microcontroller based transformer protectio
TRANSCRIPT
CHAPTER 1
INTRODUCTION
1.1 Background of study
Protection against fault in power systems (PS) is very essential and vital for
reliable performance. A power system is said to be faulty when an undesirable condition
occurs in that power system, where the undesirable condition might be short circuits,
overcurrent, overvoltage etc.
The power transformer is one of the most significant equipment in the electric
power system, and transformer protection is an essential part of the general system
protection approach. Transformers are used in a wide variety of applications, from small
distribution transformers serving one or more users to very large units that are an integral
part of the bulk power system (Anderson, 1998, P.673).
Moreover with the increasing population and their unavoidable demands,
leads to the high increase demands on electrical power. With this increase in demand of
power, the existing systems may become overloaded. The overloading at the consumer
end appears at the transformer terminals which can affect its efficiency and protection
systems. One of the reported damage or tripping of the distribution transformer is due to
thermal overload. To escape the damaging of transformer due to overloading from
consumer end, it involves the control against overcurrent tripping of distribution
transformer. Where the technology of the day has given the opportunity to use the latest
trends, and microprocessor, microcontrollers are one of the day requirements to apply in
the remote protection of the transformer.
The purpose of power system protection is to detect faults or abnormal
operating conditions and to initiate corrective action. Relays must be able to evaluate a
wide variety of parameters to establish that corrective action is required. Obviously, a
relay cannot prevent the fault. Its primary purpose is to detect the fault and take the
necessary action to minimize the damage to the equipment or to the system. The most
common parameters which reflect the presence of a fault are the voltages and currents at
the terminals of the protected apparatus or at the appropriate zone boundaries (Grigsby,
2007).
The Protective relays require reasonably accurate reproduction of the
abnormal and normal conditions in the power system for correct sensing and operation.
This information input from the power systems are usually through Current Transformer
(CT) and Voltage Transformer (VT).
Furthermore, for the past several years fuse, circuit breakers and
electromechanical relays were used for the protection of power systems. The traditional
protective fuses and electrometrical relays present several draw backs.
Alternatively, some researches were conducted on relay which can be
interfaced to microprocessors in order to eradicate the drawbacks of the traditional
protective techniques (Bashi, 2007) which led to many improvements in transformer
protection in terms of lower installation and maintenance costs, better reliability,
improved protection and control and faster restoration of outages.
Therefore a proposed solution is chosen to develop a microcontroller based
transformer overload protection prototype because the microprocessors based relays
provides greater flexibility, more adjustable characteristics, increased range of setting,
high accuracy, reduced size, and lower costs, along with many ancillary functions, such
as control logic, event recording, fault location data, remote setting, self-monitoring and
checking, etc.(Blackburn,2006).
1.2 Problem statement
An essential concern in transformer protection is the high cost of the
transformer and the relative long outage time that occurs when a large transformer fails.
The proper type of protection can often detect initial faults before they become major,
and thereby prevent major physical damage and long outage times (Anderson, 1999,
P.67)
Transformers experiences faults which leads to deterioration and acceleration
aging and failure of transformer winding resulting from insulation failures, one of the
causes is the over current. Due to overload and externally applied conditions including
over current and external short circuit causes rise in temperature of both transformer oil
and windings (Bashi, 2007).
Whenever the winding temperature raises and exceeds transformer thermal
limits, the insulation will deteriorate and may fail prematurely. Continuous thermal
overload (over temperature) might weaken the insulation of a transformer and resulting in
rapid transformer loss of life.
Over excitation (an increase in system voltage), internal faults can lead to
deterioration, acceleration aging and fault trips in transformer protection function (Reza,
2003).
Similarly, transformers must not be subjected to prolong overvoltage. For
maximum efficiency they are operated near the knee of their saturation curve, so at
voltages above 110% of rated, the exciting current becomes very high. Just a few percent
increase in voltage results in a very large increase in current. These large currents can
destroy the unit if they are not reduced promptly. (Blackburn, 2006).
However engineers and scientist have worked out various ways in which the
transformer can be protected; one of such ways is by using a relay. Therefore in order to
protect the transformer using relay, a control system idea is developed through the use of
microcontroller, hence the name of the project.
1.3 Aim and Objectives:
Aim:
The aim of this project is to design and implement a microcontroller based
transformer protection system.
Objectives:
1. To design the current and voltage sensing circuits that will be interfaced to
the microcontroller for monitoring.
2. To develop an algorithm and codes to the microcontroller which will work for under over
current, over voltage, under voltage conditions and transmit the parameters to a personal
computer.
3. To analyze and validate the performance of this system using appropriate simulation
software.
1.4 Research scope
The overall aim of the study is to build a microcontroller based transformer
protection with transformer parameters monitoring capabilities. This protection is based
on the transformer parameters fed into the ADC of the microcontroller and monitoring
the transformer parameters through PC. Immediately a fault is detected the
microcontroller taking necessary action.
Furthermore, the limitation of the entire project is divided into two. First part
of the project is to design and build the hardware of the entire system. Where a voltage
transformer of 230:160 VAC, current sensor, electromechanical relay, microcontroller,
LCD display, and finally a power supply that can generate 5VDC in order to activate the
relay circuit, the microcontroller and LCD. The second part is the development of a C
language program that will satisfy the protection of the transformer algorithm.
Correspondingly, the system development will also concentrate on elaborating
and designing a suitable transmitter module using microcontroller based circuit. An AVR
microcontroller was selected for this project because of its universal synchronous
asynchronous receiver and transmitter (USART), inbuilt ADC functionalities etc.
Finally, the highest priority is given to the software design and
implementation in order to develop a suitable algorithm that will promptly interact with
the, microcontroller and the personal computer.
1.5. Chapter outline
Chapter 1- Gives the general information about the project. The project aims and
objectives as well the problem statement.
Chapter 2-This chapter comprises of the reviews on previous works done in the field of
power transformer protection techniques and how the knowledge gained from such
reviews is deployed to meet the goals of this project.
Chapter 3-This chapter focuses in the description of the system block diagram as well as
the explanations of such block diagram with some design calculation
Chapter 4- This chapter comprises of detailed information on circuit schematic, PCB
design and microcontroller source codes.
Chapter 5- This deals with all the hardware and software testing results with a detailed
analysis on each of them.
Chapter 6- This chapter concludes the entire work performed during the project and
points out some few recommendations on improving the performance, efficiency and
reliability for systems to be designed in the future.
CHAPTER 2
LITERATURE REVIEW AND THEORETICAL BACKGROUND
2.1 Literature Review
Enormous work has been done on protection of power transformers such as:
overload voltages, overcurrent and external short-circuit etc. Some of those researches
which had been conducted, their setbacks, ambiguities and merits are as discussed in the
subsequent paragraphs.
In 2003 Ali Reza Fereidunian proposed a design which was based on a digital
differential relays for transformer protection using Walsh series and least square
Estimators. The Two estimators were been developed using the Walsh series and least
square algorithms. The transformer internal fault (short circuit) protection function was
the functionality of the differential relay. The performance of the proposed relay was
tested for internal, external faults and inrush current of the transformer. In their project
the digital differential relaying scheme comprised of filter, pre-process, data acquisition
system and a decision maker. The protective relay performs the duty of making decision
about the faulted or non-faulted situations.
The transformer internal faults and the external faults situations were tested on
the designed differential relay, and the results of these test illustrates the efficiency of the
system. Also, it was seen that both estimation algorithms perform their job correctly, but
the Walsh series acts better than least squares algorithm estimation (Ali Reza et al.,
2003).
Furthermore in 2007 S.M. Bashi et al, designed and built a microcontroller
based system for power transformer protection. The system includes facilities for
discrimination between internal fault current and magnetizing inrush current, differential
protection, over current protection has been included.
The performances of the proposed system have been examined and from the
experimental readings and observation, it was understood that the proposed system
monitors and controls the transformer when there is any fault ( Bashi et al 2007)
In 2010 V. Thiyagarajan and T.G. Palanivel proposed an innovative design to
develop a system based on microcontroller that was used for monitoring the current of a
distribution transformer in a substation and to protect the system from the rise in current
due to overloading. The protection of the distribution transformer was accomplished by
shutting down the entire unit with the aid of radio frequency communication.
The performance of the proposed system had been examined with three
various types of loading which had been added to the transformer. From the experimental
observations it was understood that, the proposed system monitors and controls the
transformer in an efficient manner. Whenever an over current was sensed by the system
while monitoring the transformer, it directs the main station to shut down the transformer
and thus it guards the unit from any serious damages (Thiyagarajan et al 2010).
In June 2006 Atthapol Ngaopitakkul and Anantawat presented an algorithm
based on a combination of discrete wavelet transformer and neural networks for detection
and classification of internal faults in a two winding three-phase transformer. The fault
conditions of the transformer are simulated using ATP/EMTP in order to obtain current
signals. The training process for the neural network and fault diagnosis decision are
implemented using MATLAB.
It was found that the proposed method gave a satisfactory accuracy and can
be useful in a development of a modern differential relay for transformer protection
scheme (Ngaopitakkul et al, 2006).
Larner et al presented a paper that attempts to review the concept of a fuse
application on high voltage Power transformers. The practical connecting of a power
transformer directly to a transmission line through fuse was discussed and was found that
the fuse presents several problems in the protection. One of the stated problem that a fuse
is that it cannot detect a fault current resulting from turn-to turn faults within the
protected transformer which can be well below the load current rating of the transformer
(Larner et al 1959).
In 2010 Mazouz et al conducted a new approach research for transformer
differential protection that ensures security for external faults, inrush and over-excitation
conditions and provided dependability for internal faults. The approach uses
programmable logic controllers (PLCs) to realize transformer differential protection.
It was concluded that the PLCs gave the protection circuits more flexibility
and makes their integration with other protection and control circuits easier. And also
found that the differential protection using PLCs provided high sensitivity for internal
faults and high stability for external faults and magnetizing inrush currents (Mazouz et al
2010).
Finally in 2000 Vaccaro et al proposed a neural diagnostic system for
transformer thermal overload protection. The research was conducted because the IEEE
power system relaying committee were lacking inaccuracy in the prediction of maximum
winding hot-spot temperature of a power transformer in the presence of overload
conditions. The proposed method was based on a radial basis function network (RBFN)
which taking in to account the load current, the top oil temperature rise over the ambient
temperature and other meteorological parameters, permits recognition of the hot-spot
temperature pattern.
The radial basis function network (RBFN) based algorithm was designed and
trained, in order to estimate the winding hot-spot transformer temperature from
knowledge of the experimental top oil temperature, weather conditions and load current
data obtained from a laboratory prototype mineral-oil-immersed transformer.
Finally, the RBFN-based algorithm for the identification of the dynamic
thermal overload in power transformers have been developed and was found that the
accuracy was improved compared with the results obtained from the IEEE power system
relay committee (Vaccaro et al 2000).
From the above review, it is finally concluded that researches done on the
transformer protection have some weaknesses; therefore by using the proposed method,
which is by using microcontroller based approach, the protection will be enhanced to a
better protection because the application of microcontroller in protecting transformer
against overcurrent and over voltage is speedily growing.
2.2 Theoretical Background
From the reviewed journals, based on S.M. Bashi work, this project plan to
design, analyse and implement the hardware of the system. Correspondingly, it is
understood that the topic of research is an advanced area of power systems protection
engineering which is normally being explored by power engineers. The purpose of the
system design is to solve complex and eradicate the problems encountered using the local
protection techniques such as fuse, circuit breakers etc.
The project is based on microcontroller transformer protection with PC based
transformer parameters monitoring capabilities. This protection is focused on the
transformer parameters feed into the ADC of the microcontroller and monitoring the
transformer voltage, current and temperature through personal computer. The voltage
transformer will be connected across a variable AC input source using an autotransformer
which can be varied from 0-250VAC. The output of the transformer (secondary) will be
connected to electric bulbs which will serve as loads. The load current will be monitored
by connecting a current sensor in series between the load and the secondary side of the
transformer. The transformer voltage will be monitored through rectifying a step down
230-12VAC transformer to a pure 5VDC and then feed to the microcontroller ADC pin
for voltage monitoring. The input of the step down transformer will be connected to
autotransformer and the output will be perfectly rectified to a pure 5VAC.
Whenever the input voltage is varied, the microcontroller shows the value of
the voltage on an LCD and also on the PC. As input voltage is varied above 230VAC, the
microcontroller detects an over voltage fault and it sends a trip signal to the voltage
protective relay for protecting the transformer and the load connected.
Similarly, the microcontroller monitor’s the load current and temperature of
transformer and displays the values on LCD and on the PC. Whenever loads are added to
the secondary side of the transformer, the current at the secondary side rise. As the load
current exceeds the rated current rating of the transformer, the temperature of the
secondary winding rises, therefore the microcontroller will send a trip signal to the
overcurrent protective relay, thereby protecting the transformer from burning.
`
CHAPTER 3
SYSTEM DESIGN
3.1 Overview
The block diagram of the system is shown in figure 3.1
Fig 3.1 Block diagram of the system
The primary of the 230:160VAC transformer is connected to a variable AC input
voltage (autotransformer), and the output is connected to a load which is usually
electrical appliances such as bulbs, electric heater etc.
At the primary side of the 230:160VAC transformer, a step down 230-12VAC
transformer is rectified to a pure 5VDC and feed into the ADC pin of the microcontroller
for monitoring the voltage of the transformer.
At the secondary side of the transformer, a current sensor is connected in
series between the load and the transformer secondary terminal for sensing, the load
current, output of the current sensor is then feed to the microcontroller ADC pin for
monitoring.
The LCD is used to display the transformer voltage, current and temperature,
similarly the personal computer is used to display the transformer parameters for
monitoring purpose.
While monitoring the transformer parameters, whenever the load current
exceeds the transformer rated current, the microcontroller detects an overcurrent faults
and it sends a trip signal to the overcurrent relay, thereby protecting the transformer from
blowing off.
Moreover, when the autotransformer secondary is varied above the specific
limit, the microcontroller detects an overvoltage faults and it sends a trip signal to the
overvoltage protective relay, thereby protecting the transformer and the loads from
blowing off.
3.2 Component details
Based on the various reviews conducted on transformer protection and the
above block diagram which was conceived out of those literature reviews conducted,
numbers of components are required in developing the protection system.
3.2.1 Microcontroller
The microcontroller is required to serve the purpose monitoring the
transformer information such as temperature, voltage and current through the LCD
display, personal computer and triggering the relay when there is any fault. Modern
power networks require faster, more accurate and reliable protective schemes.
Microcontroller-based protective schemes are capable of fulfilling these
requirements. They are superior to electromagnetic and static relays. These schemes have
more flexibility due to their programmable approach when compared with the static
relays which have hardwired circuitry.
Therefore in order to achieve this task the ATmega32 microcontroller was
chosen because of its suitability for this project such as speed, power consumption,
universal synchronous asynchronous receiver transmitter (USART) functionality, in built
ADC, and amount of RAM and ROM on the chip.
The ATmega32 is a low-power CMOS 8-bit microcontroller based on the
AVR enhanced RISC architecture. It has a High Endurance Non-volatile Memory
segments such as 32K Bytes of In-System Self-programmable Flash program memory,
1024 Bytes EEPROM, 2K Byte Internal SRAM, write/erase Cycles: 10,000
Flash/100,000 EEPROM.
The ATmega32 microcontroller I/O pins are 40 in number, and most of them can
be used as I/O pins. The input/output pins serves the purpose of connecting the ADC
chip, LED, LCD display, alarm buzzer and in this case the port A, pin one, two and three
were used to take care of ADC input since we are using three different analogue signals
one for the voltage transformer other for the current transformer and finally for the
temperature sensor.
3.2.2 Current sensor
The protection of the transformer against over current is concerned with the
detection and measurement of fault, where the measurement can be dangerous and indeed
impossible to measure if the actual load and fault currents are very large. A professional
way of avoiding these difficulties is to use the current sensor. Therefore in the block
diagram, current transformer is used to measure the load current.
The current sensor ACS756 was used because the current sensor ICs provides
economical and precise solution for AC or DC current sensing in industrial, automotive,
commercial, and communication systems. The device package allows for implementation
by the customer. Typical applications include motor control, load detection and
management, power supplies and overcurrent fault protection. The current sensor is
capable of measuring up to 50A.The monitored current values are displayed on the LCD
display and as soon the voltage transformer is overloaded the current transformer sends
the information through the ADC and the microcontroller energizes the relay, thereby
protecting the transformer.
3.2.2.1 Overcurrent protection circuit
An ammeter cannot be used in measuring the load current in this project
because an analogue signal most be fed into the ADC of the microcontroller for
monitoring the load current. A current sensor was found to be the suitable current sensing
device for this purpose. The current sensor used can measure up to 50A. The BB-
ACS756 comes with one set of dean-T connector and a 3 ways right angle pin header.
The ACS756 is power up with 5VDC and gives out voltage to indicate the direction and
current value.
The output of the current sensor is fed to Micro-controller ADC unit for
taking the necessary action. The current flowing through the CT primary can be
measured, for this purpose, digital display is provided at the output of the Micro-
controller Chip. Figure 3.2 shows the circuit diagram of the current sensing circuit.
Figure 3.2 Current sensing circuit
3.2.3 Voltage transformer
The 230VAC:12VAC step down voltage transformer is used to measure the
load voltage. The voltage transformer will pass through rectification process before fed to
the ADC. .
3.2.3.1 Secondary winding calculation
The transformer used has 120 turns of coil in the primary; therefore secondary
winding turn is calculated as shown below:
turnsN
VV
N
EE
NN
80240
1920012230120
)1.........(..................................................
2
2
2
1
2
1
The above calculation shows that the transformer has a turn ratio of 120:80 =12:8
3.2.3.2 Primary current calculation
The transformer used is a step down transformer (230VAC:160VAC). It is
known that a transformer with less turns in the secondary than in the primary would step
down the voltage, but would step up the current. The below calculation will verify that.
AI
I
II
EE
EE
NN
II
NN
667.0240160
1160240
)2.(......................................................................
1
1
1
2
2
1
2
1
2
1
1
2
2
1
The above calculation shows that the step down transformer has step up the
primary current from 0.667 to 1A at the secondary.
3.2.4 Analysis of Voltage protection circuit
3.2.4.1 Over voltage protection circuit
The over voltage and under voltage protection circuit is capable of measuring
and monitoring voltage from 200 to 250VAC. In this project the voltage can be increased
or decreased by using the autotransformer and the output of the voltage monitoring
circuit is fed to ADC converter, whenever the voltage is varied to 200VAC, the
microcontroller will detect under voltage fault and whenever the voltage is varied to
250VAC, the microcontroller detects over voltage fault, consequently the microcontroller
sends a trip signal to the relay, and the relays cuts the primary of the transformer from the
AC mains, thereby protecting the transformer.
Figure 3.3 over voltage sensing circuit.
In Figure 3.3, a step down transformer of 230-12VAC was used and was
rectified to a pure dc using the capacitor and then adjusted to voltage within 5VAC using
the potentiometer in order to be fed the analogue signal into the ADC without burning the
ADC converter.
Whenever the primary voltage of the transformer is adjusted, the secondary
voltage also changes, and based on the microcontroller program, the input voltage can be
monitor, displayed and the transformer can be protected from any over voltage fault.
3.2.4.2 over voltage protection circuit design calculation.
3.2.4.2.1. DC voltage design calculation.
The secondary voltage of the transformer is 160VAC and connected to a
bridge rectifier, therefore the DC output is approximately:
57.154.1212
)3(........................................).........7.02(2
DC
ACDC
V
VV
From equation 3, the VAC is the RMS transformer voltage and the 0.7V is the
voltage drop across the rectifier. As there are two diodes conducting for each half cycle,
therefore there will be two rectifier voltage drops.
3.2.5 Relay
The relay is an electrically controllable switch widely used in industrial
controls, automobiles, and appliances. It allows the isolation of two separate sections of a
system with two different voltage sources. For example, a +5V system can be isolated
from a 120V system by placing a relay in between them. One such relay is called an
electromechanical or electromagnetic relay EMR as shown in figure 3.4. The EMRs have
three components: the coil, spring and contacts. In Figure 3.4, a digital +5V can control a
230Vac lamp without any physical contact between them. When current flows through
the coil, a magnetic field is created around the coil (the coil is energized), which causes
the armature to be attracted to the coil. The armature’s contact acts like a switch and
closes or opens the circuit.
The relay serves as the protective device of the entire system. The relay
receives trip signal from the microcontroller and thereby cutting the transformer primary
from the input ac source hence protecting the transformer
3.2.5.1 Relay Driver Circuit
Microcontroller pins lack sufficient current to drive a relay. While the 6volts
relay’s coil needs around 12mA to be energized, the current is obtained by the V/R
expression. For example, if the coil is 6VDC and the coil resistance is 500Ω, a minimum
of 12mA (6V/500Ω = 12mA) is need to energize the relay while the microcontroller’s pin
can provide a maximum of 1-2mA current, therefore a transistor was used as relay driver
which is placed between the microcontroller and the relay as shown in figure 3.4
Figure 3.4. 230VAC lamp switched ON using microcontroller based relay
3.2.5.2 Transistor used as Driver
The transistor is used as the driver and the basic function of the driver circuit
is to provide the necessary current to energize the relay coil. The Resistor R1 is used to
set the base current for the transistor, the value of R1 should be such that when input
voltage is applied to the transistor, it is driven into saturation i.e. it is fully turned ON and
the Relay is energized. It’s important that the transistor is driven into saturation so that
the voltage drop across the transistor is minimum thereby dissipating very little power.
The protection diode in the circuit is used to protect the transistor from the
reverse current generated from the coil of the relay during the switch off time.
3.2.5.3 Transistor switching for cutoff and saturated condition
In electronic circuits, mechanical switches are not used. The switching action
is performed by the transistor with an input voltage switching the circuit as shown in
figure 3.5. When base voltage is zero, BJT will be in cut-off = 0, = (Open
switch). When base voltage is 5VDC , BJT can be in saturated (closed switch) with= = ≈ 0.2 .
Figure 3.5 simulation of transistor cut-off and saturation regions
3.2.5.4 Cutoff condition
A transistor is said to be in cutoff region when the base emitter BE junction is
not forward-biased. When is near zero, approaches zero in a nonlinear manner,
this is known as a cutoff region of operation. In this case the transistor acts as an open or
off switch.
3.2.5.5 Saturation condition
The transistor is said to be in a saturated condition when the BE base emitter
junction is in forward biased, and there is an enough base current to produce high
collector current. In this case the transistor is said to be closed or on.
Saturation:
= 0.7V, > 0 ,= − … … … … … … … … … … . (4)3.2.5.6 Design calculations of the Relay driver circuit.
3.2.5.7 Verification of transistor base Resistor value
The output from the microcontroller is required to energize the relay with a
500 Ohm coil. The supply voltage to the transistor is 5V. The microcontroller supplies a
maximum current of 2mA.
Therefore:
)5.....(..................................................5 L
FECCB I
hVR
To find the load current, the below formula is used.
mAVI
RVI
L
L
SL
125006
)6.......(......................................................................
To find the transistor current gain, the below formula is used:
Finally the is calculated since all the variables are known:
KmA
R
IhVR
B
L
FECCB
5.22500125
305
)8.......(......................................................................5
302
125
)7....(......................................................................5
mAmAh
IIh
FE
input
LFE
With the = 2.5 Ω, the closest resistor value of 2.2 Ω was chosen as
3.2.5.8 Verification of transistor in saturated region (closed) by voltage divider
Voltage divider rule states that the voltage across the resistor in a series circuit
is equal to the value of that resistor multiply by the total impressed voltage across the
series elements divided by the total resistance of the series elements.
VVK
V
VK
V
VVV
KRKR
VRR
RV
CE
CE
CE
CE
CC
c
E
CCEC
ECE
0
620
602
0?6
20
)9......(........................................
3.2.5.9 Calculating Base Current using Kirchhoff’s voltage law (KVL)
mAK
I
RVVI
VVRI
B
B
BEBBB
BBBEBB
95.12.2
7.05
)10.....(............................................................0
3.2.5.10 Calculating Collector Current using KVL
CEV
BI
mAK
I
RVVI
VRIV
C
C
CECCC
CECCCC
32
06
)11(......................................................................
3.2.5.11 Verification of transistor in cutoff region (Open)
In cut off region = ≈ 0, therefore =
VVV
VkVVRI
eqR
VVI
CE
CE
CE
CECCCC
C
CECCC
660
620
)12(......................................................................
3.2.6 Crystal oscillator
The clock circuit is an important element that is required in the system board.
This is because the microcontroller works digitally based on generated clock. The rate of
the clock is determined by a crystal oscillator that is connected to the clock logic pins.
A high speed crystal of 16 MHz is used in this project in order to avoid any
delay in terms of relay tripping ON and OFF, and monitoring of the transformer
parameters through the ADC of the microcontroller. Because the monitoring of
transformer parameters and tripping off the relay has to be very fast to avoid failure of the
entire protection system. Figure 3.6 show the crystal inscribed into the microcontroller,
with two 33 pF capacitors used to filter out external noise from interfering with the
crystal frequency
CEV
Figure 3.6 Crystal oscillator circuit inscribed in the controller
3.2.6.1 Crystal time cycle (Period) calculation.
The period of the clock cycle can be calculated by using the frequency
formula which says
nSMHz
T
FT
FT
F
5.6216
1=
1=
16MHz=
3)........(1................................................................................1=
3.2.7 Power Supply design
3.2.7.1 Power supply theory
The power supply circuit design is one of the important parts of this project,
without a power supply the electronic devices such as microcontroller, relay, alarm, LCD
etc. display will not function. Similarly a wrong power supply design will lead to the
damaging of the electronic devices used in this project.
The main power supplies needed for this project is 5VDC in order to power on
the relay and other electronic devices such as microcontroller etc. The design is done
using a transformer, bridge rectifiers, filter capacitor and a voltage regulator. Figure 3.7
shows the sequential process of designing a constant DC power supply.
Figure 3.7 Transformer power supply
In Figure 3.7 the input voltage is obtained the main 230VAC outlet and then
connected to the transformer. A step down transformer is used in stepping the 230VAC to
a 12VAC.The 12VAC serves as an input voltage to the bridge rectifier which is basically
for diodes connected where two diodes are in forward biased and the other two are in
reversed biased for each half cycles. The bridge rectifier is used in converting the 12VAC
into a dc voltage.
The filter capacitor serves as a smoother to smooth the dc voltage from the
bridge rectifier and the LM7805 is the voltage regulators which purposely stabilizes the
output voltages to 6VDC and 5VDC.
230 V, 50 HzAc
Transformer20:1
BridgeRectifier
Filter RegulatorLM78**
3.2.7.2 Power supply design calculation
3.2.7.2.1 Transformer secondary winding turns calculation
The 240VAC primary and 12VAC secondary transformer used has 120 turns
of coil in the primary; therefore secondary winding turn is calculated as shown below:
turnsN
VV
N
EE
NN
6240
144012240120
)14......(................................................................................
2
2
2
1
2
1
The above calculation shows that the transformer has a turn ratio of 120:6= 20:1
3.2.7.2.2 Transformer primary current calculation
The transformer used is a step down transformer (240VAC:12VAC). It is
known that a transformer with less turns in the secondary than in the primary would step
down the voltage, but would step up the current. The below calculation will verify that.
mAI
I
II
EE
EE
NN
II
NN
5024012
112240
)15.........(......................................................................
1
1
1
2
2
1
2
1
2
1
1
2
2
1
The above calculation shows that the step down transformer has step up the
primary current from 50mA to 1A at the secondary.
3.2.7.2.3 Verification of the sine wave characteristics displayed on the oscilloscope
Calculating Voltage peak to peak
0.38
)8.18(2.19
)16.......(......................................................................minmax
pp
pp
pp
VV
VVV
Calculating Vrms
VV
V
VV
V
VV
prms
ppp
prms
5.132
192
192
382
)17(....................................................................................................2
Calculating the period of the AC sine wave
mST
HzFF
T
20501
50
)18..(................................................................................1
Figure 3.8 shows the sine wave from a digital oscilloscope with some sine
wave characteristics parameters such as peak to peak voltages etc.
Figure 3.8 Power supply Sine Wave from a digital oscilloscope
Table 3.1
Table 3.1 show that the calculated sine wave parameters is same with the
measured results from the digital oscilloscope.
3.2.7.3 Power supply simulation
Figure 3.9 and 3.10 shows the 5VDC and 6VDC power supply simulation and
output wave forms.
Variables
Oscilloscope
values
Calculated
values
Vpp 38V 38V
Vp 19V 19V
Vrms 13.5V 13.5V
Period 20mS 20mS
Figure 3.9 Power Supply circuit simulation.
Figure 3.10 Power Supply circuit output waveforms.
3.2.8 Temperature sensing unit
The LM35 was chosen to be the temperature sensing device in this project.
The LM35 series are precision integrated-circuit temperature sensors, whose output
voltage is linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus
has an advantage over linear temperature sensors calibrated in degree Kelvin, as the user
is not required to subtract a large constant voltage from its output to obtain convenient
Centigrade scaling with a rated operating temperature range of over -55° to +150°C
These sensors use a solid-state technique to determine the temperature. That is
to say, they don’t use mercury (like old thermometers), bimetallic strips (like in some
home thermometers or stoves), nor do they use thermistors (temperature sensitive
resistors). Instead, they use the fact as temperature increases, the voltage across a diode
increases at a known rate.
Similarly, the LM35 is chosen over thermocouples because it can measure
temperature more accurately than a using a thermistor. The sensor circuitry is sealed and
not subject to oxidation, etc. The LM35 generates a higher output voltage than
thermocouples and may not require that the output voltage be amplified. Figure 3.11
shows an LM35 sensor interfaced with the AVR microcontroller.
Figure 3.11 LM35 sensor interfaced with the AVR microcontroller
The output voltage of the LM35 varies linearly with temperature. Therefore to
calculate the temperature, a general equation is shown below which is used to convert the
output voltage to temperature
Temperature ( oC) = Vout * (100 oC/V)…………………….. (19)
Hence, if Vout is 0.84V, then, Temperature = 84oC
3.2.9 Transmitter description
The transmitter section consists of the MAX232 IC and the Atmega32 TX pin.
The microcontroller is interfaced to the computer using MAX232 through RS232 serial
communication. RS232 (recommended standard 232) supports both synchronous and
asynchronous transmissions and its user data is send as a time of bits. MAX232 is an
integrated chip that converts convert Transistor–transistor logic (TTL) to RS232 and
RS232 to TTL voltage levels compatible with digital logic circuit such as the
microcontroller. The serial data sends from the microcontroller is then fed to the PC
through RS232 for monitoring purpose.
3.2.9.1 Interfacing Microcontroller and MAX232 with serial (DB9)
Max232 is an integrated circuit that has a dual driver/receiver and typically
converts signals from an RS-232 serial port to signals suitable for use in TTL compatible
digital logic circuits such as the microcontroller. The serial data sends from the PC
through RS232 gets converted to parallel data and is fed to the AVR microcontroller and
conversely. When a TTL level is fed to Max232 IC, it converts TTL logic 1 to between -
3VDC and -15VDC, and converts TTL logic 0 to between +3VDC to +15VDC and
conversely when converting from RS232 to TTL. The table below clarifies the RS232
transmission voltages at a certain logic state are opposite from RS232 control line
voltages at the same logic state.
Table 3. 2 RS232 Line Type and Logic Level
Figure 3.12: Microcontroller with Max232 interface with RS23 Interface
3.2.9.2 Interfacing serial (DB9) with PC
Currently, most PC’s have a 9 pin connector on either the side or back of the
computer. From Table 3.3 it is seen that the PC can send data (bytes) to the transmit pin
(i.e. pin 2) and receive data (bytes) from the receive pin (i.e. pin 3. The Serial port (DB9)
rs232 (recommended Standard 232) is much more than just a connector to PC because it
converts data from parallel to serial and changes the electrical representation of the data.
Rs232 line type and logic
level
Rs232 voltage TTL voltage to/from MAX
232
Data transmission (Rx/Tx)
logic 0
3V to +15V 0V
Data transmission (Rx/Tx)
logic 1
-3V to 15V 5V
Control-signal
(RTS/CTS/DTR) logic 0
-3V to 15V 5V
Control-signal
(RTS/CTS/DTR) logic 1
+3V to +15V 0V
If the connector on the PC has female pins, therefore the mating cable needs to have a
male pin connector to terminate in a DB9 connector and conversely. Data bits flow in
parallel from the PC because it uses many wires at the same time to transmit whereas
serial flow in a stream of bits from the serial connector because it transmit or receive over
a single wire. The serial port create such a flow by converting the parallel data to serial
on the transmit pin (i.e. pin 2) and conversely. The serial port has a built-in computer
chip called USART used in translating data between parallel and serial forms.
Table 3.3: RS232 pin assignments (DB9 PC signal set)
3.3 SOFTWARE DESIGN OVERVIEW.
The software design plays a very important role in the working of the entire
system; the system will not operate without the software. An algorithm needs to be
developed to enable the AVR controllers read the input and respond accordingly. The
programming language selected for this project is the C program. The C program will
enable communication between the transformer, microcontroller and PC with other
different interfaces in the system. With the software programed into it, microcontrollers
acts as brain of the whole transformer protection and transmit its parameters for
monitoring its conditions. It will send the transformer information through PC via the
RS232 serial port.
Pin 1 Input DCD Data Carrier Detect
Pin 2 Input RXD Received Data
Pin 3 Output TXD Transmitted Data
Pin 4 Output DTR Data Terminal Ready
Pin 5 Nil Nil Signal ground
Pin 6 Input DSR Data Set Ready
Pin 7 Output RTS Request To Send
Pin 8 Input CTS Clear To Send
Pin 9 Input RI Ring Indicator
The flow chart diagram developed will give an initial description of the
system software. The programs are divided into two parts which are main program and
interrupt program. The microcontroller will always loop the main program until an
interrupt occurred. When the controller receives an interrupt flag, then it will jump to
interrupt the process.
3.3.1 Flow chart of the entire system
The flowchart gives a diagram representation of the program algorithm. The
system flowchart is designed as shown below:
Figure 3.13 Flowchart description of the system
The flowchart above shows the initial description of the system program code.
The first thing the program will do is to initialize and read the ADC and the USART pins,
then sends the transformer parameters which are fed to the ADC to the personal computer
system using the UART1_Write command, then to the LCD display. The microcontroller
ADC will continuously capturing the transformer parameters, as soon as the transformer
secondary current is greater than 1A, it sends a trip signal to the overcurrent relay, and it
cuts off the load that leads to the overcurrent, thereby protecting the transformer from
burning. Same process goes to the over voltage protection, it will check whether the
transformer input voltage is greater than 230Vac, if so, it sends a trip signal to
overvoltage relay, thereby protecting the transformer.
CHAPTER 4
HARDWARE AND SOFTWARE DESIGN AND IMPLEMENTATION
4.1 Schematic diagram
4.1.1 Complete schematic diagram
As designed in chapter 3, the circuit section consists of AVR microcontroller, step
down transformer circuit for voltage sensing, current sensing circuit, relay circuits, a
temperature sensor, RS232 and the masx232 circuit.
The step down transformer used is a 230VAC to 12VAC transformer and is used
for the purpose of sensing the input voltage to the main transformer with a voltage rating
of 230VAC to 160VAC. The step down transformer is been rectified and filtered to a
pure dc which goes directly to the microcontroller ADC for monitoring the input voltage.
For the purpose of current sensing, a current transformer was used for that purpose. It
went through rectification and filtering process then directly connected to the
microcontroller ADC for monitoring the load current.
The microcontrollers send the monitored parameters to LCD display and also
transmit them to a personal computer. The transmission to personal computer was made
possible by interfacing the microcontroller with the computer using MAX232 through
RS232 serial communication. RS232 (recommended standard 232) supports both
synchronous and asynchronous transmission and its user data is send as a time series of
bits.
While monitoring the parameters, whenever a fault occurs which might be high
voltage or over current, the microcontroller sends a trip signal to the relay and thereby
protecting the transformer from burning.
Figure 4.1 Complete schematic
4.2 PCB design
4.2.1 PCB design using Software.
Earlier, testing on broad board was done and the working process of the circuit
was properly tested, problems were troubleshot and rectified. After the bread board
testing here comes the Printed Circuited Board design (PCB). Dip trace software was
used to place the components, which are joined together with multiple of tracks that gives
out the physical and electrical connections. This software was used due its neat layout
and accurate PCB layout is always the main priority section of the design
Figure 4.2 Complete circuit PCB design
The PCB layout schematics were printed on a transparent paper, where the
layouts were printed with a laser printer. Pressing iron was used to iron the transparent
paper on the PCB board systematically for about 10 minutes. The copper clads were
allowed to cool off and the transparent paper was removed from the PCB board to expose
the transferred image. A permanent marker was used to replace the missing tracks before
etching. Etching chemical (HCL acid) was poured into a squared shaped container and
the PCB board was placed inside. After that the board was cleaned with Tina chemical in
order to remove the unwanted copper and makes the board ready for drilling.
4.2.2 Soldering
After the drilling process, there comes the soldering process. Soldering attentions
need to be taken into consideration when laying out the board. Hand soldering is the
traditional method basically used for prototypes and small production stuffs. Major
impacts when laying out the board include suitable access for the iron, and thermal relief
for pads.
4.2.3 Electrical Testing and Troubleshooting
After soldering, finished PCB has to go through comprehensive checks for
electrical continuity test and shorts that might occur at time of soldering. This is achieved
by using the multimeter continuity check mode. It checks that the continuity of the tracks
if matches each other; if not a troubleshooting session has to take place in order to trace
and rectify the problem.
4.3 Software implementation
4.3.1 Programming in MikroC PRO for AVR
Microcontroller acts as the brain of the entire system. It monitors the voltage,
current and temperature of the transformer, display the parameters on LCD and PC
through RS232 serial port. Whenever a fault occurs, it automatically sends a trip signal to
the relay and thereby protecting the transformer from burning. An algorithm has been
developed which makes the microcontroller reads the input analogue signals and
responds consequently. The algorithms have been represented by the flowchart shown in
chapter three, and now the flowcharts are been interpreted into C language and complied
using the MikroC PRO for AVR. Refer to appendix for complete C program.
4.3.1.1 Brief description of the system main program
The main program is divided into multiple of parts, and the parts are the definition
of ports and pins, initialization of ADC, configuration of the serial communication
protocol ports and finally configuration of LCD display.
4.3.1.2 Initialization ports and pins.
General purpose input/ output pins of the microcontroller can be said to be the
basic and commonly used peripherals in microcontrollers. They permit the Atmega16
microcontroller pins to be used as input or outputs based on the task at hand for
monitoring and controlling devices. To use a port or pin of a microcontroller as input, a
data direction register called DDR register is used to configure the port as input. To setup
PORTB as input, DDR register is set as DDRB=0X00. And to use the PORTB as output,
DDR register setup as DDRB=0XFF.
In this project PORT A is used for Analogue digital conversion, PORT C is used
for LCD interfacing, PORT D is used for relay control, LCD control pins and serial port.
Figure 4.3 Configure input and output port
#include <built_in.h>
#define LCD PORTC
sbit RS at PORTD4_bit;
sbit EN at PORTD6_bit;
sbit RW at PORTD5_bit;
sbit relay at PORTD2_bit;
sbit relay2 at PORTD3_bit;
unsigned int voltage,current,temp,current2,current1;
int decimal,integer;float analog;
The program begins by defining PORTC for LCD, PORTD.2 for voltage relay,
PORTD.3 for current relay, PORTD.4, PORTD.5 and PORTD.6 as RS, RW and EN pins
for LCD respectively. Defining the ports gives a clear understanding of the program flow.
4.3.2 PROGRAM DESCRIPTION
The main program begins by configuring the LCD, ADC and the serial
communication port of the microcontroller.
Figure 4.4 LCD,ADC and serial port configuration
The main program is started by defining the flow of each port is the port used as
an output port or as an input port. When the port is loaded with 0xff, it signifies that the
port is used as an output port, similarly if the port is loaded with 0x00, this means the port
is used as input. As seen in figure 4.4, portd, portc and portb are defined as output
because the microcontroller gives out signals to the LCD, relays and the serial port. The
program continues with the initialization of the serial communication and setting the
speed of the communication (baud rate) as 9600bps.
void main() DDRB = 0xFF; // Set PORTB as outputDDRC = 0xFF; // Set PORTC as outputDDRD = 0xFF; // Set PORTC as outputlcd();UART1_Init(9600);com(0x01);dat(0x80);lcd_string("Transformer protect");dat(0xc0);lcd_string(" system by buga ");delay_ms(1000);com(0x01);dat(0x80);lcd_string("Aminu bugaje");voltage=temp=current;
Baud rate is the number of times a signal in a communication channel changes
state or varies. This project is designed to use 9600 baud rate which means that the
channel can change states up to 9600bps times per second.
while (1)
delay_ms(1000);
temp=ADC_Read(0x00)/4;
voltage=ADC_Read(0x01)/4;
current = ADC_Read(0x02)/4; // get ADC value from 2nd channel
com(0xc0);
lcd_string("temp:");
ascii(temp*2);
com(0x94);
lcd_string("volts:");
ascii(voltage*2);
com(0xd4);
lcd_string("current:");
asciiii((current*0.392)*2);
if(temp>20)
com(0xcb);
lcd_string("over");
if(temp<20)
com(0xcb);
lcd_string("norm");
if(current>130)
com(0xe2);
lcd_string("over--");
relay=0;
if(current<130)
com(0xe2);
lcd_string("norm--");
relay=1;
Figure 4.5 looping process of the code
The main program continues by defining temperature in ADC channel 0 of the
microcontroller, voltage in ADC channel 1 of the microcontroller and the current in ADC
channel 2 of the microcontroller. The main program continues to loop and check for the
conditions as seen in figure 4.5, whenever the voltage is greater than 230, the
microcontroller displays over-voltage on the LCD and sends a trip signal to the relay and
also if the voltage is less than 220 it displays under voltage on LCD. Similarly if the
voltage it within 220 and 230 it displays normal voltage on LCD.
The program also checks for the current and whenever it’s more than 1.1A, it
displays on the LCD and sends a trip signal to the relay and thereby protecting the
transformer from burning.
if(voltage>120)
com(0xa1);
lcd_string("over--");
relay2=0;
if((voltage<120)&&(voltage>=110))
com(0xa1);
lcd_string("norm--");
relay2=1;
if(voltage<110)
com(0xa1);
lcd_string("under--");
relay2=1;
Figure 4.6 Transmitting voltage,current and temperature values to PC
After setting the baud rate as 9600bps, this means the speed at which the
microcontroller transmits each bit per second to the personal computer. The
UART1_Write command is used to transmit the temperature, voltage and current values
to the personal computer as seen in figure 4.6
4.4. Proteus VSM for Atmega16
Proteus VSM for Atmega16 encloses everything necessary to develop; test and
almost model the embedded system designs based around the AVR Atmega16
microcontrollers.
With the exclusive feature of the Proteus simulation software, it helps in easy
development of both the system hardware and software. The Proteus design enables us to
progress in our project more rapid, giving us the ability to make hardware or software
changes which reduces hardware and software troubleshooting problems.
The project was built and tested in Proteus just by using the software prototype
components without using the physical hardware prototype. Therefore using Proteus
software, the voltage sensing circuit, current sensing circuit, temperature sensor, relays,
LED’s, LCD display and serial communication were all developed and tested as shown
figure 4.7.
UART1_Write(temp);
UART1_Write(voltage);
UART1_Write(current);
Figure 4.7 Complete circuit simulation model
4.5 Programming in Visual Basic 6.0
VISUAL BASIC is a high level programming language which evolved from the
earlier DOS version called BASIC.
BASIC means Beginners' All-purpose Symbolic Instruction Code. It is a very
easy programing language to learn. In this project, graphical user interface (GUI) was
developed using visual basic 6.0. This GUI is able to receive the voltage, temperature and
current values send by the microcontroller. The GUI can also display the transformer
information in graphical form and records the time at which the information is received.
Refer to appendix D for complete VB program.
Figure 4.8 Visual basic GUI
4.5.2 Visual basic 6.0 with ISIS 7 professional
Since the circuit diagram is already being developed to send data from the
Microcontroller to PC via RS232. It is needed to interface visual basic with the Proteus
software. Using virtual serial port Emulator Software, we were able to interface VB with
Proteus.
4.5.2.1 Visual basic 6.0 with Proteus ISIS 7 professional results
The outputs obtained from the microcontroller and transmitted to the PC via VB
GUI interface are given below. Therefore, using the virtual serial port Emulator, the
entire project was simulated perfectly. This gives us a clear idea of the hardware
implementation. The aims and objectives of this task are well achieved.
Figure 4.9 Monitoring and transmitting the transformer parameters using microcontroller
with protues software
Figure 4.10 Receiving and Monitoring transformer parameters via PC using the VB GUI
interface
4.6 Project prototype
As seen in figure 4.11, the system prototype has been developed with all
the features of a microcontroller based transformer protection as named to be the project
title. The loads are connected to the transformer secondary, and a current sensor is
connected in series with load for real time current monitoring. Based on the real time
current monitored values, the microcontroller takes decision over the relay whether to cut
off or not. The step transformer connected to the input voltage is used for high voltage
monitoring, based on the monitored voltage values; the microcontroller takes decision
over the relay. The AVR microcontroller board contains all the sub circuits on-board
including the high voltage sensing circuit, the liquid crystal display (LCD) for monitored
values display, LED’s for indication, temperature sensor, relays for protection purposes
and finally the MAX 232 and RS232 for transmitting the transformer parameters to PC.
Figure 4.11 Project prototype
It can be seen from the prototype developed that all the features of a
microcontroller based transformer protection were provided and well defined. The input
AC voltage was given through the autotransformer, the loads were connected at the
output of the transformer and the transformer parameters are monitored in personal
computer. The AVR microcontroller has on it all the sub-circuits for the transformer
protection including the liquid crystal display (LCD) for voltage, current and temperature
display of the transformer, relay driving circuits, high voltage sensing circuits, current
sensor and the transmitter circuit for real time transmission of transformer information to
personal computer
Finally, the Proteus simulation software made it easy to test, and troubleshoot the
hardware and the program which saved much of the time and reduced cost of the project.
Therefore, it can be concluded that the hardware and software implementation were
positively achieved
CHAPTER 5
RESULTS AND DISCUSSION
In order to verify the performance of the proposed microcontroller based
transformer protection system, a hardware prototype was implemented with an AVR
microcontroller ATMEGE16 with a 16MHz crystal oscillator. During this test, an
autotransformer was used for varying the input voltage of the transformer in order to
create the over voltage fault. Bulbs were used as loads to create the over current fault.
Voltage and current sensing circuits were designed for sensing the transformer voltage
and current. The validity of this project prototype is verified through this test system.
5.1 Transformer current analysis
Figure 5.1 Transformer current analyses
As in figure 5.1 when no overcurrent detected by the microcontroller through the
current sensor, the microcontroller energizes the overcurrent relay on. If loads are added
to the secondary side of the transformer, the secondary current rises. Therefore the load is
proportional to the secondary current. If the load connected does not exceed the rated
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 20 40 60
Current(A)
Time(mS)
Transformer current analysis
Series1
Normal current 1.2A Overcurrent 1.4A
Current goes to zero
Current rises to 1.2A
current of the transformer which 1.2A, the relay continues to be on. But as soon as the
load current exceeds the transformer rated current, the microcontroller sends a trip signal
to the overcurrent relay and the relay goes off., thereby protecting the transformer from
burning due to overloading. When the overcurrent is rectified, the relay goes on and
continues to allow the flow of electric current through the load.
5.2 Transformer voltage analysis
Figure 5.2 Transformer voltage analyses
As in figure 5.2, when no overvoltage detected by the microcontroller through the
voltage sensing circuit, the microcontroller energize the overvoltage relay on which
allows the flow of electric current and voltage through the transformer primary. When the
input AC voltage is varied through the autotransformer above the rated voltage of the
transformer which is 230VAC, the microcontroller detects an overvoltage condition
through the voltage sensing circuit, therefore it sends a trip signal the overvoltage relay,
and the relay cuts off the primary of the transformer from the input AC voltage thereby
saving the transformer from damaging due overvoltage. As soon as the microcontroller
-50
0
50
100
150
200
250
300
0 10 20 30 40 50
Voltage (AC)
Time (mS)
Transformer voltage analysis
Series1
Normal voltage 230VAC
Overvoltage 250VAC
relays is off,Voltage goes to zero
Voltage rises to normal 230VAC
detects normal voltage, it sends back a switch on signal to relay thereby allowing the flow
of electric current and voltage through the through transformer primary
CHAPTER 6
CONCLUSION AND FUTURE RECOMMENDATION
6.1 Conclusion
In this project, the transformer protection using a microcontroller is proposed. For
transformer voltage and current sensing, a current sensing circuit and voltage sensing
circuits were designed and the results have been verified with proteus simulation.
Hardware with an AVR microcontroller was implemented to verify the proposed
technique and the performance of the real time hardware was compared with the proteus
computer simulation. Through the transformer current analysis in figure 5.1, we can see
that the current of the transformer rises as load increases, whenever the load current goes
above the transformer rated current, the microcontroller detects an overcurrent and it
sends a trip signal to over current relay thereby protecting the transformer from burning.
As the load current goes below the rated current of the transformer, the microcontroller
detects normal there by sending an on signal to the overcurrent relay.
Moreover, through the transformer voltage analysis in figure 5.2, we can see that
the voltage of the transformer rises as the input voltage of the transformer is increased
through varying an autotransformer. Whenever the input voltage goes above the
transformer rated voltage (230VAC), the microcontroller detects an overvoltage and it
sends a trip signal to over voltage relay thereby protecting the transformer from burning.
The results indicate that the microcontroller based transformer protection achieves
numerous advantages over the existing systems in use: 1) fast response, 2) better
isolation, 3) accurate detection of the fault.
Finally, the practical results matched with the simulation perfectly, therefore the
aim and objectives of the project were all achieved successfully and project is said to be
industrious and fully automated with no manual interface required.
6.2 Future Recommendations
Any work and investigation on transformer protection is very advantageous and
challenging. Based on the present time, it can be observed that the world’s population is
increasing rapidly. Therefore demands on electricity will be high and these will lead to
demands of highly sophisticated protection devices, which will be incorporated in
transformer protection schemes.
Based on the work done in this project which protecting transformer using
microcontroller, some improvements need to be made in the future work. It was noticed
that use of current sensor prevent the protection from high performance application
because the current sensor needs some amount of time to sense the load current and
transfer the signal to the microcontroller ADC. Correspondingly, a current transformer
can be used instead of current sensor, switching semiconductor device such as thyristor
can be used instead of relay, highly advanced microcontroller such as 16bit PIC
microcontroller or a digital signal processor can be used for high speed analogue to
digital (ADC) conversion of the transformer voltage and current.
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