CONDITIONS IN POWER TRANSFORMERS:
                            A VIEW BASED ON PRACTICAL EXPERIENCE
                         V. Sokolov                                          Z. Berler, V. Rashkes
                    ZTZ-Service, Ukraine                            Cutler-Hammer Predictive Diagnostics, USA
                                                                 5421 Feltl Road, Suite 190, Minnetonka, MN 55343

Abstract: Condition-based monitoring of power transformer             The following trends can be extracted from the statistics available:
insulation should center on the prediction of a substantial drop      •    Over 70% of transformer failures, particularly in 1996-97,
in the dielectric safety margin under the impact of moisture, oil          have occurred after 20 years of service due to aging diseases.
by-products, contaminating particles, paper insulation aging          •    Transformer life is limited predominantly by accelerated
and partial discharge activity. A functional failure model of              deterioration of two components: bushings and LTC (over
power transformer insulation and possible effective methods of             50% of failures in both surveys).
the insulation condition assessment are discussed based on            •    A stable high rate (15-20%) of failures is attributed to the
practical experience.                                                      impairment of the conditions of major and minor insulation
                                                                           due to a particle contamination or the ingress of moisture
INTRODUCTION                                                               reducing the impulse withstands strength. It’s worth
During recent years the technical policy of power utilities is             emphasizing here that most of the problems have been
changing under the pressure of economic considerations. There is a         associated with the degradation of HV winding insulation that
common tendency of moving from the time-based to the condition-            is typically more sensitive to the deterioration.
based maintenance to reduce maintenance costs, to extend              •    Decomposition of cellulose can be attributed only to a low
significantly the life span of equipment and, at the same time, to         portion of failures (3-5 %) caused by an excessive aging. A
prevent possible catastrophic failures. Thorough knowledge of              deficiency of old designs is basically involved (bad cooling,
actual condition of the transformer and effective diagnostic               overinsulated coils, non-uniform current distribution in the
methods are necessary to meet these requirements.                          conductors, unforeseen stray losses, etc.), [4].
                                                                      •    Mechanical weakness and winding distortion are the cause of
This paper presents some considerations on condition-based                 10-15% of failures.
monitoring of power transformer insulation centered on a              •    Recurrent cases with localized overheating in the magnetic
prediction of a substantial drop in the dielectric safety margin           circuit due to the deficiency of old designs appear. Some of
under impact of such practically inevitable agents of degradation,         the defects do not affect directly the transformer
as moisture, particles, oil aging products and partial discharge           serviceability, but cause gassing and contamination of
activity. The paper discusses typical defects and the functional           insulation system with by-products.
failure model of HV transformer insulation as well as effective       Thus, dielectric and mechanical weaknesses seem to be the main
methods of detection and identification of its defects.               life shortening factors for transformers. The degradation of the
                                                                      dielectric safety margin appears as a really vital problem in large
CRITICAL DISEASES OF AGED TRANSFORMERS                                power transformers. Approximately 80% of transformer failures
There is a large population of aged equipment approaching the end     could be predicted and prevented if an effective diagnostics system
of the original design life. Questions arise: What’s happening with   is used. It is important to note that most of defects caused failures
aged transformers? What kind of problems may they have? What          are of reversible mode and they could have been not only detected
are the critical aging diseases?                                      but also corrected in the field.

The best means to answer these questions is the failure analysis.     DEFECTS IN COMPOSITE INSULATION SYSTEMS
Such analyses based on periodic reliability surveys were performed
by ZTZ-Service Co (the population of about 5000 units, above 100      Major Insulation
MVA, 110-750 kV), by the CIGRE WG12.18, and by Doble                  •   Excessive moisture in the cellulose insulation. This defect is
Engineering which distributed Technical Questionnaires in 1994-           inherent basically to the transformers with open-breathing
1998 the results of which have been partially published [1-3]. The        preservation system or to those which have an insufficient
comparison of the statistical data from the ZTZ-Service and Doble         sealing. Distribution of the moisture in the course of the
Engineering investigations is given in Table 1 based on [3].              transformer life is kept quite non-uniform. Most of the water
                                                                          is stored in so called “cold thin structures”, namely in the thin
        Table 1. The causes of power transformer failures                 pressboard barriers that operate at bulk oil temperature [5, 6].
                  in 1996-1997 (above 100 MVA)                        •   Oil contamination with water, particles and oil aging
Defective element                        Rate, %                          products. The most problems caused by these defects occur
                             Doble clients      ZTZ-Service clients       typically in the space “HV winding (HV bushing) –Tank”.
Bushings                     35                  45
                                                                      •   Insulation surface contamination in the forms of the
LTC                          16                  9                        adsorption of oil aging products on a cellulose surface or
Major insulation             9                   17                       deposits of conducting particles and insoluble aging products
Winding (turn, coil) aging   16                  12                       in areas of high electrical stresses. The surface contamination
Winding distortion           12                  10                       can cause a distortion of electrical field and a reduction in the
Core                         7                   7                        impulse strength of the insulation system.
Leads                        5                   -                    •   Partial discharges in weakened insulation spots.
The presence of water and impurities changes dielectric parameters      This makes it difficult to detect a defective condition of a
of deteriorated components, namely, their conductivity,                 winding’s minor insulation until a critical partial discharge or
permittivity and dissipation factor, particularly with temperature.     noticeable gas generation occurs.
The temperature effect of water on insulation characteristics of
cellulose and oil, as well the temperature effect of contaminated       FUNCTIONAL FAILURE MODEL
oil, are well known, e.g.[7]. Changes in the dielectric parameters of   A failure model has to be based on possible defects in the
the defective component(s) result in related changes in the             particular transformer component and a possible scenario of
dielectric characteristics of the whole transformer integrity, and      progression of the defective condition into a failure (the breakdown
requires further investigation to detect the “guilty” component [8].    or flashover). Table 2 summarizes typical problems with
                                                                        transformer insulation.
Defects related to excessive moisture, oil contamination or surface
contamination usually fall into the category of reversible defects.         Table 2. Typical defects and developing faults in the
The damage created by partial discharge activities is usually            components of the dielectric system of power transformers
irreversible. This type of damage usually results in carbonized         Component       Defect              Fault/Failure mode
tracks (creeping trees) that extend between the electrodes along the
                                                                        Major insulation     Excessive water          Destructive PDs
surface.                                                                                     Oil contamination        Localized tracking
                                                                                             Surface contamina-       Creeping discharge
Creeping Discharges                                                                          tion                                lead to:
Creeping discharge is likely the most dangerous failure mode that                            Abnormally aged oil      Breakdown or flashover
typically results in catastrophic failures at normal operating                               Low energy PDs
conditions. The phenomenon occurs in the composite oil-barrier                               Static electrification
insulation and progresses in four steps:                                                     discharges
                                                                                             Winding distortion
•    Partial breakdown of a gap, typically between the winding and
     the nearest pressboard barrier.                                    Minor insulation     Same as for the          Destructive PDs
•    Surface discharge in oil across the barrier (an appearance of a                         major insulation +       Localized tracking
     black carbonized mark on the barrier).                                                  Abnormally                           lead to:
•    Forcing oil and water out of the pressboard pores in the                                aged cellulose           Flashover or short-
     vicinity of sliding discharge that results in microscopic                               Bubble evolution         circuits between turns
     sparking within the pressboard. The presence of some
     excessive moisture stimulates this process.                        Lead insulation       Excessive water         Destructive PDs
                                                                                              Overheating             Gassing/bubble evolution
•    Splitting oil molecules under effect of sparking; the formation
                                                                                              Abnormally aged                     lead to:
     of hydrocarbons followed with the formation of carbonized                                Mechanical destruc-     Major insulation failure
     traces in the pressboard. This process can continue from                                 tion                    or circuit opening
     minutes to months or even years, until the treeing conductive                            Displacement
     path cause shunting of an essential part in the transformer        Electrostatic shields Mechanical disrup-      Destructive PDs
     insulation resulting in a powerful arc.                                                  tion or displace-       Gassing/bubble evolution
                                                                                              ment                               lead to:
Faulty conditions can be characterized in terms of PD activity,                               Low energy PDs          Major insulation failure
fault gas generation, and changes in the conductance, capacitance
and dielectric loss factor of a defective area. The critical level of
relevant diagnostic parameters may be evaluated using                   Typical Scenarios of Developing Insulation Failure
experimental data from [9, 10]:                                         The following typical scenarios of an insulation failure model are
     •    Oil destruction PD intensity q >300 pC;                       presented on the basis of historical cases analysis (summary of
          PD power P< 0.4 W;                                            over 200 failures):
          rate of gas generation 5-170 μlitres/Joule (different for
                                                    different oils).    •    Critical contamination of oil (typically presence of free
     •    Oil-breakdown PD intensity-q>100000 pC.                            water) + rapid change of temperature     PD appearance at
     •    Cellulose destruction (creeping discharge)                         rated voltage     breakdown.
          PD intensity q>100 –1000 pC;
          PD power P=0.1-1 W;                                           •    Surface contamination + water + rapid change of
          rate of gas generation 40-45 μl/J.
                                                                             temperature   PD appearance   flashover.

Minor (Turn & Coils) Insulation                                         •    Particles contamination + switching surge                  critical PD
•  Overheating leading to accelerated insulation aging.
•  Excessive moisture content leading to bubbles appearing at                    breakdown.
   some zones with elevated temperatures.
•  Insulation surface contamination with conducting particles           •    Water + particles contamination (or bubbles present in oil)
   and oil aging products.                                                       critical PD      Creeping discharge progressing
Above mentioned defects may cause a sharp change in electric
parameters of the minor insulation, e.g.[11]. However, it has only a    •    Surface contamination + lightning impulse                         Surface
minor impact on overall dielectric characteristics of the whole              discharge    Flashover.
transformer, due to relatively high capacitance of turn insulation.
•    Distortion of winding geometry                PD appearance           •    Levels of water content: the condition of a transformer,
         Creeping discharge progressing           Breakdown.                    which may result in an increase of the relative saturation of
                                                                                water in oil over 50% in the range of operating temperatures.
                                                                                Total water content is considered.
•    Distortion of winding geometry + Switching surge
         Flashover between coils (sometimes with restoring                 •    Water in solid insulation: the compliance with the above
     withstand strength)  Gas evolution.                                        stated level of water content. Corresponding water content in
                                                                                the barriers exceeds 1.5-2% (the estimation is made through
                                                                                dielectric characteristics).
Objectives of the Diagnostic Technique
Three main diagnostics objectives can be formulated on the basis           •    Particles in oil: the level of contamination is as per NAS
of the failure analysis:                                                        class 7 and higher; the presence of visible and conducting
•    Detection and identification of defective conditions caused by             (metals, carbon) particles.
     an accumulation of degradation agents.
•    Detection and identification of an irreversible damage to the         •    Oil aging:
     insulation (critical PD and creeping discharge).                           •    The appearance of a sludge in the period between the
•    Detection of a pre-failure state of PD activity (prior to the                   tests.
     breakdown).                                                                •    The end of the induction period (trend of accelerated
INDICATORS OF DANGEROUS INSULATION                                              •    The presence of acids and non-acid polars that accelerate
CONDITION                                                                            cellulose decomposition.
A traditional approach to the identification of critical transformer
conditions is based generally on the establishment of some value           •    Presence of bubbles in oil: symptoms of possible bubbles
for each tested, agreed-upon diagnostic parameter: fault gas                    evolution, including C2H2 generation due to high temperatures
concentration, water-in oil content, insulation power-factor, etc.              (>8000C) when the bubbles evolution is practically an
However, they are rather symptoms than characteristics of the                   inevitable phenomenon.
defective condition to be evaluated. A defective condition defines a
threat to the equipment serviceability; however, sometimes there is        •    Partial discharges: presence of PDs with q> 100-500 pC in
no any direct correlation between particular test results and the               the oil-barrier structure; symptoms of PDs revealed through
defective condition. The effective diagnostics system for                       DGA analysis.
transformer insulation has rather to reveal the threat of a critical
reduction in the dielectric safety margin of insulation.                   •    Mechanical integrity: a radial mode winding distortion that
                                                                                may change the insulation geometry. The assessment is made
Studying the models of the transformer insulation [1, 12, 13] has               through the relevant change of the leakage reactance.
shown that the dielectric safety margin of both major and minor
insulation contaminated with water is still determined by the              •    Insulation surface contamination: the symptoms assessment
dielectric withstand strength of the oil. The dangerous effect of the           is made through the change of insulation dielectric
dissolved water is determined by a sharp reduction in oil dielectric            characteristics with temperature.
strength with increase in its relative saturation, due to increase in
the conductivity of the particles available or emulsion formation in
the vicinity of a surface-active substance.                                EFFECTIVE METHODS TO ASSESS THE
For example, 30 ppm of dissolved water in oil can be a problem at
                                                                           INSULATION CONDITIONS
20oC (relative saturation ~70%) but does not affect the dielectric
strength of oil at temperature >40oC (relative saturation <30%). A         Oil Tests
low particles content improves the situation. On the other hand,           Practical experience has shown that more than 60% of the latent
polar oil-aging products may store a significant amount of water           defects have been revealed through oil tests [2]. However, the
that can transfer in a dissolved state at some elevated temperature        variations in different oil characteristics make it difficult to identify
and make the situation worse. One may emphasize that the modern            the type of the problem revealed. We have found that diagnostic
Karl Fisher method measures practically only dissolved water,              effectiveness for oil parameters could be improved by separating of
leaving the full amount of water unknown.                                  oil tests into four groups:
                                                                           •    Identification – parameters that specify the oil and remain
The evaluation of the permissible dielectric state of oil needs to be           practically unchanged during its life.
further studied. The following factors must be considered: the level       •    Aging status - parameters relevant to the aging process.
of particle contamination; presence of polar and surface-active            •    Dielectric status – parameters affecting the dielectric safety
products; value of interfacial tension of oil; limitation of percent oil        margin (water, particles, etc.);
saturation and water content in paper.                                     •    Diagnostic tests – utilization of oil as a diagnostic medium.

Based on its wide experience in designing, testing, maintenance            A test program and the protocol “Dielectric Status” includes
and refurbishing of power transformers 110-750 kV, ZTZ-Service             determining water content (before and after transformer heating);
established the following criteria for the evaluation of a dangerous       particles counting; particle identification (with a microscope);
insulation state:
measuring the breakdown voltage; measuring oil power-factors at       determined water contamination. In most of the cases the defective
20,     70,   and    900C;   DGA       (symptoms     of    PD);       condition was confirmed by the following estimation of water
resistivity measurements at 20, 70, 900C; polarization index          content through dielectric characteristics tests [11] or by the direct
measurement; IR-scanning (non-acid polars).                           measurement of water content in pressboard samples removed
                                                                      from the transformers.
The most effective diagnostic components are by-products related
exclusively to the degradation process – DGA, furans, dissolved       Fig.1 demonstrates the case of 180 MVA, 220/18 kV, GSU
metals, et al. DGA is indisputably the best detector of               transformer, with open-breathing preservation system after 18
abnormalities. However, there are still a number of unsolved          years in operation. During its Water Heat-Run Test the build-up
problems:                                                             water content in oil practically followed the rise in temperature.
•    Difference in the rate of gas generation in different oils.      The presence of free water in the transformer was determined and
•    Migration of gases between the oil and cellulose.                later confirmed by additional tests.
•    Unusual sources of gas generation.
                                                                                T oC,
•    Location of the source of gas generation.
One can expect some new benefits from advances in the DGA                                                                                      W ppm
technique, namely:                                                                                                                              %
                                                                                                 to C                    switching
•    Detection of low temperature faults (150-400 °C) using C3 –                                                         off
     C5 hydrocarbons, particularly, C4H8 buten-1.
•    Determining the temperature signature of overheated oil
     through C3-C5 hydrocarbons response.                                                                W
•    Determining the correlation between amount of gases and
     dissipated energy.

Water Heat-Run Test for In-Service Assessment of
the Level of Water Contamination
This method was described in [3, 14] .The objectives of the method
•    Assessment of the transformer health under rated conditions –
     the maximum permissible temperature.
   • Assessment of the level of water contamination using the
     build-up of water content in oil with time and temperature.
   • Assessment of possible state of water and distribution of
     water within a transformer using the rate of building-up of
     water in oil.
                                                                            Figure 1. An example of Water Heat-Run Test on an old
A loaded transformer is heated, by reducing its cooling, up to the            180 MVA, 220 kV GSU transformer (time in hours)
maximum possible temperature, to lower oil per cent saturation and            W-absolute water content, ppm, relative saturation,%
to obtain a “moisture potential” in the vicinity of insulation. The
test duration must allow “discharging” the insulation and building    Table 3 shows another case of the assessment of the level of water
up a significant amount of dissolved water in the oil. To detect      contamination in a 200 MVA, 347 kV, GSU transformer with an
water content over 1.5-2.0%, the temperature 60-75°C and test         open-breathing preservation system, after 27 years in operation.
time of 3 days has been recommended. However, practical               Following the heating of the transformer from 40 to 700C the water
experience has shown that water contamination over 2% or the          content in oil increased from 24 ppm to 44 and further to 49 ppm.
presence of free water can be assessed even using a one day test.     Water contamination > 2.5% in the thin structure was estimated.
                                                                      The further direct test on pressboard samples after draining the oil
Assuming that the main source of water contamination is a “thin”      shown a water content of 2.75 %. During drying process 36 kg of
structure of transformer insulation, the level of water content W     water was extracted
can be estimated using the equation
                                                                      Table 3. Water Heat-Run test of 200 MVA, 347 kV
                                                      ,               transformer
                                                                          time       t,   Woil               We       Estim.          Real
where W is the amount of water “discharged” from the solid                 hrs      °C    ppm      %         %         water          water
insulation into oil related to the mass of the thin structure;                                                      content in       content
We is the equilibrium moisture, which can be determined from the
absorption equation for given relative oil saturation;
                                                                          Initi     40    24.2     15        3.9                 In the 2-
[1-F(Z)] is the diffusion function [5].                                    al                                                    mm
                                                                           24       70    44.2     5.5       1.38   > 2.5%       PB-
In 1992 – 1998 the condition of over 150 transformers, rated 25 –                                 10.1        2.1   in 1000      2.75%.
1250 MVA, that came under suspicion was assessed using this                                                         kg of 2-     Extracted
“Water Heat-Run Test”. Fifty-three (53) units, predominantly of                                                     mm PB        36 kg
open-breathing design, were recognized as defective due to their           48       70    47.2    10.8        2.2                of
                                                                           72       70     49     11.2       2.24                water

Experience has shown a poor correlation between the water content             Table 4. Typical defects and diagnostic possibilities
in the oil and solid insulation at temperatures below 60-70 °C.                         for the main transformer insulation
Tests at 25-40 °C have typically led to the overestimation of the
moisture content in insulation. A special “two-steps” test at 50 and
70 °C was carried out on the 200 MVA, 347 kV, open-breathing
transformer after 27 years in service. After heating the unit from 30
to 50°C and maintaining for 24 hours, the water content increased
from 10 to 12.7 ppm only. However, a similar test at 70°C has
shown a significant rise in water up to 34 ppm. Contamination of
“thin structure” of over 2.5% was concluded.

Use of Insulation Characteristics to Assess the
Condition of Oil-Barrier Insulation
Practical experience has shown [2] that 30-35% of the problems
still can be detected through off-line tests only. Those are winding
distortions, insulation surface contamination, some bushing
problems, etc. The experience has also demonstrated that water
content in the pressboard barriers, the surface contamination level
and oil contamination can be effectively estimated using the
temperature responses of insulation power-factor and DC
insulation resistance for interwinding and winding-to-tank
insulating spaces, when taking into account the value of relative
portions of oil and solid insulation within the space. The              The model of the oil-barrier structure can be presented in
transformer insulation is a composite dielectric system, located        accordance with Figure 2,A [8]. The total current through the
between the electrodes, i.e., winding conductors, and grounded          model may be expressed as a sum of three components: the current
parts of the transformer. Dielectric measurements allow you to          through the solid insulation IP, the current through oil I0, the
determine the partial conductance of the dielectric system between
                                                                        current along the surface IS:
each accessible pair of electrodes. Sometimes the measured value
                                                                                                  I = Ip + I0 + Is
is equal to the conductance of the insulation zone between
electrodes. For instance, in the zone between the high-voltage
                                                                        It means that the full current through the composite insulation
(HV) winding (outer) and the tank, all the current from the HV
                                                                        space and any of the insulation characteristics derived from it
winding flows to ground. Sometimes the measured value is not
                                                                        depend not only on the solid insulation condition, but also on the
equal to the conductance of the insulation zone. For example, in
                                                                        conditions of the oil and the surface. Therefore, the sensitivity of
the interwinding space, in case of a severe contamination of the
                                                                        dielectric parameters to deterioration of the barrier depends on the
barrier surface, the portion of current between the HV winding and
                                                                        share of current flowing through the barrier and consequently on
the low-voltage (LV) winding flows down to the ground along the
                                                                        the relative amount of cellulose insulation in this space, i.e. on the
surface of the barrier, resulting in a decrease of the measured
                                                                        insulation structure design. Consequently all the insulation
dissipation factor.
                                                                        characteristics must be analyzed according to their
The most important components of the main transformer insulation
                                                                        This model allows us to determine the dissipation-factor at the
•    Insulation between the HV winding and the tank, including
                                                                        power frequency (or at any other frequency) as well as the dc
     the HV bushings;
                                                                        insulation resistance [8]. The third (surface) component is of
•    Insulation between the HV and the LV windings;
                                                                        practical importance only when surfaces of all the barriers are
•    Interphase insulation.
                                                                        severely contaminated. In the majority of practical cases another,
                                                                        simplified model is valid (Figure 2 B).
 These components usually have the smallest margins in the
dielectric strength, and, as a result, are the most sensitive to the
                                                                        The above approach allows us to estimate the condition of the oil
insulation deterioration. The monitoring of the solid and liquid
                                                                        within the space as well as the condition of the solid insulation
insulation in these components, i.e., the monitoring of their
                                                                        considering the relative amount of the solid insulation. For
dielectric characteristics, is a subject of great importance and one
                                                                        example, the dissipation-factor of the interwinding space at power
of the main objectives of transformer diagnostic tests. In other
                                                                        frequency can be expressed by the following simple equation:
areas of the insulation, specifically the insulation between the LV
                                                                                      tan HV-LV = Kp x tan P + K0 x tan 0
winding and the core, the margin of dielectric strength is usually
significantly higher than in the spaces that include HV winding.
Therefore, here only a high degree of deterioration can be the          The design parameters Kp and K0 can be evaluated using [7], but
cause for concern.                                                      typically K0 =0.4-0.6 and Kp =1- Ko=0.6-0.4.

The possible effectiveness in the detection of typical defects
through dielectric characteristics is characterized with Table 4.
                                                                     •     Evaluation of insulation surface contamination (including
                                                                           traces of creeping discharges) using temperature dependence
                                                                           of tan values for the interwinding space [8].

                                                                     On-Line Partial Discharge Measurements
                                                                     The recent progress in the PD measuring technique [15] has
                                                                     opened really new opportunities in an effective rejection of
                                                                     external interference, detecting weak PD signals and on-site
     IP    IS        IO               IP         IS            IO
                                                                     diagnosis of the condition of transformer insulation, quite similarly
                A                                     B
                                                                     to well established laboratory tests at the transformer factories. The
                                                                     practical experience with the application of PD Analyzers UPDA
Fig 2. The model of interwinding oil-barrier structure (A) &
                                                                     in 1998 has confirmed that this test technique provides the
simplified model of oil-barrier space “Winding-Tank” (B)
                                                                     sensitivity to PDs in field conditions of about 20 pC in power
                                                                     plants and 50 pC in the 500…750 kV substations.
Over 10 years of experience with the evaluation of insulation
                                                                     Particularly, the following problems have been identified in the
conditions through the measurement of tan and DC insulation
                                                                     transformers already tested:
resistances of relevant insulation spaces has shown that the
                                                                     •    Defective busbar isolators were found on 13.8- kV side of a
following problems can be successfully solved:
                                                                          GSU transformer;
                                                                     •    Source of PD was located in a 500-kV bushing of a 300 MVA
Estimation of the average water content (over 1-1.5%) in the
barriers using test data for interwinding space and the following
                                                                     •    The source of critical PD was detected in a 750 kV
                                                                           autotransformer (Figure 3). These PDs were caused by a
     •    Measure tan HV-LV at the elevated temperature.
                                                                           progressing creeping discharge across the 750 kV bushing
     •    Determine tan 0 of oil at the same temperature.                  insulation that was confirmed by internal inspection. The
     •    Define the design parameters Kp and K0 .                         problem was associated with water penetration through
     •    Calculate the value of the dissipation factor of the             loosed top sealing of the 750 kV lead. Internal inspection has
          pressboard tan P.                                                shown that a really catastrophic failure had been prevented.
     •    Define the water content using well-known dependence             The sketch of defects found at inspection is presented in
          for pressboards from [7].                                        Figure 4.

The examples of such water content estimation using the measured
dissipation-factor are presented in Table 5. A good correlation
between the estimated values and the results directly measured in
the samples of the pressboard, after draining the oil, has been

 Table 5. Examples of estimations of the water content in
barrier insulation through dissipation-factor measurements

                                                                         Fig 3. Results of PD measurements on the defective 750 kV
                                                                          transformer at two voltage levels (0.8-1 and 1.05 per-unit)

•   Estimation of Oil Contamination in Insulating Spaces.
    The experience has shown that in some cases the oil in the       Figure 5 demonstrates how effective was the UPDA in the rejection
    insulation spaces can be more contaminated than the oil in the   outside corona noise during PD measurements performed on 22
    sample taken from the bottom of the tank. The condition of       transformers and autotransformers rated 500 kV [15]. Another
    the oil can be better evaluated through the dissipation-factor   feature of this PD analyzer is the capability of analyzing the PD
    and dc insulation resistance in “HV-Tank” space, using the       signatures, particularly, the power dissipated in PD, and in this way
    difference in tan of the space “HV winding–Tank” measured        to utilize the diagnostic technique developed by CIGRE WG 15.01
    at two different temperatures (e.g. 60 and 300C) [8].            (TF “PD signatures”) [16, 17].
                                                                       the bushing insulation is used in many cases. The increase in the
                                                                       resistive or capacitive currents associated with the bushing
                                                                       insulation are signs of insulation deterioration: power-factor being
                                                                       more sensitive to the initial steps in insulation deterioration and the
                                                                       capacitance – to the developed defect. In the majority of cases the
                                                                       monitoring devices sum up the currents through bushing insulation
                                                                       of all three phases in the set [18, 19]. Initially the currents are
                                                                       balanced, so the output signal is close to zero. The increase in any
                                                                       one or two of the currents sets the balance off and produces an
                                                                       output signal proportional to the current increase. These devices
                                                                       are sensitive and are able to detect small increases in the current.
                                                                       The increase about 1-2% corresponds to a developing defect and to
                                                                       5-6%- to a critical defect.

                                                                       In Russia where simplified monitoring devices were widely used
                                                                       for about 30 years, they prevented about 75% of catastrophic
                                                                       failures in non-hermetically sealed bushings. The experience of
                                                                       similar monitoring hermetical bushings in the USA and Canada is
                                                                       more limited, but also is positive [18, 19].

                                                                       The method can be further improved if the dependence of the
                                                                       imbalanced current on the bushing temperature is taken into
                                                                       account. When all three bushings are in an identical insulation
  Figure 4. Sketch of the defects found in the 750-kV bushing          state, their reaction to the temperature rise or reduction is also
                                                                       identical and does not create an output signal from the device. But
                                                                       when the condition of the insulation in bushings are different their
                                                                       reaction on the temperature changes will be different as well. That
                                                                       makes the output signal dependent on the deviation of top-oil
                                                                       temperature from its value during the initial balancing. This
                                                                       dependence, together with the absolute value of the imbalance can
                                                                       be used for diagnostics. If the given imbalance is reached at the
                                                                       initial top-oil temperature, the state of the insulation is worse than
                                                                       if it is reached at a changed temperature.

                                                                       As an example, Figure 6 shows the dependence of the imbalance
                                                                       current    (current in per cent of its initial value) versus top-oil
                                                                       temperature observed during two years of monitoring on a set of
                Signals prior to the noise rejection                   138 kV bushings. With worsening the insulation state the absolute
                                                                       values of      rise, and the dependence (t) becomes somewhat
                                                                       steeper. Off-line tests confirmed worsening defects in two bushings
                                                                       of the group.

                Signals left after the noise rejection

  Figure 5. Plane projections of 3-dimensional distributions
  of pulse repetition rates obtained on a 500 kV transformer
             prior and after the noise cancellation

Monitoring Power Frequency Current through
Bushing Insulation
As oil-filled high voltage bushings are often the weakest insulating      Figure 6. Gamma versus top-oil temperature in a set of
component in transformers, their monitoring is of a special            138 kV bushings during three consecutive terms of observation
practical importance. Above-mentioned PD monitoring provided                     since September 1997 till February 1999
periodically is able to detect developing defects. Also the periodic
or permanent monitoring of the power frequency current through
Vibro-Acoustic Monitoring of Winding & Core
Clamping Force
                                                                        Condition-based monitoring of power transformer insulation has to
A reduction in clamping forces is frequently accompanied with
                                                                        be based on an extensive understanding of the processes of
winding distortions and short circuits between elements of the
                                                                        insulation deterioration. The diagnostics have to be centered on the
magnetic circuit. These defects can lead to PDs and combustible
                                                                        prediction of the substantial drop in the dielectric safety margin
gas generation, therefore monitoring the clamping forces can be of
                                                                        under impact of moisture, oil by-products, contaminating particles,
help in determining if the problem is associated with the
                                                                        partial discharge activity, tracking and creeping discharges, and
transformer insulation and in its localization. On-line vibro-
                                                                        paper aging. The functional failure model chosen highlights the
acoustic monitoring of the residual clamping forces in the core and
                                                                        most dangerous processes where attention has to be concentrated.
windings of a transformer was described in [15]. The method is
based on the measurement of the steady-state vibration at several
                                                                        The application of several, practically proven monitoring methods
points of the transformer tank and further analyzing the energy
                                                                        and multi-step test techniques provides the expert with relevant
distribution between different frequencies in this vibration. To
                                                                        interrelated information that increases the reliability of the
differentiate vibration from the windings and from the core, two
                                                                        diagnosis and allows corrective actions to be timely implemented.
load modes are used: the maximum possible load close to the rated
                                                                        In many cases corrective measures can be applied to the defective
one and no-load (or small load). The system is able to discriminate
                                                                        transformers; in other cases its catastrophic failure can be still
the separate portions attributable to the core or windings. The
                                                                        prevented, or plans made to minimize the effects of such
diagnosis is provided in the form of coefficients: when the
                                                                        catastrophic failures.
coefficient is between 1.0 and 0.9 the clamping force is good (60-
100% of the initial one), between 0.8 and 0.9 the clamping force is
satisfactory (20- 60% of the initial one), below 0.8 the condition is
critical (the clamping force is below 15-25% of the initial one).       REFERENCES
Figure 7 shows the diagnostic window of the computerized
diagnostics system during the test of a 345 kV step-up transformer.     1.   V. Sokolov "Experience with the Refurbishment and Life
The parts of the core and windings are colored: for three gradation          Extension of Large Power Transformers," Minutes of the
mentioned above colors are green, yellow and red, respectively.              Sixty-First Annual International Conference of Doble Clients,
                                                                             1994, Sec. 6-4.

                                                                        2.   V. Sokolov “Transformer Life Management Considerations”,
                                                                             Proceedings of the 1997 CIGRE Regional Meeting, 1997,
                                                                             Melbourne, Australia.

                                                                        3.   V. Sokolov “Consideration in Transformer Life Management
                                                                             - a View from Abroad”, Proceedings of the Techcon ‘99
                                                                             Annual Conference, TJ/H2b, February 18-19, 1999, New
                                                                             Orleans, LA.

                                                                        4.   P. Grestad, “Life Management of Transformers. Case Story”,
                                                                             Proceedings of the CIGRE SC12 Colloquium, 1997, Sydney.

                                                                        5.    V. Sokolov, B. Vanin “Experience with In-Field
                                                                             Assessment of Water Contamination of Large Power
                                                                             Transformers,” Proceedings of the EPRI Substation Equipment
                                                                             Diagnostic Conference VII, February 20-24, 1999, New
                                                                             Orleans, LA.
   Figure 7. The diagnosis of the core and winding residual
 clamping forces for a 345 kV, 880 MVA step-up transformer              6.   P.Griffin, V.Sokolov, B.Vanin “Consideration on Moisture
                                                                             Distribution in Transformers”, Proceedings of the 66th
                                                                             Annual International Conference of Doble Clients, April 12-
Table 6 demonstrates the effectiveness of the vibro-acoustic                 16,1999 Boston, MA.,
technology in the revealing internal defects in windings and cores.
Totally 290 power transformers rated 2-880 MVA (mainly over 50          7.   I. Gussenbauer "Examination of Humidity Distribution in
MVA), 10-500 kV were tested. 34 of them were found to have                   Transformer Models by Means of Dielectric Measurements",
problems in the core or windings. In more than 80% of cases the              CIGRE, 1980, # 15-02.
diagnosis was confirmed during an outage, and the compression
was restored.                                                           8.   V. Sokolov, B. Vanin. “Evaluation of Power Transformer
                                                                             Insulation     through     Measurement       of     Dielectric
Table 6. Results of vibro-acoustic method application to power               Characteristics”, Proceedings of the 63rd Annual International
                         transformers                                        Conference of Doble Clients, 1996, sec. 8-7.
Country Transformers Diagnosed Defect in Defect in
              tested      as defective   the core the windings          9.   P. Marutchenko, T. Morozova “Voltage versus Time
Russia        260             25          13          15                     Characteristics of Surface Discharge in Transformer Oil under
USA             12             4           4           2                     Long –Time Voltage Action”, Elektrotechnika,1978, #4, pp.
Canada         18              5           5           -                     25-28 (in Russian).
10. B. Arakelyan, E. Senkevich “Early Diagnostics of Oil-             BIOGRAPHIES
    Immersed HV Equipment”, Electricheskie stantsii, 1985, #6
    (in Russian).                                                     Dr. Victor Sokolov, a CIGRE Member and the Convenor of
                                                                      CIGRE WG 12-18, obtained his MSEE in 1962 from the Kharkov
11. T. K. Saha, M. Darveniza “The Application of Interfacial          Polytechnic University, and Ph. D. in High Voltage Technology in
    Polarization Spectra for Assessing Insulation Condition in the    1982 from the Kiev Polytechic University. Dr. Sokolov possess a
    Power Transformers”, Proceedings of the CIGRE SC12                world level expertise in all questions of designing, producing,
    Colloquium,1997, Sydney, Australia.                               testing and maintaining of power transformers. He is an author of
                                                                      numerous publications. Now he is the Technical Director of the
12. V. Ryzhenko, V. Sokolov “Effect of Moisture on the                Scientific-Engineering Center “ZTZ-Service” (Ukraine).
    Dielectric Strength of Winding Insulation in Power
    Transformers”, Electricheskie stantsii, 1981, # 9 (in Russian).   Mr. Zalya Berler, a Member of the IEEE and CIGRE, is the
                                                                      General Manager of Cutler-Hammer Predictive Diagnostics
13. Yu. Kalentyev “Investigation of Short –Term and Long-Term         Division. He obtained his MSEE from the Leningrad State
    Behavior of Oil-Barrier Insulation of HV Power transformers       University (Russia) in 1973 and held various engineering and
    in Real Operation Conditions”, Dissertation, Sant-Petersburg,     managing positions in electric utilities, including 10 years with
    Russia, 1985.                                                     Northern States Power Co, and power engineering construction
                                                                      companies. He has over 10 publications.
14. V. Sokolov, V. Vanin "In-Service Assessment of Water
    Content in Power Transformers," Proceedings of the 62nd           Dr. Viktor S. Rashkes, PE, Senior Member of the IEEE, Member
    Annual International Conference of Doble Clients, 1995, Sec.      of CIGRE, is the Manager of the Transformer Diagnostics
    8-6.                                                              Department of Cutler-Hammer Predictive Diagnostics. He obtained
                                                                      his MSEE and Ph. D. in High Voltage Technology from the
15. A. Golubev, A. Romashkov, V. Tsvetkov et al. “On-Line             Moscow Power Engineering University in 1956 and 1966,
    Vibro-Acoustic Alternative to the Frequency Response              respectively. State certified as a Senior Researcher in High
    Analysis and On-Line Partial Discharge Measurements on            Voltage Technology in Russia, a Registered Professional Electrical
    Large Power Transformers”, Proceedings of the Techcon ‘99         Engineer in the State of Massachusetts. Prior to CHPD he worked
    Annual Conference, TJ/H2b, February 18-19, 1999, New              for the Electric Power Research Institute VNIIE in Moscow where
    Orleans, LA.                                                      contributed to the creation of 787 and 1200 kV equipment and
.                                                                     transmissions, then for General Electric Co. at EPRI High Voltage
16. S. Lindgren, H. Moore. “Diagnostic and Monitoring                 Transmission Research Center in Lenox, MA. His primary field of
    Techniques for Life Extension of Transformers”, Proceedings       activity covers HV/EHV transmissions and equipment, transients,
    of the CIGRE SC12 Colloquium,1997, Sydney, Australia.             overvoltages and insulation, field and laboratory high voltage tests
                                                                      and measurements. He holds 33 patents, and has published over
17. S. Cesari, C. Hantouche, T. Muraoka, B. Pouliquen “Partial        140 papers.
    Discharge Measurement as a Diagnostic Tool”, Electra,
    December 1998, # 181.

18. Z. Berler, L. Letitskaya, V. Rashkes, P. Svy “Experience in
    the Application of the On-Line Monitoring System Using
    Power Frequency and Partial Discharges to High Voltage
    Transformer and Bushing Insulation”, EPRI Substation
    Equipment Diagnostic Conference VI, February 16-18, 1998,
    New Orleans, LA.

19. M.F. Lachman, W. Walter, S. Skinner “Experience with On-
    Line Diagnostics and Life Management of High Voltage
    Bushings”, Proceedings of the 66th Annual International
    Conference of Doble Clients, April 12-16,1999, Boston, MA.

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