction of any kind, e.g., photocopying, microfilming or storage in

electronic data processing systems, requires the explicit consent of OMICRON electronics.Reprinting, wholly or in part, is not permitted. This Application Note represents the technical status

at the time of printing. The product information, specifications, and all technical data contained

within this Application Note are not contractually binding. OMICRON electronics reserves the right to

make changes at any time to the technology and/or configuration without announcement.

OMICRON electronics is not to be held liable for statements and declarations given in this

Application Note. The user is responsible for every application described in this Application Note

and its results. OMICRON electronics explicitly exonerates itself from all liability for mistakes in this

document.


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Contents

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 Transformer....................................................................................................6

2.1 Disconnection from the Network ........................................................................................ 6

2.2 Gathering Transformer Data..............................................................................................6

2.2.1 Insulation Temperature.................................................................................................6

2.2.2 Nameplate Data ........................................................................................................... 7

2.2.3 Oil Tests....................................................................................................................... 8

2.2.4 Environmental Conditions.............................................................................................9

2.2.5 Other Information ......................................................................................................... 9

3 Connecting DIRANA to the Transformer..............................................................................9

3.1 Basic Measurement Circuit Guarding Principle ............................................................... 9

3.2 General Procedure .......................................................................................................... 10

3.3 Wiring Diagram for Various Winding Set-Ups ..................................................................12

3.3.1 Two Winding Transformer .......................................................................................... 12

3.3.2 Three Winding Transformer........................................................................................12

3.3.3 Autotransformer.......................................................................................................... 13

3.3.4 Shunt Reactor ............................................................................................................ 13

3.4 Which HV Devices Can Be Left Connected? ...................................................................14

4 Setting Up the Software....................................................................................................... 16

5 Performing the Measurement.............................................................................................. 18

5.1 Development of the Dissipation Factor Curve .................................................................. 18

5.2 Measurement Errors ........................................................................................................19

6 Interpreting the Dielectric Response in Frequency Domain ............................................. 21

7 Moisture Analysis Using DIRANA....................................................................................... 24

7.1 Principle of Moisture Analysis .......................................................................................... 24

7.2 Step by Step Guide for Moisture Analysis........................................................................ 25

7.3 Analysis of a Measurement with Limited Frequency Range............................................. 27


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8 Assessing the Analysis Results ......................................................................................... 28

8.1 Assessment According to IEC 60422............................................................................... 28

8.2 Transformer Drying..........................................................................................................29

8.3 Accuracy of Analysis Results........................................................................................... 30

8.4 Comparison to Other Moisture Measurement Techniques ............................................... 31

9 Literature .............................................................................................................................. 32


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1 Using This Document

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

dielectric response of an oil-paper-insulated power transformer using OMICRON DIRANA. Please

refer to national and international safety regulations relevant to working with 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, oreducation 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"

LAPG 1710.6 NASA "Electrical Safety"

Moreover, additional relevant laws and internal safety standards 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

Title Description

DIRANA User Manual Contains information on how to use the DIRANA

test system and relevant safety instructions.


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2 Preparing the Transformer

2.1 Disconnection from the Network

For a dielectric response measurement, the transformer needs to be switched off and then

disconnected from the network. All connections to the HV, MV and LV bushings should be

removed in a similar as to conventional dissipation factor tests. If complete disconnection is

impossible, please refer to p. 14.

After switching off it is not necessary to wait for a cool down period or for moisture equilibrium.

However, in order to avoid rapid temperature changes, the cooling system should be off during the

measurement.

2.2 Gathering Transformer Data

2.2.1 Insulation Temperature

Various data provide useful information in order to reliably assess the condition of a transformer.

The temperature of the insulation is of essential importance for moisture analysis and, therefore,

should be carefully noted. To measure this value, the oil temperature may be used. As an

example, Figure 1 depicts the temperature distribution in a large power transformer with ONAN

cooling. The top oil temperature best correlates with the average insulation temperature.

Ambient

20C

72C98C

Average

winding

83C

63C

54C

Cooling

system

74C

92C

Bottom oil Bottom winding

Top oilHot spot

Top winding

Average

oil

Figure 1: Exemple of

temperature distribution in

a large power transformer

with ONAN cooling

according to IEC 60354


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Temperature from Oil Sample

The most accurate way to determine the top oil temperature is to take an oil sample and measure

the temperature directly on-site of that oil. After opening the valve, colder oil trapped in the valve

will flow out at first, thus, wait for sufficient time in order to get a representative reading.

Temperature from Build-In Temperature Gauge

If direct sampling of the oil is not possible, the temperature of the built-in temperature gauge may

be used. This, however, may display inaccurate readings depending on the place, where the

temperature probe is located. Figure 2 depicts a build-in temperature gauge of a transformer.

Taking photographs helps for later data analysis and documentation.

Figure 2: Built-in temperature gauge displaying the top oil

temperature; in this case 42C

Temperature from Contact ThermometersAnother way to determine the top oil temperature is to place a contact thermometer on top of the

transformer tank.

Temperature from Winding Resistance Measurement

The winding temperature may also be calculated using winding resistance measurement. From the

difference between the winding resistance during the dielectric response measurement and that in

the workshop at ambient temperature the winding temperature may be calculated.

Temperature Change during Measurement

If the transformer has been switched off prior to the measurement, the temperature will slowly

decrease. Typical temperature time constants for power transformers are 1-2 Kelvin/h. Since a

dielectric response measurement takes typically less then 1 h, in maximum 3 h, the decrease in

temperature will be of minor importance. However, the cooling systems must be turned off.

2.2.2 Nameplate Data

The transformer nameplate displays the winding configuration which is essential information for

making correct connect of the DIRANA instrument. Beside this, the year of manufacture and the


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voltage ratio help to check the consistency of the subsequent moisture analysis. Taking a

photograph of the nameplate, again, helps in the documentation process.

2.2.3 Oil Tests

The Operators of power transformers depend heavily on the periodic sampling of the oil. The

screening consists of several parameters that are of particular interest with regard to the dielectric

response measurement:

Acidity

High acidity reflects paper and oil aging and often increases the oil conductivity. It takes a

certain quantity of an alkaline material to neutralize these acids. A standard method that is

used to find this quantity neutralization number, is to mix potassium hydroxide (KOH) with

the acid/oil until it is neutralized, and is measured in milligrams of KOH per gram of oil.

[ASTM D974, D664, D1534]. New oils have an acidity below 0.05 mg KOH / g oil. It

increases with aging to 0.5 and above. The conductivity of oil is influenced by acids and

given in Siemens per meter, that is S / m or 1/ / m. New oils have around 0.05 pS/m and a

conductivity of above 20 pS/m at ambient temperature points on a progressed aging state.

Water in oil

Since the water content in oil in ppm strictly depends on temperature, no levels of permitted

moisture concentration based on ppm can be given. By applying the water content in oil

(ppm) and the sampling temperature (C) to a moisture equilibrium diagram (Figure 3) a

very rough estimation of moisture content in paper can be made. Since aging of oil and

paper shifts the equilibrium curves, this method overestimates moisture in paper. This

especially applies if the acidity and / or oil conductivity are high. To overcome the influence

of oil aging, water saturation in oil (%) instead of water content in oil (ppm) can be used [3].

0

1

3

4

5

0 10 20 30 40 50

Moisture in oil / ppm

Moistureincellulose/%

2

Solubility in oil / ppm20 50 80 120

260

500

880

0C 20C 30C 40C

60C

80C

100C

Figure 3: Moisture equilibrium

curves based on moisture

content in oil in ppm (redrawn

according to the original source

[1]). Note, that these diagrams

usually overestimate moisture

content in paper.


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Moistureinagedpressboard/%

Moisture relative to saturation / %

0

1

2

3

4

5

6

0 10 20 30 40

21C

40C60C

80C

Figure 4: Moisture equilibrium

diagram based on moisture

saturation in oil [2]

2.2.4 Environmental Conditions

In case the transformer was out of service, the ambient temperature helps to judge about the

accuracy of the build-in temperature gauge.

The relative humidity in the air and possible rain should be noted. Wet bushings increase the guard

current and may lead to a negative dissipation factor. This is especially important, if an insulation

system to ground is measured (CLor CH).

2.2.5 Other Information

If available, take note of the setup of the main insulation (number and diameter of barriers and

spacers). Since the position of the tap changer may influence the high frequency portion of the

dissipation factor trace, note the tap changer position.

3 Connecting DIRANA to the Transformer

3.1 Basic Measurement Circuit 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, which is mostly exposed to electromagnetic disturbances. Guarding is required to

prevent disturbances due to unwanted current paths as caused by dirty bushings and unwanted

electromagnetic fields.

Figure 11 illustrates the principle of guarding. Without guarding, the ammeter measures the current

through the insulation volume Ivolandthe unwanted current over the insulation surface Isur. After

applying a guard wire, the unwanted current Isurwill bypass the ammeter and flow directly to the

voltage source.


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Insulationunder test

~

A

IVolISur

IVol + ISur

Insulation

under test~

A

IVolISur

IVol

ISur

Figure 5: A dielectric response measurement without guarding (left) and with guarding (right)

Figure 6 illustrates the guarding principle for a power transformer. Here the currents over dirty

bushings will not be measured by the instrument. Additionally, the transformer tank and the

shielded measurement cables will prevent electromagnetic field coupling.

Guard

CHLCL

LV HV

A

Voltage source

Current sense 1AInstrum

ent

CH

=

IVol

ISur

IVol

ISur

Figure 6: Guarding principle applied to a

power transformer

3.2 General Procedure

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

Please refer also to the user manual.

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 transformer tank.


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2. Next, connect all HV bushings to each other. Do the same for all LV bushings.

3. After this, connect the cable for the voltage output (yellow) to the HV bushings and the

cable for the input channel (red) to the LV bushings.

4. Connect the guard of bothmeasurement cables to the transformer tank. Insure a good

connection, avoid lacquered surfaces or corroded metal.

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


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3.3 Wiring Diagram for Various Winding Set-Ups

3.3.1 Two Winding Transformer

To determine the water content of the main insulation, the capacitance between HV and LV

winding CHL provides the most valuable information. The wiring diagram is the same for three

phase transformer as for single phase (Figure 7).

A

A

V

Output

CH1

CH2

single channel mode

CH1: UST-A CHL - Measurement of the main insulation (LV - HV)

CHL

CL CH

Figure 7: Typical measurement set-up for a two winding transformer

3.3.2 Three Winding Transformer

For a three winding transformer with HV, MV and LV winding (or tertiary winding), both current

measurement channels can be used simultaneously. Figure 8 depicts this connection. The

measurement voltage is applied to the winding which is located in between the other two windings.

Capacitance measurements help to identify the location of the windings.

CHT

CLT CHL

CT CL CH

A

A

V

Output

CH1

CH2

dual channel modeCH1: UST-A CHL - Measurement of the insulation LV- HV

CH2: UST-B CLT - Measurement of the insulation LV- TV

Figure 8: Typical measurement set-up for a three winding transformer


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3.3.3 Autotransformer

For an autotransformer, the measurement voltage should be connected to the (internally

connected) HV and LV winding and the input channel to the tertiary winding. In case, the tertiary

winding cannot be reached, use the same connection as for the shunt reactor, p. 13.

single channel modeCH1: UST-A CHT - Measurement of the insulation HV- TV

A

A

V

Output

CH1

CH2

CHT

CT CH

Figure 9: Measurement set-up for an autotransformer with tertiary winding

3.3.4 Shunt Reactor

A shunt reactor contains only one single winding per phase, not two, as for normal power

transformers. Instead of measuring the capacitance between windings CHL, the capacitance of thesingle winding to ground CHwill be measured. Therefore, the guarding technique (voltage to HV,

current from LV and guard to tank) cannot be used. Since the capacitance of the bushings to tank

will not be guarded, the measured losses will be higher than the losses of the internal capacitance

CHonly. Depending on the condition of the bushing (surface wetness, dirt), this will result in an

overestimation of moisture content. A typical overestimation compared to the "true" moisture

content (m.c.) is 15 %; that means for example 2 % m.c. + 15 % = 2.3 % m.c.. Figure 10 displays

the corresponding wiring diagram. In order to minimize disturbances, the voltage here is applied to

the tank and the current is measured at the bushings. Depending upon the on-site conditions, theconnection with fewer disturbances might also be to apply the voltage to the bushings and

measure the current at the tank.


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CH

single channel mode

CH1: GST-A CH - Measurement of the insulation HV - Tank/Core

A

A

V

Output

CH1

CH2

Figure 10: Measurement set-up for a shunt reactor or an autotransformer without tertiary winding

3.4 Which HV Devices Can Be Left Connected?

It is the best practice to completely disconnect the transformer from the network. However, if a

complete disconnection is impossible, it must be distinguished between CHL and CH/CL

measurements. The capacitance between the windings CHLprovides most valuable information for

subsequent moisture determination. The high and low voltage winding capacitances to ground (CHand CL) are only useful if a measurement of CHLis impossible.

Effect of Remaining HV Devices on Guarded CHL- Measurements

While measuring the capacitance between windings CHL (voltage output to HV-winding, current

input to LV or MV winding, guard to tank), 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 still-connected devices should have low capacitances and losses compared to the

transformer insulation; otherwise high guard currents may cause a negative dissipation

factor (p. 19).

Avoid electromagnetic field coupling since the still-connected devices might act as

antennas.


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If these requirements are fulfilled, the instrument can attain the same accuracy as that of a

complete disconnection. Figure 11 illustrates the effect of a still connected HV device C Extwhich is

connected to ground / guard.

ACurrent input

Guard/Ground

CLT CHLCT

MVLV HV

~

Voltage source

CExtCH

ACurrent input

Guard/Ground

CLT CHLCT

MVLV HV

~

Voltage source

CExtCH

Figure 11: Wiring diagram for a

three winding transformer

having an external device CExt

still connected

Effect of Remaining HV Devices on Not Guarded CH/CL- Measurements

For CHand CLmeasurements (voltage output to HV/LV-winding, current sense to tank, guard to LV

or MV winding if available) the still-connected HV devices will increase the losses and thus lead to

an overestimation of moisture. This is especially ture for insulations having losses or impedances

in the range of the transformer insulation.

~

Current sense

Guard

Ground

CLT CHLCT

MVLV HV

A

Voltage source

CExtCH

Figure 12: Wiring diagram for CH

- measurement having an

external device CExtstill

connected

A measurement is of little value or impossible in case of

Wet or dirty bushings

Cables with paper / oil insulation

Voltage transformers

Whereas the following devices will have a minor influence:


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Short PE cables

Short bus bars with high resistance to ground

Other devices having low losses compared to the transformer insulation

4 Setting Up the Software

1. Connect DIRANA to one 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. Press the button "Configure Measurement".

3. By clicking the drop-down-list, choose the configuration that fits to your measurement

specimen. You may also refer to the corresponding wiring diagram in order to connectDIRANA to the transformer.

4. Click the settings tab and then enter 100 Hz as stop frequency. The section below will

explain the required frequency ranges.

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


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Required Stop Frequency for Subsequent Moisture Analysis

For moisture analysis, the very low frequencies of the dissipation factor curve are required. The

dissipation factor plotted over frequency shows a typical s-shaped curve (Figure 13). With

increasing moisture content, temperature, or aging, the curve shifts towards higher frequencies.

Moisture particularly influences the low frequency part. A change in the high frequency part occurs

only for high water content. The middle part of the curve with the steep gradient reflects oil

conductivity. Insulation geometry determines the local maximum or "hump" on the left-hand side of

the steep gradient. In order to determine the moisture content of the insulation, the measurement

should provide the point of inflection on the left-hand side of the area dominated by insulation

geometry. There, the properties of the cellulose insulation and its water content dominate.

high

low

oil

conductivity

moisture and aging

of cellulose

high

high

low

low

0,001 0,01 1 100

0,001

0,01

0,1

1

1000

Frequency [Hz]

Dissipationfactor

insulationgeometry high

low

oil

conductivity

moisture and aging

of cellulose

high

high

low

low

0,001 0,01 1 100

0,001

0,01

0,1

1

1000

Frequency [Hz]

Dissipationfactor

insulationgeometry

Figure 13: Interpretation scheme of a dissipation

factor curve providing discrimination between

the influences of moisture, aging, oil

conductivity and insulation geometry

The position of the area influenced by moisture in cellulose and, consequently, the frequency

range required for the specific insulation depends on the condition of the insulation. Dry or cold

insulations require measuring down to very low frequencies, i.e. 100 Hz. For hot or highly

conductive insulations, the stop frequency can be much higher; e.g. 0.1 Hz.

As the condition of the transformer to be measured is unknown in most cases, set the stop

frequency to the lowest value, i.e. 100 Hz. Then, observe the dissipation factor curve during the

measurement and stop the measurement when the "hump" and the point of inflexion on its left-

hand side appear. See also the measurement example below.

Note that for elevated temperatures the "hump" will not be as distinct as in Figure 13. The

dissipation factor trace does not show such a clear local maximum, but rather, a slight point of

inflexion (Figure 24).


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5 Performing the Measurement

5.1 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 14: Dissipation factor curve starting at the

high frequencies. The table at the top displays the

values for the curser position, currently for power

frequency.

Figure 15: Dissipation factor curve after transition

from time to frequency domain at 0.1 Hz.

Figure 16: Complete dissipation factor curve.

Sufficient data for subsequent moisture analysis

were already available at 0.0005 Hz, corresponding

to 40 minutes measurement time. The

measurement could have been stopped at this

point.

During the measurement, DIRANA can be disconnected from the computer and the measurement

will continue offline. After reconnection to the computer, the measurement results are loaded into

the DIRANA software and displayed in the graphical view pane.


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The measurement can be stopped when the "hump" and the point of inflexion on its left-hand side

are measured; please refer to the explanations for Figure 13.

5.2 Measurement Errors

Voltage Source Overload

If the instrument is not able to reach the desired voltage, an error message will indicate the

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.

Input Overflow

In case the software displays an input overflow error, check that the transformer and the DIRANA

have the same reference potential. Usually this error appears when the transformer tank is on a

floating potential. Connect the transformer tank to the ground terminal on the rear panel of the

DIRANA (p. 10).

Negative Dissipation Factor

The dissipation factor curve may turn negative at high frequencies, see Figure 17. Reasons for this

problem may be at first a high guard impedance, at second a small measured capacitance in

conjunction with a large guard capacitance, at third high guard currents (dirty bushings) and at

fourth the inductivity of coils.

CHL

f/Hz0.001 0.010 0.100 1.000 10.000

DF

0.0050.0100

0.0500.100

0.5001.000

Figure 17: 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 transformer tank.


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Try to decrease the guard currents (clean bushings, disconnect all devices which are

possibly still connected to the transformer).

Check ratio of capacitances (measure adjacent windings only).

Ensure a proper connection of the DIRANA housing to the reference potential, usually the

transformer tank.

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 18 illustrates a dip caused by a remaining polarization. For this transformer, 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.

f/Hz0.0010 0.0100 0.1000 1.0000 10.0000 100.0000

DF

0.007

0.010

0.020

0.030

0.050

0.070

0.100

0.200

0.0001

Figure 18: Dip at the transition from

frequency domain (FDS) to time

domain (PDC)

To solve this problem,

Depolarize the dielectric by connecting the terminals of the HV and the LV bushings to each

over and to the transformer 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 the resistance test of the

dielectric.

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 19 shows disturbances on

the time domain current for 600-1100 s measurement time as an example. They cause


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disturbances in dissipation factor for the low frequencies. Generally, the disturbances in time

domain will appear in frequency domain depending on their frequency spectrum.

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

Figure 19: Time domain current with disturbances at around 1000 s (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 CL-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 "Switch Frequency" to the same value as the "Stop Frequency", e.g. 100 Hz. Note

that this increases the time duration for the measurement substantially.

6 Interpreting the Dielectric Response in Frequency Domain

The dielectric response of oil-paper-insulated power transformers 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.


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Superposition of Dielectric Properties

Figure 20 displays the dissipation factor of only pressboard with a moisture content of 1, 2 and 3 %

measured at 20C.

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

3%

2%

1%

Figure 20: Dissipation factor of pressboard only

having moisture content of 1, 2 and 3 %

Figure 21 shows the dissipation factor of only oil with a conductivity of 1 pS/m measured at 20C.

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.

f/Hz0.001 0.010 0.100 1.00 10.00 100.00

DF

0.0001

0.001

0.01

0.10

1.00

10.0

Figure 21: Dissipation factor of oil only having a

conductivity of 1 pS/m at 20C

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 ten

Hertz.


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f/Hz0.001 0.010 0.100 1.000 10.000 100.000

DF

0.01

0.03

0.10

0.30

Pressboard

Insulation

Geometry

Oil

conductivity

Pressboard

Figure 22: Dissipation factor of pressboard and

oil together with the interfacial polarization effect

(insulation geometry)

Figure 22 displays the dissipation factor of pressboard having 1 % moisture content and oil

togetherwith the interfacial polarization effect (insulation geometry). The insulation geometry (ratio

of oil to pressboard) determines the interfacial polarization effect. The frequency range of 1000-

10 Hz is dominated by the pressboard. Oil conductivity causes the steep slope at 1-0.01 Hz. Theinterfacial polarization (insulation geometry) determines the local maximum or "hump" at 0.003 Hz.

Finally, the properties of pressboard appear again at the frequencies below 0.0005 Hz. The

frequency limits correspond to Figure 22, but will vary in a wide range with moisture, oil

conductivity, temperature and amount of conductive aging by-products.

Moisture especially increases the losses in the low frequency range of the dielectric response of

pressboard. Thus, data on the left-hand side of the area dominated by interfacial polarization

(insulation geometry) are required for a reliable moisture determination. The point of inflexion onthe left hand side of the area dominated by insulation geometry must be reached.

Since pressboard also dominates the high frequency area above 10 Hz in Figure 22, 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 20 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.

If geometry data of the transformer are known, it is not necessary to measure down to these low

frequencies. For example, for Figure 22, the measurement could be stopped at 0.001 Hz.

Influence of Moisture and Temperature

For increasing moisture content and oil conductivity, the curve shifts toward higher frequencies, but

the shape remains similar. Figure 23 depicts the dissipation factor over frequency for 3 % moisture

content and 10 pS/m oil conductivity. Figure 24 illustrates the influence of temperature on the same


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insulation system. At 50C the losses of pressboard along with the oil conductivity increase while

the shape of the curve remains similar.

f/Hz0.001 0.010 0.100 1.000 10.000 100.00

DF

0.01

0.02

0.10

0.20

1.00

Figure 23: Dissipation factor of an oil-paper-

insulation with pressboard having 3 % moisture

content and oil with a conductivity of 10 pS/m

f/Hz0.001 0.010 0.100 1.000 10.00 100.00

DF

0.01

0.05

0.10

0.50

1.00

5.00

10.00

Figure 24: Dissipation factor at 50C of an oil-

paper-insulation with pressboard having 3 %

moisture content and oil with a conductivity of

43 pS/m

For the measurement as shown in Figure 23, sufficient data for subsequent moisture analysis wasavailable at 0.0021 Hz, corresponding to a measurement time of 14 minutes. At this frequency the

only properties of pressboard appear, which is the prerequisite for accurate moisture analysis.

Finally, for the elevated temperature of 50C of Figure 24, the measurement could have been

stopped at 0.01 Hz.

7 Moisture Analysis Using DIRANA

7.1 Principle of Moisture Analysis

Moisture determination is based upon a comparison of the transformer's dielectric response to a

modelled dielectric response. A so-called XY model combines the dielectric response of

pressboard as taken from a database with that of oil with regard to the insulation temperature. A

fitting algorithm rearranges the modelled dielectric response and delivers moisture content and oil

conductivity. The software will automatically compensate for the influence of conductive aging by-

products. Figure 25 depicts the programming flowchart of the analysis algorithm.


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Measurement Data base

Temperature

Oil XY-model

moisture content,

oil conductivity

Comparison

44C Model Curve

f/Hz0.01 0.10 1.00 10.00 100.0

DF

0.005

0.01

0.02

0.05

0.10

0.20

0.50

1.00

Figure 25: Programming flowchart of the analysis algorithm

7.2 Step by Step Guide for Moisture Analysis

1. Select the Measurement

Select the desired measurement in the measurement

collection, and open the moisture assessment window by

clicking on the Assessment button.

2. Enter Variables

For temperature compensation, type the insulation

temperature into the corresponding field. For this

measurement, it was 44C.

The fields barriers (X - ratio of barriers to oil) and spacers

(Y - ratio of spacers to oil) determine the insulation

geometry. If data with a sufficient frequency depth were

measured (3-5 points on the left hand side of the hump),

the software would automatically calculate these values.

Therefore, no numbers have to be entered.

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.


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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.

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 themodel curve and the measurement curve. Figure 26 displays the result of the automatic curve

fitting.

Figure 26: Assessment screen

after automatic curve fitting

In this example, the automatic curve fitting gives the result of 1.7 % moisture content, 9.3 pS/m oilconductivity, 20 % barriers and 14 % spacers.

4. Optimizing the Moisture Analysis by Hand

As the low frequencies on the left-hand side of the "hump" reflect moisture, a good fitting of this

area should be observed. In this respect, Figure 26 leaves some room for improvements. Since

insulation geometry causes the hump, decreasing the amount of barriers to 12 % gives a better

fitting of this area. Consequently, the moisture content must be adjusted to 1.5 %.

The automatic assessment gave a different result than the optimization by hand because the lower

limit for barriers was set to 20 %. By setting the limit for barriers to 10 % in the "Advanced Limits

for Automatic Assessment" tab, the automatic assessment comes to the same result as the

optimization by hand.


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Figure 27: Assessment screen

after optimization by hand

7.3 Analysis of a Measurement with Limited Frequency Range

The moisture analysis reaches a high accuracy, if data of the low frequencies dominated by

pressboard are available (Figure 22). Figure 28 depicts an example with a limited frequency range

where the measurement was stopped too early; the low frequency properties of pressboard are

invisible.

f/Hz0.01 0.10 1.00 10.0 100.0

DF

0.005

0.010

0.020

0.050

0.100

0.200

0.500

Figure 28: Dissipation factor curve without

information on the left hand side of the "hump"

To analyze such a measurement, some estimation of the geometric conditions will help. Set the

geometry condition to fixed values of X = 30 % and Y = 20 %. The amount of barriers to oil X

typically ranges from 15 to 55 % and of spacers Y from 13 to 24 %. Usually, older transformers

contain a higher ratio of pressboard to oil (Figure 29). To estimate the ratio of solid to liquid

insulation, one may also look at the high frequencies of 100-1000 Hz.

After this, perform the automatic assessment and, if necessary, some optimization by hand as

described above. For the example of Figure 28, the assessment result is then depicted in Figure

30.


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0

10

20

30

40

50

60

70

1960 1970 1980 1990 2000 2010

Year of manufacture

BarriersXin% 22 kV 65 kV

110 kV 220 kV

400 kV 500 kV

autotransformer

Figure 29: Ratio of barriers to

oil X for various transformers

depending on year of

manufacture.

Figure 30: Assessment result

for a measurement with limited

frequency range

8 Assessing the Analysis Results

8.1 Assessment According to IEC 60422

The DIRANA software assesses the moisture concentration based on the classifications given in

the IEC 60422 "Mineral insulating oils in electrical equipment Supervision and maintenanceguidance". The categories are:

Category Moisture content in % Color

Dry below 2,2

Moderately wet 2,2-3,7

Wet 3,7-4,8

Extremely wet above 4,8


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IEC 60422 provides categories for moisture contamination of power transformers based on

moisture saturation. Moisture saturation can be converted into moisture content using sorption

isotherms (Figure 31). The IEC rates moisture saturations of more than 6 % as "moderately wet",

which is equivalent to a moisture content of approximately 2.2 %. In this area the water molecules

become more and more active, increasing the dangerous effects of water. At this level,

maintenance actions such as drying should be considered, taking into account the importance and

future operation of the transformer. Figure 31 shows the relationship between moisture content and

moisture saturation and illustrates the categories of IEC 60422 in order to assess the results

analyzed by DIRANA.

Moistureco

ntent[%]

Moisture saturation [%]

0

1

2

3

4

5

10 20 30

21C

80C

Moderately

wet

Dry

Wet,

> 30 %extremelywet

Moisture

conta

minatio

n

Figure 31: Moisture sorption isotherm for a

cellulose material relating moisture saturation to

moisture content with categories according to

IEC 60422

8.2 Transformer Drying

Basically there are three approaches for the drying of power transformers: off-site oven drying, on-

site drying and on-line drying.

Off-Site Oven Drying

Off-site oven drying is the traditional drying technique used for new transformers in the factory.

High temperature applied together with low pressure dry the insulation. However, for an already

installed transformer, the transportation to a workshop can be very expensive. Additionally, the

transformer will be off-line for a considerable length of time.

On-Site Drying

For on-site drying techniques, the transformer will be left in the substation. Low frequency heating

of the winding in combination with vacuum is one common on-site drying technique. A second

technique uses hot oil spray together with vacuum. Both techniques are very effective but have the

disadvantage that the transformer will be out of service during the maintenance action.


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On-Line Drying

Here on-line drying systems dry the oil through continuous circulation. The transformer can be left

in service and the oil will regain its dielectric withstand strength very quickly. As the oil contains

only a very small amount of water, typically half of 1 % of all the water in the transformer, this

method of drying the solid insulation will take the long time of months up to years. Additionally,

there is a risk that the inhibitors in the oil may be inadvertently removed.

DIRANA can validate the efficiency of drying methods. Drying methods will at first affect the outer

layers of the cellulose insulation and thus cause an inhomogeneous moisture distribution. In order

to obtain a more realistic moisture distribution for moisture analysis by DIRANA, the transformer

should be in operation and reach at least a top oil temperature of 50C. This procedure causes a

homogenous moisture distribution and a reliable moisture analysis result.

8.3 Accuracy of Analysis Results

The analysis software will reliably calculate moisture content if the following conditions are fulfilled:

Materials consist of oil and oil-impregnated paper/pressboard. An analysis of transformers

without oil is possible as well; however the cellulose materials must be oil-impregnated.

Measurement data on the left-hand side of the "hump" are available (Figure 22). This

makes the analysis independent from the geometric setup and the oil conductivity of the

specific insulation. The software will calculate the insulation geometry; the user doesn'thave to enter the data.

No "direct" oil connection between the windings, at least one winding must be fully covered

with paper/pressboard. This condition is surely fulfilled at voltages above 20 kV. In the other

case the large influence of the oil gap might hide the properties of pressboard and paper. If

this occurs, the dissipation factor curve will not have the specific shape of Figure 22.

The following conditions influence the accuracy of moisture analysis:

Very high temperature:

Cellulose materials have different temperature dependent behavior. The temperature

compensation of the software will perfectly compensate the influence of temperature if the

materials inside the transformer are the same as in the data base. As this is rarely the case,

an increase in temperature of 30 K can lead to an underestimation of moisture content of

0.5 %. For example, the moisture analysis indicated 2.5 % moisture content for a

transformer measured at 50C. Here the "true" moisture content may range from 2.5-3 %

depending on the temperature characteristic of the material used.


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Low temperature:

Temperatures below 10C involve the problems of a reliable temperature measurement and

of the temperature dependent behavior of the cellulose materials used in the particular

transformer.

High oil conductivityAn oil conductivity of more then 20 pS/m at ambient temperature points on conductive

aging by-products. These by-products increase the dielectric losses in a similar way as

water and may lead to an overestimation of moisture. Without compensation the

overestimation can be up to 1.5 % moisture content. DIRANA compensates for this

influence, however an overestimation of moisture of up to 0.3 % may occur.

8.4 Comparison to Other Moisture Measurement Techniques

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 overestimatesmoisture

content in paper . This especially applies if the acidity and / or oil conductivity are high.

Dielectric Response Methods

For the recovery voltage method RVM, the CIGR 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" [4].

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.

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


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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 [5]. The laboratory measuring the water content of paper samples may use adifferent instrument and procedure as the one used to scale the DIRANA data base.

Consequently, the indicated moisture content might differ to a certain extend.

Sample position

The temperature distribution inside a power transformer causes a moisture distribution. The

cold insulation structures (construction elements) accumulate water and the hot structures

(winding paper) are drier. DIRANA will indicate an average moisture content of the barriers

and spacers operated at oil temperature and the winding paper.

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

9 Literature

[1] 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.

[2] 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


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[3] M. Koch, M. Krger: Moisture Determination by Improved On-Site Diagnostics, TechCon

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

[4] S. M. Gubanski et al.: Dielectric Response Methods for Diagnostics of Power

Transformers CIGR Task Force 15.01.09, Technical Brochure 254,Paris, 2004

[5] M. Koch, S. Tenbohlen, J. Blennow, I. Hoehlein: Reliability and Improvements of Water

Titration by the Karl Fischer Technique Proceedings of the XVth International Symposium

on High Voltage Engineering, ISH, Ljubljana, Slovenia, 2007