International Journal of Pure and Applied Mathematics
Volume 118 No. 24 2018
ISSN: 1314-3395 (on-line version)
url: http://www.acadpubl.eu/hub/
Special Issue
http://www.acadpubl.eu/hub/
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 Transmission
Dist.2
Mumbai,Maharashyra State,India2
prakaqshkumargore@gmail.com
gore.prathamesh@gmail.com
May 26, 2018
Abstract
In any of the industry flashovers in electrical system
are quite often and generally handled casually. But when
any of the vulnerable auxiliary or MCC become out of
service on account of loose connection, it is difficult to
handle the situation as the emergency systems of the
process industry gets locked which simultaneously may be
associated with huge revenue loss. All attempts should be
made to minimize such electrical flashovers by adopting
suitable technical measures.
Key Words:switchgear,mcc,bimetallic.
1
INTRODUCTION
The electricity is the most convenient and versatile form of energy
as far as its application is concerned and therefore has entered all
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the nooks and corners of the human activity. However, we tend
to overlook the hazards electricity poses and fail to treat it with
the respect it deserves. Electricity is often referred to as a silent
killer, because it cannot be tasted, seen, heard, or smelled and so
it is essentially invisible. Poor electrical connections can be very
efficient at overheating, because they can generate a high wattage
over a small area for a long period of time. If the watt density
is high enough, the connection will glow. Also an oxide actually
forms at the (loose) contact area, and the resistance of the oxide
causes the I2 R power dissipation. If we’re lucky, proper enclosing
of these connections will keep an occurrence like this from igniting
nearby combustibles, such as wood and cellulose insulation. Even
though a poor connection is hazardous, there is currently no way
to detect a poor connection as soon as it begins. While working in
various power projects for a prolonged period, numbers of electrical
failures have been witnessed on account of various reasons. These
failures not only damaged the equipment but on many occasions
also caused the generating unit trip outs resulting in to longer down
times of generating unit / equipment along with huge revenue loss
to generating companies. The present paper discusses the loose
connection problems and their preventions in low tension high
tension switch gear and drives of thermal power stations.
2
IMPORTANT SWITCHGEARS
The Thermal Power Stations have got number of switchgears
depending upon the unit size. Some important switchgear of 62.5
MW 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 station
transformers no. 1/2.
• 3.3 KV HT coal handling plant switchgears A/B fed from 3.3
KV HT STATION Bus A/B.
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• 415 V LT station bus fed from 3.3 KV HT station bus A/B
through 1000 KVA transformers.
• 415 V CW MCC workshop MCC fed through 415 V LT
station bus.
ii) Unit Switchgears:• 3.3 KV HT unit bus A & B fed from only 7.5 MVA unit
auxiliary transformer.
• 415 V LT unit bus A & B fed from 3.3 KV HT unit bus A
through 750 KVA transformer.
• 415 V LT boiler MCC Turbine MCC fed from 415 V LT unit
bus.
B. POWER HOUSE; 2X210 MW UNITS:i) Station Switchgears:• 6.6 KV HT station bus CA & CB from 40 MVA station
transformers 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 station
bus CA & CB.
• 415 V LT station bus A/B fed from CA & CB through 2 MVA
transformers.
• 415 V LT station bus A/B fed from DA & DB through 2 MVA
transformers.
• 415 V LT station bus A/B for CW, WT plant, fuel oil pump
houses; separate buses .
ii) Unit Switchgears:• 6.6 KV HT unit bus A/B fed from 15 MVA unit auxiliary
transformers A/B.
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• 415 V LT unit service switchgears (USS) A/B fed from 6.6
kV unit bus through 2 MVA transformers.
• 415 V LT turbine MCC A/B, boiler MCC A/B fed from
respective USS buses.
• 415 V LT normal emergency bus fed from USS bus A and
during emergency from diesel generator set.
These switchgears feed power to different drives of various
auxiliaries. Many times flash over took place in switchgears,
MCCs, different 3.3 kV, 6.6 kV, 415 V buses due to
loose/improper joint connections.
With the maintenance
experience, methods to monitor electrical joints have been
developed and brought the failures within controllable limits.
In the ensuing paragraphs monitoring of electrical joint failure has
been explained.
3
FAILURE
DUE
TO
POOR
TERMINATION
AND
LOOSE
CONNECTIONS IN ELECTRICAL
SYSTEMS
While analyzing the electrical failures, it revealed that about 20 to
25% of the total failures were the result of joint failures due to
poor termination and loose connections. In general, the poor
termination/loose connection in an electrical system causes
overheating 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 and
cables, etc.
• Burning and charging of the support insulator which if not
controlled in the early stage may lead to insulator catching
fire and flash over finally terminating to a phase to phase or
phase to ground fault.
• Disconnection in the system causing interruption of power or
further damage to other electrical equipment. Especially the
3 phase induction motors which happen to be the back bone
of power industry are very much susceptible to single phasing
due to disconnections.
• Flash over in the system especially in the equipment dealing
higher current/higher voltage.
• Mal operation of protective equipment like blowing of fuses,
operation of thermal overload relays and other protective
relays, auto control loops including flashover in the CT
secondary circuit.
• Loose connections could lead initiation of fire in the building.
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• In switchgears, the loose connections increase the overall
temperature of the switchgear which for a reliable operation
is limited by permissible temperature rise of various
materials (such as insulated material) and adjacent
equipment. Thus reducing life of equipment and increasing
chances 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, breakers
and isolators. Each of the elements forming an electrical joint has
its inherent resistance. Whenever current flows to the system, it
causes heating of the joints depending upon the joint resistance.
The heat so generated is to be distributed to the surroundings by
radiation and conduction in order to keep the temperature within
permissible limits. Hence the equipment like cables, bus bars,
clamps, etc. are so designed that in addition to carry the rated
currents they also help in dissipating the heat generated at joints.
In case of poor termination the heat generated at the joints is
more than the dissipation capacity and causes local heating, thus
an increase in temperature.
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4
THE
CAUSE
TERMINATION:
OF
Special Issue
POOR
The common causes of poor termination are generally due to
following 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 one
stud reducing the contact area. This results due to problem
in 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 oversize
hole or by matching the lug palm leading to inadequate cross
sectional area of the lugs for transfer of current.
• Use of lower thickness nuts causing reduction in contact area
in case of clamp connection on studs coming out from
transformers.
• Space between links and terminal being taken care by
bolts/studs not meant for carrying the current.
• Formation of oxide layer or corroded layer leading to higher
resistance.
• Looseness in the terminal screw, bolts, nuts, etc.
• Bimetallic/plating not used for termination of aluminum over
copper, copper based alloys.
• Large size aluminum cables terminated in motor terminal box
where limited surface area is available due to copper motor
leads and brass screw.
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Normally as per practices prevailing, all efforts are made to ensure
that the joints are properly made but there is no easy check to
ascertain right at initial stage before putting the equipment in
service that the joint is perfect. After the system is in operation
for some time, the loose joints are identified by deterioration
pattern of cables /links and terminals. Due to overheating
tarnishing of metals, charring of insulating supports, and change
in color of copper links such as purple and bright red are also
observed. At some locations, especially in case of motor terminal
boxes in which the joints have been overheated during service, it
is difficult to pinpoint the defective component responsible for the
overheating. This is because almost all the components of a joint
appear to have overheated. In large power distribution system
where a huge amount of power is being consumed by motors used
to drive various auxiliaries, it is essential to minimize the outages
due to electrical fault arising of improper/loose electrical joints.
Hence it is felt essential to identify the loose connection in an
electrical 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 rated
current and comparing the same with figures available for similar
joints under laboratory test or with those measured earlier on site
is one method to identify the poor joints. This method imposes
difficulty in field as for measurement of millivolt drop as
measurements are to be done in live conditions which is not safe
and particular or the suitable size equipment are required to inject
rated current at the joints to carry out the measurements in dead
condition. In view of above an easy method to identify the poor
joints/terminations is the measurement of contact resistance of
joints. While the equipment are under shutdown the contact
resistance at various joints may be measured easily without
disturbing the system and compared with others or with the
available expected values which shall indicate the difference
between a healthy joint and a poor joint. The contact resistance
values for important joints/terminations may be measured and
the data may be generated to keep a record for reference purpose.
This practice of measurement of joint resistance is adopted during
the three consecutive years at one of the Thermal power station
and the results obtained are quite encouraging in identifying the
status of the joints and reducing the joint failures.
Case Studies:
Some of the case studies are given below:Case Study 1
Equipment: Boiler feed pump motors of 210 MW BHEL thermal
units.
Capacity: 4000 kW, 6.6 kV, 400 A
History: There have been number of failures at the terminal box
of BFP motor. In almost all the feed pump motors such failures in
alternate years were reported. Investigation revealed that the
problem was due to high joint resistance as in each phase single
core 1000 square mm aluminum cables and the motor connection
copper lead were terminated at a single stud provided on an
insulator. Joint was modified by providing a separate copper plate
and 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 of
0.25-0.30 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-2
Equipment: Boiler Auxiliary Booster Pump motor capacity:
90KW, 415 V, 155 A.
History: The motor was reported to be comparatively hotter. The
motor is connected to switchgear with 3 core 185 sq.mm.
Aluminum cable. The motor terminal was opened and it was
observed that B Phase joint appeared comparatively hot. It could
not be identified whether the heating was due to loose connection
or due to problem in lug. It was decided to measure the resistance
between the conductor and lug to ascertain condition of the lugs
and termination at switchgear end appeared ok.
• The resistance between cable conductors and lug at motor
end:
R = 0.08 milliohm
Y = 0.08 milliohm
B = 5.51 milliohm
• With the cable connected at switchgear and the 3 Phases of
the cable at the motor end were shorted and phase to phase
resistance at switchgear end was :
Between R-Y: 16.32 milliohm
Between Y-B: 22.04 milliohm
Between B-R: 22.08 milliohm
It is observed from the above readings that B phase joint is
having higher lug to cable resistance and the effect is significantly
reflected on the complete cable resistance. The joint was attended
and the resistance results noted is:
Between R-Y: 16.32 milliohm
Between Y-B: 16.46 milliohm
Between B-R: 16.36 milliohm
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At one of the thermal power plant, the contact resistance of
different types of electrical joints has been taken and the
achievable and acceptable values of contact resistance of some of
the 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 and
similar data can be generated for other installations.
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CONCLUSION
The measurement of contact resistance of different types of
electrical joints and updating their records has helped in
minimizing the electrical flashovers in the Power Plants where the
authors have worked. This has not avoided only the break downs
but simultaneously reduced emergency maintenance hours and
also 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 Firing
Properties of Polyvinyl Chloride Covering Cords at Elevated
Temperatures, Bull. Japan Assn. of Fire Science and
Engineering 33:2, 25-29 (1983).
[4] Bland, B., Electrical DamagesCause or Consequence?
Forensic Sciences 29, 747-761 (1984).
J.
[5] Ettling, B. V., Ignitability of PVC Electrical Insulation by
Arcing, IAAI-Oregon Chapter Newsletter, 6 (Mar. 1997).
[6] Kinoshita, K., Hagimoto, Y., and Watanabe, N., Investigation
Reports and Igniting Experiments on the Electrical Causes of
Many Fires Started after the Big Earthquake in Kobe Area
in 1995, published in Urgent Study Reports on the HanshinAwaji Big Earthquake, Science and Technology Agency of
Japan, 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., Phenomenon
of 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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International Journal of Pure and Applied Mathematics
[9] Meese, W. J., and Beausoliel, R. W., Exploratory study of
Glowing Electrical Connections (NBS BSS 103), [U.S.] Natl.
Bur. Stand., Gaithersburg, MD (1977).
[10] Hijikata, T., and Ogawara, A., Research on Thermal
Phenomena of Twist Joint Point of PVC Insulated Flexible
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Science and Engineering 204-205 (1992).
[11] Mitsuhashi, N., Yokoi, Y., Nagata, M., and Isaka, K.,
Concerning the History of Deterioration in Insulated Electric
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BIOGRAPHY
P.K.GORE
Graduated in electrical engineering from G.S.T.I. Indore and
subsequently, 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 years
in different power projects and achieved remarkable experience in
the field of operation, maintenance, construction, erection and
commissioning of thermal power plants. He headed the biggest
power plant of the state for about three years. He also worked in
private power plant in the capacity of senior Executive Director
and completed the erection commissioning work of 2x60 MW
Nanjing, China make Thermal units. Presently he is working as
Assistant Professor in electrical section of D.Y.Patil College of
Engineering, Akurdi; Pune
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PRATHAMESH GORE
Graduated in electrical engineering from NIT Calicut. He has
worked in LT Construction-Power Transmission Distribution unit
for about 5 years in different power projects and achieved
remarkable 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 at
765/400 kV PGCIL substation at Jabalpur. Presently he is
pursuing his MBA from NMIMS, Mumbai.
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