Presentation Taylor GridCo

7/29/2019 Presentation Taylor GridCo

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FAILURE ANALYSIS OF GRIDCo

SUBSTATION TRANSFORMER TO

ENHANCE MAINTENANCE

PERFORMANCE

Joseph BenjaminTaylor, MSc. MSc. MGhIE, AreaManager, 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, M139PL, UK e-mail: [email protected]
mailto:[email protected]@gmail.commailto:[email protected]@gmail.commailto:[email protected]:[email protected]:[email protected]@gmail.commailto:[email protected]@gmail.com


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Abstract

This article uses such conventional reliability analysis asFailure Mode and Effect Analysis (FMEA), Fault TreeAnalysis (FTA) and interconnection with ReliabilityBlock Diagram (RBD)/ and or Logic Diagram (LD) to

analyze the failure of an oil-filled step-down powertransformer

FTA, RBD and/or LD and the more detailed FMECA,Failure Mode Effect and Criticality Analysis inter-relatewith FMEA as maintenance as well as design tools tofacilitate decision on maintenance requirements, andthereby addressing maintainability

Analysis technique is typically demonstrated in theapplication to transformer failure analysis in this paper


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USES OF FMEA, FTA, RBD and/or LD As maintenance and design tools to address

maintainability, to analyze, review and explainsystem failure for instance

Recommend actions to reduce the likelihood of thefailure occurring and identify improvement

opportunities. Definition:Maintainabilitythe ability of an item,

under stated conditions of use, to be retained in, orrestored to, a state in which it can perform itsrequired functions, when maintenance is performed

under stated conditions and using prescribedprocedures and resources [BS 4778]

- Probability that required maintenance action will besuccessfully completed in a given time period [Dhillon

1999]


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PROFILE OF GHANA GRID COMPANY LIMITED

(GRIDCo) & VOLTA RIVER AUTHORITY (VRA) Ghana Grid Company Limited (GRIDCO) is a wholly Government-owned

company established in the year 2008 [4] to operate and manage thetransmission 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 circuitkilometres of transmission lines spread throughout the country

Operates a Supervisory Control and Data Acquisition, SCADA and aninterconnected grid [4-5].

The VRA operates hydro and thermal power stations and is currently agenerator of electricity following power sector restructuring.

Prior to restructuring, VRAs power transmission assets were maintainedand operated by a separate department in VRA- the Transmission Systemswhich constituted the core of GRIDCo


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THE FAILURE - PREAMBLE

On the evening of Sunday, September 16, 2007 at about 16:20hours, a 3-Phase, 3-winding, Westinghouse-make powertransformer manufactured in 1973 with rated capacity25/33MVA, and voltage 161/34.5/11.5kV wasengulfed in fire,and ultimately got burnt [6] at a substation in Tarkwa about 300kilometres west of the capital, Accra

The transformer is Oil-filled, ONAN/ONAF cooling, fitted withradiators, bushings and conservator tank with Buchholz relay

The fire outbreak was traced to insulation deterioration of 125volts direct current (125VDC) control cable inside the chamber ofBuchholz relay housing.

Prior to the fire outbreak maintenance work had been done onthe Buchholz relay to replace a section of 125VDC control cableaccessed by maintenance personnel to have been deteriorated.


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Figure 1:

Cut view of Buchholz Relay

[14]


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Figure 2: General arrangement of Buchholz

relay with cover removed showing front & rear

views [15]


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OBSERVED LIMITATION IN

ENGINEERING ANALYTICAL TOOLS Perceived absence of appropriate reliability

engineering tool in GRIDCo to aid in analysingequipment 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 bestpractice uses such conventional reliability analysis asFMEA [2-3], FTA and interconnection with RBD and/orLogic Diagram, LG to analyse equipment failure [2-3].

Definition: Reliability is the probability that a failurewill not occur in a particular time [Dhillon, 1999]


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Brief introduction, origin, strengths &

limitations of FMEA, FTA & RBD FMEA was developed in the 1950s, 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 knownand/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 andreducing cost [2]

Limitations of traditional FMEA :

Not suitable for applications where critical combinations of component failuresneed to be revealed, because it considers one component at a time and assumesall other components to be functioning perfectly [2].

Directed more towards analysis of existing systems [2, 8] and does not concentrateon proposing designing out excellent systems. However [9-11] address theselimitations.


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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 1970s *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 otherevents ]- 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 ORand 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 designalternatives to be evaluated(5) as retention of knowledge-base


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Brief introduction, origin, strengths &

limitations of FMEA, FTA & RBD- Continued

RBD is a process used to break down high level reliability requirements for thewhole plant to those needed for individual systems or items

All systems can be broken down into a combination of series and parallelreliabilities, and RBD combines both [2-3]

RBD describes the effect of a failure of a component on the system as a whole, orvice versa

RBD also describes a system as a number of functional blocks interconnected inaccordance with the failure effect of each block on the system reliability as awhole,( and contrast with a block schematic diagram of the systems functionallayout )

RBD recognizes series and parallel failure behaviours as two principal failurebehaviours [2-3]

Other Strength of RBD : - Simple to construct

Models simulations at any level of component details as might be necessitated bythe particular model, and like FTA facilitates decision on maintenancerequirements.

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


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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 inTarkwa in Ghana.

Transformer is fitted with radiators, bushings, conservatorwith Buchholz relay

A typical RBD is used to model, and analyse the failure,beginning with the consideration of the plant hierarchysince FMEA analyses the hardware, functions of the systemor a combination

Application of the analysis technique begins by consideringthree levels of plant hierarchy consisting of system,subsystem and component levels through FMEA of thesingle component failure and completing the analysis withthe interconnection of RBD and FTA


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Figure 3: Typical simple Plant

Hierarchy of Tarkwa substation [12]


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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 hierarchyof the plant structure

Considers the equipment class- the transformers through theequipment 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 orfunctional 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 subfunction of production transforming alternating voltage for powertransmission


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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)plantstructure, (2) the hierarchy of the equipment class i.e.transformer

Considers function of transformer, assesses potentialfunctional failure, failure mode- the station ground, cause-the Buchholz relay and effect of failure- the fact that powercan not be supplied to customers, as well as the system ofcurrent controls and action

FMEA as discussed here and shown in Figure 4 considersone component failure at a time

Considers the station DC ground fault as a failure caused bythe one component - buchholz relay

Assumes all other components to be functioning perfectly


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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 differentcomponents- the Buchholz relay, bushing, and defined top event- thetransformer 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 - cableinsulation breakdown, non functioning of relay, bushing vibration

The fault tree construction then proceeds level by level till all fault eventshave been developed to the prescribed resolution further down to reachthe basic fault events thermal effect, bare cable contact, maintenanceaction, 125VDC battery, no transformer oil in relay to actuate relay sensor,design deficiency, internal source or external means that could causebushing vibration, bushing crack, overloading or voltage regulation whichcould result from mal operation of the transformer

Basic fault events are analysed and recommendations made as to actionsnecessary to reduce the likelihood of the failure occurring, as well as

identifying improvement opportunities


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Figure 6: Typical Reliability

Block Diagram for Tarkwa

transformer failure


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Discussion of Typical representation

of RBD as shown in Fig 6

A typical simple reliability block diagram (RBD) ofthe failure of a transformer at Tarkwa substationas shown in Figure 6 and discussed in this Sectioninterconnects the FTA represented in Figure 5

RBD as shown in fig. 6 describes the transformerfailure as consisting of a number of functionalblocks - the Buchholz relay fault, designdeficiency, bushing failure and mal operation.

In a logical sense Figure 6 is a simple modelling ofthe system failure logic showing the logicalconnection between components of the system.


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RECOMMENDED ACTION TO REDUCE

LIKELIHOOD OF FAILURE OCCURING & FOR

IMPROVEMENT OPORTUNITIES Continuous improvement, Review and update of maintenance strategy,

policy and inputs to maintenance function

Develop proactive maintenance techniques as Condition-basedMaintenance etc.

Develop maintenance techniques that take advantage of available toolsand techniques as FMEA, FMECA, FTA, RBD, criticality approaches etc.

Review and update Technical procedure for replacement/modification of125VDC cable if such a procedure exists. Otherwise consider developingone.

Approval to undertake replacement/repairs and/or modification requirestreamline and centralisation, if such a policy exists. Otherwise considerdeveloping one

Provide Parallel/redundant protections for 125VDC control cable forBuchholz relay

Install a back-up 125VDC battery bank to increase reliability of protectionsystem

Provide appropriate and specific training, workshops and seminarstailored to suit the requirement of maintenance and operating staff


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CONCLUSIONS

Using conventional reliability analysis such as FMEA, FTA and interconnection withRBD the transformer failure has been analysed

The cause of a combination of two events - insulation deterioration of 125VDCcontrol cable and maintenance action i.e. design, as well as maintenanceperspective 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 systemthrough an alternative parallel path using a combination of back-up 125VDCsupply and protective relay

Parallel components are inherently more reliable since system failure occurs whenall components have failed, however capital costs are required.

Quick fixes, generally reactive in nature and underlines fire fighting only solvesymptoms rather than root causes of problems as was the case with themaintenance action that contributed to the transformer failure

By designing systems that incorporate FMEA, RBD, and FTA tools and techniquesthe maintenance function could be improved to become more proactive


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CONCLUSIONS- CONTINUE

Present work has been limited to using tools of FMEA thatconsidered 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 aretechniques available to calculate for simple trees once the failurelogic has been modelled using FTA, as well as for complex trees

especially for multiple component failures of basic events, whichneed to be looked at and applied to the analysis of the transformerfailure.

There is also Failure Mode Effect and Criticality Analysis, FMECA,which is the more detailed form of FMEA that combines FMEA andCA, Criticality Analysis

There are other tools such as Root Cause Analysis, (quality) CauseAnalysis Tools as Fishbone (Ishikawa) diagram, Pareto Chart andScatter diagram that have not been considered in the present work,but which can be applied to analyse the transformer failure


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END OF PRESENTATION

THANK YOU


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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 ReliabilityCentred 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 forQuality, 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 SystemSafety, 2002, 76 (1), 1-10.

12. Taylor, J.B. Operation and Maintenance Strategy review of Volta River Authority (VRA) and the GhanaGrid Company Limited (GRIDCo), M.Sc. Dissertation, University of Manchester, Manchester, U.K., 2010.

13. Accident on July 25, 2000 at La Patte dOie in Gonesse (95) to the Concorde registered F-BTSC operatedby AirFrance (English translation). Bureau dEnquetes et dAnalyses pur la Securite delAviation Civile,Ministere de lEquipement des Transports et du Logement, France, 2002.

14.http://electricalandelectronics.org/2009/03/19/buchholz-relay/, Last accessed October, 2010.

15.http://www.transformerworld.co.uk/buchholz.htm Transformer world website of Rothside TechnologyLimited, Bury St Edmunds, Suffolk, UK Last accessed October, 2010.

16.http://www.atvus.in/Catalog/Buchholz relay magnetic swich.pdf , Last accessed October, 2010.
http://electricalandelectronics.org/2009/03/19/buchholz-relay/http://www.transformerworld.co.uk/buchholz.htmhttp://www.atvus.in/Catalog/Buchholz_relay_magnetic_swich.pdfhttp://www.atvus.in/Catalog/Buchholz_relay_magnetic_swich.pdfhttp://www.transformerworld.co.uk/buchholz.htmhttp://electricalandelectronics.org/2009/03/19/buchholz-relay/http://electricalandelectronics.org/2009/03/19/buchholz-relay/http://electricalandelectronics.org/2009/03/19/buchholz-relay/

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