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Discuss about the Design of an Overheating Transformer Protection System.

Motivation

The break of insulation between turns in the windings of the transformer result in flow of small current in the windings. If this fault is unattended and the transformer is not isolated from the power grid, the coil impedance will decrease. This results in continuous increase of the current and causes a rupture in the dielectric strength of the oil. This will lead to the formation of electric arc which decomposes to vaporize the oil and forms gas bubbles. Hence the pressure of oil in the tank increases and on exceeding the limit, the tank ruptures.  Fire results due to the exit of the gas and liquid from the tank. The transformer fire greatly affects the lives, safety and surroundings. The transformer fire accidents happen frequently worldwide leading to both men and material loss.

Transformer Faults lead to excessive damage and ruin which incur high costs to repair and modify the operation of existing system. With the beginning of digital era, conventional solid state and electromechanical relays are getting replaced with microcontroller which enables reliable, secure and fast operation. This has motivated to take a work in the protection of overheating of transformer automatically using microcontroller based automatic system.

The major components of transformer include the primary and secondary windings, transformer core which is a ferromagnetic material through which there is magnetic permeability from primary to secondary winding, dielectric material which may be oil, wood or gas with poor conductivity,  tank containing the dielectric material, the core and the windings and the 

conservator to store the transformer oil.

The protecting mechanisms available in the current scenario for overheating of transformer are Circuit breaker to prevent damage originated by excess current, thermal relay which operates when the liquid temperature exceeds the threshold, over current relay operating in presence of any short circuit, liquid level meter which monitors the level of the liquid in the tank, differential relay which operates if the difference in current between the primary and secondary windings exceeds the threshold, lightning arrestors for preventing heavy voltage surges, pressure relief mechanism to release the increase in pressure due to arcing, Buchholz relay operating in the event of gas accumulation in the tank and pressure relay operating on pressure accumulation in the tank.

When a transformer is switched on to a power system, the inrush currents may occur whose magnitude and period depends on the size of the power transformer, the source impedance and others.  The inrush currents may be limited by employing current limiter circuits, pre-charge circuits and switching relays.

Objectives

The operation of a circuit breaker in transformer protection is explained here.

Circuit Breaker Operation Circuit

Figure 1 Circuit Breaker Operation Circuit

Whenever there is fault, the secondary current in the current transformer increases which actuates the relay. Contact2 is now closed and the tripping coil is energized by the d.c. source. This will start the mechanism of circuit breaker and moves the moving contact away from the fixed contact of the circuit breaker. The contacts are now opened making the circuit breaker tripped by the relay. An electric arc is formed between the contacts which is extinguished. Manual tripping is done by closing the contact1.

Objectives

The objectives of this project are classified into primary and secondary objectives based on the time restrictions. Primary objectives concentrate on reaching the goal of the project while the secondary objectives aid for perfection of the project.

To make detailed analysis of the construction, types, applications and operation of different types of transformers.

To evaluate the various factors that cause overheating of transformers.

To keep rapport with industries to survey the past transformer failures and also to study the safety precautions followed.

To analyze the existing methods in preventing overheating of transformers.

To study the operation of microcontrollers and its application circuits.

To come up with ideas to design a new method to automatically sense the abnormal condition and take preventive action against overheating using microcontroller.

To design the hardware circuit for the prevention of overheating of transformer

To develop the algorithms for different modules like sensing current, sensing temperature and others

To implement the software and integrate it with the hardware circuit.

To test the circuit under different operating conditions and demonstrate the working of the method

Electricity plays a vital role in our day-to-day life. Transformers play the most vital role in supplying continuous power supply to our homes and industries. Transformers are employed in transmission substation at power plant and in power substation of the distribution grid.  Large transformers are employed in transmission substation of the power plant which convert the generators’ voltage in the range of thousands of volts to extremely high voltages suitable for long distance transmission in the rage of several hundred thousands of volts. After the power supply reaches the distribution grid, once again the voltage is stepped down in transformers of power substations to make it suitable for distribution to houses. Hence failure of transformer may result in huge loss. The replacement is tedious, time consuming, laborious and also accounts for unnecessary investment. Hence automatic prevention of overheating of transformer and thereby avoiding its failure is a smart technique and has more significance.

Proposed Approach

Proposed Approach

The proposed approach is explained in the following steps.

A complete literature survey on the sources for transformer overheating is to be done. A research on the current protecting mechanism of the power transformer, the relay mechanisms, the protecting electronic circuits, current, voltage and power ratings is to be done. Based on the survey, microcontroller based system for automatic protection of transformer is to be implemented. The plan of the project is to sense line currents and line voltages using current transformer and voltage transformer and to measure the temperature using temperature sensor. If the measured current and voltage values exceed the already stored threshold value, the transformer will be tripped by the relay circuit. If the temperature exceeds, a cooling fan will be turned on to cool the transformer. Also, the measured values of current, voltage and temperature will be displayed in the LCD.

Microcontroller development board

Relay Circuit

Cooling Fan

Distribution Transformer

Current Transformer

Voltage Transformer

LCD display

Temperature Sensor

Load

Microcontroller Integrated Development Environment (IDE)

Computer System

Microcontroller is essentially a computer on a chip used for specific applications like home appliances, vehicles, machinery control, robotic control, medical appliances, wireless transceivers, and other devices. The constituents of microcontroller comprise a processor, crystal and timing oscillators, timers/counters, ADC/DAC, memory, PWM and other peripherals.

The supply current is continuously sensed by the current transformer which steps down the current. Then the current is converted into voltage and then rectified by the rectifier which is fed to the Analog input pin of ADC of microcontroller. Similarly, the supply voltage is step down by the voltage or potential transformer and rectified by the rectifier which is given to the ADC of microcontroller. The thermistor or LM35 is used as the temperature sensor. The output voltage of the temperature sensor will be in the range of milli volts and is proportional to the temperature being sensed. The analog value is fed as input to the Analog to Digital Conversion channel of the Microcontroller. The digital outputs of ADC are compared with the previously stored threshold values in the microcontroller. If the measured values exceed the threshold values stored, it indicates the abnormal condition. Then the tripping circuit is activated which cuts off the load. Also, if the temperature exceeds the predefined value, the cooling fan will be switched on by the relay circuit. The measured values will be displayed in the LCD.

Equipments Required

An extension of the project to alert the persons at distant location through mobile phones using GSM is planned if time is available. GSM automatically sends the measurements like current, voltage and temperature from the transformer to the maintenance authorities at distant places using wireless transmission and able to control.

The risk assessment matrix is given in Attachment 2. One of the main risks associated with the project is preparation of the detailed report on the power transformer, causes of overheating, protection schemes and circuits and other parameters. This will require considerable amount of time to collect the data through various sources, interpret the data, analyze it and deriving conclusions from it. But considering the risk in it, it is very low as the group work will improve the efficiency with less fatigue. Another risk is constructing the hardware circuit by dealing with electronic components including transformers, soldering equipment, sensors and others. They have to be handled carefully and common protocols like wearing gloves and shoes are essential while assembling the circuit. Hence there is very low rating of risk assessment for the project.

The project commenced on semester 1 of 2016. The topic was first selected with a detailed analysis. Literature survey was done about the project. Data related to the project were collected from various sources including information from power station, online resources, maintenance industry and discussion with experts. The collected data is included in the progress report.

Conclusion

Power transformers are essential and expensive devices in power supply transmission and distribution. Transformer failures mainly occur due to failures in insulation between the windings. This occurs due to failure of insulation between the adjacent windings and also due to overloading. This over heats the transformers and subsequent increase in fluid pressure, resulting in serious fire hazards including explosion. Hence automatic prevention of overheating is an essential and challenging task. Microcontrollers are the modern electronic systems with advanced features of fast real time processing and data acquisition. Hence, an automatic prevention system for transformer failure is proposed and to be implemented.

The proposed method is highly reliable since any abnormal condition is automatically sensed within a fraction of second and accordingly, preventive action is taken like tripping of the load. It requires no manual operation. It greatly avoids the chances of transformer failures and accidents.

References

Tay, H., & Swift, G. W. (1985). On the Problem of Transformer Overheating Due to Geomagnetically Induced Currents. IEEE Power Engineering Review PER-5(1), 48-49.

Ying, Z. F., Chen, Y. Y., He, W. C., Wu, J. J., & Feng, K. (2012). A New Extension Method in Diagnosis of Transformer Overheating Fault. AMR Advanced Materials Research, 562-564, 1757-1761. 

Bhumiwat, S. A. (2013). Identification of overheating in transformer solid insulation by polarization depolarization current analysis. 2013 IEEE Electrical Insulation Conference (EIC). 

Talhi, M., Flazi, S., & Fofana, I. (2014). Impact of local overheating and electrical discharge on the streaming electrification of transformer oil. 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL).

Zhang, Y., Yan, B., Cao, F., Xie, D., & Zeng, L. (2011). Analysis of eddy current loss and local overheating in oil tank of a large transformer using 3-D FEM. 2011 International Conference on Electrical Machines and Systems.

Franklin, A. C., Franklin, D. P., & Stigant, S. A. (1983). The J & P transformer book: A practical technology of the power transformer. London: Butterworths.

Winders, J. J. (2002). Power transformers: Principles and applications. New York: Marcel Dekker.

Bimbhra, P. S. (2003). Electrical machinery: Theory, performance and applications. New Delhi: Khanna.

Dar, I. R. (n.d.). Experimental Testing of Microcontroller Based Protection for Three Phase Power Distribution Transformer.

Protection of Distribution Transformer using Arduino Platform. (n.d.). Retrieved August 22, 2016, from https://sci-int.com/pdf/15757681051 A 403-406--ADIL NASEEM1 -EE-protection using arduino (1).pdf

Risk Assessment of Transformer Fire Protection in a Typical New Zealand High-Rise Building (n.d.). Retrieved August 22, 2016, from https://ir.canterbury.ac.nz/bitstream/handle/10092/1223/thesis_fulltext.pdf;sequence=1

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