In this paper we have presented a low cost, simple and robust mechatronic system for the measurement of temperature in extreme conditions. It is specially designed for the protection of high potential parts of power transformers and switch gears under faulty operating conditions

Ajmal, Ms T

Prasad, D

Rehman, Mahfoozur

Saad, M R

English

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   Proceedings of International Conference on Man-Machine Systems 2006 September 15-16 2006, Langkawi, Malaysia  AN ELECTRO-OPTICAL TECHNIQUE   FOR THE MEASUREMENT OF TEMPERATURE OF HIGH POTENTIAL BODIES    Ms T. Ajmal1, Dr. Mahfoozur Rehman2, D. Prasad3 and M. R. Saad3  1 Electronics Engineering Department, Z H College of Engineering and Technology, AMU, Aligarh, INDIA E-mail: tahmina_ajmal@yahoo.com 2 School of Electrical & Electronic Engineering, USM Engineering Campus, 14300 Nibong Tebal, Pinang, MALAYSIA                                                                                 Email: mahfoozur@eng.usm.my                                                                                                   3Deptt. of Electronic and Communication, JMI, New Delhi 4Department of Space Science, Lulea University of Technology, Box.812, S-98128, KIRUNA, SWEDEN E-mail: misbah.rehman@gmail.com  Abstract In this paper we have presented a low cost, simple and robust mechatronic system for the measurement of temperature in extreme conditions. It is specially designed for the protection of high potential parts of power transformers and switch gears under faulty operating conditions. It comprises of two parts i.e., temperature sensing part cum transmitter which contains one thermistor, one LED, current limiting resistance and 5V source which is well insulated and placed on the high potential part to monitor the temperature and indication system /receiver which consists of photo detector, current to voltage converter, amplifier and the indication system and is kept at appreciable distance from the high potential body. Output of the system is recorded and analyzed when potential of the hot body is increased from 50V to 250V. Keywords Remote measurement, Optical Fiber, temperature measurement, thermistor, optical technique 1. Introduction The measurement and/or regulation of temperature is an important everyday application of transducers. Accurate temperature sensing mechanisms are also present in living organisms. Measurement and regulation of temperature are important functions in a wide variety of consumers, industrial and medical applications, covering a wide range of temperatures of interest. Its importance further increases when it has to be made under injurious conditions where the conventional methods fail. One of the burning topics is the measurement of temperature of high potential parts of the useful equipments like transformers generators, switch gears which play important role in the stable operation of such machines /equipments. Because, increase in temperature, beyond a certain limit, is due to over loading, friction and sparking in side the machine or chip. Some transducers are available, for non contact measurement, like fluoroptic temperature sensor and laser based temperature measuring system [1]. These transducers are costly as well as have complicated circuits and have limited accuracy.  The system, to be used for the purpose must be capable of operating under extreme conditions, without affecting the real temperature, as well as must have the added feature of remote sensing with continuous monitoring at low cost. In this paper we have presented a low cost, simple and robust technique which can be used in extreme conditions. It is specially designed for the measurement of temperature of high potential parts of power transformers and switch gears. It has mainly two parts i.e., transmitter and receiver. The transmitter is installed over the high potential body during observation, while receiver is kept at a remote place for safe study and recording of temperatures. 2. Approach and methodology Basically we want to measure the temperature of the body, at a high potential; in such a way that measuring instrument as well as the observer should not be in danger. For this reason we have employed optical isolation between the hot body and the display unit. To have remote measurement which becomes necessary in such cases, we have used optical fiber to transmit the data efficiently. Schematic diagram of the proposed system is shown in fig 1. In this system, basic temperature sensor is a thermistor which is connected in series with a 5V supply, an LED and a current limiting resistance. Output of the LED is taken through optical fiber to a remote place where it is optically coupled to a photo detector whose output is fed to a current to voltage converter, an amplifier and display system. When temperature of the hot body increases, resistance of the thermistor decreases and current passing through the LED increases. This increase in current increases the output optical power of the LED which is efficiently connected to a photo detector through multimode optical fiber and hence the output current of the photo detector increases which   Proceedings of International Conference on Man-Machine Systems 2006 September 15-16 2006, Langkawi, Malaysia    Figure 1- Schematic Diagram of the Temperature Measuring System  is converted into voltage with the help of current to voltage converter and amplified suitably to be interfaced with the display system. Transmitter circuit consists of thermistor, LED, suitable resistance and a 5V dc supply in a close circuit. Thermistor touches the hot body, and its resistance decreases as temperature increases. Hence current increases through the LED and resulting optical power, from the LED, increases. Expression for optical power may be given as follows.  Expression for LED forward current, IF, may be written as follows [2];    tthfwFRRVVI+−=                                                                                                (1)               where, Rth= 148R0 x 10-8. e4000/T is the resistance of thermistor for an ambient temperature of 250C and  Vfw ,  is the forward voltage of the LED and RT is the total resistance. When this current passes through LED, it produces an optical power which may be given by following expression [3’4]:  ]10148[/4000800TTfwFReRVVKKIP+−==−                (2)      where K is a constant and IF is the forward current of the LED. LED is coupled with the optical fiber and hence optical power passes through optical fiber to the photo diode on the other end as shown in Fig.1. Some power will be lost during transmission. However a portion of it will be converted into current by the photo diode and it will be directly proportional to optical power P0, coupled to it and the detector photo current Id may be given by following expression [5].  0RPI d α=                                                                       (3) Where R is the responsivity of the photo detector and α is the loss factor due to transmission. Substituting the value of IF and P0 from Eq. 1 & 3, we get, ]10148[/400080 TTfwdReRVVRKI+−=−α              (4)     With the help of the current to voltage converter it will be converted into an output voltage V0, given by following expression: 0.0 RIV d=                                                                        (5)                   Where, R0 is the feedback resistance of the current to voltage converter. Experimental results obtained with the system show that the non- linear ties of LED and photodiode are taken care of by the nonlinearity of the thermistor.  3. Experimental methods and results The circuit of the thermistor and LED, shown in Fig.1, is fabricated on a small PCB. LED No.MFOE71 which can be nicely coupled with multimode, plastic optical fiber is used as the light source. Thermistor resistance and external resistance are so chosen that LED operates on the linear part of the characteristic between optical power and forward current of the LED. Hence operating current of, 30mA, at ambient temperature of 250C, is chosen for  the LED. The PCB of the transmitter is so installed that thermistor remain in touch with the hot, high potential body. Optical fiber, after proper polishing, is installed in the LED and the other end is fitted inside the plastic case of photo diode, which is placed at a distance of 10 meters from the high potential body initially. PCB of the receiver circuit consists of a photo diode No.MFOD71, a current to voltage converter, an inverting amplifier and power supply to bias the Opamps and photo diode. Output voltage of the receiver is connected to a precision grade multimeter (Tektronix). Temperature of the high potential body is slowly increased and output of the receiver is recorded. Simultaneously, temperature of the hot body is measured    Proceedings of International Conference on Man-Machine Systems 2006 September 15-16 2006, Langkawi, Malaysia  with the help of a mercury thermometer and output of the receiver is calibrated in terms of temperature. Potential of the high potential body is increased in steps of 50V and experiments are repeated and average of a number of readings is finally entered in the table.. Results obtained are given in Table-1   Table 1- Experimental Results Output Voltages (Volts) Temp   0C by Merc. Therm 50 100 150 200 250 16 0.600 0.600 0.602 0.604 0.605 22 0.635 0.637 0.640 0.640 0.641 30 0.710 0.710 0.712 0.711 0.712  38 0.790 0.790 0.792 0.792 0.793 46 0.880 0.879 0.879 0.882 0.882 53 0.940 0.939 0.941 0.941 0.942 68 1.090 1.092 1.093 1.098 1.100 74 1.150 1.150 1.155 1.158 1.158 82 1.240 1.247 1.246 1.250 1.252 94 1.330 1.328 1.335 1.337 1.340 115 1.450 1.455 1.458 1.459 1.462  Calibration Curve00.20.40.60.811.21.41.60 50 100 150Temperature By Mercury ThermometerVolts50 Volts 100 Volts 150 Volts200 Volts 250 Volts Figure 2 – Calibration Curve   The data obtained with the measuring system is given in Table-1 and graphical relationship is shown in Fig.2. It is appreciably a straight line relationship which shows that the output voltage is directly proportional to the temperature in the measurement range which is a highly desirable feature in a Mechatronic System. It can be easily adjusted to disconnect a system by operating a relay in a substation, under working conditions, before a serious fault may occur. It is a very important feature. It is observed that the effect of variation of potential of the hot body is nearly negligible because most of the components are insulated and signal is transmitted through optical fiber which has isolated the high potential body from the receiver. System is robust as well as precise in the measurement of temperature of high potential bodies up to the voltages of the order of 250V, according to the experiments.  4. Conclusions  A proto-type model of Electro-optical temperature measuring system is designed, fabricated and tested. Initially it is tested up to domestic supply voltages (50 to 250V) only due to the limitations of the laboratory. In this range, change in output voltage with change in temperature is appreciably constant. Long distance transmission can be tried with LASER and mono mode optical fiber and further work can target the testing of this system at higher voltages up to 11000V. It may have another very good application in the case of costly chips which are used in circuits and get heated up due to a number of reasons. This arrangement may also be used to save them before getting damaged due to over heating by disconnecting them before this situation.  References  [1]   Doebelin E.o., Measurement system: Application and design, Mc Graw Hill, 1990 [2] Wilson, J. and HawkesJ.F.B,2001 Optoelectronics. Second edition , Prentice-Hall Of India 2001 , pp139. [3] Senior J.M., 2001 Optical Fiber Communications, Second Edition, pp399 [4] Straus, J., 1978, Linearized Transmitters for Analog Fiber links, Laser Focus (USA), 14(10) pp54-61 [5] Soclof, S., 1990, Applications of Analog Integrated Circuits, Eastern Economy Edition, Prentice-Hall, Inc., Englewood Cliffs N. J., USA, pp 481                                    

An Electro-Optical Technique For The Measurement Of Temperature Of High Potential Bodies.

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