37 CHAPTER-3 ON-LINE MONITORING OF TRANSMISSION LINE USING GSM TECHNOLOGY 3.1 INTRODUCTION Power sector is facing severe energy losses right from Generation to distribution, The technical losses in generation can be well defined and innovations are on to scale down these losses. The severe losses on account of Transmission and Distribution are indefinable but cannot be quantified with the sending end parameters. This illustrates the involvement of non-technical parameters in T&D of electrical energy. T & D losses are of greater concern for the Indian Electrical Industry (IEI) since their magnitude is huge when compared to other developed countries. The present T & D losses which include unaccounted energy loss are between 18 to 32% amounting to a financial loss of Rs 1,000,000 Cr PA. These losses are on account of improper handling of transmission and distribution system. If we can save even 1% out of the above losses it will be a definite help to the power sector in specific and the environment at large. As per The Energy and Resources Institute (TERI) [59], energy losses occur during the process of supplying electricity to consumers. The total T & D losses are combination of technical and non-technical losses. The technical losses are due to energy dissipated in the conductors and equipment used for transmission, transformation, sub-transmission and distribution of power. These losses are inherent in a system and can be reduced to a best level. The non-technical (commercial) losses are caused by pilferage, defective meters, and errors in meter reading and in estimating unmetered supply of energy.
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37
CHAPTER-3
ON-LINE MONITORING OF TRANSMISSION LINE USING GSM
TECHNOLOGY
3.1 INTRODUCTION
Power sector is facing severe energy losses right from Generation to
distribution, The technical losses in generation can be well defined and innovations
are on to scale down these losses. The severe losses on account of Transmission and
Distribution are indefinable but cannot be quantified with the sending end parameters.
This illustrates the involvement of non-technical parameters in T&D of electrical
energy. T & D losses are of greater concern for the Indian Electrical Industry (IEI)
since their magnitude is huge when compared to other developed countries. The
present T & D losses which include unaccounted energy loss are between 18 to 32%
amounting to a financial loss of Rs 1,000,000 Cr PA. These losses are on account of
improper handling of transmission and distribution system. If we can save even 1%
out of the above losses it will be a definite help to the power sector in specific and the
environment at large.
As per The Energy and Resources Institute (TERI) [59], energy losses occur
during the process of supplying electricity to consumers. The total T & D losses are
combination of technical and non-technical losses. The technical losses are due to
energy dissipated in the conductors and equipment used for transmission,
transformation, sub-transmission and distribution of power. These losses are inherent
in a system and can be reduced to a best level. The non-technical (commercial) losses
are caused by pilferage, defective meters, and errors in meter reading and in
estimating unmetered supply of energy.
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Reduction of losses will have healthy economics and also a positive step to
preserve environment, however it is often expensive and difficult to reduce technical
losses. Replacement of old equipment with the latest is one way to reduce technical
losses. The Life of system components can be increased substantially if the faults are
sensed through an effective, speedy and highly sensitive wireless communication
System. Current method being used to assess the damage on the transmission grid is
by visual inspection. Due to dispersion of transmission lines over hundreds of miles, it
is difficult to sense the fault by visual inspection or by using traditional methods. In
order to acquire different parameters and deliver them to the control centers, it is
required to install data acquisition systems (DAS) and various sensors in
predetermined towers and communicate via wireless network. For efficient
monitoring and control, a robust and fast communication system is required [60]. This
chapter discusses how the different data is acquired and delivered to the control center
for loss analysis and to initiate corrective measures by using different sensors and
wireless communication. The proposed method is highly useful for the future
deregulated power system and smart grid applications.
There are many ways to collect the information i.e., SCADA, PLC, optical
fiber, etc. However, it is impossible to use these systems in many places in the power
system (remote places) and also costly. A wireless solution is thus sought. There are
wide smart grid applications [41] using wireless communication. The GSM SMS and
ZigBee communication are proposed [61] for monitoring over head conductors. The
concept of using wireless sensors has proposed [62] for substation automation .The
Y.Yang, F.Lambert and D.Divan were first proposed the use of sensor networks to
monitor overhead transmission lines [63, 64]. R. A. Leon, V. Vittal, Y. Yang et al [65,
66] introduced the importance and implementation of sensors in power grid
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monitoring. The proposed model consists of sensors like; voltage sensing transformer
is used for acquiring change in voltage, Thermister for change in temperature and
Accelerometer for cable sag & tilt due to overloading and climatic conditions.
Wireless GSM is used to deliver and collect the data. Fig.3.1 shows the single line
diagram of power system network using wireless communication.
Fig.3.1. Single line diagram
3.2 LINEAR NETWORK MODEL
This section explains the proposed linear network model. Fig.3.2 shows an
example of long overhead transmission line where the number of towers in-between.
The distance between two primary substations can be 50 kilometers. On the other
hand distance between two towers can be 0.25 to 0.5 kilometer depending on actual
needs and geographical constraints.
To monitor the transmission line and acquire the changes in the power system
line with respect to time, different sensors listed in Table 3.1 are used. In general all
sensors are analog in nature and deliver output in the form of voltage, current,
resistance, etc. All the signals will be fed for conversion to achieve signals which can
be perfectly suitable for computing using state of art embedded technology.
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Fig.3.2. Power system network with sensors
The embedded technology collects data and converts it for soft computing.
Appropriate software can compute these signals and creates control, information, data
logging and communicating signals.
The primary parameter which can change during all the conditions is voltage
and it determines the energy loss. The change in voltage is directly proportional to
losses to certain extent. The second parameter is current which gives information
about real usage. Once, if the current goes high beyond the set limit, it creates sag
(voltage sag) and power becomes infinite. So, entire thing will be considered as loss.
Unless we have a proper tool to monitor, this cannot be identified and solved. A real
relationship of usage verses loss can be arrived only from the magnitude of current.
The change in temperature occurs on transmission line due to various reasons;
it starts from simple overloading, continuous overloading, and climate changes. All
this creates skin effect and makes them to elongate from its original length and
thereby forms physical sag between two poles (Towers) [67, 68]. The proportional
increment of cable length will increase I2R loss and voltage drops in the transmission
lines.
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To analyze the parameter called sag, created because of frequent change in
temperature and it is essential to overcome on long term basis. Sag will not be created
at the time of installation. During the course of time with the changes in temperature
and climate sag will be created automatically and increase the loss. More sag can
create tilting of transmission line during climatic turbulences. The effect of sag can be
reduced by maintaining the temperature within the limit.
The model is designed by considering voltage sensing transformers,
accelerometers and temperature sensors. These are placed at 25% and 75% of the total
distance and close to the poles/towers. Each sensor sends data at regular intervals
[65]. Data acquisition system also placed on the poles / towers to collect the data from
the sensors. The data obtained on transmission lines are sent through GSM lines to the
control centre in the substation.
At the receiving point (control centre) all GSM data will be received from
various parts of transmission line and fed to a single computer. Analysis is made
regarding the losses at various points and will be displayed. All the collected data will
be logged safely for future or present comparison of utilization factor (UF).
The UF based demand management will reduce various burdens on
transmission line and keeps devices perfect. The proposed scheme presents data like
power handled by tower, losses between tower to tower, loss percentages at each
tower, overall efficiency of transmission line. As a decision making system, the
scheme will deliver control outputs in the form of digital and will be converted into
RS-232 standard. The RS-232 data will be converted into GSM signals and passed to
distribution end. Thus, wireless communication provides a major contribution to
reliable network operation and efficient energy management.
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Table 3.1.Types of sensors for monitoring
Monitoring parameters Type of sensor
Cable tilt Accelerometers
Inclination Accelerometers
Temperature Temperature sensor
Extension & Strain Strain sensor
Cable position Accelerometers
Current Magnetic field Sensors
Magnetic field Magnetic field Sensors
Power Quality Graph Magnetic field Sensors
3.3. STRUCTURE OF LABORATORY MODEL
The structure of the proposed laboratory model for on-line analysis of
transmission line is shown in Fig.3.3. The model is a scaled down version of the
original system using low voltages. The change in voltage, current, temperature and
angle are scale down values of real variables. The same circuits can be employed for
higher version installation at the field areas. There are no changes required except
enclosures for the DAS circuit. The proposed technology consists of the following
seven major categories to meet on-line challenges and acquiring transmission line
data:
� Instrument Transformers
� Signal conditioning circuits (SC)
� Accelerometers
� Temperature Sensors
� Embedded microcontroller (EMC)
� Software required
� Communication network
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Fig.3.3.Block diagram representation of the laboratory model
3.3.1 Instrument Transformers
Instrument transformers are used in the measurement and control of
alternating current circuits [10]. These are essential to step down the high voltage /
current into measurable low voltage / current for measuring purpose. Isolation with
ratio metric reduction will be done by these types of transformers.
There are two distinct classes of instrument transformers:
(1) Potential transformer
(2) Current transformer
Potential Transformers
The potential transformer operates on the same principle as that of a power or
distribution transformer. The main difference is that the capacity of a potential
transformer has ratings from 100 to 500 volt amperes (VA). The low voltage side is
usually wound for 110 V. The high voltage primary winding of a PT has the same
voltage rating as that of the primary circuit. Assume that it is necessary to measure the
voltage of a 3.3kV, single phase line. The primary of the PT is rated at 3.3kV and the
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low voltage secondary is rated at 110V. The ratio between the primary and the
secondary winding is: 3300/110 or 30/1.
Current Transformers
Current transformers are used so that ammeters and the current coils of other
instruments and relays need not be connected directly to high voltage lines. In other
words, these instruments and relays are isolated from high voltages. CTs also step
down the current in a known ratio. The use of CT allows using relatively small and
accurate instruments, relays and control devices of standardized design in the
measuring circuits.
The existing transmission line need not be modified or reconfigured, because
the CT is noncontact primary (Clamp type or tong type) type. The secondary winding
has the standard current rating of 5A; therefore the ratio between the primary and
secondary current is xxx/5A.
3.3.2 Signal Conditioners
These devices are made up of semiconductor operational amplifiers. Signal
conditioners are more reliable. The output of secondary isolation system will be AC in
nature, must be rectified, conditioned and calibrated as per the requirement of
conversion circuits. These circuits are Op-Amp based full wave precision rectifiers
(or) absolute rectifiers. These circuits meet overall standards of measurements. The
prime objectives of these devices are to rectify, filter, setting up the calibration limits,
protecting the high voltage hazards, protecting the inputs and outputs. Output of these
circuits will be pure DC in nature.
3.3.2.1 Voltage Sensing
Voltage sensing circuit is shown in Fig.3.4. It consists of bridge rectifier; it
can be used to convert AC to DC. A1 is an inverting unity gain amplifier. A2 is
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inverting summing mixed gain amplifier. During positive half cycle the Op-Amp A1
produces an output of 0.454V. Op-Amp A2 produces an output of 0.908V across the
path having gain of –2 and an output of –0.454V across the path having a gain of –1.
Thus, the resultant output voltage is 0.454V. It can be amplified to require voltage by
varying the trim pot. The 500K trim pot is adjusted so that a full scale output voltage
of 5V is produced. A capacitor is connected to A2 so that it acts as an integrator.
Hence output voltage is a pure DC voltage it is then given to ADC.
Fig.3.4 Voltage sensing circuit diagram
3.3.2.2 Current Sensing
Current sensing circuit diagram is shown in Fig.3.5. Current sensing is very
similar to the voltage sensing, instead of potential divider a shunt to be used to
convert current into voltage. Once current is converted into voltage, Full wave
precision rectifier (FWPR) can be directly used and output will be 0-5V
corresponding to the minimum to maximum CT value of 0-5Amps.
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Fig.3.5 current sensing circuit diagram
3.3.3 MEMS Accelerometer
MEMS Technology is a well known abbreviation for Microelectromechanical
systems even though; there exist various names for MEMS technology such as micro
machines etc. MEMS Accelerometer accurately detects and measures acceleration,
tilt, shock and vibration in performance-driven applications. In industry it detects the
power, noise, bandwidth and temperature specifications and earthquake detection in
geotechnical engineering (Bernstein, 1999). Since there are various types of sensors
for various applications, there is a need to select the right sensors which fit to the
intended applications. In our work we have used MEMS Accelerometer to measure
transmission line tilt and sag.
We have used three axes MEMS accelerometer which provide voltage output
for the change in X, Y, Z axes. No need to connect signal conditioner because it
produces 0 to 5V for the change in physical directional changes.
3.3.4 Temperature Sensors
Transmission lines are heated due to over load and climatic conditions. When
line current increases, the conductor heats up, elongates, and the line sag increase. If
the line is operated beyond its maximum design temperature, the line sags may violate
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design clearances. Use of the Transmission Line Monitoring System for dynamic
ratings allows utilities and transmission operators to develop and apply line ratings in
real time, based on actual weather conditions instead of fixed, conservative
assumptions. By using temperature sensors and wireless communication, over-head
lines and cables are monitored, analyzed, and visualized with one system. In the
proposed model Thermister is used as a temperature sensor which is inexpensive, easy
to use and adaptable. Temperature is the most important parameter during high
current flow on the cables, possibility of losses will be very high and can be detected
using temperature sensor and appropriate tripping action can be performed to save
energy.
3.3.5 Embedded Microcontroller
Generally A/D converters are interfaced with the microprocessor using a
separate interfacing IC namely programmable peripheral device. This requires large
hardware circuit. But in the proposed design, state of art embedded system technology
is used to reduce lot of hardware. These devices consist of packed hardware inside;
any devices can be brought down to the front end and can be used. Output from signal
conditioning circuits is connected to this circuit for A/D application. Simultaneously
all analog data are fed and digitized data are sent to computer as RS-232 signals. The
digitized data is to be decoded for real values. The expected speed of this device is
9600 baud rate. It proposes middle end embedded microcontroller like pic16F877A,
which consists of 8 channel 10bit ADC with lots of additional features. These devices
require very minimum supporting hardware’s like clock and reset circuits externally.
The embedded circuit diagram is shown in Fig.3.6. The circuit consists of
1) Power supply
2) Clock circuit
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3) PIC16F877A
4) Reset circuit
5) RS232 circuit
Fig.3.6 Embedded circuit diagram
3.3.5.1 Power Supply Circuit
Irrespective of the technological growth one must construct a reliable power
source for embedded controller. A 230V/12V step down transformer and bridge
rectifiers are used to convert into DC. A constant voltage regulator LM7805 with
necessary filters is used to produce constant 5V given to embedded circuit.
Irrespective of the change in voltage and current, output voltage will be kept constant
at 5V.
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3.3.5.2 Clock Circuit
10 MHz crystal as a resonator made up of Quartz is used in this work to meet the
requirements. Generally crystal oscillator is made up of quartz whose crystalline
structure will not be changed under any circumstances on physical changes. 10 MHz
crystal oscillator is used to produce constant frequency. Further, it can be divided
inside the microcontroller as per the requirement of the operation to achieve exact
timing. The general disadvantage of the crystal, it may produce abnormal clocking at
some conditions, which can be eliminated using appropriate low pass filter coupled
across crystal and connected to ground. Crystal is connected across 13 and 14 pins.
3.3.5.3 PIC16F877A Microcontroller
Microprocessors brought the concept of programmable devices and made
many applications of intelligent equipment. Most applications which don’t need large
amount of data and program are tended to be costly and consist of a lot of peripherals.
These drawbacks lead to the use of microcontroller, which is a true computer on a
chip. This is heart of the work, which collects the data, passes to the computer and
takes control action. To perform various operations and conversions required to
switch, control and monitor the devices a processor is needed. In this research a
PIC16F877A Microcontroller is used. The pin-Diagram of the Microcontroller is
shown in Fig.3.7. The features and external requirements are discussed below.
Features
• High-performance RISC (Reduced Instruction Set Controller) CPU
• Only 35 single word instructions to learn
• All single cycle instructions except for program branches which are two cycle
• Operating speed: DC - 20 MHz clock input and DC - 200 ns instruction cycle
• 4K x 14 words of Program Memory (EPROM)
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• 256 x 8 bytes of Data Memory (RAM)
• Interrupt capability (up to 14 internal/external interrupt sources)
• Eight level deep hardware stack
• Direct, indirect, and relative addressing modes
• 12-bit multi-channel Analog-to-Digital converter On-chip absolute band gap