A SYSTEM AND A PROCESS TO MONITOR, IDENTIFY, AND RECTIFY CABLE
TERMINATION JOINT HOT SPOTS IN ELECTRIC POWER DISTRIBUTION NETWORKS
Field of invention:
This invention generally relates to detecting points of possible fire hazard in an
electrical power distribution network (EPDN). More particularly, it relates to
continuously (24x7) monitoring the EPDN in an industrial set up where a number
of low tension (LT) panels are used and power is distributed through PCC &
MCC.
Background of invention:
A potential problem in electrical distribution system such as Power Control
Centers, Motor Control Centers is short circuit triggering fire. Short circuit
usually takes place due to disintegration of insulation between different phases of
power cable or between a phase and ground. Insulation disintegrates due to variety
of reasons, one of most common causes being burning of insulation due to
excessive heat.
Cable insulation is exposed to high temperature due to the creation of the so called
hot spots at cable end termination. For example, hot spots occur at the bolted type
joints in the power conductors and cable terminals, and are often attributable to
poor workmanship or loosening of joints because of vibrations over time. In
another example, when aluminium bus bars are terminated with copper lugs, the
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difference in thermal coefficients of expansion of copper and aluminium leads to
joint loosening.
Heated joints transfer heat to the cable insulation which starts burning if the
temperature reaches above 90 degrees centigrade. Cable insulation burning results
eventually into short circuiting. This could produce sparks which could lead to
fires.
There are two widely known methods of locating loose connections in EPDNs –
checking joints with spanner during a network shutdown (offline Method) or
checking for hot spots without shutting down the network by using devices such
as a temperature gun or an IR scanner (online method).
Both methods have advantages and disadvantages. While the offline method is
direct, reliable and safe it requires production shutdown. It also is more time
consuming, tedious and more labor oriented, and therefore cannot be used on
large networks on frequent basis. Online method is quick and requires less
manpower than the offline method, however, it is not safe since it exposes the
operator to high voltages when safety barriers are removed on energized
equipment.
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The key drawback of both methods is that they cannot monitor occurrence of,
detect or communicate hotspots on 24x7 basis in large networks (>10000 joints)
without human intervention.
A comparison of the known state of art with the present invention shown below:
Point of
comparison
Known state of art 1
Checking Loose
joints manually with
Spanner.
Known state of
art 2 – Checking
loose joints by
IR-Scanner
Thermography
Temperature
Gun.
Present
Innovation –
Loose joint
identified by
Automation
Production
Loss
High- since this can
be done only during
shutdowns.
Nil – since no
shutdown required
Nil – since no
shutdown required
Risk of
electrocution
Nil – since done
offline when network
is switched off.
High – since done
online hence
chances of
electrocution.
Nil – Since location
of the overheated
joint is known, only
that feeder can be
switched off for
checking loose
connection.
Manual
efforts
required to
execute
Highly Laborious –
checking all joints
with spanner for
loose connection is
an extremely
laborious job in a
huge network
Moderately
Laborious –
Compared to the
offline method,
this method
requires less
human efforts:
even though all
joints still have to
be checked,
checking them
with these devices
require less
efforts.
Almost Nil –
efforts required to
identify loose joint
is minimal since
location of loose
joint is pin pointed
by the system.
Time
Required to
execute
Highly Time
Consuming
Moderately Time
Consuming
Almost Nil:
Virtually no time is
required since loose
joints are
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automatically
identified by the
system.
Frequency
of checking
the EPDN
Very Low: can be
done only once or at
the most twice in a
year during the
scheduled shutdown
periods. Joint may
overheat and fail
between any two
checks.
Low: Can be done
more times since
shutdown not
required.
Nevertheless since
it is moderately
time consuming
and laborious it
can’t be
performed
continuously.
Joint may
overheat and fail
between any two
checks.
Extremely High
24x7: All joints are
monitored
simultaneously &
continuously hence
no chances of
failure of joint in
between.
Objectives & Advantages of the invention:
One of the objectives of the present invention is to detect the occurrence of and
communicate the location of an overheated cable joint from an EPDN by
monitoring the EPDN continuously for 24x7 without any human intervention
from any part of world through wireless network.
Another object of the present invention is to disconnect the feeder from the mains
to isolate supply to the overheated joint.
Brief Description of Drawings:
Fig. 1 shows a general arrangement of Electrical distribution, where power supply
is distributed to various sections.
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Fig 2 shows the block diagram of invention showing the Control Unit in detail.
Fig 3 shows cross section of control panel where thermal sensors are mounted
near cable joints.
Fig 4 shows electrical circuit diagram arrangement for interconnecting sensors,
push buttons to the Control Unit.
Fig 5 shows technology scheme with various components of the system.
List of parts:
1. Electric Power Distribution
Network (EPDN)
2. 3-phase power supply
3. Transformers
4. LT distribution panel (LT
panel)
5. Control unit
6. Power recipients
7. Fault identification unit
8. Power supply to the Control
unit
9. Programmable Logic Unit
(PLC)
10. Modem interface unit, 10A -
antenna
11. GSM unit
12. Alarm indicator
13. Thermal sensors, 13A –
power cables, 13B - joints
14. Cable end termination
15. Circuit breaker
16. Push buttons
17. Message receiving centres
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Summary of invention:
The present invention provides a system of detection of occurrence of overheating
and the location of overheated joint in an EPDN. The detection of overheated
joints is carried out using sensors which are provided near cable joints. These
sensors are wired up as an input to the control unit. In the event of a joint
overheating, changes in the input from the sensor to the control unit are identified.
Control unit gives signal to the Modem to transmit preprogrammed messages to
the assigned mobile numbers. The recipient of the messages, which may be
located anywhere in the world, is thus notified about the panel which included the
overheated joints. Depending upon mode of further action (automated or manual)
this information will include either the exact joint location or be limited to
specifying the affected panel location.
In the auto mode the control unit gives command to automatically shut off the
concerned circuit breaker to isolate the overheated joint from the EPDN. In the
manual mode determination of the exact location of the affected joint is done
through testing of each compartment of the panel, for example with the help of
push button assisted mechanism. The affected circuit is switched off manually.
Detailed description of invention:
In a typical EPDN (1) power supply (2) is distributed through a set of
transformers (3) to various recipients of power through LT distribution panels (4)
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(or simply LT panels). Figure 1 shows a typical EPDN (1) with a 3 phase high
tension (HT) power supply (2), a power transformer which converts supply from
HT to low tension (LT), a control unit, an LT distribution panel (4) which supplies
power to various feeders (not shown). It also shows a power supply recipient (6)
in the form of a machine which is connected to the LT panel (4). In an EPDN (1),
there could be hundreds of such LT panels (4), each containing dozens of cable
end terminations (14). The present invention, which will be described in detail, is
applicable to vast EPDNs (1) of this kind.
Figure 2 shows a schematic or a block diagram of the invention. It shows a fault
identification unit (7), a control unit (5), and an alarm indicator (12). The fault
identification unit (7) in turn comprises a set of thermal sensors (13), a fault
identification mechanism operated by a set of push buttons (16).
As shown in Figure 3, thermal sensors (13) are installed on power cables (13A)
near their joints (13B) in LT panel (4). The sensors (13) may be of any type
including but not limited to RTD, or thermostat, or thermocouples which are fixed
to cable with use of any means like cable ties etc. The sensors (13) are mounted at
any suitable distance but preferably not more than 5 inches from the joint on cable
insulation so that heat generated at the joints reaches the sensor. They may be
located at any position which allows sensing of temperature of the joints without
delay.
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The joints (13B) will typically be at a higher temperature than the temperature
sensed by the sensor. There will be thus a temperature gradient between the
sensed temperature and the actual joint temperature. For the safety of the EPDN
(1) it is considered that an alarm is raised based on the cable temperature at the
sensor location rather than the actual joint temperature. A gradient of 5 oC is
considered as practical with the sensor (through the signal sending mechanism of
the invention) being designed to send a ‘change of status’ signal when the cable
temperature at the sensor location reaches 70 oC. This would suggest that the
maximum joint temperature for the related joint would be no greater than 75 oC at
the time a ‘change of status’ signal has been sent out.
In the case the joint temperature comes down, another ‘change of status’ signal is
sent out, typically when the cable temperature drops to below 67 oC on its surface
(which would indicate that the temperature at the surface of joint drops to below
72 oC).
Figure 4a shows part of the fault identification unit (7). Sensors (13) are mounted
near each joint (13B). In the manually operated version of the invention, all
sensors (13) are connected in series. In the case of automated version of the
invention, the sensor circuit arrangement is as shown in Figure 4b.
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In the case of the manual version, a pushbutton (16) is wired in parallel
connection to each of the sensors (13) in any given LT panel compartment. All
sensors (13) are in series and wired as an input to the Control unit (5).
As shown in Figure 2, the control unit has a Programmable Logic Controller
(PLC) (9), a power supply (8), a modem interface unit (10), and a GSM unit (11).
The PLC (9) is powered by a 24v DC supply from power supply unit (8). The
power supply unit (8) itself runs on a 230v AC supply obtained from external
single phase source. The PLC (9) can be of any make but must have Modem
(Modulator Demodulator) interface (10) in it. The modem interface unit (10)
connects the modem with the PLC (9). The modem has a separate power supply
and provision for a GSM unit (11) which has a GSM-based SIM card. The modem
also has an antenna (10A) which is placed out the body of Control unit (5).
In the manual version of the invention, determination or identification of the
feeder line the joints (13B) of which have become overheated (or faulty) is carried
out by checking out the sensor circuit for each joint (13B). The checking process
involves accessing each of the sensor circuits. Rather than opening of the LT
panels (4), which poses health risk, pushbuttons (16) are provided on the outside
of the LT panel (4), typically on its backside. One pushbutton is provided for each
of the joints inside the panel. The pushbuttons (16) are connected to the sensors
(13) as indicated in Figure 4a. There is also an alarm indication arrangement in the
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form of a Pato lamp provided with a buzzer. The Control unit (5) also has a ‘fault
acknowledgement’ button to reset alarm once the faulty joints are identified.
For Auto mode of operation of the present invention (see Figure 4b) each sensor
(13) is wired independently to the control unit as a separate input. Also the control
unit is connected with a circuit breaker trip coil.
Operation of invention:
During the life of an EPDN (1), joints (13B) of an EPDN (1) system could
become loose due to various reasons. Loose joints (13B) at the cable end
terminals have an air gap between the cable ends and the parts they are connected
to. Since air is bad conductor of electricity, contact resistance at the loose joints
(13B) increases from its normal value. Increased contact resistance causes more
I2R-type losses which leads to generation of heat at loose joints (13B). Under
these circumstances, if the heating continues, the joint temperature continues to
rise. If the joint temperature reaches 75 oC, the system of the invention is so
designed that the sensor surface temperature may exceed 70 oC, thereby causing
the sensor to change its state.
When sensor changes its state, the PLC, which monitors the signal as an input,
registers the change. The PLC then selects a message such as “XYZ LT Panel –
Temperature too High” and commands the modem for the transmission of the
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message to the preprogrammed message receivers which are typically in the form
of mobile phones.
The modem of the modem interface unit (10) via SIM card of the GSM unit (11)
transmits the change of state messages to the message receiving centres (17) in the
vicinity – typically the mobile phone service provider’s mobile tower in the
vicinity. Through the GSM mobile network the messages are next delivered to
devices such as the mobiles phones of the maintenance staff or any other
concerned persons or monitoring system which are located within or outside the
plant anywhere in the world.
As an additional feature, the PLC (9) will also trigger an alarm indicator (12)
which is a Pato light mounted on the control unit. Pato light will continue to flash
optionally with a buzzer till either the sensor (13) resets (indicating a changed
state) due to temperature of overheated joint falling down to within an acceptable
limit or due to an intervention in the form of pressing of the pushbutton (16)
across the activated sensor, thereby bypassing the sensor circuit path which is
broken during the faulty state.
When maintenance person within factory receives the SMS (which is specific to
the LT panel in the manual mode of the invention), he will, as the first point of
action, typically first go to the central maintenance department to collect thermal
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gun. Next, he will go to specific LT panel which contains the faulty joints. A
single LT panel may contain a number of separate compartments for feeder.
Identification of exact compartment in which there is an overheated joint is a
challenging and time consuming task. The present invention facilitates this
through a very simple electrical circuit activation protocol (see Figure 4a).
The faulty joint identification process requires the maintenance person to press all
pushbuttons of the LT panel one by one while monitoring Pato light on the control
unit. In the case of a faulty joint, when the corresponding pushbutton is pressed,
the Pato light stops lighting. Once the compartment containing the faulty joint is
identified, the maintenance person first switch off the circuit breaker of the
compartment containing the faulty joint followed by opening the compartment
and checking the temperature of joints with thermal gun. By checking all six
joints with thermal gun he can easily and precisely identify the overheated joint
(s).
The present invention thus allows short-listing relatively small number joints (for
example, 500 or less) as suspected faulty joints from a large number of joints (for
example, 10,000) that may exist in an EPDN. With the provision of a simple
electrical circuit and by putting in place a simple checking protocol involving
pressing pushbuttons from outside the compartment, the invention makes it
feasible for the operator to further short list the number of faulty joints to, say, 6.
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Finally by scanning with thermal gun he can precisely pin point single overheated
joint(s).
In automated version of the invention, identification of cable joint is done
automatically and the SMS specific to the location of the faulty joint like “ Joint R
phase incoming, compartment 1, panel XYZ is overheated” is issued to the
concerned staff. Here the message is not only panel specific but also identifies the
actual joint.
The automated version also facilitates tripping off of the circuit breaker. Thus the
power supply to the overheated joint is automatically cut off and further damage
to the joint and consequently to the panel and ultimately potentially to the EPDN,
is avoided.
To cover the entire setup of LT panels multiple control units are installed across
the factory. Each control unit can take input from 8 LT panels where it can
provide panel-specific messages. For the automated version of the invention,
PLCs are provided with as many input channels as the number of joints in the
panel.
Another unique feature of this innovation is a ‘self-health check’ facility for
control unit. There is a test button provided on the control unit. When test button
is pressed, PLC executes a program which sends a test message like “Temperature
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SMS Unit at XYZ Location within acceptable limit” to all the concerned
authorities. The same message is also sent automatically by control unit at
predefined time of a day. Thus the system checks itself and intimates about its
healthiness on its own. This eliminates the need of manually monitoring the
working of control units which are remotely located; also if any of the control unit
stops working, it can be identified very easily since in that event SMS will not be
received from that Unit at predefined time.
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Claims:
1. A system to monitor, identify, and rectify cable termination joint hot spots
in electric power distribution networks, said networks distributing
electrical power through a transformer to various applications through low
tension panels, each of said panels containing a number of power cable
end terminations, characterized in that said system comprises thermal
sensors which are installed on power cables near their joints in the low
tension panels and an alarm indicator that raises an alarm based on the
temperature sensed by said sensor and the gradient between the
temperature sensed by said sensor and the actual joint temperature.
2. A system as claimed in claim 1 wherein said electrical power distribution
network distributes power through a transformer to various sections
through low tension panels to which power is supplied through a three
phase high tension power supply, a power transformer which converts
supply from high tension to low tension, a control unit, an low tension
distribution panel which supplies power to various feeders, and a cable
chamber.
3. A system as claimed in claim 2 wherein said thermal sensors are of any
type selected from a group comprising RTD, thermostat, and
thermocouples, said sensors being mounted at a distance whereby the heat
generated at the joints reaches the sensor and at a position which allows
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sensing of temperature of the joints in a predetermined time, preferably in
less than one second.
4. A system as claimed in claim 3 wherein a temperature gradient between
the sensed temperature and the actual joint temperature is maintained and
an alarm is raised based on the cable temperature at the sensor location
rather than the actual joint temperature.
5. A system as claimed in claim 4 wherein said temperature gradient is of 5 oC.
6. A system as claimed in claim 5 wherein is considered as practical with the
sensor (through the signal sending mechanism of the invention) being
designed to send a ‘change of status’ signal when the cable temperature at
the sensor location reaches 70 oC. This would suggest that the maximum
joint temperature for the related joint would be no greater than 75 oC at the
time a ‘change of status’ signal has been sent out.
7. A system as claimed in claim 6 wherein said sensor sends a first ‘change
of status’ signal when the cable temperature at the sensor location reaches
a temperature of 70 oC, and wherein said sensor sends a second ‘change of
status’ signal in the case the cable drops to below 67 oC.
8. A system as claimed in claim 7 wherein said sensors are mounted near
each joint, and wherein all sensors are connected either in series or in
parallel configuration.
9. A system as claimed in claim 8 wherein in the case where said sensors are
connected in series, a pushbutton is wired in parallel connection to each of
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the sensors in any given panel compartment, and wherein all sensors are
wired as an input to control unit.
10. A system as claimed in claim 9 wherein said control unit has a
programmable logic controller (PLC) with a modem interface to connect a
modem with said PLC.
11. A system as claimed in claim 10, wherein in the case of where said sensors
are in a series configuration, the determination of the feeder line the joints
of which have become overheated (or faulty) is carried out using a
checking process where sensor circuit for each joint is checked for
temperature.
12. A system as claimed in claim 11, wherein said checking process involves
accessing each of the sensor circuits by operating said pushbuttons and
where in the case of an overheated sensor is detected, an alarm is raised.
13. A system as claimed in claim 8, wherein said sensors are in a parallel
configuration, each said sensor being wired independently to said control
unit as a separate input, and wherein said control unit is connected to a
circuit breaker trip coil, and further wherein said modem transmits a
message regarding state of said joints to preprogrammed message
receivers.
14. A process to monitor, identify, and rectify cable termination joint hot spots
in electric power distribution networks, said process comprising the steps of:
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- providing a system to monitor, identify, and rectify cable termination
joint hot spots in electric power distribution networks as claimed in
any of claims 1 to 13 and comprising thermal sensors which are
installed on power cables near their joints in the low tension panels and
an alarm system that raises an alarm based on the temperature sensed
by said sensor and the gradient between the temperature sensed by said
sensor and the actual joint temperature
- causing said sensors to change its state in response to the joint
temperature reaching 75 oC, and registering said changed stage on said
programmable logic controller
- transmitting a message through said modem to said pre-programmed
message receivers,
- in the case where said sensors are in a series configuration, identifying
faulty joints by operating each said pushbutton
- rectifying detected faulty joints.
15. A process to monitor, identify, and rectify cable termination joint hot spots
in electric power distribution networks as claimed in claim 14, wherein in
the case where said sensors are in a parallel configuration, said step of
identifying faulty joints is carried but by automated sending of fault
indicative messages to pre-programmed message receivers.