24th International Conference on Electricity Distribution
Glasgow, 12-15 June 2017
Paper 1308
MANAGING ON-LOAD TAP CHANGER LIFE CYCLE IN TNB DISTRIBUTION
POWER TRANSFORMERS
Young Zaidey YANG GHAZALI
Tenaga Nasional Berhad – Malaysia
young@tnb.com.my
ABSTRACT
TNB has developed an Asset Management (AM)
Framework in accordance with ISO 55001:2014. One of
the key elements of the framework is the asset life cycle
management. This paper presents TNB experience in
managing the On-Load Tap Changers (OLTC) over their
complete lifecycle to improve performance and
reliability. Using failure mode, effect and criticality
analysis (FMECA) based on the previous failure data,
lifecycle management strategies for OLTC have been
identified. It covers the main activities over the lifecycle
stages namely design, operation and maintenance.
Adoption of new technology in the design, enhancement
in condition assessment and effective implementation of
condition based maintenance were amongst the strategies
implemented. Finally these strategies were validated
through field evaluation and lifecycle cost analysis prior
to the successful implementation throughout TNB.
Most power transformers studies indicated that the main
cause of power transformers failures is the OLTC [1]
since it has mechanical parts that are in constant
movement. In TNB distribution network, there are seven
cases involving permanent damage of OLTC since 2005.
In most failure cases, it has caused prolonged power
interruption, often due to severity of damage and timely
replacement, as the result of unavailability of spares and
incompatibility with the existing design. The root causes
of failures are often attributed to degradation and
carbonization of oil as well as contact problems as
illustrated in Fig 2.
INTRODUCTION
Presently, there are 1,260 power transformers rated upto
33kV with On-Load Tap Changer (OLTC), installed in
TNB medium voltage distribution network with capacity
ranging from 7.5 MVA up to 30 MVA. About 70% of the
power transformers and their respective OLTC are aged
between 15 and 25 years. Out of the total population,
95% of the OLTC is of in-tank oil-immersed selector
switch type with transition resistors or also known as oil
switch type OLTC as shown in Fig. 1, which combines
both functions of tap selector and diverter switch in one
oil-filled compartment. The typical number of tap change
operation of the OLTC in TNB distribution ranged
between 2000 and 5000 tap change per year.
Fig. 1: In-tank oil-immersed selector switch type OLTC with
transition resistors used in TNB distribution
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Fig. 2: Root cause analysis of typical OLTC failures in TNB
distribution power transformers
During normal tap change operation, switching arcs occur
in oil due to the making and breaking of currents. These
arcs cause degradation and carbonization that
contaminate and reduce the dielectric strength of the oil
[2]. The carbonization over time causes accumulation of
carbon deposits on the surface of the fiberglass cylinder
that resulted in the formation of electrical treeing.
Fig. 3: Formation of electrical treeing on the cylinder’s surface
resulted in tracking and arcing between phases
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24th International Conference on Electricity Distribution
Glasgow, 12-15 June 2017
Paper 1308
Without proper maintenance, the electrical treeing
develops into tracking and finally phase-to-phase arcing
in the OLTC. This is the common mode of failure found
in most of the cases resulting in the permanent damage of
the OLTC involved. On the other hand, coking or low
conductivity film buildup was often observed on fixed
and roller contacts during OLTC maintenance. This film
consists of layer of pyrolytic carbon formation that bond
to the oxide layer which was formed as the result of
surface oxidation of the contacts [2]. In some cases,
coking has led to erosion or contact ware and pitted
marks can be visibly observed. This occurs as the result
of continuous overheating, as coking worsen over time
due to increased contact resistance [3]. Prolonged
condition of such phenomenon may result in excessive
arcing that could trip the transformer protection.
Table I: FMECA based on common findings of OLTC failures
Failure
Mode
Arcing
between
phases
Excessive
arcing
between
contacts
Failure
Effect
Failure
Failure
P C
Mechanism Consequence
Tracking as
• Permanent
the result of
tracking on
Degradation & accumulation Transformer
fibreglass
trip by
H carbonization of carbon
cylinder
protection H H
of oil
deposits on
relays
• Damage of
the surface
metal parts
of cylinder
Overheating
• Damage of
Transformer
due to
contact
Coking of
trip by
M
increased
M M
contacts
protection
• Contact
contact
relays
wear off
resistance
S
Failure
Cause
Note: S=Severity of failure, P=Probability
C=Criticality, H=High, M=Medium, L=Low
of
failure,
Based on the FMECA in Table 1, TNB has identified the
key activities at various lifecycle stages that can
contribute to mitigate the causes of failure and improve
the performance and reliability of power transformers as
a whole. The lifecycle management strategies for OLTC
based on these key activities are further discussed in the
following subchapters.
Fig. 4: Erosion and pitted marks on the roller contacts (left) as
well as carbon buildup on the fixed contacts (right) were
observed during maintenance
Realizing the needs to reduce the risk of OLTC failures,
asset management strategies for OLTC have been
proposed and implemented over its complete lifecycle.
Thus, this paper describes TNB approach and experience
in managing the lifecycle of OLTC in distribution power
transformer to improve its performance and reliability.
Fig. 5: Lifecycle activities of an asset
FAILURE
MODE
EFFECT
CRITICALITY ANALYSIS
AND
A failure mode effect and criticality analysis (FMECA)
as shown in Table 1 has been used to determine the most
appropriate asset lifecycle management strategy for
OLTC based on the actual findings on OLTC failures [4].
FMECA evaluation on other components of the OLTC
such as motor drive and oil surge relay are not discussed
in this paper since severity of failure of these components
are still low. In addition, these components follow the
standard design based on the manufacturer and type of
OLTC. Furthermore, the operation and maintenance
requirement of these components only involve visual
inspection and functional check which are very minimal.
OLTC
LIFECYCLE
STRATEGIES
MANAGEMENT
The complete lifecycle of an asset in accordance with
Asset Management requirements of ISO 55001:2014 is
illustrated in Fig. 5, where the activities at every stage of
the lifecycle are also shown.
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Improving Design and Specification
Adoption of Vacuum Switch OLTC to Mitigate the
Degradation and Carbonization of Oil in the OLTC
Compartment as well as Coking of Arcing Contacts
Degradation and carbonization of oil as described above
are the most common causes of failure that has resulted in
failure and permanent damage of the OLTC. In order to
mitigate this problem, TNB has adopted the use of
vacuum switch OLTC, shown in Fig. 6, that confines
switching in interrupted vacuum bottles. As the results,
this helps to prevent contamination of oil due to
carbonization and hence lower the rate of oil degradation
due to switching arcs. Furthermore, with the absence of
oil inside the vacuum switch, formation of low
conductivity film and deposition of carbon that lead to
coking on the contacts’ surface will no longer occur.
With the arc quenching property of the vacuum switch,
contact erosion and thus contact wear is minimized and
thus reduces maintenance costs. Prior to the adoption of
the use of vacuum switch type OLTC in 2011, a field trial
was conducted in 2007 to evaluate its performance.
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24th International Conference on Electricity Distribution
Glasgow, 12-15 June 2017
Paper 1308
validate and compare the net present value of the total
lifecycle cost of using vacuum switch type OLTC
together with self-dehydrating breather, and the oil switch
type OLTC. The LCCA of the two systems shown in
Table II reveals that the use of vacuum switch type
OLTC together with self-dehydrating breather could
generate saving of more than RM 600,000 a year for the
entire transformer population, not including saving on the
avoidance of loss revenue due to OLTC failure.
Fig. 6: Vacuum switch type OLTC used in TNB distribution
power transformers
Adoption of Free Maintenance Self-Dehydrating
Breather to Mitigate the Degradation of Oil in the
OLTC Compartment
Degradation of oil in the OLTC is not only influenced by
the switching arcs that occur due to making and braking
of currents during on-load tap change operation, but also
affected by the presence of moisture in the oil. Even with
the use of vacuum switch type OLTC, moisture can still
be presence due to leakage or in most cases due to lack of
maintenance of the OLTC dehydrating breather.
Fig. 8: Use of maintenance free self-dehydrating breather
Table II: Comparison of the net present value of the total
lifecycle cost over the life span of 40 years per OLTC system
Total
OLTC System
Operation &
Initial Cost
Lifecycle
Maintenance
(RM)
Cost
Cost (RM)
(RM)
Oil switch type OLTC
& conventional
160,814.00
breather
Vacuum switch type
OLTC & self196,145.00
dehydrating breather
69,652.23
230,466.23
15,198.46
211,343.46
Enhancing Methods for Condition Assessment
Fig. 7: Lack of maintenance of silica gel in the dehydrating
breather of the OLTC
In order to mitigate this problem, TNB has adopted the
use of maintenance free self-dehydrating breather as
depicted in Fig. 8. The breather has a heating element
mounted within the container to heat the desiccant at
selected intervals with temperature sensor to monitor the
correct operation of the heater. A moisture sensor
measure the humidity of the air to ensure only dehydrated
air goes through the piping into the OLTC conservator.
Both sensors are controlled by an electronic controller.
The condensed moisture formed on the surface of the
container will be expelled outward by gravity. A field
trial on the use of the self-dehydrating breather was
conducted in 2007 prior to its adoption in 2011.
Lifecycle Cost Analysis
Prior to the adoption on the use of vacuum switch type
OLTC together with maintenance free self-dehydrating
breather, a lifecycle cost analysis (LCCA) over the
expected transformer life span of 40 years is performed to
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The adoption of vacuum switch OLTC is only applied for
the new power transformers. Therefore, for the existing
in-service power transformers, emphasis is given on the
utilization (operation) and maintenance lifecycle stages
which are discussed below.
Condition Assessment Using Oil Quality Analysis to
Detect Degradation of Oil in OLTC
The implementation of the condition assessment for
OLTC by means of oil quality analysis involved
breakdown voltage & moisture content has started since
2007. Table III summarizes the oil quality indicator limits
used for OLTC.
Table III: Condition indicators for OLTC oil quality analysis
Property
Breakdown
Voltage (kV)
Water Content
(ppm)
Limits
Good
Fair
Poor
Bad
> 50
40 to 50
30 to 39
< 30
< 15
15 to 30
31 to 45
> 45
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24th International Conference on Electricity Distribution
Glasgow, 12-15 June 2017
Paper 1308
Condition Assessment Using Dissolved Gas Analysis
(DGA) to Detect Coking of Contacts
DGA has been applied to assess the condition of the
OLTC since 2007. However, the health condition of the
OLTC then was not truly understood due to unavailable
guide for interpretation of DGA results. In 2011, with the
issue of IEEE Std. C57.139-2010 [5], a more enhanced
condition assessment method using DGA data was
formulated to interpret the health condition of the OLTC.
The statistical model used follows the method described
in Annex B of IEEE Std. C57.139-2010 to determine two
types of condition indicator limits i.e gas concentration
and gas ratio limits. The model was initially based on
DGA data up to the year 2011 taken from 503 units of
oil-immersed selector switch type OLTC classified as
“ARAB” type OLTC in accordance with IEEE Std.
C57.139-2010 classification scheme.
The gas concentration limits is defined in terms of a
statistical outlier limit identifying extreme values of gas
concentration suspected to be the results of faults or
unusual stresses. The gases that are used as indicators to
discriminate between normal and faulty conditions are
C2H2, C2H4 and CH4 together with the sum of CH4, C2H6
and C2H4 called the total dissolved heating gases
(TDHG). The upper outlier limits U1, U2 and U3 were
calculated for each set of gas concentration data. U3 is
introduced to represent the most extreme values requiring
the highest attention (U3 = Q3 + 4.5IQR where Q3: Third
quartile, IQR: Interquartile range). Table IV summarizes
the gas concentration limits for OLTC.
Table IV: OLTC gas concentration limits
Concentration (ppm)
Generic
CH4
C2H4
C2H2
TDHG
C ≤ U1
C ≤ 2083
C ≤ 3522
C ≤ 14598
C ≤ 7000
U1 > C ≤ 2083 > C 3522 > C ≤ 14598 > C ≤ 7000 > C ≤
U2
≤ 3254
5569
23004
11029
Condition
Codes
Normal
(1)
Caution
(2)
U2 > C ≤ 3254 > C 5569 > C ≤ 23004 > C ≤ 11029 > C ≤ Warning
U3
≤ 4423
7617
31410
15058
(3)
C > U3
C > 4423
C > 7617
> 31410
> 15058
Danger
(4)
Note: C=gas concentration
The gas ratios that are used as indicators to discriminate
between normal and faulty conditions are C2H4/C2H2 and
TDHG/C2H2. The gas ratio values of non-faulty OLTC
operating under normal conditions are described by the
percentiles of the lognormal distribution representing the
nonoutlier gas ratios. The gas ratio limits calculated for
the study are based on 90th (C090), 95th (C095), and 99th (C099)
percentiles of the lognormal distribution with significance
level of 0.1, 0.05 and 0.01 respectively. Table V
summarizes the gas ratio limits for OLTC.
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It should be noted that, the ratio limits in Table V are
applied only when any of the gas concentration has
reached “Warning” limit or “Condition 3” in Table IV.
Table V: OLTC gas ratio limits
Ratio
Generic
C2H4 / C2H2
R ≤ C090
R ≤ 0.378
Condition
Codes
TDHG/C2H2
R ≤ 0.684
C090 > R ≤ C095 0.378 > R ≤ 0.480 0.684 > R ≤ 0.843
C095> R ≤ C099 0.480 > R ≤ 0.743 0.843 > R ≤ 1.234
R > C099
R > 0.743
R > 1.234
Normal
(1)
Caution
(2)
Warning
(3)
Danger
(4)
Note: R=gas ratio
Table VI gives the overall interpretation to facilitate the
understanding on the condition of the arcing contacts
based on the results of the gas concentrations and ratios.
Table VI: Interpretation of DGA results for OLTC
Results
Interpretation
Condition Condition
Codes
Indicator
R = 1 and
Good  Normal OLTC operation
C≤2
 Light coking or deterioration of
arcing contacts, or
R = 1 and
Fair  Unusually high frequency of tap
2<C≤4
change operation or high load current
causing heating of transition resistors
1<R≤3
 Coking or increased deterioration of
and
Poor
arcing contacts
2<C≤4
R = 4 and
 Heavy coking or severe deterioration
Bad
2<C≤4
of arcing contacts
Table VII compares the outcome of the DGA results on
745 units of OLTC interpreted using the above method
with the same DGA results using the Duval’s Triangle
Method for Load Tap Changers [6].
Table VII: Comparison on the number of problematic OLTCs
detected using TNB condition assessment method derived from
IEEE Std. C57.139-2010 and Duval’s Triangle for OLTC
TNB OLTC
Assessment
Method
OLTC
Duval’s
Triangle
Method
59
55
No. of severe thermal faults with
moderate or heavy coking
41
22
No. of thermal faults in progress
with light coking or heating of
transition resistors
18
33
Fault Identification
Total no.
detected
of
possible
faults
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24th International Conference on Electricity Distribution
Glasgow, 12-15 June 2017
Paper 1308
Effective Implementation of Condition Based
Maintenance
Based on the enhanced condition assessment method
above, an overall health condition of the OLTC is derived
as shown in Table VIII that provides recommended
actions for mitigations as tabulated in Table IX.
The assessment was validated through visual inspection
of the OLTC condition. Fig. 9 shows typical observations
found during maintenance. Validation of the assessment
was also carried out by means of Dynamic Current
Measurement using Dynamic Winding Resistance
technique and it is discussed elsewhere [7].
Table VIII: Overall health condition of the OLTC
DGA
Results
Overall Health
Condition
Oil Quality Analysis Results
Good
Fair
Poor
Bad
Good
1
2
3
3
Fair
2
2
3
3
Poor
3
3
3
4
Bad
3
3
4
4
Table IX: Recommended mitigating actions based on the
overall health condition of the OLTC
Condition Condition
Codes Indicator
1
2
3
4
Recommended Actions
Continue oil sampling at 12 month
interval
Continue oil sampling at 6 month
Caution
interval
Inspect for leaks & condition of silica
gels. Conduct internal inspection on
Warning
arcing contacts. Perform overhaul &
replace oil.
Inspect for leaks & silica gels’ condition.
Danger Possible replacement of arcing contacts.
Perform overhaul & replace oil.
Normal
Table X: Example of maintenance planning sheet based on the
criticality of the OLTC health condition
Fig. 9: Surface erosion with pitted marks and coking of the
roller contacts (left) and carbon deposits on the cylinder (right)
were observed during OLTC maintenance at several sites
CONCLUSIONS
Managing asset lifecycle is the main focus of asset
management system starting from the creation of the
asset, utilization, maintenance up to its retirement. Thus,
each aspect of the lifecycle activities must be strategized
to optimize the usage of the asset and to strike a balance
between cost, risk and performance of the asset. This
paper has presented all possible mitigating actions that
have become the OLTC asset management strategy at
various stages of its lifecycle. Based on the FMECA,
three main lifecycle activities have been identified for the
implementation of the lifecycle strategy which includes
design, operation and maintenance of OLTC. Finally,
validation through field evaluation and lifecycle cost
analysis was performed to evaluate for technical and
economic feasibility of the proposed technologies and
methodology, prior to the successful adoption of all the
strategies throughout TNB since 2011.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
Based on the health condition and recommended
mitigating actions, more effective maintenance planning
can be executed where maintenance work can now be
prioritized according to the criticality of the OLTC
condition as shown in the example given in Table X.
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[7]
R. Jongen, et. al., 2007, “A statistical approach to
processing power transformer failure data”, CIRED 19th
International Conference, Paper 546
J.J. Erbrink, E. Gulski, et. Al, 2008, “Advanced on-site
diagnosis of transformer OLTC”, IEEE International
Symposium on Electrical Insulation, 252-256
J.J. Erbrink, E. Gulski, et. al, 2010 “Condition assessment
of OLTC using dynamic resistance measurements,”
International Conference on HV Engineering, 433-436
BS 5760-5 “Guide to failure modes, effects and criticality
analysis (FMEA and FMECA)”
IEEE Std C57.139-2010, “IEEE Guide for Dissolved Gas
Analysis in Transformer Load Tap Changers”
M. Duval, 2008, “The Duval triangle for load tap
changers, non-mineral oils and low temperature faults in
transformers”, IEEE Electrical Insulation Magazine, Vol.
24, No. 6, 22-29
Mohd Shahril,Yasmin Hanum, Young Zaidey, et.al, 2013
“Diagnosis of OLTC via Duval Triangle Method and
Dynamic Current Measurement”, Malaysian International
Tribology Conference, Procedia Engineering 68, 477- 483
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