Power transformers

1. J. N. Karamchetti, M. Tech., FIE,
Sr. Faculty (P&E)
Engineering Staff College of India
Over view of Failure of
Transformers – Remedial
Measures

2.  Transformer is of strategic importance in a
substation.
 Its Performance depends on:
 Working environment,
 Quality in designing,
 Manufacturing.
 A transformer which is properly maintained
and which is working under normal
operating conditions, in all probability gives
satisfactory performance through out its life.
In fact is serves longer than its expected life,
which is required by utilities.

3.  The following parameters contribute to the
normal conditions for operation of a
transformer:
 Rated voltage and rated current with
permissible limits,
 Temperature of oil and winding not
exceeding the prescribed values,
 Proper functioning of accessories and
protective devices,
 Free from external faults such as line
breakdowns, equipment breakdowns.

4.  In oil filled transformers paper (or cellulose)
material along with oil forms major insulation,
 Paper and paper board insulation immersed in oil
and subjected to temperature for longer periods,
lose mechanical strength,
 Dielectric strength remains until the paper is
charred, when free carbons become conducting,
or too brittle to withstand mechanical shocks.
 Further complications results with the liberation of
free water due to decomposition, which causes
deterioration in dielectric strength of oil. De-
polymerization of insulation takes place when
deterioration starts.

5.  Aging of transformer depends on the dielectric
performance of the insulation system.
 Insulation deterioration is related to temperature.
 The higher the temperature the faster is insulation
deterioration.
 When transformer operates below normal
temperature loss of life of insulation is less than
normal.
 When the operating temperature is greater than
normal, loss of life is higher than normal.

6.  Consequently a transformer may be safely
operated for a time at about normal
temperature provided the loss of insulation
life during this period is adequately
compensated by operation for a sufficiently
long time at temperature below normal.

7.  Proper maintenance of transformer includes
proper upkeep of solid and liquid insulation
to their specified levels with marginal and
permissible variations.
 This is possible, by proper operation such
as maintaining the load current and voltage
and oil and winding temperatures at their
rated levels and not exceeding these levels.
 Paper or oil dielectric have varying degree
of sensitivity to degradation upon over
loading, ingress of moisture, improper
handling and storage affecting life.

8.  Specifications for Design of Transformers
 Quality Aspects, OLTC
 Testing of Transformers,
 Insulation Resistance,
 Polarization Index,
 Capacitance and Tan delta,
Course Content

9.  Characteristics of Insulating Oil,
 Reclamation of Oil,
 Condition Monitoring of Oil,
 Dissolved Gas analysis (DGA)
Course Content

10.  Protection of Power Transformers,
 Differential Protection,
 Restricted Earth Fault,
 OL+EF Protection,
 Earthing of Transformers,
 Thermovision &
 Residual Life Assessment of Transformers
Course Content

11. Transformer - Maintenance
 Useful life period of Equipment of same
design and rating is not the same
Depends on:
 the individual loading pattern
 Maintenance Practices,
 Short Circuits currents, duration and no of
times
 Hence they have different residual life

12. Introduction – Maintenance
 Preventive Maintenance, predictive
maintenance, therefore, needs to be
adopted to keep the equipment
continuously in service for optimum output.
 This necessitates Condition Based
Maintenance to provide advance
information about the health of the
equipment for planning the need based
maintenance/ overhaul.

13.  Causes of failure of equipment requires
thorough investigations to avoid recurrence.
 Based on the results of analysis decide
Preventive Action and Corrective Action
 Acceptable norms/permissible limits during
maintenance tests on the equipment as per
relevant Indian/International Standards
should be strictly adhered to.
Root Cause Analysis

14. Preventive to Predictive
the Future of
Transformer Maintenance

15.  Condition Monitoring may be defined as
predictive method making use of the fact that
most equipment will have a useful life before
maintenance is required.
 The application and development of special
purpose equipment, the means of acquiring
the data and the analysis of that data to
predict the trends.
Condition Monitoring and Diagnostic
for Power Transformers

16.  Initial Stage of a condition monitoring
programme consists of establishing the base
line parameters and then recording the actual
base line (or finger prints) values.
Condition Monitoring and Diagnostic
for Power Transformers

17.  The next stage is the establishment of routine
testing of plants and equipment observing the
running condition and assessing the
parameters previously determined for the
baseline. These readings are then compared
with the fingerprint and the state of the
present plant condition can be determined
from the absolute figures. The rates of
degradation and an assessment of the likely
to failure can be estimated from the trend.
Condition Monitoring and Diagnostic
for Power Transformers

18. Condition Monitoring of Transformers
 Pre commissioning / Factory test results,
 A complete database of the test results,
 History of the equipment,
 A thorough knowledge on the testing is
required for carrying out any assessment,
 Test results compared with the pre-
commissioning / factory test results to asses
the present condition.
 Calibration of test instruments is also to be
ensured for reliability of assessment.

19. Benefits of Condition Monitoring can
be summarized as below:
 Reduced maintenance costs,
 Results provide a quality – control feature,
 Limit the probability of destructive failures, leading
to improvements in operator safety and quality of
supply,
 For assessing possibility & severity of any failure
and consequential repair activities,
 Provides information on the plant operating life,
enabling business decision to be made either on
plant refurbishment or replacement.

20. Condition Monitoring Techniques

21.  Dissolved gas analysis of Transformer oil is based
on the breakdown of the molecules of oil locally, in
certain types of electrical faults such as ionization,
heating, arcing and pyrolysis of cellulose.
 It is a powerful diagnostic technique for ‘on line
monitoring’ the internal conditions of Transformers
due to its capability to detect defects in the early
stages before they develop into major faults.
 In this method it is possible to check whether a
transformer is subjected to a normal amount of
aging and heating or whether there are incipient
defects.
Dissolved Gas Analyser

22.  When the oil comes in contact with a fault it
will start to break down.
 Dissolved gas analysis is the process of
extracting and measuring these byproducts.
 The byproducts come in the form of low
molecular weight hydrocarbons and are the
symptom of the problem.
 As can be seen in Figure, the oil is broken
down into different combinations of gases
depending on the fault type.
Dissolved Gas Analyser

23. Oil Breakdown Products

24.  the operating status Paper of transformers is
furan analysis.
 Furans are produced from the breakdown of
the solid insulation within transformers.
 Figure below shows breakdown of a paper in
a transformer. As can be seen CO and CO2
are generated.
Furans

25.  But it has been noted that these gases can be
produced in transformers at elevated levels
even when there is no paper degradation.
This could be due to some other components
used in the manufacturing.
 It has been shown that glucose is formed only
when the paper degrades.
 The glucose then converts into furans,
Furans

26. Paper Breakdown Products

27.  The gases underlined are the combustible
gases we look at to predict, and know the
course of transformer failure through DGA
in their presence in percentage, or parts
per millions of concentration. Below are a
few charts to represent and show some of
the concentration of combustible gases.
 These evaluations come about because of
certain types of transformer failure that will
show up in DGA tests.
Dissolved Gas Analyser

28. A large number of tests are available but there
are only 4 or 5 that provide the information
needed to evaluate the condition of the oil.
Should one or more properties of the oil fall
below an accepted norm, disastrous events
can occur.
Oil as an insulator it must be able to withstand
a large amount of electrical stress without
failure.
Oil as Insulator

29. This is determined by its dielectric strength.
Should an oil’s dielectric strength fall below a
certain level, a flashover can occur.
Low breakdown voltages indicate that an oil is
contaminated with conductive particles and
or free water.
Moisture in solution will not be detected in this
test as good results can be obtained with a
dissolved water content greater than 50
ppm.
Oil as Insulator

30. Questions & Answers
Case Study – 1 Follows: