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Joseph Benjamin Taylor, MSc. MSc

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Joseph Benjamin Taylor, MSc. MSc. MGhIE, Area Manager, NNS, Ghana Grid Company Limited (GRIDCo); , Jyoti K. Sinha Lecturer, Course Director, MEAM MSc. School of MACE, University of Manchester, M13 9PL, UK

2 Abstract 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 transformer 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 opportunities. 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 1999]

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

5 THE FAILURE - PREAMBLE 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.

6 Figure 1: Cut view of Buchholz Relay [14]

7 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]

9 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 limitations.

10 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

11 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 requirements. Limitation is that it considers only one component failure [2-3] even though there could be many-component failures like the Concorde failure [13].

12 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

13 Figure 3: Typical simple Plant Hierarchy of Tarkwa substation [12]

14 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 transmission

15 Figure 4: Typical FMEA for a single component failure of Transformer [12]

16 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. transformer Considers function of transformer, assesses potential functional failure, failure mode- the station ground, cause- the 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

17 Figure 5: Typical representation of a simple fault tree of the Tarkwa transformer failure [12]

18 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

19 Figure 6: Typical Reliability Block Diagram for Tarkwa transformer failure

20 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 one. 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 system Provide appropriate and specific training, workshops and seminars tailored to suit the requirement of maintenance and operating staff

22 CONCLUSIONS 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 failure. 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


25 REFERENCES 1.Profile of Volta River Authority, Corporate Diary, 2008.
2. O’ Connor, P.D. T. Practical Reliability Engineering, 2002, John Wiley & Sons, Chichester.  3.Smith D.J. Reliability, Maintainability and Risk, Practical Methods for Engineers including Reliability Centred Maintenance Safety-related Systems, 1997, Butterworth Heinemann 4.Ghana Grid Company limited (GRIDCo), Corporate Business Plan and Budget, 2008 5.Orientation Course for Newly Employed VRA Staff- Brief on Transmission Systems Department, 2003. 6. Report on 9T2 Tarkwa Power Transformer Failure , Takoradi Area, Transmission Systems Department, VRA, 2007 7.Smith D. J. Reliability, Maintainability and Risk, Practical Methods for Engineers, 1993, Butterworth Heinemann. 8. Stamatis, D.H. Failure Mode and Effect Analysis: FMEA from Theory to Execution, American Society for Quality, 1995. 9. Aven, T. Reliability and Risk Analysis, Elsevier, Oxford, 1992. 10. Pillay, A. and Wang, J. Modified failure mode and effects analysis using approximate reasoning. Reliability Engineering and System Safety, 2003, 79 (1), P69-85. 11. Price, C. J. and Taylor, N. S. Automated multiple failure FMEA. Reliability Engineering and System Safety, 2002, 76 (1), 1-10. 12. Taylor, J.B. Operation and Maintenance Strategy review of Volta River Authority (VRA) and the Ghana Grid Company Limited (GRIDCo), M.Sc. Dissertation, University of Manchester, Manchester, U.K., 2010. 13. Accident on July 25, 2000 at La Patte d’Oie in Gonesse (95) to the Concorde registered F-BTSC operated by AirFrance (English translation). Bureau d’Enquetes et d’Analyses pur la Securite del’Aviation Civile, Ministere de l’Equipement des Transports et du Logement, France, 2002.  14., Last accessed October, 2010.  15. Transformer world website of Rothside Technology Limited, Bury St Edmunds, Suffolk, UK Last accessed October, 2010.  16., Last accessed October, 2010.

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