failure due to poor termination & loose connections in

Failure Due To Poor Termination &Loose Connections in Electrical Systems

P.K.Gore 1, Prathamesh Gore 2,Asstt.Professor (Electrical), D.Y.P.C.O.E.,Akurdi,Pune1

Former Executive Director, M.P.State Electricity Board1

Pune,Maharashtra State,India1

Pursuing MBA course in NMIMS,Mumbai2

Former Site Engr , L & T const.-Power TransmissionDist.2

Mumbai,Maharashyra State,India2

[email protected]@gmail.com

May 26, 2018

Abstract

In any of the industry flashovers in electrical systemare quite often and generally handled casually. But whenany of the vulnerable auxiliary or MCC become out ofservice on account of loose connection, it is difficult tohandle the situation as the emergency systems of theprocess industry gets locked which simultaneously may beassociated with huge revenue loss. All attempts should bemade to minimize such electrical flashovers by adoptingsuitable technical measures.

Key Words:switchgear,mcc,bimetallic.

1 INTRODUCTION

The electricity is the most convenient and versatile form of energyas far as its application is concerned and therefore has entered all

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International Journal of Pure and Applied MathematicsVolume 118 No. 24 2018ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

the nooks and corners of the human activity. However, we tendto overlook the hazards electricity poses and fail to treat it withthe respect it deserves. Electricity is often referred to as a silentkiller, because it cannot be tasted, seen, heard, or smelled and soit is essentially invisible. Poor electrical connections can be veryefficient at overheating, because they can generate a high wattageover a small area for a long period of time. If the watt densityis high enough, the connection will glow. Also an oxide actuallyforms at the (loose) contact area, and the resistance of the oxidecauses the I2 R power dissipation. If we’re lucky, proper enclosingof these connections will keep an occurrence like this from ignitingnearby combustibles, such as wood and cellulose insulation. Eventhough a poor connection is hazardous, there is currently no wayto detect a poor connection as soon as it begins. While working invarious power projects for a prolonged period, numbers of electricalfailures have been witnessed on account of various reasons. Thesefailures not only damaged the equipment but on many occasionsalso caused the generating unit trip outs resulting in to longer downtimes of generating unit / equipment along with huge revenue lossto generating companies. The present paper discusses the looseconnection problems and their preventions in low tension hightension switch gear and drives of thermal power stations.

2 IMPORTANT SWITCHGEARS

The Thermal Power Stations have got number of switchgearsdepending upon the unit size. Some important switchgear of 62.5MW Units and 210 MW Units size Power House are listed below:-

A. POWER HOUSE; 5X62.5 MW UNITS:-i) Station Switchgears:-

• 3.3 KV HT station bus A/B fed from 30 MVA stationtransformers no. 1/2.

• 3.3 KV HT coal handling plant switchgears A/B fed from 3.3KV HT STATION Bus A/B.

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International Journal of Pure and Applied Mathematics Special Issue

• 415 V LT station bus fed from 3.3 KV HT station bus A/Bthrough 1000 KVA transformers.

• 415 V CW MCC workshop MCC fed through 415 V LTstation bus.

ii) Unit Switchgears:-

• 3.3 KV HT unit bus A & B fed from only 7.5 MVA unitauxiliary transformer.

• 415 V LT unit bus A & B fed from 3.3 KV HT unit bus Athrough 750 KVA transformer.

• 415 V LT boiler MCC Turbine MCC fed from 415 V LT unitbus.

B. POWER HOUSE; 2X210 MW UNITS:-i) Station Switchgears:-

• 6.6 KV HT station bus CA & CB from 40 MVA stationtransformers no. 3 & 4

• 6.6 KV HT station bus DA DB fed from CA & CB.

• 6.6 KV HT bus A/B for coal handling plants fed from stationbus CA & CB.

• 415 V LT station bus A/B fed from CA & CB through 2 MVAtransformers.

• 415 V LT station bus A/B fed from DA & DB through 2 MVAtransformers.

• 415 V LT station bus A/B for CW, WT plant, fuel oil pumphouses; separate buses .

ii) Unit Switchgears:-

• 6.6 KV HT unit bus A/B fed from 15 MVA unit auxiliarytransformers A/B.

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• 415 V LT unit service switchgears (USS) A/B fed from 6.6kV unit bus through 2 MVA transformers.

• 415 V LT turbine MCC A/B, boiler MCC A/B fed fromrespective USS buses.

• 415 V LT normal emergency bus fed from USS bus A andduring emergency from diesel generator set.

These switchgears feed power to different drives of variousauxiliaries. Many times flash over took place in switchgears,MCCs, different 3.3 kV, 6.6 kV, 415 V buses due toloose/improper joint connections. With the maintenanceexperience, methods to monitor electrical joints have beendeveloped and brought the failures within controllable limits.

In the ensuing paragraphs monitoring of electrical joint failure hasbeen explained.

3 FAILURE DUE TO POOR

TERMINATION AND LOOSE

CONNECTIONS IN ELECTRICAL

SYSTEMS

While analyzing the electrical failures, it revealed that about 20 to25% of the total failures were the result of joint failures due topoor termination and loose connections. In general, the poortermination/loose connection in an electrical system causesoverheating at the joints which further leads to failure such as:-

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Fig.1. 6.6 KV BHEL Motor Terminal Connector

• Heating of connected adjacent clamps, nut/bolts, bus andcables, etc.

• Burning and charging of the support insulator which if notcontrolled in the early stage may lead to insulator catchingfire and flash over finally terminating to a phase to phase orphase to ground fault.

• Disconnection in the system causing interruption of power orfurther damage to other electrical equipment. Especially the3 phase induction motors which happen to be the back boneof power industry are very much susceptible to single phasingdue to disconnections.

• Flash over in the system especially in the equipment dealinghigher current/higher voltage.

• Mal operation of protective equipment like blowing of fuses,operation of thermal overload relays and other protectiverelays, auto control loops including flashover in the CTsecondary circuit.

• Loose connections could lead initiation of fire in the building.

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• In switchgears, the loose connections increase the overalltemperature of the switchgear which for a reliable operationis limited by permissible temperature rise of variousmaterials (such as insulated material) and adjacentequipment. Thus reducing life of equipment and increasingchances of failures.

Fig.2. 6.6KV Unit Switchgear

The electrical joints are made by copper/aluminum cable lug,clamps, studs, link and bus connection by contactors, breakersand isolators. Each of the elements forming an electrical joint hasits inherent resistance. Whenever current flows to the system, itcauses heating of the joints depending upon the joint resistance.The heat so generated is to be distributed to the surroundings byradiation and conduction in order to keep the temperature withinpermissible limits. Hence the equipment like cables, bus bars,clamps, etc. are so designed that in addition to carry the ratedcurrents they also help in dissipating the heat generated at joints.In case of poor termination the heat generated at the joints ismore than the dissipation capacity and causes local heating, thusan increase in temperature.

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4 THE CAUSE OF POOR

TERMINATION:

The common causes of poor termination are generally due tofollowing reasons:-

• Lugs improperly crimped to the cables.

• Lugs of higher than recommended size used for termination.This results loose cable to lug joint.

• More than two higher size cables being terminated at onestud reducing the contact area. This results due to problemin tightening the bigger size cable on single stud.

• Proper size washers are not being used.

• The palm surface area of the lugs reduced by drilling oversizehole or by matching the lug palm leading to inadequate crosssectional area of the lugs for transfer of current.

• Use of lower thickness nuts causing reduction in contact areain case of clamp connection on studs coming out fromtransformers.

• Space between links and terminal being taken care bybolts/studs not meant for carrying the current.

• Formation of oxide layer or corroded layer leading to higherresistance.

• Looseness in the terminal screw, bolts, nuts, etc.

• Bimetallic/plating not used for termination of aluminum overcopper, copper based alloys.

• Large size aluminum cables terminated in motor terminal boxwhere limited surface area is available due to copper motorleads and brass screw.

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Normally as per practices prevailing, all efforts are made to ensurethat the joints are properly made but there is no easy check toascertain right at initial stage before putting the equipment inservice that the joint is perfect. After the system is in operationfor some time, the loose joints are identified by deteriorationpattern of cables /links and terminals. Due to overheatingtarnishing of metals, charring of insulating supports, and changein color of copper links such as purple and bright red are alsoobserved. At some locations, especially in case of motor terminalboxes in which the joints have been overheated during service, itis difficult to pinpoint the defective component responsible for theoverheating. This is because almost all the components of a jointappear to have overheated. In large power distribution systemwhere a huge amount of power is being consumed by motors usedto drive various auxiliaries, it is essential to minimize the outagesdue to electrical fault arising of improper/loose electrical joints.Hence it is felt essential to identify the loose connection in anelectrical system.

Fig.3. 415V Motor terminal Box

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5 METHODS TO IDENTIFY POOR

JOINTS:

Measurement of millivolt drop across a joint by passing ratedcurrent and comparing the same with figures available for similarjoints under laboratory test or with those measured earlier on siteis one method to identify the poor joints. This method imposesdifficulty in field as for measurement of millivolt drop asmeasurements are to be done in live conditions which is not safeand particular or the suitable size equipment are required to injectrated current at the joints to carry out the measurements in deadcondition. In view of above an easy method to identify the poorjoints/terminations is the measurement of contact resistance ofjoints. While the equipment are under shutdown the contactresistance at various joints may be measured easily withoutdisturbing the system and compared with others or with theavailable expected values which shall indicate the differencebetween a healthy joint and a poor joint. The contact resistancevalues for important joints/terminations may be measured andthe data may be generated to keep a record for reference purpose.This practice of measurement of joint resistance is adopted duringthe three consecutive years at one of the Thermal power stationand the results obtained are quite encouraging in identifying thestatus of the joints and reducing the joint failures.

Case Studies:Some of the case studies are given below:-Case Study 1Equipment: Boiler feed pump motors of 210 MW BHEL thermalunits.Capacity: 4000 kW, 6.6 kV, 400 AHistory: There have been number of failures at the terminal boxof BFP motor. In almost all the feed pump motors such failures inalternate years were reported. Investigation revealed that theproblem was due to high joint resistance as in each phase singlecore 1000 square mm aluminum cables and the motor connectioncopper lead were terminated at a single stud provided on aninsulator. Joint was modified by providing a separate copper plateand terminating the motor end and cable end lug by the nut

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bolts. The joint resistance was reduced from an initial value of0.25-0.30 milliohm to 0.020-0.028 milliohm in a particular case.After the modified arrangement in all the motors of Power house,not a single failure at the motor terminal has been reported.Case study-2Equipment: Boiler Auxiliary Booster Pump motor capacity:90KW, 415 V, 155 A.History: The motor was reported to be comparatively hotter. Themotor is connected to switchgear with 3 core 185 sq.mm.Aluminum cable. The motor terminal was opened and it wasobserved that B Phase joint appeared comparatively hot. It couldnot be identified whether the heating was due to loose connectionor due to problem in lug. It was decided to measure the resistancebetween the conductor and lug to ascertain condition of the lugsand termination at switchgear end appeared ok.

• The resistance between cable conductors and lug at motorend:

R = 0.08 milliohmY = 0.08 milliohmB = 5.51 milliohm

• With the cable connected at switchgear and the 3 Phases ofthe cable at the motor end were shorted and phase to phaseresistance at switchgear end was :

Between R-Y: 16.32 milliohmBetween Y-B: 22.04 milliohmBetween B-R: 22.08 milliohm

It is observed from the above readings that B phase joint ishaving higher lug to cable resistance and the effect is significantlyreflected on the complete cable resistance. The joint was attendedand the resistance results noted is:

Between R-Y: 16.32 milliohmBetween Y-B: 16.46 milliohmBetween B-R: 16.36 milliohm

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At one of the thermal power plant, the contact resistance ofdifferent types of electrical joints has been taken and theachievable and acceptable values of contact resistance of some ofthe electrical joints are tabulated here for reference:-

Fig.4. 4000KW, 6.6KV boiler feed pump motor

The above data is generated for a particular power station andsimilar data can be generated for other installations.

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6 CONCLUSION

The measurement of contact resistance of different types ofelectrical joints and updating their records has helped inminimizing the electrical flashovers in the Power Plants where theauthors have worked. This has not avoided only the break downsbut simultaneously reduced emergency maintenance hours andalso down times of machines along with huge amount of revenue.

References

[1] Wikipedia ,High resistance connections

[2] Dr.V Babrauskar., How Electrical faults leads to ignition.

[3] Nagata, M., and Yokoi, Y., Deterioration and FiringProperties of Polyvinyl Chloride Covering Cords at ElevatedTemperatures, Bull. Japan Assn. of Fire Science andEngineering 33:2, 25-29 (1983).

[4] Bland, B., Electrical DamagesCause or Consequence? J.Forensic Sciences 29, 747-761 (1984).

[5] Ettling, B. V., Ignitability of PVC Electrical Insulation byArcing, IAAI-Oregon Chapter Newsletter, 6 (Mar. 1997).

[6] Kinoshita, K., Hagimoto, Y., and Watanabe, N., InvestigationReports and Igniting Experiments on the Electrical Causes ofMany Fires Started after the Big Earthquake in Kobe Areain 1995, published in Urgent Study Reports on the Hanshin-Awaji Big Earthquake, Science and Technology Agency ofJapan, Tokyo (1995).

[7] Hotta, E., On the Phenomenon of Glowing Connections, J.Japan Assn. for Fire Science 24:1, 52-58 (1974).

[8] Hagimoto, Y., Kinoshita, K., and Hagiwara, T., Phenomenonof Glow at the Electrical Contacts of Copper Wires, Natl. Res.Inst. of Police Science ReportsResearch on Forensic Science 41,30-37 (Aug. 1988).

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[9] Meese, W. J., and Beausoliel, R. W., Exploratory study ofGlowing Electrical Connections (NBS BSS 103), [U.S.] Natl.Bur. Stand., Gaithersburg, MD (1977).

[10] Hijikata, T., and Ogawara, A., Research on ThermalPhenomena of Twist Joint Point of PVC Insulated FlexibleCords, Summary of 1992 Annual Mtg. of Japan Assn. of FireScience and Engineering 204-205 (1992).

[11] Mitsuhashi, N., Yokoi, Y., Nagata, M., and Isaka, K.,Concerning the History of Deterioration in Insulated ElectricWires and Fire Hazards, J. Japanese Assn. for Fire ScienceEngrg. 31, No. 1, 11-19 (1981).

BIOGRAPHY

P.K.GORE

Graduated in electrical engineering from G.S.T.I. Indore andsubsequently, M. Tech. in Heavy Electrical Equipment from M. A.C. T. Bhopal. He has worked in M.P.P.G.C.L. for about 34 yearsin different power projects and achieved remarkable experience inthe field of operation, maintenance, construction, erection andcommissioning of thermal power plants. He headed the biggestpower plant of the state for about three years. He also worked inprivate power plant in the capacity of senior Executive Directorand completed the erection commissioning work of 2x60 MWNanjing, China make Thermal units. Presently he is working asAssistant Professor in electrical section of D.Y.Patil College ofEngineering, Akurdi; Pune

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PRATHAMESH GORE

Graduated in electrical engineering from NIT Calicut. He hasworked in LT Construction-Power Transmission Distribution unitfor about 5 years in different power projects and achievedremarkable experience in the field of operation, maintenance,construction, erection and commissioning of electrical substations.He headed the construction site of 8 bays of 765 kV level at765/400 kV PGCIL substation at Jabalpur. Presently he ispursuing his MBA from NMIMS, Mumbai.

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