With an ageing population of transformers it is important for a Utility
to have a well defined replacement strategy which is closely backed by an
appropriate condition monitoring strategy. The following is a report that I compiled for 6 power stations, most of which were in their half life. This report was done on 2001 but the salient concepts are still relevant.
This report highlights the
long-term strategy to manage the ageing generator transformer fleet within a Power Utility. It first concentrates on establishing an estimated
remaining life of the transformers and comparing this to the strategic life of
plant plans for the stations. From this, recommendations are given on the
management method for the transformers concerned.
The transformers for A and B are in a satisfactory condition. Since
these stations have a strategic life up to 2011, these transformers will be
able to operate until then. No major refurbishment plans are proposed for these
transformers other than the proper maintenance as prescribed by Generation.
The transformers for F and E are presently in the latter stages of
their life and have an estimated ±10 years life.
The strategic remaining station life for F and E are 47 and 48 years
respectively. This remaining age difference is too high for life extension
methods to be implemented, thus either rewinds or new transformers are proposed
for these stations according to end-of-life of the respective transformers. The
timing of these rewinds or replacements are highlighted in section 5.3 and the
bar chart for strategic planning for the transformers in appendix 2.
The transformers for D are still relatively new but their insulation
has shown signs of significant degradation. Proper oil preservation systems
need to be implemented as soon as possible to maintain the proper controlled
environment within the transformer to stabilise its ageing process. The strategic
station life is up to 2048 with an estimated transformer life of ±18 years. This means that these transformers will also have to be
either rewound or replaced around 2019.
The C transformers have an average remaining life span of 18 years and
the proposed strategic life for the station is up to 2035. With a remaining age
difference of 17 years, it would be of advantage to apply life extension
methods to these transformers to prolong the life for a few years and then
rewind/refurbish these transformers to achieve the remaining life. This would
be the most cost-effective program when considering the remaining station life.
The decision for replacement or refurbishment of a transformer is both
strategic and financial. Much thought need to be given to the future
requirements of the plant before such a task can be carried out.
LIFE ESTIMATION OF TRANSFORMERS
Lifetime
evaluation of any equipment is related to its ageing process. The principle
factor influencing ageing and life expectancy is thermal stress. The life
duration of the transformer is assumed to be the life duration of the paper
insulation. Abnormal events are normally associated with chemical reactions of
pyrolysis, oxidation and hydrolysis taking place within the insulation
material. These reactions are accelerated by increases in temperature and in
the concentration of oxygen and moisture.
Transformer oil
is also affected by chemical degradation. Oxidation can cause acid compounds
and sludge to form causing a reduction in dielectric strength and impairing
cooling.
There has been a
trend evident for large GSU transformers to begin to fail after 18-20 years in
service. This trend is evident in many areas of the world and is not simply due
to the thermal ageing of the insulation, but due to the effects of system
events that accelerate ageing with a consequent reduction in the electrical and
mechanical strength of the windings.
According to the
OEM, the expected life of transformers is considered to be ± 35 years. This will be used as a basis of end-of-life of the transformers.
From the table 1, the proposed remaining life is assumed according to this
criterion.
Table 1
Expected
Insulation Life – Using Furanic Method
(Extract from Guideline for the extension of
substations, research project by EPRI)
One of the most
dependable and reproducible measures of paper ageing is the degree of
polymerisation (DP) of the cellulose. Cellulose is the principle constituent of
insulating papers used in power transformers and the cellulose molecule is made
up of a long chain of glucose rings. The DP is the average number of these
rings in the molecule. When paper is new the DP is typically between 1000 and
1400, but as the paper ages thermally, the bonds between rings begin to break
and the average length of the chain is reduced. The shortening of the chains is
also associated with diminished mechanical properties (tensile strength, burst
strength, elongation to rupture). A DP value of 200 is generally felt to
represent the level, at which “useful mechanical properties” of the paper are
lost, so this may be used as an insulation life end-point.
Once a DP value has been determined, a
translation is required to establish the relative age of the insulation. The
reduction of DP with ageing time does not follow a linear pattern. It is very
rapid at first and then becomes more gradual. Shroff and Stannett observed that
if any set of ageing data is plotted with DP on a logarithmic scale and time on
linear scale straight-line results for the portion of the data after the initial
rapid drop-off are obtained. The zero time intercepts for the straight lines
range from DP = 700 to 900 for sets of test data at different temperatures and
by various investigators. All of the data can be grouped by normalising it
based on the time required for DP to reduce to 200 (the life end-point). The
resultant graph is shown in figure 1.
Figure 1
This is admittedly an approximate method
for assessing the condition of the transformer insulation, but it produces an
answer that should be of adequate quality to make engineering judgements. For
example a transformer with insulation DP = 300 would not be a good candidate
for a partial rewind because about 70% of the insulation life has already been
consumed.
The following Table 2 illustrates the
application of this criterion on the transformers to determine the approximated
insulation life. The average curve with DP = 800 at 0 normalised life was used
to estimate the normalised life.
The value for C GSU trfr 4 has been
estimated to 20 years remaining insulation life due 1994 refurbishment of this
transformer after it had failed thus not giving true DP results.
Table 2
Table 3
CONDITION ASSESSMENT
When determining the probability of
trouble free service for so many additional years it would be unwise to base
the evaluation only on thermal ageing. The decision should be made in terms of
different service record aspects as investigated below:
The most widely
used screening technique is by Dissolved gas analysis, with Carbon monoxide and
Carbon dioxide being the key gases to evaluate. Experience has demonstrated
that under normal operating temperatures, the rate of evolution of carbon
dioxide typically is 7-20 times higher than that for carbon monoxide, but
ratios down to five may be considered normal.
The ANSI/IEEE
C57.104, Guide for the Interpretation of gases generated in power transformers
gives the following method. Absolute dissolved gas levels are provided for four
status conditions ranging from normal operation to modest concern (investigate)
to major concern and finally to imminent risk of failure. The CO2 and CO levels
(PPM) for each status are as follows:
Table 4
An unusually low CO2/CO ratio with small
amounts of gas present could be indicative of a developing problem that could
be corrected.
The following is a table of the CO2 and CO
results for the GSU transformers. It is an average of the results over the past
5 years to give a realistic distribution of results
Table 5
From table 5 the transformers that were
most affected by insulation breakdown are; E GSU transformers 4 and 3, F GSU
transformers 1 and 2, C GSU transformer 1 and D GSU transformers 1 and 2. The
reasons could be that of thermal degradation or high oxygen and moisture
levels.
Although a transformer might have been
designed and tested to withstand mechanical stress resulting from external
faults, the latter must nevertheless be considered as an ageing factor. The
clamping force of many transformers will be reduced over time due to the
insulating material shrinking by means of being compacted by continuous
vibrations.
These results are based on historical
data which are not available for estimation. It is however assumed that the
number and effects, of the short circuit conditions for the transformers are of
average quantity and level. That of major events have been recorded as such
under thermal effects and insulation breakdown.
Repeated surges will usually not have a
cumulative damaging effect on the insulation. However, in the presence of
moisture or contamination deposits, repeated surges or over voltages may cause
permanent damage in highly stressed areas of the insulation structure or ignite
partial discharges that will persist under service voltage.
The basic approach to condition
assessment is to use the oil as the primary indicator of the internal condition
of the transformer. This assessment is accomplished as follows:
Insulation life
is a function of thermal ageing of the system. It is reasonable to expect that
the transformer insulation systems will last the expected 30-40 years at full
load if the system is maintained in a good condition. Failure or end of life
occurs as the result of a variety of mechanisms including poor oil preservation
systems. Thus, it is necessary to examine the causes of deterioration of the
paper and oil properties.
Insulation can degrade from the exposure
to high levels of oxygen and water at normal operating temperatures. This
degradation can occur in older transformers having oil expansion tanks with no
rubber bags to prevent oxygen from entering the tank or failure to maintain the
dehydrating breathers so that water also enters the oil through the expansion
tank.
As part of the life extension of
transformers, EPRI recommends that the levels of O2 be controlled to
a maximum of 2000 PPM.
Table 6 below shows the levels of O2
of the GSU transformers as an average over the past 5 years and indicates that
all the GSU transformers are above the EPRI recommended limit of 2000 PPM for
oxyGSU This indicates that the existing free breathing system is not adequate
to prevent a controlled oxygen environment within the transformer. This further
emphasises the importance of the installation of a conservator bag. Of concern
are the transformers of D and C since these transformers will have to remain in
service for the longest after a decision is taken for replacement/refurbishment.
Proper oil preservation systems need to be investigated for these transformers.
Table 6
The EPRI recommended maximum limit being
2000 PPM
The ageing process is minimal if the oil
and paper are kept dry, the oxygen content is nominal, and the hot-spot
temperatures are not above standard allowances. If the transformer temperature
does not exceed the rating of the paper, it is possible to load the transformer
at or above its rating depending on the ambient without significant loss of
life. Failure can result from thermal
ageing if the transformer is overloaded to the extent that the hot-spot
temperatures above the rating of the paper exists for long periods of time.
transformers are run to full load and
all transformers are kept within the specified temperature limits. The
transformers for F have shown signs of operating at temperatures higher than
the rest of the transformers but these levels have being within the upper limit
of 90° C above which the life of the insulation could be drastically
affected.
It has been recognised that every
internal inspection or relocation of the transformer involves a risk of
mechanical damage. Experience indicates that a transformer that has been
trouble free so far is more likely to remain so than one that has required
on-site repair or needed to be relocated.
Of concern would be that of C GSU
transformer 4 where it has experienced a failure in the past resulting in it
being repaired and moved.
The above
condition assessment establishes that the transformers have been maintained in
a satisfactory condition from the resources and technology that was available.
It however also points out the effects of oxygen on the insulation life and the
level of oxygen within the transformers. When compared to world standards and
recommendations these levels are too high. It is worthwhile investigating and
implementing methods of control of the levels of oxygen within the transformer.
These include the installation of conservator bags within the expansion tank of
the transformer.
POSSIBLE SOLUTIONS/ACTION PLAN
The bar chart in Appendix 2 clearly illustrates the long-term plant
strategy and the role the GSU transformers would have to play. From the table 7
below the remaining life of the power stations after the operating life of the
existing GSU transformers has been exceeded are summarised. This indicates that
B and A power stations would not require further planning for transformers
since they will outlive the station operating life requirement. For D, F and E replacement/refurbishment
strategies will definitely have to be planned. For C, life extension
methods/refurbishments could be investigated to try to prolong the life of the
existing transformers to last the full life of the station. These will be
discussed further.
Table 7
The transformer
life is shortened by a number of events. Taking action to prevent failure from
any of these causes is a method for extending life. Controlling the
characteristics of the internal transformer system such as controlling the
oxygen and water contents will ensure that the maximum designed life is
attained. A combination of maintaining the insulation in a good condition with
proper loading of the transformers will ensure longer expected life of the
transformers. These will be discussed below.
The figure 2 below illustrates the
action plan needed to reduce oxygen and moisture levels in the transformer.
This is recommended by EPRI in the guideline for the life extension of
substations:
Figure 2
EPRI recommends
that the oxygen content of the oil be controlled to a maximum of 2000 PPM. As
illustrated in section 4.4, the GSU transformers have a high exposure to oxygen
and have been this way since initial installation. Thus, methods of control
will have to be implemented to reduce the levels of oxygen. One of these methods
is the installation of a transformer bag in the conservator tank. This project
has already been investigated and is being implement for D. Investigations will
have to be carried out for the other stations. Based on the life of plant
strategy, the installation of bags for F and E will have to be investigated
under the refurbishment/replacement plans for the transformers. Since life
extension plans will have to be investigated/implemented for C, the transformer
conservator bags will also be highly recommended in this case.
To prolong to lives of the existing
transformers it is of extreme importance to have timely and high quality
maintenance. In the long term, as a minimum, the following points should be
noted:
· Oil pumps are to be replaced
with refurbished pumps or new pumps having improved bearings.
· The tap changer must be
regularly serviced and inspected. Parts that are worn and parts that have a
history of problems are to be replaced. Internal leads are to be inspected and
the tape replaced if needed.
· Bushings are to be timeously
tested for power factor. If a high value is reached, replacement will have to be
investigated.
· Inspect and test control and
protective devices. If signs of deterioration and imminent failure are evident,
these need to be replaced. The control wiring is to be replaced if it shows
signs of severe deterioration.
· The oil properties including
dielectric, power factor and water content are to be checked. If the properties
are out of line, the oil must be passed through processing equipment where the
oil is filtered, dehumidified and degassed.
· Repair or replace gaskets that
are leaking.
The proper
maintenance and testing of the transformers need to be carried out according to
IEEE and standards.
The condition of the various transformer
system components must be carefully considered when making decisions on major
repair or replacement of transformers. Careful considerations should be given
to expectations for major repairs that are to be performed by a repair
facility:
1. What is the expected life after
repair? Is the expected life after repairing the same as for a new transformer?
Is it the expected original life minus the service up to the time of repair? It
is not realistic to expect the same total life after repair as for a new
transformer.
2. The condition of the paper
insulation is of obvious importance when making repair decisions. If all
indicators are positive (O2 and H2O have been low, CO and CO2 are low, the
transformer has been maintained in a good condition, there has not been
excessive overloading, and the furans are low ) the probability is that the
condition of the insulation is good and that repair is appropriate from this
viewpoint.
3. If the indicators are
questionable or negative, it is recommended that a sample of insulation be
removed for degree of polymerisation (DP) tests. The following guidelines based
on DP analysis can be used:
DP < 200 The paper is near the end of its useful
life and repair of such windings is not recommended.
400 < DP < 600 Reuse of the coils is questionable
unless the repair is to see limited service.
DP > 600 Some life has been removed but most of
the useful life remains so that reuse of the coils is usually satisfactory.
4. The degree of refurbishment justified depends upon the age and
condition of the transformer. The options are:
- To rewind the transformer,
- To rewind with replacement of bushings and tap changer,
- To a redesign using modern techniques to adapt the existing core and tank and bring the
- refurbished transformer up to the modern standards of a new transformer.
The long-term plant strategy for
transformers (Appendix 2) puts C in a unique situation. There is a possibility
to prolong the life of the transformer but this may not be enough to last the
end-of-life of the station. It may not be cost effective to replace the
transformers since the remaining life of the station would not be enough to get
a return on investment. Since refurbishment/rewind only offers limited life
when compared to that of a new transformer (due to the ageing of the magnetic
circuit and materials of the remaining parts) it may be an option to be
investigated when the capability of the existing components (insulation, tap
changer, bushings) of the transformers have been exhausted.
The refurbishment/replacement of
transformers is suggested for D, F and E. This decision is based on the station
long-term plant strategy and the condition of the transformers. The long term
plant strategy for transformers (appendix 2) illustrates the timing of such a
replacement plan. This refurbishment/replacement program is capital intensive
and must be included in the long term plant budgeting.
The spare transformers within also plays
an important role. One spare transformer exist for A and B. One spare
transformer exist for F. For E a spare set of windings is available in the
stores.
The spare transformer for F plays a very
important role in that it can be assembled and made ready for a swap during one
of the routine outages and the existing one be rewound/refurbished or replaced.
This can then replace the GSU transformer of the other unit.
The spare set of windings for E can be
used as a model for the manufacture of further windings. These can be
manufactured now while the existing transformers are still in operation. This
will reduce the down time when these transformers are rewound.
CONCLUSION
The average remaining life of the
transformer has been estimated using the design age and the estimated
insulation life (illustrated in Table 3). This gives a realistic estimated life
that can be used for the future strategic planning of the transformers within the
Utility.
The condition
assessment tries to establish the present condition of the transformers and how
it was maintained in the past. It helps to identify deficiencies in the method
of maintenance and areas in which concentration needs to be given to prolong
the life of the transformers. From this, it has been established that the
transformers have a high concentration of oxygen, which is the main factor in
the deterioration of the paper insulation. This is due to the fact that there
were no conservator bags installed from initial installation. From a financial
and strategic basis, it is suggested that conservator bags be installed in the
transformers at D. It is also highly recommended that conservator bags be
installed at C since it is of utmost importance in the long term to prolong the
life of these transformers, since only a refurbishment will eventually be
needed instead of a replacement.
It is beneficial
to prolong the life of existing equipment for as long as possible before the
option of replacement is visited, however the replacement should be planned
adequately to ensure continuous operation of the plant. This will also hold
true for the GSU transformers within Utility. As far as possible the
maintenance and operating characteristics of the transformers must be within
proper standards and limits respectively.
The strategic
plan for the GSU transformers may be found in the bar chart in appendix 2. This
highlights that the transformers at A and B don’t need to be replaced or
refurbished since they will outlive the station life if proper maintenance is
carried out on these transformers.
It is suggested
that the F spare transformer be refurbished and replace GSU transformer 1.
Thereafter refurbishing/replacing GSU transformer 1 and replacing GSU
transformer 2 with this.
Further
investigations will have to be made into using the spare windings available for
E in pre-manufacturing windings for a rewind, thus reducing the down time of
refurbishment.
The D GSU
transformers must be refurbished around 2017. Life extension plans need to be
implemented at C as soon as possible and refurbishment’s around 2015.
This gives an estimated guide for the
long term planning and timing of events for the GSU transformers within the
Utility. The condition of these transformers must however be continually
monitored and evaluated since any adverse events could affect the above
estimation and this needs to be properly analysed and managed.
RECOMMENDATIONS
- Install conservator bags in the GSUs transformers at D (March 2000).
- Install conservator bags in the GSUs transformers at C (2001/2).
- Carry out the recommended maintenance as per - generation requirements on all GSU transformers.
- Refurbish Spare F transformer (2003)
- Swap spare F transformer with GSU trfr. 1 at F (2004)
- Refurbish/replace F GSU trfr. 1 (2005)
- Swap F GSU trfr.2 with GSU trfr. 1 (2006/2007)
- Refurbish/replace F GSU Trfr. 2. Keep as spare.
- E - Use Spare set of windings to pre-manufacture windings for rewind / replacement (2004)
- E - GSU trfr. 4 replace windings/refurbish/replace (2006)
- E - GSU trfr. 3 replace/refurbish (2007)
- E - GSU trfr. 2 replacement/refurbish (2009)
- E - GSU trfr. 1 replacement/refurbish (2010)
- C - GSU trfr.1 Refurbish/replace (2015)
- C - GSU trfr. 3 refurbish/replace (2016)
- C - GSU trfr. 4 refurbish/replace (2017)
- C - GSU trfr. 2 refurbish/replace (2018)
- D - GSU trfr. 1 refurbish/replace (2017)
- D - GSU trfr. 2 refurbish/replace (2018)
- A & B - maintain transformer - will last station life
ABBREVIATIONS
A A Power Station
B B power Station
C C Power Station
CH4 Methane
C2H2 Acetylene
C2H4 Ethylene
C2H6 Ethane
CO Carbon
Monoxide
CO2 Carbon
dioxide
O2 Oxygen
D D Power Station
DGA Dissolved
Gas Analysis
DP Degree
of Polymerisation
E E Power Station
EPRI Electric
Power Research Institute
F F
Power Station
GSU Trfr. GSU Transformer
H2 Hydrogen
H2O Water
OEM Original
equipment manufacturer
PPM Parts
per million
TDCG Total
Dissolved Combustible Gas
BIBLIOGRAPHY
- Lifetime Evaluation of Transformers, Working group 09 of study committee 12, Electra No. 150 October 1993.
- DJ Allan and A White, Life Management of Power Plants, IEE Conference Publication No. 401, 1994
- Guidelines for the Life Extension of Substations, TR-105070 Research project 2747-09, EPRI, April 1995.
- IEEE Guide for the Interpretation of Gases Generated in Oil-immersed Transformers, IEEE Std C57.104-1991.
- IEEE Guide for the Loading Mineral-Oil-Immersed Transformers, IEEE Std C57.91-1995.
- Life Extension Program for Older Substation Transformers, Wallace J. Penner, Entergy Services, Inc. Doble Engineering Company, 1994.
- Oil Cooled Power Transformer and Reactor Refurbishment, CE Odendaal, ABB Powertech Transformers (Pty) Ltd.
APPENDIX 1
APPENDIX 2
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