Joseph Benjamin Taylor, MSc. MSc. MGhIE, Area
Manager, NNS, Ghana Grid Company Limited
(GRIDCo); e-mail: [email protected];
[email protected]
Jyoti K. Sinha Lecturer, Course Director, MEAM MSc.
School of MACE, University of Manchester, M13
9PL, UK e-mail: [email protected]
• This article uses such conventional reliability analysis as
Failure Mode and Effect Analysis (FMEA), Fault Tree
Analysis (FTA) and interconnection with Reliability
Block Diagram (RBD)/ and or Logic Diagram (LD) to
analyze the failure of an oil-filled step-down power
• FTA, RBD and/or LD and the more detailed FMECA,
Failure Mode Effect and Criticality Analysis inter-relate
with FMEA as maintenance as well as design tools to
facilitate decision on maintenance requirements, and
thereby addressing maintainability
• Analysis technique is typically demonstrated in the
application to transformer failure analysis in this paper
• As maintenance and design tools to address
maintainability, to analyze, review and explain
system failure for instance
• Recommend actions to reduce the likelihood of the
failure occurring and identify improvement
• Definition: Maintainability – the ability of an item,
under stated conditions of use, to be retained in, or
restored to, a state in which it can perform its
required functions, when maintenance is performed
under stated conditions and using prescribed
procedures and resources [BS 4778]
• - Probability that required maintenance action will be
successfully completed in a given time period [Dhillon
• Ghana Grid Company Limited (GRIDCO) is a wholly Government-owned
company established in the year 2008 [4] to operate and manage the
transmission assets of the Volta River Authority (VRA) including the 69kV,
161kV, & 225kV substations.
• GRIDCo has transmission assets, comprising over 43 transformer and
switching substations, and covering approximately over 4,000 circuit
kilometres of transmission lines spread throughout the country
• Operates a Supervisory Control and Data Acquisition, SCADA and an
interconnected grid [4-5].
• The VRA operates hydro and thermal power stations and is currently a
generator of electricity following power sector restructuring.
• Prior to restructuring, VRA’s power transmission assets were maintained
and operated by a separate department in VRA- the Transmission Systems
which constituted the core of GRIDCo
• On the evening of Sunday, September 16, 2007 at about 16:20
hours, a 3-Phase, 3-winding, Westinghouse-make power
transformer manufactured in 1973 with rated capacity
25/33MVA, and voltage 161/34.5/11.5kV was engulfed in fire,
and ultimately got burnt [6] at a substation in Tarkwa about 300
kilometres west of the capital, Accra
• The transformer is Oil-filled, ONAN/ONAF cooling, fitted with
radiators, bushings and conservator tank with Buchholz relay
• The fire outbreak was traced to insulation deterioration of 125
volts direct current (125VDC) control cable inside the chamber of
Buchholz relay housing.
• Prior to the fire outbreak maintenance work had been done on
the Buchholz relay to replace a section of 125VDC control cable
accessed by maintenance personnel to have been deteriorated.
Figure 1:
Cut view of Buchholz Relay
Figure 2: General arrangement of Buchholz
relay with cover removed showing front & rear
views [15]
• Perceived absence of appropriate reliability
engineering tool in GRIDCo to aid in analysing
equipment failure [12]. If there were such a tool it is
not known to the author.
• Quick fixes of problems and solutions are observed to
be the norm.
• For a typical system failure current maintenance best
practice uses such conventional reliability analysis as
FMEA [2-3], FTA and interconnection with RBD and/or
Logic Diagram, LG to analyse equipment failure [2-3].
• Definition: Reliability is the probability that a failure
will not occur in a particular time [Dhillon, 1999]
Brief introduction, origin, strengths &
limitations of FMEA, FTA & RBD
FMEA was developed in the 1950’s, as a systematic method that appeared under
different names, to analyse technical systems’ failure.
FMEA is an ‘engineering technique used to define, identify, and eliminate known
and/or potential failures, problems, errors, and so on from the system, design,
process, and/or service before they reach the customer’ [8]
FMEA is a simple analysis method to reveal possible failures and to predict the
failure effects on the system as a whole’ [9].
FMEA is a valuable starter in the preparation of RBD on the basis that each failure
mode is related to its effect on the systems’ output
Strengths include prevention planning, identifying change requirements and
reducing cost [2]
Limitations of traditional FMEA :
Not suitable for applications where critical combinations of component failures
need to be revealed, because it considers one component at a time and assumes
all other components to be functioning perfectly [2].
Directed more towards analysis of existing systems [2, 8] and does not concentrate
on proposing designing out excellent systems. However [9-11] address these
Brief introduction, origin, strengths & limitations
of FMEA, FTA & RBD- Continued
Origin of Fault tree Analysis, FTA - is traced to Bell telephone laboratories in 1962 and to
Boeing in the 1970’s [2].
FTA is a reliability/safety design analysis technique, which graphically describes the
combinations of events leading to a defined system failure mode called the top event
Shows the logical relation between system failure, i.e. a specific undesirable event within a
system [which constitutes the top event of the tree], and failures of different components
of the system [which constitute the basic fault event of the tree].
The basic (input) fault events could either be “independent” (round-shaped) [of other
events ]- event requiring no further development or “dependent” (kite –shaped)-event that
depends on lower events, but not developed further downwards.
Conventional reliability analysis using FTA involves a number of logical possibilities, two
main logical symbols and two gates-the ‘OR’ and an ‘AND’ gates [2-3 and 7], and are also
based on details of plant structure in a static condition
Limitations: They are proven expensive in designing solutions because of the sheer
quantum and volume of data involved in analysis.
Strengths of FTA and also RBD [2-3 and 7] lie in their uses in variety of applications,
namely; (1) facilitates decision on maintenance requirements,
(2) to prepare diagnostic routines, (3)define reliability and safety of failure modes and
effects (4)design built-in-test, fault indications and redundancies (5)enables design
alternatives to be evaluated(5) as retention of knowledge-base
Brief introduction, origin, strengths &
limitations of FMEA, FTA & RBD- Continued
RBD is a process used to break down high level reliability requirements for the
whole plant to those needed for individual systems or items
All systems can be broken down into a combination of series and parallel
reliabilities, and RBD combines both [2-3]
RBD describes the effect of a failure of a component on the system as a whole, or
vice versa
RBD also describes a system as a number of functional blocks interconnected in
accordance with the failure effect of each block on the system reliability as a
whole,( and contrast with a block schematic diagram of the systems functional
layout )
RBD recognizes series and parallel failure behaviours as two principal failure
behaviours [2-3]
Other Strength of RBD : - Simple to construct
Models simulations at any level of component details as might be necessitated by
the particular model, and like FTA facilitates decision on maintenance
Limitation is that it considers only one component failure [2-3] even though there
could be many-component failures like the Concorde failure [13].
Application of FMEA, RBD & FTA Tools
for failure analysis of transformer
• Application of FMEA, RBD & FTA is demonstrated in Figures
3-5 to analyse and review failure of a 3-Phase, 3-winding,
oil-filled, ONAN/ONAF, transformer at GRIDCo substation in
Tarkwa in Ghana.
• Transformer is fitted with radiators, bushings, conservator
with Buchholz relay
• A typical RBD is used to model, and analyse the failure,
beginning with the consideration of the plant hierarchy
since FMEA analyses the hardware, functions of the system
or a combination
• Application of the analysis technique begins by considering
three levels of plant hierarchy consisting of system,
subsystem and component levels through FMEA of the
single component failure and completing the analysis with
the interconnection of RBD and FTA
Figure 3: Typical simple Plant
Hierarchy of Tarkwa substation [12]
A typical simple plant hierarchy of Tarkwa
substation- Discussion of Fig. 3
• Figure 3 is a typical simple plant hierarchy of the Tarkwa substation
• The failure analysis using RBD begins by considering the hierarchy
of the plant structure
• Considers the equipment class- the transformers through the
equipment subclass or unit - the power transformer and continues
down to the maintainable item- the Buchholz relay
• Maintainable item is either repaired or replaced during the life of
the transformer
• The replaceable item- the Buchholz relay is viewed as a structural or
functional unit of a system or equipment, the transformer
• The Buchholz relay is considered as an entity for investigation
• A diagrammatic representation of a Buchholz relay is shown in
Figures 1-2
• The equipment class or unit, the power transformer performs a sub
function of production transforming alternating voltage for power
Application of FMEA tool to analyze the
transformer failure- Discussion of Figure 4
• Figure 4 reflects a typical FMEA
• FMEA examines all of the possible failures of the
transformer and design taking into account (1)plant
structure, (2) the hierarchy of the equipment class i.e.
• Considers function of transformer, assesses potential
functional failure, failure mode- the station ground, causethe Buchholz relay and effect of failure- the fact that power
can not be supplied to customers, as well as the system of
current controls and action
• FMEA as discussed here and shown in Figure 4 considers
one component failure at a time
• Considers the station DC ground fault as a failure caused by
the one component - buchholz relay
• Assumes all other components to be functioning perfectly
Discussion of Typical representation
of FTA as shown in Fig. 5
• A typical application of FTA to the transformer failure at Tarkwa substation
is demonstrated in Figure 5
• It shows the logical relations between failure events of the different
components- the Buchholz relay, bushing, and defined top event- the
transformer failure
• Four different lower level failures (lower level events) are examined Buchholz relay, bushing, mal operation & design deficiency
• These are in turn logically related to different lower level failures - cable
insulation breakdown, non functioning of relay, bushing vibration
• The fault tree construction then proceeds level by level till all fault events
have been developed to the prescribed resolution further down to reach
the basic fault events – thermal effect, bare cable contact, maintenance
action, 125VDC battery, no transformer oil in relay to actuate relay sensor,
design deficiency, internal source or external means that could cause
bushing vibration, bushing crack, overloading or voltage regulation which
could result from mal operation of the transformer
• Basic fault events are analysed and recommendations made as to actions
necessary to reduce the likelihood of the failure occurring, as well as
identifying improvement opportunities
Figure 6: Typical Reliability
Block Diagram for Tarkwa
transformer failure
Discussion of Typical representation
of RBD as shown in Fig 6
• A typical simple reliability block diagram (RBD) of
the failure of a transformer at Tarkwa substation
as shown in Figure 6 and discussed in this Section
interconnects the FTA represented in Figure 5
• RBD as shown in fig. 6 describes the transformer
failure as consisting of a number of functional
blocks - the Buchholz relay fault, design
deficiency, bushing failure and mal operation.
• In a logical sense Figure 6 is a simple modelling of
the system failure logic showing the logical
connection between components of the system.
• Continuous improvement, Review and update of maintenance strategy,
policy and inputs to maintenance function
• Develop proactive maintenance techniques as Condition-based
Maintenance etc.
• Develop maintenance techniques that take advantage of available tools
and techniques as FMEA, FMECA, FTA, RBD, criticality approaches etc.
• Review and update Technical procedure for replacement/modification of
125VDC cable if such a procedure exists. Otherwise consider developing
• Approval to undertake replacement/repairs and/or modification require
streamline and centralisation, if such a policy exists. Otherwise consider
developing one
• Provide Parallel/redundant protections for 125VDC control cable for
Buchholz relay
• Install a back-up 125VDC battery bank to increase reliability of protection
• Provide appropriate and specific training, workshops and seminars
tailored to suit the requirement of maintenance and operating staff
Using conventional reliability analysis such as FMEA, FTA and interconnection with
RBD the transformer failure has been analysed
The cause of a combination of two events - insulation deterioration of 125VDC
control cable and maintenance action i.e. design, as well as maintenance
perspective has been observed as the mode of the failure
Failure resulted in inconvenience to customers, was a catastrophe to the plant and
environment, led to high cost to the utility
Failure could have been possibly prevented if re-cabling of the Buchholz relay
control cable had been completed or if redundancy had been built into the system
through an alternative parallel path using a combination of back-up 125VDC
supply and protective relay
Parallel components are inherently more reliable since system failure occurs when
all components have failed, however capital costs are required.
Quick fixes, generally reactive in nature and underlines fire fighting only solve
symptoms rather than root causes of problems as was the case with the
maintenance action that contributed to the transformer failure
By designing systems that incorporate FMEA, RBD, and FTA tools and techniques
the maintenance function could be improved to become more proactive
• Present work has been limited to using tools of FMEA that
considered only one component failure to model FTA & RBD.
• FTA has been used only to analyse the failure and not to evaluate it.
• For future work based on different techniques and tools, there are
techniques available to calculate for simple trees once the failure
logic has been modelled using FTA, as well as for complex trees
especially for multiple component failures of basic events, which
need to be looked at and applied to the analysis of the transformer
• There is also Failure Mode Effect and Criticality Analysis, FMECA,
which is the more detailed form of FMEA that combines FMEA and
CA, Criticality Analysis
• There are other tools such as Root Cause Analysis, (quality) Cause
Analysis Tools as Fishbone (Ishikawa) diagram, Pareto Chart and
Scatter diagram that have not been considered in the present work,
but which can be applied to analyse the transformer failure
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