HVDC TRANSFORMER INSULATION: OIL CONDUCTIVITY

646
HVDC TRANSFORMER INSULATION:
OIL CONDUCTIVITY
Joint Working Group
A2/D1.41
January 2016

HVDC TRANSFORMER INSULATION:
OIL CONDUCTIVITY
JWG A2/D1.41
Members
A. Küchler, Convenor (DE), U. Piovan, Secretary (IT), M. Berglund (SE), G. Chen (UK), A. Denat (FR),
J. Fabian (AT), R. Fritsche (DE), T. Grav (NO), S. Gubanski (SE), M. Kadowaki (JP), Ch. Krause (CH),
A. Langens (DE), S. Mori (JP), B. Noirhomme (CA), H. Okubo (JP), M. Rösner (DE), F. Scatiggio (IT),
J. Schiessling (SE), F. Schober (DE), P. Smith (UK), P. Wedin (SE)
Corresponding Members
I. Atanasova‐Höhlein (DE), Ch. Perrier (FR)
Invited Experts
S. Jaufer (CH)
Former Members
T.C.S.M. Gupta (IN), S. Ingebrigtsen (NO), M. Koch (AT), M. Saravolac (FR), S. Uhrig (AT), H. Yu (CN)
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ISBN : 978-2-85873-349-1

HVDC transformer insulation: Oil conductivity
Page 3
HVDC transformer insulation: Oil
conductivity
Table of Contents
EXECUTIVE SUMMARY................................................................................................................................ 5
ABBREVIATIONS ......................................................................................................................................... 7
1 INTRODUCTION ....................................................................................................................................... 7
1.1 HVDC Transformer Insulation....................................................................................................................................7
1.2 Results of JWG A2/B4.28........................................................................................................................................9
1.3 Scope of Work for JWG A2/D1.41......................................................................................................................9
2 CONDUCTION MECHANISMS, MEASUREMENT TECHNIQUES AND STANDARDS ................................. 11
2.1 General Theories for Electric Conduction Mechanisms in Transformer Oil..................................................... 11
2.1.1 Drift and Diffusion Theory ...........................................................................................................................................................11
2.1.2 Electric Conduction in Frequency Domain.................................................................................................................................12
2.1.3 Electric Conduction in Time Domain ...........................................................................................................................................15
2.2 Existing Measurement Techniques and Standards ............................................................................................. 18
2.2.1 IEC and ASTM Standard Procedures........................................................................................................ 19
2.2.2 Approach A: Step Response Measurements............................................................................................ 23
2.2.3 Approach B: Evaluation of Oil Resistivity Measurement Methods (IEC 60247 and 61620)........... 26
2.2.4 Approach C: On-site Determination of Oil Conductivity by Dielectric Response Analysis............... 29
2.3 Round Robin Test Results (RRT 1)...........................................................................................................................30
2.3.1 Aims of RRT 1............................................................................................................................................... 30
2.3.2 Measurement Results...................................................................................................................................32
2.3.3 Conclusions of RRT 1.................................................................................................................................... 33
3 CONCEPT FOR OIL CONDUCTIVITY MEASUREMENTS........................................................................... 34
3.1 Characterisation of an Oil Curve (Three Stress Points Characterisation Concept) ....................................... 34
3.2 Recommended Test Procedure for Oil Conductivity Measurements................................................................ 36
3.2.1 General Requirements for Oil Conductivity Measurement ...................................................................36
3.2.2 Test Arrangement ........................................................................................................................................ 37
3.2.3 Sample and Preparation............................................................................................................................ 38
3.2.4 DC Conductivity Determination ................................................................................................................. 38
3.3 Verification of Concept by Round Robin Test Results (RRT 2).................................................................. 39

Page 4
4 OIL CONDUCTIVITY VALUES AT PRODUCTION AND IN SERVICE......................................................... 43
4.1 Campaign of Measurements on Transformers (RRT 3) ...................................................................................... 43
4.2 Oil Conductivities prior to Dielectric Testing and prior to Transformer Energisation on Site ...................... 44
4.3 Oil Conductivities at Different Stages of Transformer Life .............................................................................. 47
4.4 Discussion of the Measurement Results of RRT 3................................................................................................. 48
5 CONCEPT FOR OIL-IMPREGNATED PRESSBOARD CONDUCTIVITY MEASUREMENTS........................... 50
5.1 Characterisation of Dielectric Properties by Currents....................................................................................... 50
5.2 Recommended Test Procedure for Pressboard Conductivity Measurements ................................................. 53
5.2.1 Test Arrangement (Test Vessel, Electrodes, Samples, Measuring Circuit, Power Supply) ................ 53
5.2.2 Preparation of Test Vessel, Samples, Electrodes, Oil............................................................................ 54
5.2.3 Required and Optional Test Parameters (Electrical Stress, Time of Electrification, Temperature) . 54
5.2.4 Description of Measurement ...................................................................................................................... 54
5.2.5 Contents of Report....................................................................................................................................... 55
5.3 Verification of Concept by Round Robin Test Results (RRT 4)........................................................................... 55
6 DIELECTRIC TEST EFFECTIVENESS AND RELIABILITY.............................................................................. 59
6.1 Field Calculation Results......................................................................................................................................... 59
6.1.1 Introduction................................................................................................................................................... 59
6.1.2 Models for Simulations................................................................................................................................ 59
6.1.3 Material Properties..................................................................................................................................... 60
6.1.4 Assumptions Used for Simulations .............................................................................................................61
6.1.5 Sets of Simulations Performed................................................................................................................... 61
6.1.6 Quantities Reported.................................................................................................................................... 62
6.1.7 Results and Comments................................................................................................................................. 62
6.2 Impact of Oil Conductivity Values on Test Effectiveness and Reliability........................................................ 65
6.2.1 Oil Conductivity ........................................................................................................................................... 65
6.2.2 Duration of PR Test...................................................................................................................................... 65
7 CONCLUSIONS AND SUGGESTIONS...................................................................................................... 66
BIBLIOGRAPHY/REFERENCES.................................................................................................................... 67
ANNEXES .................................................................................................................................................. 70
ANNEX A: Recommendations for the measurement of DC-oil-conductivity....................................................... 71
ANNEX B: Recommendations for the measurement of conductivity of oil-impregnated pressboard.................. 82

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EXECUTIVE SUMMARY
This is the report of CIGRE JWG A2/D1.41 (HVDC transformer insulation: Oil conductivity), which has been initi-
ated by CIGRE JWG A2/B4.28 in order to continue CIGRE activities on looking into performance and reliability in
service of HVDC converter transformers.
In AC dielectric test, there is a one to one proportionality between the test level and the corresponding dielectric
stress. The AC stress on the winding during induced test compare with the stress during normal service condition is
not influenced by the type of mineral oil, as the relative permittivity of various mineral oil is relatively similar. This is
not the case for polarity reversal (PR) test and for DC test because the dielectric stress depends not only on
voltage but also on polarisation time and on oil conductivity, which can be significantly different depending on the
type of mineral oil.
To investigate this matter, CIGRE JWG A2/B4.28 carried out a series of simulations of PR tests of a HVDC insula-
tion system considering different oil conductivities and different duration of the test. These simulations showed that
(a) a longer duration of the PR test (360/360/180 min) would make the test more effective in terms of test and ser-
vice stresses and that (b) a change in oil conductivity between test and service may have very significant effects on
the polarisation time and the stress distribution (amplitude and duration) among the oil and the other insulating
materials. It is possible that this change is such that dielectric stress during the test has values below the ones
during service. Thus, the test would be not severe enough. Therefore, JWG A2/B4.28 had agreed not to
recommend a modification of the existing PR test due to practical and logistic constraints as well as a heavy de-
pendence on the oil quality [1].
JWG A2/B4.28 judged that the issue of oil conductivity in relation to dielectric testing effectiveness had first priority.
Oil conductivity was found to be the dominant factor and, considering that there is no standard procedure which is
consistently applied to the measurement of oil conductivity, JWG A2/B4.28 submitted to CIGRE a term of reference
(ToR) for the initiation of the new JWG A2/D1.41 to address this issue. It was recommended that the priority shall
be assigned to measuring oil conductivity throughout the transformer lifecycle and evaluating the effect of the oil
conductivity during the design and design review stage.
The scope of work for the new working group was summarised in the following items: (1) Literature review on con-
duction mechanisms, (2) review of techniques and standards for measurement of conductivity of liquids that are
representative for the conditions in a HVDC insulation system, (3) recommendation for oil sampling and handling of
samples, (4) guidance for evaluation and interpretation, (5) recommendations for a simple and representative test
of oil quality to be used for acceptance tests and during service, (6) recommendations for measurement of conduc-
tivity of oil-impregnated pressboard, (7) determination of oil-conductivity values of HVDC transformers at production
and in service during a campaign of measurements, (8) analysis of the impact of the values found in respect of
dielectric test effectiveness and reliability and (9) suggestions for new standards, if possible.
The work of the group is described in the following chapters and sections of the TB:
Chapter 2 gives a report on conduction mechanisms in oil which depend on a number of parameters such as tem-
perature or electrical stress (Section 2.1). Also, existing measurement techniques and standards (Section 2.2) and
the results of a first Round Robin Test (RRT 1) are described which was performed with unused oils in order to see
whether existing techniques and standards can be used for the required test of oil quality (Section 2.3). The inves-
tigation showed that the measurement parameters according to IEC 60247 and IEC 61620 are not defined very
precisely or measuring electrical stresses and measuring times are not relevant for the conditions in a HVDC
transformer insulation system. Therefore, several different procedures are commonly in use and were tested in
RRT 1. The results were somewhat worrying as the spread among different methods and different laboratories was
as high as several orders of magnitude.
The JWG concluded that measurement parameters must be defined more precisely. Chapter 3 describes the so-
called “Three Stress Points Characterisation Concept” that was developed for that purpose and that shall charac-

Page 6
terise the complex conduction behaviour of mineral insulating oil just by the measurement of three characteristic
values (Section 3.1). It was a challenge to find a concept which both is simple enough for application and relevant
enough for determination of dielectric test and services stresses in the transformer. Recommendations for a simpli-
fied test procedure were based on this concept (Section 3.2) which then was verified by a second Round Robin
Test RRT 2 with unused and used mineral oils (Section 3.3). The results show a tremendous improvement. Never-
theless, the spread was still in the range of one order of magnitude.
In Chapter 4, the third Round Robin Test RRT 3, which is a campaign of measurements for the determination of oil
conductivities at transformer production and in service, is described (Section 4.1). For that purpose, oil conductivi-
ties were measured prior to dielectric factory testing, prior to energisation on site and after many years of service
operation. The discussion of the results shows that oil quality prior to testing and prior to energisation seem to be
comparable and that increase of oil conductivity during service life cannot be excluded (Section 4.2).
Chapter 5 transfers the concept of oil conductivity measurements to the measurement of oil-impregnated press-
board conductivity. For that, it was considered that polarisation processes and therefore polarisation currents play
an important role (Section 5.1) and appropriate recommendations for testing were developed (Section 5.2) which
were verified by a fourth Round Robin Test RRT 4 with oil-impregnated pressboard samples (Section 5.3).
Chapter 6 describes the calculation of dielectric stresses based on the material properties measured in the RRTs
both for oil and pressboard (Section 6.1). The electrical-stress calculations show the impact of oil conductivity val-
ues on dielectric test effectiveness and test reliability.
Finally, Chapter 7 discusses conclusions and suggestions:
Oil-conductivity shall be measured according to the recommendations in Annex A of the TB (Recommendations for
the measurement of DC oil-conductivity) in order to characterise the oil-curve by three stress points. Oil-
impregnated pressboard conductivity shall be measured according to the recommendations in Annex B in the TB
(Recommendations for the measurement of conductivity of oil-impregnated pressboard).
Oil conductivity shall be measured after testing, after commissioning and throughout the lifetime of a transformer.
The conductivity of the oil from the transformer during factory acceptance test shall be in the same range
(difference less than a factor of 10) as the conductivity of the oil from the transformer right after commissioning in
order to achieve relevance of the test. In case of exceeding this limit, agreement shall be found between
manufacturer and purchaser based on information about insulation system robustness discussed at design review
stage.
The physical behaviour of oil is not sufficiently described just by conductivity. Ion drift and space charge may have
a complex influence on electrical-stress distribution. Therefore, material models based on the RC model
(conductivity/permittivity model) only give first order approximation results. In oils with a very low conductivity more
physical processes beyond conduction alone come into play when trying to determine the effect of an applied
electrical stress. Examples of such physical processes are ion generation (strongly non-linear for high electrical
stress) and ion transport, charge injection/interaction at the electrodes and at the pressboard interface or
electrohydrodynamic (EHD) movement of oil.
Field calculations based on the RC material models indicate that insulation system time constants can be long in
comparison with standard test durations, especially for low-conductive oils. In order to achieve an effective test,
electrical stresses can be increased by increasing the PR test voltage, by increasing the test duration or by a
combination of both. This has been discussed in the previous working group A2/B4.28.
JWG A2/D1.41 agrees that polarisation times are getting longer for low-conductive oils. Yet, the group has no clear
quantitative indication on how to change the PR test.

Page 7
Abbreviations
AC alternating current
DC direct current
HVDC high voltage direct current
PB pressboard
PR polarity reversal
RRT round robin test
1 Introduction
1.1 HVDC Transformer Insulation
HVDC power transmission has been applied for a long time and utilised in electric power transmission systems. In
recent years, its application area has been expanding to modern power systems including long distance power
transmission, distributed power generation, energy storage, off-shore wind farms, localised sophisticated power
systems and so on. In these systems, the newly introduced HVDC systems are to be operated in unison with the
surrounding conventional AC power systems, by introducing AC-DC power conversion apparatuses.
In such conversion systems, the main apparatuses are HVDC converter transformers, which are arranged at both
ends of the DC power transmission lines. The HVDC converter transformer consists of steel cores, windings and
electrical insulation materials. For cooling and electrical insulation purposes, insulating oil is used with solid insula-
tion such as oil-impregnated pressboard (PB). The insulation materials used are the same as in a conventional AC
transformer, however, the applied electrical stresses are different due to the addition of the DC stress components
superimposed to AC and impulse voltages. Especially in oil, very different field distributions have been observed
[2].
Under AC and impulse voltage applications, the electrical-stress distribution can be determined by the permittivity
of the oil and PB materials. Conversely, for steady state DC voltage application, the electrical stress can be
determined by the electrical conductivity of the insulating materials. In general, insulating oil has a lower permittivity
(dielectric constant) and a higher conductivity than pressboard, which makes the AC voltage more concentrated in
the oil and the DC voltage more concentrated in the pressboard. The electrical conductivities of the insulating oil
and PB show a temperature dependence and an electrical-stress dependence, which is dependent on the duration
of the stress application with certain time constants. In addition, the conductivity of the insulating oil and impregnat-
ed PB is expected to change due to thermal degradation from long-term transformer operation.
Generally, in oil and PB composite insulation systems, the electrical-stress distributions are dependent on the
complex permittivity ε
*
of equation (1.1-1) and obtained by solving the following equations (1.1-2) and finally equa-
tion (1.1-3).
∗
j (1.1-1)
div div ∙ (1.1-2)
div ∙ j ∙ 0 (General case) (1.1-3)
div ∙ 0 (DC and low frequency) (1.1-4)
div ∙ 0 (AC and high frequency) (1.1-5)

Page 8
Where : permittivity, ε*
: complex permittivity, : conductivity of the materials, q : accumulated charge density, :
angular velocity of applied AC voltage, E: electrical-stress vector, J: conduction current density vector.
Note: Equations (1.1-1) and (1.1-3) are given in a frequency-domain representation.
At the instant of DC voltage application, the electrical stress can be determined by permittivity ratio using equation
(1.1-5) and can gradually change, depending on the time constants, finally leading to the DC steady state shown by
equation (1.1-4) for conductivity. The electrical-stress behaviour under DC polarity reversal (PR) voltage can be
calculated using equation (1.1-3) for both permittivity and conductivity. The thusly-calculated equipotential lines
under the above different voltage conditions are shown in Figure 1.1-1 for a parallel-lapped PB arrangement in
insulating oil. In the case of steady state DC voltage condition in Figure 1.1-1, the DC electrical stresses are not
concentrated in the insulating oil but in the PB volume, depending on the conductivity ratio of the insulating oil and
impregnated PB. Moreover, because of the accumulated charges on the PB surface , PR is shown to be the most
critical condition in the electrical insulation performance of the HVDC converter transformers. Figure 1.1-2 shows
one example of the electrical-stress distributions in HVDC converter transformers and the difference between the
electrical-stress distributions of DC and AC voltage is obvious.
Figure 1.1-1: Electrical-stress distribution
under different voltage applications for
oil/PB composite systems with a high
conductivity ratio [3]
Figure 1.1-2: Difference of electrical-stress
distribution in a HVDC converter
transformer between AC and DC voltage
applications [4]

Page 9
The permittivity is weakly dependent on the temperature, but the conductivity of the insulation materials is
strongly dependent on the temperature, as well as the thermal degradation of the materials. It can be concluded
that knowledge of the conductivity values of both oil and pressboard is important, recognizing the influencing
factors such as temperature, time, electric stress etc. as well as transformer operation conditions.
1.2 Results of JWG A2/B4.28
In AC dielectric tests there is a one to one proportionality between the test level and the corresponding dielectric
stress. For example, during the induced voltage test, the transformer is fed with double the rated voltage to induce
double the rated dielectric stress between turns in all windings. Moreover, the ratio between the stresses during the
induced voltage test and the stresses during normal AC service condition is not influenced by the type of the
transformer mineral oil used during the test and in service as the relative permittivity of various mineral oils differs
little.
This proportionality between voltage and stress, and independence from mineral oil type is not generally given for
the PR (and DC) test because the dielectric stress depends not only on voltage but also on a) polarisation time and
b) oil conductivity, which can be significantly different depending on the type of mineral oil.
To investigate this matter, CIGRE JWG A2/B4.28 [1] carried out a series of simulations of PR tests of a HVDC
bushing insulation system considering different oil conductivities and different durations of the test.
These simulations showed that:
a) A longer duration of the PR test (360/360/180 min) would make the test more effective in terms of test and
service stresses.
b) A change in oil conductivity between test and service may have very significant effects on the polarisation
time and the stress distribution (amplitude and duration) among the oil and the other insulating materials. It
is possible that this change is such that dielectric stress during the test has values below the ones during
service. Thus the test would be not severe enough.
Therefore JWG A2/B4.28 had agreed not to recommend a modification of the existing PR test due to practical and
logistic constraints as well as a heavy dependence on the oil quality.
JWG A2/B4.28 judged that the issue of oil conductivity in relation to dielectric testing effectiveness had first priority.
However, this task was outside the scope and competences of JWG A2/B4.28. The JWG submitted to CIGRE a de-
tailed term of reference (ToR) for the initiation of a new A2/D1 JWG to address this issue.
1.3 Scope of Work for JWG A2/D1.41
The Terms of Reference for JWG A2/D1.41 briefly describe the background and the scope of work:
As already mentioned, JWG A2/B4.28 showed the impact of conductivity variations of combined oil/solid insulation
system components on design and reliability of HVDC converter transformers. The effectiveness of existing
standard dielectric DC and PR tests in respect of oil conductivity and polarisation time has been reviewed. Oil
conductivity was found to be the dominant factor and, considering that there is no standard procedure, which is
consistently applied to the measurement of oil conductivity, this could result in dielectric stresses during the test
lower than in service.
Therefore JWG A2/B4.28 recommended that the priority shall be assigned to measuring oil conductivity throughout
the transformer lifecycle and to evaluating the effect of the oil conductivity during the design and design review
stage.

Page 10
The scope of work was summarised in nine items:
1. Review available literature on conduction mechanisms in organic liquids, in systems with uncovered and
covered electrodes.
2. Review techniques and standards for measurement of conductivity of liquids, and how representative they
are for conditions in an oil gap in a composite HVDC insulation system.
3. Give recommendation of sampling and handling of samples of oil taken from service.
4. Guidance for evaluation and interpretation.
5. Advise on possibilities for a simple and representative test of oil quality to be used by suppliers, OEMs and
end users. This is to be used for acceptance tests and during service, if possible.
6. Advise a test procedure for measurement of conductivity of oil-impregnated pressboard.
7. Perform a campaign of measurements to determine oil-conductivity values of HVDC transformers at
production and in service.
8. Analyse the impact of the values found in respect of dielectric test effectiveness and reliability.
9. Suggestions for new standards, if possible.
The work of the group was performed according to the following chapters and sections:
Chapter 2 gives a report on conduction mechanisms in oil which depend on a number of parameters such as
temperature or electrical stress (Section 2.1). Also, existing measurement techniques and standards (Section 2.2)
and the results of a first Round Robin Test (RRT 1) are described which was performed with unused oils in order to
see whether existing techniques and standards can be used for the required test of oil quality (Section 2.3).
Chapter 3 describes the so-called “Three Stress Points Characterisation Concept” that was developed by the group
in order to characterise the complex conduction behaviour of mineral insulating oil just by the measurement of
three charcteristic values (Section 3.1). It was a challenge to find a concept which is both simple enough for
application and relevant enough for the dielectric test and services stresses in the transformer. Recommendations
for a simpified test procedure were based on this concept (Section 3.2) which then was verified by a second Round
Robin Test RRT 2 with unused and used mineral oils (Section 3.3).
In Chapter 4, the third Round Robin Test RRT 3, which is a campaign of measurements for the determination of oil
conductivities at production and in service, is described (Section 4.1). For that purpose, oil conductivities were
measured prior to dielectric factory testing, prior to energisation on site and after many years of service operation.
The results show that oil quality prior to testing and prior to energisation seem to be comparable and that an
increase of oil conductivity during service life cannot be excluded (Section 4.2).
Chapter 5 transfers the concept of oil conductivity measurements to the measurement of oil-impregnated
pressboard conductivity. For that, it was considered that polarisation processes and therefore polarisation currents
play an important role (Section 5.1) and appropriate recommendations for testing were developed (Section 5.2)
which were verified by a fourth Round Robin Test RRT 4 with oil-impregnated pressboard samples (Section 5.3).
Chapter 6 describes the calculation of dielectric stresses based on the material properties measured in the RRTs
both for oil and pressboard (Section 6.1). The electrical-stress calculations show the impact of oil conductivity
values found on dielectric test effectiveness and test reliability. Finally, Chapter 7 discusses conclusions and
suggestions.
Additional information is given in the Bibliography and in the Annexes which contain the documents
“Recommendations for the measurement of DC oil-conductivity” and “Recommendations for the measurement of
conductivity of oil-impregnated pressboard”.

Page 11
2 Conduction Mechanisms, Measurement Techniques and Standards
2.1 General Theories for Electric Conduction Mechanisms in Transformer Oil
2.1.1 Drift and Diffusion Theory
The mechanisms of the electric conduction in transformer oil are usually explained by the ionic drift and diffusion
theory. As an external electrical stress is applied, motion of charge carriers can lead to electric current. These
charge carriers may be blocked by metal electrodes and a charge layer can be formed at metal/liquid interface.
Due to the non-linearity of ionic drift and diffusion, it is not practical to seek an accurate analytic solution of the ionic
transport equations. However, with proper simplifications, an approximate solution can be obtained.
In a thermodynamic equilibrium the volume conductivity is composed of the sum of the products of ion mobilities
µi, and their charge densities qi
(2.1-1)
Assuming an equilibrium in electrolyte AB dissolving in a perfectly insulating liquid
↔ (2.1-2)
the concentration of positive and negative ions n+, n- may be expressed in terms of their concentration  as well as
the recombination and dissociation constants KR and KD
(2.1-3)
With elementary charge e and the volume charge densities being qi = ni e, the conductivity can be written in terms
of the mobility and the recombination/dissocation constants
(2.1-4)
Considering a particle in an electric field moving according to Stokes Law, the ion mobility can be obtained from
Walden’s Rule, which states that the product of mobility µ and viscosity at infinite dilution is constant and can be
represented as
6
(2.1-5)
where q is the ionic charge and r represents Stokes radius of the ion.
Mobility of ions in oil varies with temperature, following an Arrhenius type dependence [5]. The average mobility of
ionic carriers µ, estimated from the transient current following the application of a step voltage in a small gap is in
the range of µ = 10-9
m2
/Vs [6], which is in line with the estimation based on Walden’s rule.

Page 12
2.1.2 Electric Conduction in Frequency Domain
As soon as an electrical stress is applied to the oil, the positive and negative charge carriers start to move under
the electric force. Once a local charge imbalance is induced, polarisation will be created. The polarisation, P, and
the electrical stress, E, can be related by
∙ ∗
1 ∙ (2.1-6)
Here, 0 is the vacuum permittivity and ∗
is the complex permittivity of the material. When the frequency is below
103
Hz, the dielectric loss contributed by electronic, ionic and orientational polarisation can be neglected [7]. Thus,
the complex permittivity measured in the low frequency range is determined by the motion of charge carriers.
(1) Different conduction models for liquid
The model of Poisson – Nernst – Planck (PNP) is commonly used to describe the redistribution of charge carriers
that is induced by the external field. The PNP model is based on the continuity equations for charge carriers and
the Poisson equation. Different solving approaches have been made by taking into account different boundary
conditions. Jaffe et al. assumed that the ionic current is proportional to the variation of the density of the ions in the
vicinity of the electrode [8]. Later, Macdonald suggested that a resistance can be added in parallel to the liquid bulk
to describe the non- ideal character of the electrodes, which is known as GPNP model [9]. Barbero et al. found that
this model is equivalent to Jaffe’s model if the conduction current through the electrode is small when compared
with the displacement current [10]. Another model, which assumes that the current density on the electrode is
proportional to the local electric field, has also proved a good agreement with the measured experimental results of
hydro-solutions [11]. Sawada claimed that the space charge polarisation effect has not been included in the
dielectric constant of the Poisson equation, and therefore he proposed a modified model based on a new Poisson
equation, which is called the PNPS model [12].
All these models were developed to explain the conduction mechanism in electrolytic liquids. Modification needs to
be made in order to adapt these models in the study of transformer oil. First, the recombination and dissociation
effect can be negligible in electrolytic solutions, whilst the transformer oil can be considered as a weak electrolyte
and only a small amount of its molecules can be dissolved into free ions. Thus, the dissociation and recombination
effect should not be ignored in the analysis of dielectric properties of transformer oil. Second, the boundary
condition of the transformer oil/metal interface may be different from that used in the study of electrolytic liquids [8],
[9], [11], [12], [13]. Third, because transformer oil is a mixture of paraffin, naphthene and aromatic compounds,
charge carriers with different mobility are to be expected, which increases the complexity of using the PNP model.
(2) Application of Poisson –Nernst –Planck model in transformer oil
Let us consider a parallel metal electrode system filled up with transformer oil and assume the slow charge carriers
are distributed evenly between the two electrodes and two types of charge carriers that have the same properties
except for their polarity are mobile, the density of positive charge carriers and negative charge carriers can be
written as:
(2.1-7)
where, and are the densities of the positive charge carriers and negative charge carriers, and µ+ and µ- are
the mobilities for positive and negative charge carriers, respectively.
The electric field is subject to the Poisson equation
, , ,
(2.1-8)

Page 13
When the flow of liquid can be ignored, the density of positive and negative charge carriers can be denoted as
,
, ,
, , ,
(2.1-9)
,
, ,
, , ,
(2.1-10)
where, D and D are the diffusion coefficients for positive and negative charge carriers and is the
recombination coefficient.
Assuming all charge carriers will be blocked at the interface, the current flow through the circuit can be calculated
from the change of the induced charge at the electrode which is attracted by the space charge in the bulk. This
induced charge due to space charge in the bulk can be expressed with the distance l between the electrodes as
[14]
, , (2.1-11)
When the external voltage that is applied upon the two electrodes is a simple sinusoidal voltage sin ,
the relative dielectric permittivity involves electrode polarisation and can be written as [14]
2
sin

/
2
cos/
(2.1-12)
where, f is the frequency, S is the surface area of the electrode,  is the angular frequency.  is the relative
permittivity at high frequency. These equations can be easily obtained by integrating the current flow through a
pure ohmic sample that has a constant conductivity and permittivity [15].
(3) Existence of injected charge carriers in transformer oil under a low electric field?
It has been noticed that the frequency responses of transformer oil share several common features. The real part
of the complex permittivity would increase when the frequency goes low, whilst the imaginary part decreases with
the frequency with a slope approximate to -1 [16]. According to the simulation results based on the PNP model,
there would always be a peak in the imaginary part of the complex permittivity in a frequency range from 10-3
Hz -
104
Hz [7], which could not be found in the experimental work. Therefore, there should be other charge
transportation processes involved.
Felici et al. proposed an ionic injection model and claimed that the charge carriers can be generated in the vicinity
of the electrode. The injection of charge carriers can be described by means of a two-step process: A) charge
carriers are created in a region that is close to the metal/liquid interface; B) these newly generated charge carriers
are extracted from that region and drift into the bulk. The injection in insulating hydrocarbon liquid has been studied
and an analytical solution has been obtained as [17]
2 ∙ 2
(2.1-13)

Page 14
with

16
(2.1-14)
where, iq is the charge density that is considerably far away from the electrode,
0
iq is a constant related to the
nature of the electrode and the liquid, 1K is the modified Hankel function and kb is the Boltzmann constant.
A theoretical study of the frequency domain polarisation in liquid has been carried out by Coelho. In his theory, the
distribution of charge carriers was assumed to be linear. For a completely blocked electrode system where no
charge transfer to the electrodes occurs, the frequency dependent complex permittivity can be denoted as [18]

1
tanh
(2.1-15)
with
1
τ
1
(2.1-16)
The polarisation caused by the ionic injection effect has been theoretically studied by assuming the field is evenly
distributed between the two metal electrodes. the polarisation caused by the injection can be described as [19]
Δ 0
Δε ∙
2
, if
2
1
Δε , if
2
1
(2.1-17)
If the injected charge carriers are fast enough so that they can reach the electrodes in a half wave of the alternating
field, the polarisation caused by the injection can only contribute to the imaginary part of the complex permittivity.
Later, Yuan and his colleagues proved that this conclusion is still valid in the presence of blockable charge carriers.
An analytical expression for the frequency domain polarisation of transformer oil has been given as [20]

1
tanh
(2.1-18)
with
1
1
(2.1-19)
The curves of the frequency dependent complex permittivity of transformer oil that are calculated from simulation
and measured experimentally are shown in Figure 2.1-1 [20]. The dashed lines, the solid lines and the points
represent the calculated results based on Coelho's expression, the calculated results based on Yuan's expression

Page 15
and the experimental results, respectively. The values of the real part of the complex permittivity that are calculated
based on the PNP model are higher than the values obtained experimentally when the frequency is below 1 Hz.
The curves for the imaginary part of the complex permittivity that are simulated using the PNP model reach a peak
as the frequency decreases. On the other hand, the simulated values using the modified PNP model can give a
better fit to the experimental results. However, the existence of these fast injected charge carriers is not clear and
experimental verification is needed.
Figure 2.1-1: Theoretical and experimental results of the complex permittivity of transformer oil
at different temperatures in °C. The dotted lines are calculated from Coelho's expression, whilst
the solid lines are calculated from Yuan's expression. [20]
2.1.3 Electric Conduction in Time Domain
The time domain measurement of conduction processes includes polarisation and depolarisation current (PDC)
measurements.
(1) PDC principle theory and extended Debye model
The PDC measurement is an effective method in the time domain to investigate the slow dielectric polarisation in
insulating materials. When an electrical stress is applied on the test medium, the current density can be denoted as
(2.1-20)
Where is the response function of the dielectric material. If the sample can be treated as a homogeneous
material, the electrical stress can be replaced with the external voltage . Then the current in the
external circuit can be written as
(2.1-21)
where 0
C is the (geometrical) capacitance of the electrode arrangement in vacuum.
If the test object is completely discharged and a step voltage with the following characteristics is applied during a
charging time ,

Page 16

0, for 0
, for 0
0, for
(2.1-22)
It is easy to notice that the current will be zero when 0t  . The polarisation current contributed from the DC
conductivity and the different polarisations can be expressed as
(2.1-23)
Once the sample is short circuited when pt t , the relaxation of the polarisations builds up the depolarisation
current, which can be described as
(2.1-24)
If the charging time pt is long enough so that the entire system has entered a quasi-equilibrium state, ( )pf t t is
negligible and the depolarisation current is determined by .
The extended Debye model, which is shown in Figure 2.1-2, is successfully used for the descripiton of solid
insulating materials, e.g. [21], [22]. According to this model, the polarisation current can be denoted as
exp (2.1-25)
Similarly, the depolarisation current can be calculated as
1 exp
exp
(2.1-26)
Figure 2.1-2: Extended Debye model [22]
(2) Oil model and the polarisation current
The conduction and polarization mechanisms in transformer oil differ significantly from that in solids. It has been
reported that the depolarisation current of transformer oil is much lower than the polarisation current; hence,
Küchler stated that the polarisation in transformer oil is mainly induced by the conduction and not by the
polarisation [23]. According to the extended Debye model, the difference between the polarisation current and
the depolarisation current approaches a constant value. However, there is no experimental support for
this conclusion in oil. Thus, the extended Debye model has to be used with caution in transformer oil research.

Page 17
Küchler suggested a simpler oil model, to describe time dependence of the polarisation current. It assumes that the
initial amount of charge carriers is diminished by the already transported amount charge by the following equations
[23]
0 (2.1-27)
∙ ∙
(2.1-28)
Where is the mobile charge in the oil  is the mobility of ions, l is the distance between two electrodes.
Obviously, the solution to the oil model gives an exponential decrease of current, which is in a good agreement
with the experimental results.
(3) Depolarisation current and its correlation with frequency domain measurement
As part of the charge carriers in transformer oil cannot pass the electrode surface, these charge carriers will
accumulate in the vicinity of the electrodes and form charge layers. Once the external electric field is removed,
these blocked charge carriers will start to diffuse back into the oil bulk. The depolarisation current measurement
can provide information about charge density and mobility of charge carriers.
Yuan et al. have studied the depolarisation process with the assumption that the depolarisation is mainly affected
by the charge diffusion. The depolarisation current can be written as [24]

8 2 | |
exp (2.1-29)
in which, is the total charge in the charge layer, is the diffusion coefficient for the a-th charge carriers and l is
the drift length of the charges.
When the quantity of the charge carriers that are adsorbed by the electrode is equal to that drifting to the charge
layer, the system of oil/electrode reaches an equilibrium state. The conductivity contributed by the charge layer can
be denoted as:
2
2 2
4 ∆
(2.1-30)
where ab is a constant related to the liquid and t is the time to reach the equilibrium state. If the values of the
conductivity and mobility are substituted in the modified PNP model given in Section 2.1.2, the relative complex
permittivity can be calculated. The frequency dependence of the permittivity obtained theoretically and
experimentally is shown in Figure 2.1-3.
The calculated plots do not differ much for both temperatures and they are in accordance with the experimental
observation. The calculated complex permittivity using Yuan's expression [20] is slightly higher than the
experimental data. As Coelho’s model does not take account of the ionic adsorption at the electrode, the real part
of the complex permittivity calculated using Coelho’s model can be expected to be higher than the actual value.
Besides, the expression [25] used to calculate the depolarisation current is not correct in a rigid way. When the
charge carriers are not evenly distributed between the electrodes, the internal field is distorted by the presence of
the space charge. As the knowledge about the distribution of the charge carriers in the vicinity of the electrode is
still limited and due to the non-linearity of the ionic drift and diffusion model, it is not practical to obtain an analytical
solution for the depolarisation current and this equation is only an approximation. The depolarisation current
comprises both current induced from ionic drift and ionic diffusion. However, the ionic drift effect has not been

Page 18
considered in Yuan's work [25], thus, the calculated value may also be higher than the actual value, which can
result in a higher value of the permittivity.
Please note, if the field is low, the equation (2.1-29) for the depolarisation current is not valid as the newly
generated charge carriers are unable to reach the electrode within a short time. When the field is high, there are
both injected charge carriers and dissociated charge carriers in the mineral oil. These injected charge carriers
might react with the dissociated charge carriers or the neutral molecules and generate new charge carriers.
Therefore, the analysis method proposed in Yuan's study [25] is only valid for a medium electric field.
Figure 2.1-3: Comparison between theoretical and experimental value of the permittivity of
transformer oil at temperatures of 30 °C and 90 °C
2.2 Existing Measurement Techniques and Standards
Prior to the development of test procedures for the determination of oil conductivity, existing measurement tech-
niques and standard procedures are summarised in Section 2.2.1, and three different approaches covering differ-
ent aspects are described in Section 2.2.2, 2.2.3 and 2.2.4:
Approach A (“Step Response Measurements”, Section 2.2.2): Conductivity measurements are performed similar to
the physical conditions in insulation systems during transformer testing and service operation (electrical stress,
time, temperature). By means of voltage step-response measurements on the test sample, measurements shall
provide all the system information that is suitable to derive the quantities of interest and to describe the physics of
conduction processes and relevant parameter dependences.
Approach B (“Measurements according to IEC 60247, IEC 61620 and ASTM D1169”, Section 2.2.3): Conductivity/
resistivity measurements are performed according to any of the given standard procedures.
Approach C (“On-site Dielectric Response Measurements on Transformers”, Section 2.2.4): Conductivity calcula-
tions are performed from non-intrusive dielectric response measurements on the transformer (not on a material
sample). They shall give oil-related diagnostic information on the insulation system.
The three measuring approaches A, B and C are based on different purposes and have different advantages and
disadvantages. Therefore, it seems that none of the methods can replace the others completely, if all purposes are
considered.

Page 19
2.2.1 IEC and ASTM Standard Procedures
IEC and ASTM standards describing conductivity or resistivity measurements are summarised in Table 2.2-1 (for
liquids [26], [27], [28]) and in Table 2.2-2 (for solids [29], [30]). Volume conductivity/ resistivity is regarded here.
Standard IEC 61620, 1998-11 IEC 60247, 2004-02 ASTM D1169 – 09
Title
Insulating liquids –
Determination of the dielectric dissi-
pation factor by measurement of the
conductance and capacitance –
Test method [26]
Insulating liquids –
Measurement of relative permittivity,
dielectric dissipation factor (tan )
and d.c. resistivity [27]
Standard Test Method for Specific
Resistance (Resistivity) of Electrical
Insulating Liquids [28]
Scope
Quantities
Dielectric dissipation factor
(Conductivity measurements are
used for dissipation factor determi-
nation only)
Relative permittivity, dielectric
dissipation factor (tan ),
d.c. resistivity
Specific resistance (resistivity)
Remark: By definition, conductivity is
related to an initial current density
during a very short period of time
(initial conductivity).
Remark: By definition, d.c. resistivity
is related to “steady-state current
density” (clause 3.3), note that this
cannot be determined within 60 s.
Remark: By definition, d.c. resistivity
is related to a current density “at a
given instant of time” (clause 3.1.1)
Application
Particularly suited for highly insulat-
ing liquids, complement to IEC
60247
Primarily for reference tests on un-
used liquids, also applicable to liq-
uids in service
Tests on unused liquids, liquids in
service and liquids subsequent to
service
Method *)
Current measurement, trapezoidal
voltage
Current measurement with dc volt-
age
Current measurement with dc volt-
age
Electrical Stress *)
< 0.1 kV/mm
(voltage 10 … 100 V, d = 1 … 4mm)
0.05 … 0.25 kV/mm
(unless otherwise specified)
0.2 to 1.2 kV/mm (according to
mutual agreement, upper limit in
order to avoid ionization)
Time of electrification *)
0.45 … 5 s (trapezoidal square
wave, f = 0.1 to 1 Hz, rise time 1 to
100 ms)
1 min
1 min direct polarity /
5 min short circuit /
1 min reversed polarity
Temperature Ambient or elevated temperature Ambient or elevated temperature According to mutual agreement
Testcell
General
Three terminal configuration, as
recommended in IEC 60247
Any cell that meets general require-
ments (5.1.1 to 5.1.5), figures give
examples
Any cell that meets general require-
ments (Annex A1.), figures give
examples
Configuration Three terminals
Two terminals or three terminals
(guard)
Two terminals or three terminals
(guard)
Electrode distance d = 4 mm (example) d = 1 or 2 mm (examples) d = 2.54 mm (example)
Surface-volume
ratio
Low, e.g. 2.6 cm-1 (i.e. d > 4 mm) Low, e.g. < 5 cm-1 (i.e. d > 2 mm)
Large enough to provide enough
current
Electrode material As recommended in IEC 60247
Stainless steel (no chemical interac-
tion with liquids and cleaning materi-
als, e.g. stainless steel, gold, nickel,
rhodium)
Stainless steel (no chemical interac-
tion with liquids and cleaning materi-
als, e.g. stainless steel, gold, nickel,
rhodium)
*) In the case of resistivity/ conductivity measurements only
Table 2.2-1: Standards with references to the measurement of “conductivity” or “resistivity” of
insulating liquids

Page 20
Standard IEC 60093, 1980-01 ASTM D257 – 07
Title
Methods of test for volume resistivity and surface resis-
tivity of solid electrical insulating materials [29]
Standard Test Methods for DC Resistance or Conduct-
ance of Insulating Materials [30]
Scope
Quantities
Volume and surface resistances,
corresponding resistivities, conductances and con-
ductivities
DC insulation resistance (volume and surface re-
sistance), corresponding resistivities, conductances
and conductivities
Remark: By definition, volume resistivity is related to
“steady-state current density” (clause 2.2).
Note that this often cannot be determined within the
times given below.
Remark: By definition, resistivity and conductivity are
related to a current without respect to time (clause 3.1).
Note that this is not correct because of polarisation in
solids; the adjective “apparent” is generally applied to
resistivity/ conductivity values therefore (5.3).
Application Tests on solid insulating materials Tests on (solid) insulating materials
Method *) Current measurement with dc voltage Current measurement with dc voltage
D.c. voltage *)
100, 250, 500 V and 1, 2.5, 5, 10, 15 kV
(100, 500 and 1000 V are the most frequently used
values)
100, 250, 500 V and 1, 2.5, 5, 10, 15 kV
(500 V unless otherwise specified, higher values for
tests near operating conditions or to increase sensitivity,
appendix X1.5.1)
Time of electrification *)
Values are taken at 1, 2, 5, 10, 50, 100 min until steady
state is reached. If steady state is not reached within
100 min, the volume resistance is reported as a function
of electrification time. (Note that this is not correct be-
cause of polarisation in solids. The adjective “apparent”
would be necessary here.)
1 min (standard, unless otherwise specified),
conductance-time curve up to many hours, in case of
polarisation, appendix X1.4. (Note that this is an “appar-
ent conductance” only, because of decreasing polarisa-
tion currents.)
Temperature and
humidity
As during conditioning As during conditioning
Testsetup
General
Many different arrangements according to the shape of
the sample or the test setup
Many different arrangements according to the shape of
the sample or the test setup
Configuration
Two terminals or three terminals (with guarding if possi-
ble)
Two terminals or three terminals (with guarding if possi-
ble)
Electrode distance Dependent on sample geometry Dependent on sample geometry
Surface-volume
ratio
Dependent on sample geometry Dependent on sample geometry
Electrode material Many different methods of surface electrode application Many different methods of surface electrode application
*) In the case of volume resistivity/ conductivity measurements only
Table 2.2-2: Standards with references to the measurement of “volume conductivity” or “volume
resistivity” of solid insulating materials
a) The Terms “Conductivity” and “Resistivity”
The quantities conductivity  (or ) and resistivity  are reciprocal to each other,  = 1/. Nevertheless, the term
“conductivity” (or “resistivity” respectively) is defined in different ways for the different standards:
Initial conductivity: IEC 61620 (for liquids) relates “conductivity” to an initial current density during a very short time
of electrification (initial conductivity) at very low electrical stress. The standard does not intend primarily to describe
conductivity measurements, initial conductivity is introduced as an auxiliary quantity for the calculation of dielectric
dissipation factor. In the limit of very low stress and short time, this is also referred to as “equilibrium conductivity”.

Page 21
Steady-state conductivity: IEC 60247 (for liquids) and IEC 60093 (for solids) relate “resistivity” to a to steady-state
current density. This definition coincides with the steady-state electrical stress in HVDC insulation systems.
Unfortunately, the standards propose times of electrification which are too short to reach steady-state conditions.
Apparent conductivity: ASTM D1169 (for liquids) and ASTM D257 (for solids) relate “d.c. resistivity” to a current
density “at a given instant of time”. In the case of liquids, this definition is correct, but the proposed time of
electrification gives undefined values between initial and steady-state values. In the case of solids, very long-lasting
polarisation currents are superimposed to the conduction currents. Therefore, the calculated “conductivity” must be
denoted as “apparent conductivity”.
b) Relation to Transformer Test and Service Conditions
Compared to the transformer test and service conditions, the standard measuring procedures for oil conductivities
are performed both with very short times of electrification and at very low electrical stress, as shown in Figure 2.2-
1. Therefore, standard measurements do not provide all the information which is necessary for the characterisation
of HVDC insulation materials. Basically, initial values (valid in a time range up to a few seconds) and one-minute
values (neither representative for initial nor for steady-state conductivities) are determined by today’s standards.
This is not sufficient: There is a superposition of conduction and polarisation currents (mainly for solids), and
conductivities can change with time and stress history (mainly for liquids). Furthermore, conductivities are
measured at low electrical stress only, which does not yield a realistic situation for highly stressed materials,
electrical-stress dependences are neglected. Today’s standard conductivity measurements cannot be satisfactorily
related to transient and steady-state conductivities in insulation systems under HVDC test and service conditions.
According to IEC 60093 and ASTM D257, volume resistivities (and conductivities resp.) of solid insulating
materials are measured by application of a constant and comparatively low DC voltage (0.1, 0.5 or 1 kV,
sometimes up to 15 kV). Current values are taken at predefined times, i.e. at 1, 2, 5, 10, 50 or 100 min after voltage
application. In reality, these currents are caused by polarisation of the material and by time-dependent and voltage-
dependent charge conduction processes. Therefore, simply using one conductivity value determined from just one
measured current, as proposed in this standard, is a too simple approach, as nonlinearities and very long lasting
polarisation processes in solid dielectric materials are misleadingly neglected. These kinds of measurements must
be called “apparent conductivity” measurements therefore [31], [32]. ASTM D257 takes this into account and refers
both to the measurement of “apparent conductance curves” and to higher test voltages up to 15 kV for
measurements “near operating conditions” (see [30], appendix X1.5.1).
In liquid insulating oil, polarisation processes can be neglected and measured currents are proportional to
conductivity. IEC 60247 and ASTM D1169 propose to apply a low electrical stress (< 0.25 kV/mm and < 1.2 kV/mm
resp.) to the liquid and to measure the conductivity at an early instant, already at t = 60 s after electrification. It must
clearly be noted that oil conductivity changes during the time of voltage application because of charge carrier drift
processes (which can last for significantly longer periods of time) and because of charge carrier generation,
injection and recombination. Therefore, a conductivity value, which is derived from a measurement at an early and
predefined time, is normally very different from a steady-state conductivity.
IEC 61620 proposes to measure dielectric properties by means of a trapezoidal test voltage at very low electrical
stress. Permittivities are calculated from the displacement current when the voltage changes and conductivities are
calculated from the conduction current as long as the voltage is constant. Thereby, only the initial oil conductivity
value at very low electrical stress is determined, which is different from conductivities during transient and steady-
state conditions.
c) Reproducibility
IEC 60247 and ASTM D1169 do not give clear guidelines for electrical stresses and times of electrification which
would be necessary for reproducible results. Furthermore, conductivity values are dependent on the stress history
of the oil sample because of ion drift processes. Reproducible measurements would either require a determination
of an electrical pre-conditioning prior to the measurement or sufficient relaxation time without any stress prior to the
measurement, in order to reach thermodynamic equilibrium of the ions. ASTM D1169 proposes to average two

Page 22
measurements with both polarities in order to compensate for polarity-related effects. However, this is not enough;
it was shown by Liebschner that a conditioning needs a number of subsequent polarity reversals [33].
IEC 61620 proposes a procedure that gives reproducible results, because of a fast changing trapezoidal voltage
with a half-cycle duration that is significantly lower than the transit-time of the ions in the oil gap. Thereby, an initial
conductivity value at low electrical stress is determined which can be a reproducible fingerprint for comparisons
between different oils. Nevertheless, the initial conductivity is different from the steady-state and transient
conductivities in insulation systems and it is not clear whether a relation can be established.
Another possibility to get reproducible results would be the measurement of steady-state conductivities. This would
be nearer to operating conditions, but it is not yet part of a standard. In the following sections, possible approaches
are discussed.
0.1s 1 s 1h 1d 1m 1y1min
6
5
4
3
2
1
Test-
and
service stresses
in insulation systems
t
in kV/mmE
Proposed measuring procedure "A"
ASTM D1169
IEC 60247
IEC 61620

E

t
Dependence of conductivity
Dependence of
oil conductivity on
field strength
(schematic)
on time (schematic)
Time of electrification Conductivity
Electric field stress
(t)
Time-dependent conductivity at low field strengths
IEC 61620
ASTM D1169
IEC 60247
Steady-state conductivity
Initial conductivity
Transition
IEC 61620
IEC 60247
ASTM D1169
steady-state
Figure 2.2-1: Typical dependences of conductivity on time and electrical stress (top and right)
and comparison of measurement procedures with test and service stresses
in liquid insulation materials (mineral oil)

Page 23
2.2.2 Approach A: Step Response Measurements
There is no standard for the measurement of oil conductivities that guarantees both reproducibility and
comparability with HVDC oil-conductivities under test and service conditions. Therefore, it was discussed to
measure the dielectric system response of the oil by a voltage step response measurement in time domain at
different electrical stresses, Figure 2.2-2: Thereby, the whole range of interest can be covered:
Time of electrification: From 1 s to steady-state conditions
Electrical stress: From low electrical-stress conditions up to 10 kV/mm
Temperature: As desired, e.g. 20, 50 and 90 °C
Note: A stablized voltage source is a prerequisite for the measurement of currents that must not be influenced by
displacement currents due to changing voltages.
Figure 2.2-2: Step-response measurements with different electrical stresses for the determination
of standard and non-standard conductivities for a mineral-oil sample at room temperature
(normalized diagram)
All kinds of standard and non-standard conductivity values can directly be read from the curves:
(1) “Conductivity” according to IEC 61620: The initial conductivity can be read at t = 1 s from a current curve at a
low electrical stress. There is no electrical-stress dependence in the low electrical-stress region. Values are
reproducible.
Note: During the first
seconds, displace-
ment currents from
the high-voltage
source are super-
imposed.

Page 24
(2) “Conductivity” according to ASTM D1169 and IEC 60247: The conductivities can be read at t = 60 s from a
current curve at a low electrical stress. Due to the ion drift processes, the values are strongly dependent on the
selected electrical stress, on the oil-gap width and on stress history. These standards do not provide reproducible
results therefore.
(3) Steady-state conductivity (non-standard): The electrical-stress dependent steady-state conductivities can be
read as soon as the currents reach their steady states. Values are reproducible and they are related to the
conductivities in a HVDC insulation system.
(4) Physical conduction behaviour with time (non standard): The time-varying conductivities can be analysed, if a
deeper insight into the ion drift processes and the transient behaviour of the insulating systems is required.
A comprehensive description of the proposed method was given during the CIGRE Session 2010 [32].
The test cell is designed both for liquid and solid materials, in order to allow comparable measurements for all
kinds of insulating materials, Figure 2.2-3 (left). Flat solid material samples are put directly between the flat guard
ring electrodes, liquids are measured between the same electrodes being separated by spacers between the guard
ring and the high-voltage electrode, and it is also possible to measure composite or layered materials [34]. The
concept is to use the same test cell and the same test procedure for all materials which have to interact in a HVDC
insulation system. The test cell has a high-voltage design. The electrodes are immersed in the sample oil in order
to prevent partial discharges up to 30 kV/mm (solid materials) and 10 kV/mm (liquids). In contrast to existing
standards, the edge radius at the measuring electrode is 2 mm in order to avoid local field enhancements, which
might cause locally enhanced currents due to the nonlinear conductivity at high electrical stress.
material sample
Glass vessel
Oil
High-voltage terminal High-voltage electrode
Guard ring
Temperature sensor
Glass plate
Measuring cable
additional weight,
Measuring electrode
Oil gap /
Spacers
optional for oil,
required for PB
0
Test voltage
Conduction and polarisation current
Depolarisation current
t
t
i t( )p
i t( )d
tc
v
(1) (2) (3)
i t( )c +
Figure 2.2-3: High-voltage test cell, test voltage profile and current system response [21], [37]
Note: The knowledge about the effective area of the measuring electrode and the thickness of the sample is re-
quired to calculate the conductivity. However, the correcting equation of the effective area is developed based on
the assumption that the measuring electrode had sharp corners and the field distribution within a guarded electrode
system has not been carefully studied [35], [36]. CIGRE JWG A2/D1.41 proposes to increase the edge radius of
the measuring electrode to prevent this local field enhancement effect. It has been reported that the field distortion
might be very serious at the edge of the measuring electrode and the guard electrode [32]. Yuan and his col-
leagues have studied this effect by means of field simulation [19]. The electrode system used in their simulation
and the gap among these the three electrodes are illustrated in Figure 2.2-4.
The field distributions at the edge of the guarded electrode and the measuring electrode are calculated under the
condition that the variables g, h and r are selected to be 0.5 mm, 2 mm and 0.1 mm are shown in Figure 2.2-5. As
seen from Figure 2.2-5, the electric fields at the edge of the guard electrode and the measuring electrode can be
seriously distorted and the maximum field in this electrode system can be much higher than the average field
between the measuring electrode and the high voltage electrode. There are three ways to reduce the maximum
field: (1) reduce the distance h between the measuring electrode and high voltage electrode, (2) increase the edge
radius r of the measuring electrode and guard electrode, (3) decrease the gap g between the measuring electrode

Page 25
and guard electrode. It is proposed by [19] to estimate the maximum field in a guarded electrode system using the
empirical expression:
0.1 1
1.5 1.3
1 (2.2-1)
Extra caution must be taken when using the correction equation provided in ASTM D 257. As the edge of the
measuring electrode is rounded, the electrical field between the measuring electrode surface and the high voltage
electrode is no longer homogeneous and a significant error can be caused if the three parameters, h, r, g, are not
chosen appropriately [19].
Figure 2.2-4: Guarded electrode system and gap among the three electrodes [19]
Figure 2.2-5: Field distortion at the edges of guard electrode and measuring electrode [19]
In order to obtain the dielectric response of a material, it is proposed to record step response measurements in
time the domain which are similar to the HVDC test and service voltage stresses, as shown in Figure 2.2-5 (right).
The proposed test procedure consists of three phases: (1) A depolarisation (relaxation) phase without field in order
to discharge any remaining polarisation, (2) a polarisation phase with the DC test voltage applied and (3) a
depolarisation phase without voltage (terminals short-circuited).
g
h
r

Page 26
In phase (2) the sum of conduction and polarisation currents and in phase (3) the depolarisation current are
measured. For evaluation purposes, they can be depicted in a log-log diagram which provides an excellent
visualisation of short, medium and long term processes in the insulation and which allows for direct comparison of
polarisation and depolarisation currents [21], [37]. Additionally, the voltage can be reversed, in order to investigate
materials and systems under the conditions of polarity reversal (PR) [31], [33].
The evaluation of the measured currents can be performed in several ways [32]: In case of oils, the current is
directly proportional to the time-dependent conductivity and it is analysed according to Figure 2.2-2, depolarisation
currents can normally be neglected. The analysis includes determination of conductivities according to standards,
determination of steady-state HVDC conductivities and transient ion drift analysis. In case of solids, it is important
to distinguish between comparatively low conduction currents and comparatively high polarisation currents. This
can be done by means of the depolarisation current and the so called charge difference method CDM [38].
HVDC insulation materials can be characterised for a wide range of applications, using a series of step response
measurements performed with electrical stresses of 1, 3 and 10 kV/mm for liquids (for solids additionally 30
kV/mm) and with temperatures 20, 50 and 90 °C. Polarisation and depolarisation times shall be long enough to
gain the relevant data for HVDC applications. In many cases, quasi steady-state conditions are reached within
1000 s for oils. For solids often more than 10000 s are required.
Advantages of the proposed method:
1. A set of step-response measurements provides the complete dielectric system information (including
conductivity, polarisation, nonlinearities, drift processes, standard and non-standard quantities).
2. Measurements can cover the whole range of relevant electrical stresses, times and temperatures (comparability
with real HVDC stress situations).
3. Steady-state conductivities can be determined for every stress condition (comparability with real long-term
HVDC stresses).
4. Initial conductivities can be determined (comparability with IEC 61620).
5. One-minute conductivities can be determined (comparability with IEC 60247 and ASTM D1169).
6. Ion drift processes (ion transit times) can be analysed from the conductivity curves (comparability with transient
HVDC stress situations).
7. Solid materials can be measured with the same method (comparability of all materials in an insulation system).
The Term of References of JWG A2/D1.41 demands that the impact of the measured conductivity values on
dielectric test effectiveness and transformer reliability shall be analysed (see ToR scope - point 8). The proposed
method gives all the information that is necessary for this purpose.
2.2.3 Approach B: Evaluation of Oil Resistivity Measurement Methods (IEC 60247 and 61620)
In this section a brief study of two different IEC standard methods for measurement of oil conductivity is performed.
The two standards are IEC 60247 and IEC 61620, both dedicated to the task of measuring conductivity in dielectric
liquids. The majority of the information with regards to the standards and their procedures and backgrounds are
drawn from the standards themselves. Descriptions of the methods are focused on use on transformer oil.
At first, the purpose of the measuring method is described: In the standard IEC 60247, it is stated that “the methods
are primarily intended for making reference tests on unused liquids. They can also be applied to liquids in service in
transformers, cables and other electrical apparatus.” In addition to that it is stated that “permittivity, tan  and
resistivity, either separately or together, are important indicators of the intrinsic quality and degree of contamination
of an insulating fluid. These parameters may be used to interpret the deviation from desired dielectric
characteristics and the potential influence on performance of equipment in which the fluid is used.”

Page 27
For the standard IEC 61620 it is said that “this International Standard describes a method for the simultaneous
measurement of conductance G and capacitance C enabling the calculation of the dielectric dissipation factor tan 
of insulating liquids. The proposed method applies both to unused insulating liquids and insulating liquids in service
in transformers and in other electrical equipment. The standard is no substitute for IEC 60247; rather it
complements it insofar as it is particularly suited to highly insulating liquids and it recommends a method of
measurement for these liquids. This method allows values of the dielectric dissipation factor as low as 10–6
at
power frequency to be determined with certainty. Moreover, the range of measurements of tan  lies between 10–6
and 1 and can be extended up to 200 in particular conditions”.
Based on that it is clear that several methods that to some extent are similar are available, and that additional
methods (IEC 61620) have been developed in order to address difficulties experienced with previous methods (IEC
60247).
To summarise it can be stated that the international standards for oil conductivity measurement have been
developed with the purpose of evaluating dielectric liquids from a quality point of view, where for example
repeatability is an important feature as well as simplicity of the method. Experiences gained from developing the
standards are described in the sections further on.
Method
In both IEC 60247 and IEC 61620 measurements can be performed in similar geometries. The measuring cells are
vaguely described in IEC 60247 and IEC 61620 are based on those described in IEC 60247 . In principle,
measuring cells could take on many different forms and still be within the requirements of the standards. One
example is the oil gap length in the cell which could vary. Examples of typical measuring cells are readily given in
the standards.
One possible measurement setup described in IEC 60247 (Annex C, Figure 5) is shown in Figure 2.2-6.
Figure 2.2-6: Description of one measurement setup in IEC 60247
The differences among IEC 60247 and IEC 61620 mainly display in the voltage applied to a sample, resulting in
different electrical stresses within the sample during a resistivity measurement.
In IEC 60247 an electrical stress of 0.25 kV/mm is recommended. This, combined with the typical cell designs
shown in the standard, gives rise to a typical test voltage of 500-1000 V. The maximum voltage of the standard is
2000 V. The time of application of voltage (and thereby measurement time) is required to be 60 s.
For resistivity measurements, IEC 61620 differs from IEC 60247 mainly in the voltage level applied to the sample
and thereby the electrical stress in the sample. In IEC 61620 a voltage of 10 V to 100 V is recommended leading to
an electrical stress in the sample of less than 0.1 kV/mm. The motivation for selecting low voltages is given in the
coming sections. The time of application of voltage (and thereby measurement time) is required to be 0.45 – 5 s.

Page 28
Effectiveness, Experiences and Advantages/Disadvantages
The intention of the described measurement methods IEC 60247 and IEC 61620 is to be able to characterise
liquids with a measurement method that produces consistent results and is simple of perform. The methods
described are widely used and commercially available apparatuses are readily supporting the standards.
If the different methods are studied individually, some input can be obtained directly from the standards
themselves, summarising the experience of the use of the methods. In IEC 60247, it is stated that
“the conventional resistivity as measured by this standard is generally not the true resistivity. Application of a
d.c. voltage will change the initial characteristics of the liquid with time, due to charge migration. The true
resistivity can only be obtained at low voltage, immediately after application of the voltage. This standard uses
a relatively high voltage for an extended time and the result will generally be different from that from IEC
61620. Measurements of resistivity of liquids to this standard, depends on a number of test conditions,
namely:
….
b) Magnitude of the electrical stress
The resistivity of a given specimen may be influenced by the applied stress. For results to be comparable,
measurements shall be made with approximately equal voltage gradients and with the same polarity. The
gradients and the polarity shall be noted.
c) Time of electrification
Upon the application of d.c. voltage, the current flow through the specimen decreases due to the sweep of
charge carriers to the electrodes. The conventional arbitrary time of electrification is 1 min. Variation in the
time of electrification can result in appreciable variation in the test results. (Some high viscosity fluids may
require considerably longer electrification time … .)”
Shortly it can be said that flaws with regards to the standard IEC 60247 have been identified for high resistivity
liquids, and that was also the background to the creation of the standard IEC 61620. The basic problems of IEC
60247 are that it lacks repeatability and consistency within the standard mainly due to the high electrical-stress
levels in the samples and the low level of standardisation of the measurement cells (geometry). The relatively high
voltage and electrical stress combined with freedom of geometry gives rise to a situation where the same samples
measured in different setups within the standard produce different apparent resistivity values. When applying
different geometries (mainly oil gap length) allowed by the standard, combined with the requirement of 0.25 kV/mm
in electrical stress, the conduction mechanisms in transformer oil are affected, and measured current (and thereby
the deduced resistivity) will be different. Further information to why this is the case, can be gained below in the
section devoted to the physical background of the standards.
IEC 61620 has devised a test method that addresses the flaws of IEC 60247 by measuring at lower voltages and
electrical stresses. The result of this is that the method becomes geometry independent and the true material
property can be measured regardless of what measuring cell is applied. Use of a lower voltage also typically
simplifies performing the measurement from a point of view of practical handling.
Background
The background to the method in IEC 60247 is engineering based. In principle it advises to apply a voltage across
a material sample and to measure the current, whereas the resistivity can be calculated. As can be seen below,
some major physical phenomena which make interpretation of the test result more complex are ignored.
A relatively detailed physical background to the phenomena affecting conduction in dielectric liquids is given in IEC
61620. The connection between the physical background and the formulation of the method in IEC 61620 can be
readily made based on this. It is stated in the standard that the equilibrium of the liquid is not disturbed if applied
voltage and electrical stress are sufficiently small. In addition, several physical phenomena such as field-enhanced
dissociation, charge injection and EHD are briefly described and the general conclusion is that the influence of
these on the characterising measurements of the material itself can be eliminated if the electrical stress and
voltage is small (< 0.1 kV/mm and hundreds of volts). The information is given in Annex C of IEC 61620.

Page 29
Conclusions
It is desirable that a method for measuring resistivity of transformer oil should be easy to use, reproducible and
consistent. The same sample measured in different setups allowed by the standard should produce the same
result. IEC 60247 does not fulfil that relatively simple requirement for highly resistive transformer oils. That said, the
methods described in this particular standard can still be useful for comparing samples evaluated in exactly the
same setup or for evaluating liquids with lower resistivity than new transformer oil. IEC 61620 on the other hand,
addresses the challenges identified for transformer oil and recommends a robust and consistent method while at
the same time retaining the flexibility of the build up of the measurement setup given in IEC 60247.
2.2.4 Approach C: On-site Determination of Oil Conductivity by Dielectric Response Analysis
This section describes the on-site determination of oil conductivity by dielectric response analysis. Dielectric
response analysis measures dielectric properties of insulation systems in the frequency domain or in the time
domain over a very wide frequency or time range and calculates condition variables like oil conductivity and solid
insulation conductivity by use of mathematical modeling. It is applied as a non-intrusive on-site technique for
periodical assessments of the aging condition of insulation systems, particularly power transformers.
A dielectric response measurement is a three terminal measurement that includes the output voltage, the sensed
current and a guard. The guarding technique ensures an undisturbed measurement even at onsite conditions with
dirty outdoor insulators and electromagnetic interferences. For two winding transformers, after disconnection from
the network, the voltage output of the instrument is connected to the HV winding, the current input to the LV wind-
ing and the guard to the tank. The test can be performed in time domain by applying a DC voltage, the correspond-
ing technique is called Polarisation and Depolarisation Current Analysis (PDC Analysis), and in frequency domain
by applying an AC voltage leading to Frequency Domain Spectroscopy FDS or Dielectric Frequency Response
DFR. Both test techniques reflect the same fundamental polarisation and conduction mechanisms and can be
combined. They were developed for dielectric diagnoses on high-voltage equipment [39], [40].
For a time domain test (PDC), most interesting information is obtained for longer measuring times up to a few
thousand seconds. The pressboard barriers are charged via the oil ducts after voltage step application. Therefore,
the initial current value and the main time constant of the current curves are directly related to the conductivity of oil
in the main ducts. The polarisation and depolarisation current and the difference between polarisation and de-
polarisation curves at longer times are directly related to the conductivity of the paper/pressboard part of the insu-
lation system and its water content, Figure 2.2-7 (left).
Figure 2.2-7: Main features of PDC and FDS response in an oil-paper barrier system [40]
In the frequency domain (FDS, DFR), the dissipation factor plotted vs. frequency shows a typical s-shaped curve.
With increasing oil conductivity, moisture content, temperature or aging the curve shifts towards higher frequencies.
The middle part of the curve with the steep gradient reflects oil conductivity. Insulation geometry conditions
0.1
10
100
1000
1 10 100 1000 10000
Time (s)
Current(nA)
1
low
high
low
oil
conductivity
moisture of
cellulose
and aging
highIdep
Ipol
insulation
geometry
0.001
0.01
0.1
1
10
0.0001 0.001 0.01 0.1 1 10 100 1000
Frequency (Hz)
Dissipationfactor
high
low
high
high
low
low
moistureof
cellulose
andaging
insulation
geometry
oil
conductivity
moisture of
cellulose,
aging

Page 30
determine the "hump" left of the steep gradient. Moisture influences the low and the high frequency parts, Figure
2.2-7 (right).
Oil conductivity strongly influences the dielectric response and thus can be calculated by means of mathematical
modelling. Today the so-called XY-model, as proposed by various CIGRÉ working groups, is commonly accepted,
[39], [40]. It represents the volumetric fraction of two materials in one insulation system. In the cylinder-shaped
core-type transformer insulation all pressboard barriers and insulation paper are transformed into one single barrier
with relative thickness X. All spacing strips of the cylindrical insulation form one with width Y. All the oil ducts form
one oil duct of relative thickness 1-X and width 1-Y. Based on this model, the complex permittivity ε(ω) of a multi-
layer insulation can be calculated and the oil conductivity can be estimated from the dielectric response by auto-
matic curve fitting [41].
Calculating the oil conductivity of power transformers from the dielectric response provides the following
advantages:
1. The test result (oil conductivity) is related to the oil between two windings.
2. The test can be performed on site without the need of taking an oil sample.
3. Interferences due to sampling are excluded (contamination, transportation).
4. Influences of the laboratory test are excluded (sample handling, influence of the test cell material).
5. The test result is repeatable with diagnostic equipment from various vendors.
The method implies the following disadvantages:
1. The electrical stress in the oil is only a few volts per millimeter; voltage-dependent characteristics cannot be
investigated.
2. The dielectric response allows for calculation of a single oil-conductivity value only, thus time-dependent effects
cannot be observed.
3. Information about transformer insulation geometry (X, Y) is required.
2.3 Round Robin Test Results (RRT 1)
2.3.1 Aims of RRT 1
The first round robin test (RRT 1) of JWG A2/D1.41 was performed to compare the different existing methods
which are used in various international laboratories to determine the conductivity of mineral oil. Sixteen laboratories
participated in RRT 1 worldwide, using two types of unused inhibited mineral oil which are differentiated by the
sequential letters A and B: Oil A and Oil B. Table 2.3-1 gives an overview of the different applied methods,
electrical stresses, temperatures and times of electrification.
The following methods were used during RRT 1. For frequency domain measurements, conductivity was calculated
from complex permittivity or dissipation factor:
(1) Polarisation/Depolarisation Current (PDC) analysis
The measurement and evaluation of the dielectric response is one possible way to diagnose the condition of an
insulation system. Polarisation and Depolarisation Current (PDC) testing is a non-destructive dielectric testing
method to determine the conductivity of insulations and to get information about water content and ageing of the
dielectric material. This diagnostic technique is based on a time domain measurement. After a DC step voltage
application, the PDC analysis can provide information about the insulations conductivity within the initial periods
(some seconds).

Page 31
(2) Very low frequency (VLF) method (0.01 to 0.1 Hz)
As long as the frequency of an alternating voltage is kept low enough (≤ 0.1 Hz) the applied voltage can be seen as
pulsating direct voltage, because dc phenomena (e.g. space charges) also occur at this state. This method is used
to measure insulation losses, i.e. the insulation dissipation factor or tan at different frequencies. Usually the range
of frequencies is between 0.01 and 0.1 Hz.
(3) Frequency Domain Spectroscopy (FDS)
The Frequency Domain Spectroscopy (FDS) generalises the conventional measurement of tan and complex ca-
pacitance or permittivity at different frequencies, traditionally performed at power frequency. The measurement of
the dielectric dissipation factor is stated to be independent of the geometry of insulation systems, assumed that the
insulation structure is homogeneous and isotropic.
(4) Square wave method (0.5 Hz)
According to IEC 61620 the parameters tan , capacitance and resistance of an insulating material can be
determined corresponding to the displacement current and conduction current, resulting from the application of a
square wave voltage. The conductivity can be calculated out of the measured parameters.
Electrical stress in V/mm
0.5 1.33 2.5 3 10 20 50 100 250 300 1k 3k 6k 7k 10k
Temperaturein°C
25 FDS (1)
3600 s for whole
spectrum
(1 mHz to 1 kHz)
AC 0.01 Hz
200 s for
whole
spectrum
(100 Hz to
0.01 Hz)
Rect.
0.5 Hz
30 s
FDS (2)
240 s for
whole
spectrum
(0.01 Hz to
100 Hz)
1000 s
Rect.
0.5 Hz
3600 s
Rect.
0.5 Hz
60 s
3600s
Rect. 20 s 1000 s
10800 s
86400 s
60 s (6)
86400 s
1000 s
10800 s
AC 50 Hz
60 s (3)
1000 s
10800 s
86400 s
1000 s
10800 s
1000 s
10800 s
AC 0.1 Hz
AC
0.01 Hz
10800 s
30 60 s
40 FDS (1)
50 FDS (2) Rect.
0.5 Hz
60 s
60 s
55 FDS (1)
60 86400 s 86400 s 86400 s
70 FDS (1) 60 s
75 FDS (2) Rect.
0.5 Hz
60 s
90 FDS (1) AC 0.01 Hz
200 s for
whole
spectrum
(100 Hz to
0.01 Hz)
FDS (2) 1000 s
Rect.
0.5 Hz
3600 s
Rect.
0.5 Hz 60
s
1000 s
10800 s
86400 s
60 s (6)
86400 s
10800 s AC 50 Hz
60 s (3)
1000 s
10800 s
86400 s
1000 s
10800 s
1000 s
10800 s
AC 0.1Hz
AC
0.01 Hz
10800 s
Table 2.3-1: Overview of the different used methods for determining the apparent conductivity of
mineral oil in RRT 1. The measuring time is given in seconds in the case of PDC measurements,
different methods are labelled accordingly

Page 32
2.3.2 Measurement Results
The conductivity measurement results were categorised in four different electrical-stress ranges and two tempera-
ture levels at different measuring times (from seconds to several hours):
0 - < 0.1 kV/mm at 25 or 90 °C
0.1 - < 1 kV/mm at 25 or 90 °C
1 - < 4 kV/mm at 25 or 90 °C
> 4 kV/mm at 25 or 90 °C
Figure 2.3-1 exemplarily shows the results for Oil A measured with different methods at 0.1 to < 1 kV/mm at 25 °C
and 90 °C respectively. Figure 2.3-2 and Figure 2.3-3 show the results for Oil B measured with different methods at
different electrical stresses and at 25 °C. If available, the water content is given in ppm to the according measure-
ments.
Figure 2.3-1: Apparent conductivity of Oil A measured with different methods at 0.1 to <1 kV/mm
at 25° C (left) and at 90 °C (right) with information about water content (if available)
Figure 2.3-2: Apparent conductivity of Oil B measured with different methods at 25° C and
at <0.1 kV/mm (left) and at 0.1 to <1 kV/mm (right) with information about water content
(if available)
4.6
3.4
3.4
1.6
1.6
13.8
4
13.1
1.6
1.6
7
8
8
2
1E‐16
1E‐15
1E‐14
1E‐13
1E‐12
1E‐11
1E‐10
1 10 100 1000 10000 100000
σappin S/m
t in s
Conductivity of Oil A at 25°C, 0.1 to <1kV/mm
water content is given next to each symbol in ppm
DC
Rect.
Median
4.6
3.4
3.4
1.6
1.6
13.8
4
9
13.1
1.6
1.6
6.3
8.3
2
1E‐16
1E‐15
1E‐14
1E‐13
1E‐12
1E‐11
1E‐10
1 10 100 1000 10000 100000
σappin S/m
t in s
Conductivity of Oil A at 90°C, 0.1 to <1kV/mm
DC
Rect.
Median
99 9 9
9
2
1E‐16
1E‐15
1E‐14
1E‐13
1E‐12
1E‐11
1E‐10
1 10 100 1000 10000 100000
σappin S/m
t in s
Conductivity of Oil B at 25°C, 0 to <0.1kV/mm
DC AC LF
FDS Rect.
Median
6.5
4.7
1.9
1.9
1.9
1.9
14.1
8 8
11.2
1.9
1.9
14.3
12.4
10
2.2
1E‐16
1E‐15
1E‐14
1E‐13
1E‐12
1E‐11
1E‐10
1 10 100 1000 10000 100000
σappin S/m
t in s
Conductivity of Oil B at 25°C, 0.1 to <1kV/mm
DC
Rect.
Median

Page 33
Figure 2.3-3: Apparent conductivity of Oil B measured with different methods at 25° C and
at 1 to <4 kV/mm (left) and at >4 kV/mm (right) with information about water content (if available)
2.3.3 Conclusions of RRT 1
In general, a very high spread between the methods was observed.
It was assumed that oil conductivity measurement results are related to measurement parameters such as electri-
cal stresses, time of electrification or water content. At elevated temperatures, higher conductivities were found
than at room temperature and the spread was smaller at 90 °C compared to the measurements at room tempera-
ture.
Note: Mineral oil is a natural product and can have small variations between oil batches of the same type of oil.
However, the observed spread in the measurement results cannot be attributed to such huge differences of up to
three orders of magnitude.
As the influence of the measurement parameters could not be quantified within RRT 1, another test was suggest-
ed. In order to find a common concept and to harmonise the different measuring methods, measurement parame-
ters and procedures had to be defined accurately. To carry out another expedient round robin test (RRT 2), the
procedures for the cleaning of the measurement cell and for the measurement itself were defined according to
Chapter 3 and Annex A.
5.2
2.9
1.9
11.2
1.9
17.4
15.9
1E‐16
1E‐15
1E‐14
1E‐13
1E‐12
1E‐11
1E‐10
1 10 100 1000 10000 100000
σappin S/m
t in s
Conductivity of Oil B at 25°C, 1 to <4kV/mm
DC
Median
1.9
1.9
1E‐16
1E‐15
1E‐14
1E‐13
1E‐12
1E‐11
1E‐10
1 10 100 1000 10000 100000
σappin S/m
t in s
Conductivity of Oil B at 25°C, >4kV/mm
DC
AC LF
Median

HVDC TRANSFORMER INSULATION: OIL CONDUCTIVITY

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