See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/225721183
Lifetime management of power transformers
Article in e & i Elektrotechnik und Informationstechnik · December 2003
DOI: 10.1007/BF03053972
CITATIONS
READS
17
3,469
3 authors, including:
C. Sumereder
Fachhochschule Joanneum
73 PUBLICATIONS 612 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
BB-CLEAN (Interreg Alpine Space) View project
EAS-Lab View project
All content following this page was uploaded by C. Sumereder on 17 October 2014.
The user has requested enhancement of the downloaded file.
CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
LIFE TIME MANAGEMENT OF POWER TRANSFORMERS
Christof SUMEREDER, Michael MUHR, Bernhard KÖRBLER
Graz University of Technology - Austria
sumereder@hspt.tu-graz.ac.at
INTRODUCTION
Power Transformers belong to the most
important and most cost-intensive equipment of
the electrical power transmission and
distribution. Occurring an error in a transformer
an interruption of the electrical power supply of
far areas is connected, but also large economic
losses are caused beyond that. Their operation
must be therefore a continuous and error free
power supply over decades as secured as
possible. Occurring errors and thus itself
announcing possible failures of the transformers
should be detected therefore in time, in order to
initiate suitable measures for error correction. A
continuous entry of the isolation status can
prevent thus large errors, which extend service
life of the system and which optimise
maintenance. Thus apart from higher system
availability an avoidance of unplanned
disconnections and expensive repairs is
achieved.
The emphasis of this contribution is to present
methods in comparison of well-known
procedures for condition evaluation, which permit
a substantially more exact and more reliable
predicate about the current status of the isolation
system of liquid-isolated power transformers.
Degradation of the paper can cause the
transformer to fail by several mechanism: the
brittle paper can break away from the
transformer windings and block ducts; water is a
product of degradation and builds up in the
paper, reducing its resistivity; in the extreme,
local carbonising of the paper increases the
conductivity to cause overheating and conductor
faults. [2]
For this reason it is important to know about the
condition of the transformer and the operating
condition. Taking a look to the well known law of
Montsinger [3] the reduction of the estimated life
time can be calculated due to the 8 degree
formula:
υ − 90° C
Thermal Aging = 2
8° C
… (1)
Rising the average life time temperature for 8
degrees over the maximum allowable operating
temperature the estimated life time of the paper
oil insulation system is reduced for the half. The
average life time temperature can be calculated
with the load duration curve of the transformer
(see diagram 1).
CONDITION EVALUATION
The basis for a condition evaluation of electrical
power equipment is the knowledge about the
characteristics of the insulation system and their
behaviour. According to the definition and
terminology of maintenance [1] following
measures can be distinguished: inspection,
extended inspection, overhaul and servicing. The
condition of a transformer is dependent on the
operation condition and the number and amount
of the maintenance measures and their intervals.
AGE AND AGING MECHANISM
The aging condition of the insulation properties
can be expressed in the mechanism for aging
where the degradation of the cellulosic material,
formation of furan products and effects of oxygen
and water were essential.
TUG_Sumereder_A1
Session 1 Paper No 35
Diagram 1: Load Duration Curve
With this average life time temperature the life
time reduction can be determined with the
Montsinger curve of diagram 2. For example the
maximum allowable operating temperature is
90 °C and the average life time temperature is
98 °C, the total life time reduction is 50 %. This
means that the estimated life time for a
transformer of 50 years is reduced to 25 years if
the maximum allowed operating temperature is
exceeded in average for 8 °C.
1
CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
Diagram 2: Temperature – Life Time Phase Diagram, Montsinger Law
In the case of a life time reduction the condition
curve of diagram 2 has to be adapted adequate.
The condition curves describes the condition of
the equipment starting at 100 % (new) and going
down to 0 % (end of life time). It can be
calculated by building a mathematical model. In
formula 2 the condition curve is calculated with
an exponential function third degree.
C AGE = Exp(a +
CAGE
LT
AGE
a, b, c
1
* AGE 3 ) … (2)
3
b − c * LT
age condition factor
expected useful technical lifetime
age
constants
MAINTENANCE STRATEGY
In dependence of the maintenance strategy the
point of time and scope of maintenance
measures were different. In former years the time
based maintenance (TBM) with predefined
intervals rooted in empirical feedback was
executed. In recent days of the liberalized
electricity market the condition based
maintenance (CBM) where the maintenance is
driven by the technical condition of the
equipment is applied.
TUG_Sumereder_A1
Session 1 Paper No 35
In diagram 3 this two strategies were opposed.
At TBM the measures increase the condition in
constant maintenance periods. After 65 years the
critical level is reached. Applying the CBM
maintenance measures were done when the
diagnosis level is obtained. The condition never
can get under this diagnosis level. At the end of
life time the costs for maintenance can get very
high and the risk of a transformer fault is rising
according to the bath tube curve.
Reaching the critical level respectively the
estimated end of life time a risk evaluation has to
be done. When a transformer fails, loads served
by the transformer will usually be interrupted. In
many industrial applications the loss of a
transformer will stop production until the
transformer is replaced. The replacement
transformer may be a spare or it may have to be
leased. [4] In any case a risk evaluation and
economical consideration must be made on
weather to purchase a new transformer or to
operate the aged transformer.
The factors to be considered in the risk
evaluation are:
• Failure Statistics
• Determination of Repair Costs
• Repair Time and Procedure
• Evaluation of Interrupt Costs
• Philosophy of Company
2
CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
Diagram 3: Maintenance Strategies
•
TECHNICAL DIAGNOSTICS
Technical diagnostics can be divided into electrical,
mechanical, chemical, acoustical and optical measuring
methods. Most sensors of monitoring systems are a
combination of physical measuring methods e.g. the
sensor for oil analysis is electrochemical type. Beside
the oil analysis also electrical measurements were
applied, the partial discharge diagnosis gives
information about hot spots in the insulation system.
In addition to Publication 60422 the IEC published as
well two other very useful Publications for transformer
predictive maintenance:
•
Publication 60567 for the analysis of dissolved
and free gasses.
•
Publication 61198 for the analysis of dissolved
furans. [5]
Gassing in transformers are of course since long time
well-known, the Buchholz Relais is in use since the
beginning of oil immersed transformer technology.
OIL ANALYSIS
In power transformers a combination of liquid and solid
insulant is used to fulfil the requested demands on
insulation and cooling. For the evaluation of the quality
of this insulant combination a large number of analysis
can be performed. The most important tests for mineral
insulating oil were describes in IEC Publication 60422.
This standard distinguish three categories depending
upon the purposes of the analysis:
•
Group 1 are routine tests, recommended when
there are not suspicious of fault in the
equipment.
•
Group 2 are complementary tests which should
be performed after some abnormal results have
been found.
TUG_Sumereder_A1
Group 3 are special investigative tests.
Session 1 Paper No 35
The Buchholz Relais is equivalent to an integrative
measuring system, the gassing can only be detected in
periodical intervals. New electronic gassing measuring
systems are operated in an online monitoring modus.
Most of the sensors gain a signal with electrochemical
measuring methodes. The advantage is that the
development of gassing can be set in dependence of
transformer load and time. This technology enables to
observe the condition of the insulation system in relation
of transformer stress and to optimise the operating
condition to reduce overload and of to prevent
degradation of oil and cellulose insulation.
ELECTRICAL DIAGNOSTICS
Due to failures or partial discharges the transformer oil
is enriched with failure specific gasses. The
3
CIRED
17th International Conference on Electricity Distribution
conventional partial discharge measurement according
to IEC 60270 “High-voltage test techniques - Partial
discharge measurements” enables an exact
measurement of the partial discharge activity, but to
localise the void alternative methods have to be applied.
Beside the acoustical method the optical method offers
the location of the partial discharge source.
•
Barcelona, 12-15 May 2003
Condition Evaluation: With technical diagnosis
or monitoring systems technical parameters of
transformers can be measured
First optical partial discharge detection sensors were
developed and tested. Readiness for marketing will take
time but the operating principle can be explained:
A partial discharge impulse emits an optical spectrum
which is dependent of the surrounded medium. The
emission spectrum of the electromagnetic impulse
consists of ultraviolet, visible and infrared frequencies.
The emitted light can be coupled into a fibre optic cable
using an optic lens system or fluorescent fibres. The
signals can be converted to electric impulses, amplified,
digitalized, filtered and data processed on an PC
system.
The optical sensors can be posed at any point of the
insulation system due to the flexible behaviour and
potential free character of the fibre optic cables.
MONITORING SYSTEMS
The dielectric diagnosis is a very important aid to
prevent the risk of an insulation fault, which could occur
a long interruption in the power supply. The duration of
power failures are a very important criterion for the
quality of a grid, customers do not accept a fall out of
the mains and also demand best quality at low costs.
For this reasons the permanent online monitoring of
insulation systems get more and more important.
Working with permanent online systems the transformer
can stay in operation during the measurements for the
diagnosis runs. The measuring intervals depend on age,
importance or condition of the machine and the
diagnosis equipment can be shared under several
cascade sets.
SUMMARY
The key to find the optimal life time management for the
operation of power transformer is to consider three
groups of measures according to figure 1:
•
Maintenance Strategy: different measures and
time intervals depending on the company
philosophy
•
Age and Aging Mechanism: the degradation of
the insulation system is dependent of the
operations system and overload times
TUG_Sumereder_A1
View publication stats
Session 1 Paper No 35
Figure 1: Life Time Management
On the basis of these informations the engineers can do
a risk evaluation and if necessary dispose about value
received measures.
Applying life time management as described by using
diagnosis and monitoring systems over the whole life
time period the condition of the power transformer can
be evaluated, maintenance strategies and the volume of
measures can be adapted flexible to minimize the life
cycle costs and the risk of long servicing and fall out
time. The reinvestment can be delayed for several years
and the point of renewal can be appreciated much
better.
LITERATURE
[1] EN 13306: 2001 08 01 “Maintenance Terminology”
[2] R.J. Heywood, A.M. Emsley, M. Ali “Degradation of
cellulosic insulation in power transformers”, IEE Proc.Sci. Meas. Technol. Vol. 147 No. 2, March 2000, p. 8690
[3] V.M. Montsinger “Loading transformer by
temperature” AIEE transactions, Bd. 49, 1930, p. 776792
[4] C.T. Walters, “Failed Transformers: Replace or
Repair?” Pulp and Paper Industry Technical
Conference, 1993, Conference Record p. 127-129
[5] A. Pablo, “Diagnostic”, Proceedings of the Meeting
My Transfo, Torino 15th-16th October 2002
4