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Microcontroller based transformer protectio


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Microcontroller based transformer protectio

  1. 1. 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.
  2. 2. 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).
  3. 3. 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
  4. 4. 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.
  5. 5. 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.
  6. 6. 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.
  7. 7. 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
  8. 8. 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).
  9. 9. 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
  10. 10. 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
  11. 11. 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. `
  12. 12. 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.
  13. 13. 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.
  14. 14. 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.
  15. 15. 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. 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 BBACS756 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
  16. 16. measured, for this purpose, digital display is provided at the output of the Microcontroller 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. . Secondary winding calculation The transformer used has 120 turns of coil in the primary; therefore secondary winding turn is calculated as shown below: N1 E  1 .......... .......... .......... .......... .......... .........( 1) N2 E2 120 230 V  N2 12 V N2  19200  80 turns 240
  17. 17. The above calculation shows that the transformer has a turn ratio of 120:80 =12:8 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. N1 I  2 .......... .......... .......... .......... .......... .......... .......... .( 2 ) N2 I1 N1 E  1 N2 E2  E1 I  2 E2 I1 240 1  160 I1 I1  160  0 . 667 A 240 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 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.
  18. 18. 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. over voltage protection circuit design calculation. DC voltage design calculation. The secondary voltage of the transformer is 160VAC and connected to a bridge rectifier, therefore the DC output is approximately: V DC  V AC  V DC  12  2  ( 2  0 . 7 )......... .......... .......... .......... .......... ( 3 ) 2  1 . 4  15 . 57 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.
  19. 19. 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 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
  20. 20. Figure 3.4. 230VAC lamp switched ON using microcontroller based relay 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. Transistor switching for cutoff and saturated condition In electronic circuits, mechanical switches are not used. The switching action = 0, = 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 (Open
  21. 21. = = ≈ 0.2 . switch). When base voltage is 5VDC , BJT can be in saturated (closed switch) with Figure 3.5 simulation of transistor cut-off and saturation regions 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. 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:
  22. 22. = − = 0.7V, >0 , … … … … … … … … … … . (4) Design calculations of the Relay driver circuit. 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: RB  VCC  hFE .......................................................(5) 5 IL To find the load current, the below formula is used. IL  VS .............................................................................(6) RL IL  6V  12mA 500 To find the transistor current gain, the below formula is used: h FE  5  h FE  5  IL I input .......... .......... .......... .......... .......... .......... .......... ....( 7 ) 12 mA  30 2 mA Finally the is calculated since all the variables are known: RB  VCC  hFE .............................................................................(8) 5 IL RB  5  30  2500  2.5K 5  12mA
  23. 23. = 2.5 Ω, the closest resistor value of 2.2 Ω was chosen as With the Verification of transistor VCE 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. VCE  RE  VCC ..............................................(9) RC  RE R E  0 K Rc  2 K  VCC  6V VCE  ? 0  6V 2K  0 0  6 2K  0V VCE  VCE VCE Calculating Base Current I B using Kirchhoff’s voltage law (KVL) I B R B  V BE  V BB  0.......... .......... .......... .......... .......... .......... .....(10 ) IB  IB  V BB V BE RB 5  0 .7 2 .2 K   1.95 mA Calculating Collector Current using KVL
  24. 24. VCC  I C RC  VCE ......................................................................(11) IC  IC  VCC  VCE RC 60 2 K  3mA Verification of transistor In cut off region IC  = VCE ≈ 0, therefore in cutoff region (Open) = VCC  VCE ......................................................................eq(12) RC I C RC  VCC  VCE 0  2k  6  VCE 0  6  VCE VCE  6V 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
  25. 25. Figure 3.6 Crystal oscillator circuit inscribed in the controller Crystal time cycle (Period) calculation. The period of the clock cycle can be calculated by using the frequency formula which says 1 ........................................................................................(13) T F = 16MHz 1 T= F 1 T=  62.5nS 16 MHz F =
  26. 26. 3.2.7 Power Supply design 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. 230 V, 50 Hz Ac Transformer 20:1 Bridge Rectifier Filter Regulator LM78** 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.
  27. 27. Power supply design calculation 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: N1 E  1 .......... .......... .......... .......... .......... .......... .......... .......... ......(14 ) N 2 E2 120 240V  N2 12V N2  1440  6turns 240 The above calculation shows that the transformer has a turn ratio of 120:6= 20:1 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. N1 I  2 .......... .......... .......... .......... .......... .......... .......... .........( 15 ) N2 I1 N1 E  1 N2 E2  E1 I  2 E2 I1 240 1  12 I1 I1  12  50 mA 240
  28. 28. The above calculation shows that the step down transformer has step up the primary current from 50mA to 1A at the secondary. Verification of the sine wave characteristics displayed on the oscilloscope Calculating Voltage peak to peak V pp  Vmax  Vmin .............................................................................(16) V pp  19.2  (18.8) V pp  38.0 Calculating Vrms Vrms  V pp 2 ....................................................................................................(17) 38  19V 2 2 Vp 19    13.5V 2 2 Vp  Vrms Vp  Calculating the period of the AC sine wave 1 .......... .......... .......... .......... .......... .......... .......... .......... ..(18) F F  50 Hz 1 T   20 mS 50 T Figure 3.8 shows the sine wave from a digital oscilloscope with some sine wave characteristics parameters such as peak to peak voltages etc.
  29. 29. Figure 3.8 Power supply Sine Wave from a digital oscilloscope Oscilloscope Calculated Variables values values Vpp 38V 38V Vp 19V 19V Vrms 13.5V 13.5V Period 20mS 20mS Table 3.1 Table 3.1 show that the calculated sine wave parameters is same with the measured results from the digital oscilloscope. Power supply simulation Figure 3.9 and 3.10 shows the 5VDC and 6VDC power supply simulation and output wave forms.
  30. 30. 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
  31. 31. 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
  32. 32. 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. 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.
  33. 33. Rs232 line type and logic Rs232 voltage TTL voltage to/from MAX level 232 Data transmission (Rx/Tx) 3V to +15V 0V logic 0 Data transmission (Rx/Tx) -3V to 15V 5V logic 1 Control-signal -3V to 15V 5V +3V to +15V 0V (RTS/CTS/DTR) logic 0 Control-signal (RTS/CTS/DTR) logic 1 Table 3. 2 RS232 Line Type and Logic Level Figure 3.12: Microcontroller with Max232 interface with RS23 Interface 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.
  34. 34. 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. 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 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.
  35. 35. 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
  36. 36. 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.
  37. 37. 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.
  38. 38. Figure 4.1 Complete schematic
  39. 39. 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
  40. 40. 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.
  41. 41. 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. 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. #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; Figure 4.3 Configure input and output port
  42. 42. 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. void main() { DDRB = 0xFF; // Set PORTB as output DDRC = 0xFF; // Set PORTC as output DDRD = 0xFF; // Set PORTC as output lcd(); 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; 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.
  43. 43. 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.
  44. 44. while (1) { delay_ms(1000); temp=ADC_Read(0x00)/4; voltage=ADC_Read(0x01)/4; current = ADC_Read(0x02)/4; 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; } // get ADC value from 2nd channel
  45. 45. 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 4.5 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.
  46. 46. UART1_Write(temp); UART1_Write(voltage); UART1_Write(current); 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.
  47. 47. 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
  48. 48. 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. 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.
  49. 49. Figure 4.9 Monitoring and transmitting the transformer parameters using microcontroller with protues software
  50. 50. 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.
  51. 51. 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
  52. 52. 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 Transformer current analysis 1.6 Normal current 1.2A Overcurrent 1.4A 1.4 Current rises to 1.2A 1.2 1 Current(A) 0.8 Series1 Current goes to zero 0.6 0.4 0.2 0 -0.2 0 20 40 60 Time(mS) 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
  53. 53. 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 Transformer voltage analysis 300 Overvoltage 250VAC 250 200 Voltage rises to normal 230VAC Normal voltage 230VAC Voltage (AC) 150 Series1 100 50 relays is off,Voltage goes to zero 0 -50 0 10 20 30 40 50 Time (mS) 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
  54. 54. 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
  55. 55. 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.
  56. 56. 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. Reference Books Badri ram and D N Vishwakarma (1995) power system protection and switch gear New delhi: Tata Mc Graw hill. Frank D. Petruzella (2010) Electric motors and control systems 1st ed. New york: McGraw-Hill
  57. 57. J. Lewis Blackburn , Thomas J. Domin (2006). Protective Relaying Principles and Applications . 3rd ed. United States of America: CRC press Leonard L. Grigsby (2007). The Electric Power Engineering Handbook. 2nd ed. United States of America: CRC press. P. M. Anderson (1998). Power system protection. New York: John Wiley & Sons, Inc. P.673. Smarajit Ghosh, (2007). Electrric Machines 1st Edn. India: Dorling Kindersley Journals Ali Reza Fereidunian, Mansooreh Zangiabadi, Majid Sanaye-Pasand, Gholam Pournaghi, (2003) ‘Digital Differential Relays For Transformer Protection Using Walsh Series And Least Squares Estimators’. CIRED (International Conference on Electricity), pp. 1-6. Atthapol Ngaopitakkul and Anantawak kunakorn (2006), ‘Internal Fault Classification in Transformer Windings using Combination of Discrete Wavelet Transforms and Backpropagation Neural Networks’ International journal of control, automation and systems, 4(3), pp. 365-371. Mazouz A. Salahar Abdallah R. Al-zyoud (2010), ‘Modelling of transformer differential protection using programmable logic controllers’ European journal of scientific research, 41(3), pp. 452-459. Pankaj Bhambri, Chandni Jindal, Sagar Bathla (2007), ‘Future Wireless TechnologyZIGBEE’ Proceedings of national conference on challenges, pp. 154-156. R. A. LARNER and K. R. GRUESEN, (1959). Fuse Protection or High-Voltage Power Transformers, pp.864-873.
  58. 58. S.M Bashi, N. Mariun and A.rafa (2007). ‘Power Transformer protection using microcontroller based relay’, Journal of applied science, 7(12), pp.1602-1607. V.Galdi, L.lppolito, A.piccolo and A.Vaccaro (2000) ‘Neural diagnostic system for transformer thermal overload protection’ Electric Power Applications, IEE Proceedings, 147 (5), pp. 415 - 421 . V.Thiyagarajan & T.G. Palanivel, (J2010) ‘An efficient monitoring of substations using microcontroller based monitoring system’ International Journal of Research and Reviews in Applied Sciences, 4 (1), pp.63-68.