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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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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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, 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" • 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 20°C 72°C _98°C Average winding 83°C 63°C 54°C Cooling system 74°C 92°C

Bottom oil Bottom winding

Top oil Hot 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 42°C

Temperature from Contact Thermometers

Another 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

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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/Ω / 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 M o is tu re i n c e ll u lo s e / % 2 Solubility in oil / ppm 20 50 80 120 260 500 880 0°C 20°C 30°C 40°C 60°C 80°C 100°C

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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M o is tu re i n a g e d p re s s b o a rd / %

Moisture relative to saturation / % 0 1 2 3 4 5 6 0 10 20 30 40 21°C 40°C 60°C

80°C 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 (CL or 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 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.

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Insulation under test

~

A IVol ISur IVol + ISur Insulation under test

~

A IVol ISur 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 C_HL C_L LV HV A Voltage source Current sense 1 A In s tr u m e n t C_H

=

I

_Vol

I

_Sur

I

_Vol

I

_Sur

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 both measurement 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)

C_HL

C_L C_H

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 mode

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

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

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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 mode

CH1: 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 the single winding to ground CH will 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 CH only. 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, the connection 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 CHL provides most valuable information for subsequent moisture determination. The high and low voltage winding capacitances to ground (CH and CL) are only useful if a measurement of CHL is 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 CExt which is connected to ground / guard.

A

Current input Guard/ Ground CLT CHL CT MV LV HV

~

Voltage source C_Ext CH

A

Current input Guard/ Ground CLT CHL CT MV LV HV

~

Voltage source C_Ext

CH 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 CH and CL measurements (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 C_LT C_HL C_T MV LV HV

A

Voltage source CExt

C_H Figure 12: Wiring diagram for CH

- measurement having an external device CExt still

connected

A measurement is of little value or impossible in case of • Wet or dirty bushings

• Cables with paper / oil insulation • Voltage transformers

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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 connect DIRANA to the transformer.

4. Click the settings tab and then enter 100 µHz as stop frequency. The section below will explain the required frequency ranges.

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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] D is s ip a ti o n f a c to r insulation geometry 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] D is s ip a ti o n f a c to r insulation geometry

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 µHz. 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 µHz. 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/Hz 0.001 0.010 0.100 1.000 10.000 DF 0.005 0.0100 0.050 0.100 0.500 1.000

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/Hz 0.0010 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/Hz 0.0010 0.0100 0.1000 1.0000 10.0000 DF 0.020 0.050 0.100 0.200 0.500 1.000 2.000 t/s 2 5 10 20 50 100 200 500 1000 I/A 0.0000005 0.0000007 0.0000010 0.0000020 0.0000030 HV+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 µHz. 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 20°C. f/Hz 0.001 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 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.

f/Hz 0.001 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 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 ten Hertz.

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f/Hz 0.001 0.010 0.100 1.000 10.000 100.000 DF 0.01 0.03 0.10 0.30 P re s s b o a rd In s u la ti o n G e o m e tr y O il c o n d u c ti v it y P re s s b o a rd

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 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 1000-10 Hz is dominated by the pressboard. Oil conductivity causes the steep slope at 1-0.01 Hz. The interfacial 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 on the 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 50°C the losses of pressboard along with the oil conductivity increase while the shape of the curve remains similar.

f/Hz 0.001 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/Hz 0.001 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 50°C 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 was available 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 50°C 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/Hz 0.01 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 44°C.

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 the model 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 oil conductivity, 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/Hz 0.01 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 B a rr ie rs X i n % 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 maintenance guidance". The categories are:

Category Moisture content in % Color

Dry below 2,2

Moderately wet 2,2-3,7

Wet 3,7-4,8

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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. M o is tu re c o n te n t [% ] Moisture saturation [%] 0 1 2 3 4 5 10 20 30 21°C 80°C Moderately wet Dry Wet, > 30 % extremely wet Moist ure cont amina tion

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 50°C. 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't have 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 50°C. 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 10°C 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 conductivity

An 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 overestimates moisture 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 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" [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 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 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”, Cigré SC A2 & D1 Colloquium, Brugge, Belgium 2007

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[3] M. Koch, M. Krüger: “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” CIGRÉ 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

DIRANA Application Guide - Measuring and Analyzing Power Transformers

DIRANA

Application Guide

Measuring and Analyzing the Dielectric

Response of a Power Transformer

Contents

1 Using This Document

2 Preparing the Transformer

3 Connecting DIRANA to the Transformer

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4 Setting Up the Software

5 Performing the Measurement

6 Interpreting the Dielectric Response in Frequency Domain

7 Moisture Analysis Using DIRANA

8 Assessing the Analysis Results

9 Literature