DESIGN AND IMPLEMENTATION OF AN AUTO-TEMP CONTROL SYSTEM FOR DISTRIBUTION TRANSFORMER

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INTRODUCTION
A transformer can be referred to as a power transmission device system that basically converts one voltage level into another voltage level without altering the frequency of the system. During this conversion process, losses occur in the core and windings of the said system. These losses are generally termed heat losses. As a result of these losses in the transformer. the output power of the transformer drops and returns a bit less than its input power. The increase in the rating and capacity of a transformer directly results to an increase in its generated heat. There are different mediums of cooling system conversant with this power system device, they basically include: air, water, oil mediums of cooling systems. The oil and water-cooling mediums of cooling systems in the past years has been done manually. Due to its manual operation characteristic, it is faced with the disadvantage and challenge of not recognizing over heating (Bharathidasan, et al., 2019).
Transformer design most takes into consideration the thermal behavioral aspect. Therefore, precise and proper temperature calculations to a reasonable extent guarantees good quality, performance and long-life expectancy of transformers. The temperature gradient between conductor and oil consists of a gradient inside the solid winding insulation and a gradient inside the boundary layer at the winding surface must be optimal. The gradient inside the solid insulation depends on the thickness of the enamel, paper insulation and oil pockets between conductor and paper wrapping. The heat transfer at the winding surface of a transformer is determined by the cooling conditions approach.
The two basic approaches mostly in use are: The Natural Convective Cooling (ON) and The Forced Convective During the operation of the transformers, heat losses occur and the windings temperature get heated up. Heat losses in the transformer include the losses in iron core, due to the magnetic induction and the copper losses that occur as a result of the flow of electrical current through the windings of the transformer. It is paramount to set up a medium of external cooling system to reduce the heating up of the transformer winding temperature. While the standard average temperatures for the standard-class dry transformers are 80ºC, 115 ºC and 150 ºC, the temperatures of the hottest point reach 150 ºC, 185 ºC and 220 ºC respectively. The expected life of transformers at various operating temperatures is not exactly known. (Buyukbicakci et al., 2014). Amuthan et al., (2017) designed a quadrant cooling type transformer, in this medium, the transformer is cooled by the exterior fan using solar energy. The fans are located in different quadrants with an angle difference of 45 degrees in each quadrant. This method is synonymous to the ONAF method in which air is blown on the cooling fin of the transformer from four angles thereby generating swift cooling. The fans are directly connected to solar panels in this case therefore constant cooling occurs whenever UV rays comes up.

LITERATURE REVIEW
Tekade and Rakhonde (2014) created a microcontroller-based cooling system for transformers. Transformer

METHODOLOGY
This section of the article explains the method (i.e. the design consideration and construction) employed in solving the initial stated problems. For the construction of the prototype, the electrical components listed below were employed in the design following due consideration and calculation process. Components are as follows;

DESIGN CONSIDERATIONS AND CALCULATION REQUIREMENTS
The different stages involved in the design of the automatic temperature control system of a distribution transformer are as follows: 1. Power supply. 2. Microcontroller circuit 3. Temperature sensor 4. LCD display unit 5. Buzzer 6. Control buttons 7. Fan The capacitance of a capacitor suitable to smoothen out an AC voltage with such ripple voltage can be calculated using the expression below C = 0.7 * I / VRPP * F Where, I = max output current = 0.75A F = Pulsating DC Frequency = 2 x AC voltage frequency Calculating Peak to peak ripple voltage, VRPP = Peak to peak ripple voltage VRPP = 2 x Vrpp (6) VRPP = 2 x 3.11V VRPP = 6.22V C = Capacitance of a capacitor, C = (0.7*0.75A) /( 6.22*100Hz) C = 0.0008441F C = 844.1µF An 844.1µF capacitor is not feasible, and as such a 1000µF electrolytic capacitor will be employed. The effect of a 1000µF capacitor on the ripple voltage at the output of the rectifier circuit can be calculated using the expression as follows: Change in VRPP = 0.7 * I / C * F (7) Change in VRPP = 0.7 * 0.75 / 1000 µF * 100Hz = 5.25V VRPP = 5.25V / 2 VRPP = 2.625V

VOLTAGE REGULATOR
The LM7805 IC voltage regulator can supply a regulated power at 5V and a maximum current of 1A. The LM78XX voltage regulator series can effectively operate when the voltage supplied to its input is at least 2V greater than its output voltage. LM7805 whose output is 5V, the minimum input voltage can is calculated as follows;

MICROCONTROLLER POWER SUPPLY
The microcontroller is supplied with a stable 5V at its Vdd pin 13 and 32 while it is grounded at its Vss pin at 12 and 31 respectively.

OSCILLATOR
The fastest and the maximum working operation of the microcontroller is achieved when it is driven by a 20MHz oscillator as seen from the microcontroller's datasheet. Microcontroller instruction execution speed is the rate in terms of time in which the microcontroller will execute the commands stored in its program memory.
Microcontroller's instruction execution speed = (1 / Frequency) * 4 cycles. At a frequency of 20MHz, the execution speed of the microcontroller can be calculated as; (1 / 20000000) * 4 cycles = 20µS A 16pF capacitor is feasible and as a result a 10pF ceramic capacitor is employed for the design. This A/D Converter module can also operate in sleep mode in which clock is derived from its internal RC oscillator. Following points may help you to understand the concept of reference voltages.

ADC MODULE OF THE PIC MICROCONTROLLER
Analog to Digital Converter (ADC) is a device that converts an analog quantity (continuous voltage) to discrete digital values. Most of the PIC Microcontrollers have built in ADC Module. In this project, PIC16F877A is used.
When the ADC input is -Vref, result will be 0000000000 When the ADC input is +Vref, result will be 1111111111 Resolution of ADC = (+Vref --Vref)/(2 -1), which is the minimum voltage required to change the ADC result by one bit.

TEMPERATURE SENSOR
The LM35 temperature sensor has three terminals pins by which it is connected. Pin 1 is the input pin, pin 2 is the output pin and pin three is the ground pin. Input pin (pin-1) is connected to 5V supply, the output pin (pin 2) is connected to an analog input pin (RA0) of the microcontroller and pin 3 of the LM35sensor is grounded.

LCD UNIT
The LCD is powered by the system 5V

Figure 4: LCD Connection
The connection of the LCD screen is as follows; RESET (RS) pin is connected to pin 33 of the microcontroller. ENABLE (E) is connected to pin 34 of the microcontroller. Data pin4 (D4) was connected to pin 35 of the microcontroller. Data pin5 (D5) was connected to pin 36 of the microcontroller. Data pin6 (D6) was connected to pin 37 of the microcontroller. Data pin6 (D7) was connected to pin 38 of the microcontroller.

BUZZER
The buzzer switch was connected to pin 16 of the microcontroller.

Figure 5: Buzzer connection
The capacitor C4 serves as a voltage holding capacitor, to keep the LED active for a short time even after the voltage supply is turned off. The voltage across the capacitor, which is time dependent, can be found by using Kirchhoff's current law, where the current discharging the capacitor must equal the current through the load (in this case the internal resistance of the LED serves as the load). This results in the linear differential equation C.dv/dt + V/R= 0 Where, C = the capacitance of the capacitor.
Solving this equation for V yields the formula for exponential decay: V(t) = V0 x e -t/RC (9) Where V0 is the capacitor voltage at time t = 0. The time required for the voltage to fall to is called the RC time constant and is given by: τ=RC4.
Where τ is measured in seconds, R in Ohms and C in Farads. To achieve a voltage, hold time of 0.4sec, and taking the internal resistance of the LED bulb to be 820Ω (measured value at room temperature), we can compute the required capacitor as follows: τ = 0.4s R = 820 Ω C4 = τ / R C4 = 0.4s /820 Ω C4=

465.11µF
A 465.11µF capacitor not feasible, thus a 470µF electrolytic capacitor is employed in the design.

CONTROL BUTTONS
The control buttons S2, S3 and S4 which indicate increment, decrement and OK respectively were connected to pins 20, 21 and 22 of the microcontrollers respectively in an active low mode using the conventional 10Ω pull up resistors.

FAN
The fan employed in the design is a 5V DC motor with a maximum current rating of 100mA. The circuit components are soldered firmly on the vero board which proved mounting surface for them. The vero board is a continuous type which provide room for the use of neater circuit and less use of jumpers. The process of construction is as follows;

CONSTRUCTION
Vero board type is chosencontinuous vero board is used in this case. Simulated circuit from Proteus is brought in view as guide.
Each component is mounted and soldered rightly as shown in the working circuit diagram or placed according to choose following the right connection for electric path on the vero board. The circuit is tested for continuity using a multimeter. The continuity test is very important as points where isolation is required may be bridged and this can lead to a total destruction of components as electronic components are very sensitive. Points where continuity are needed are also tested as a break in conduction can lead to malfunction or total failure of the circuit.

Fig: General Circuit Design of the Prototype Casing Layout and Dimensions
The device is packaged in a transparent case of dimension 13.2cm x 19.5cm x 8.2cm. With cut outs on its surface for LCD screen, navigation buttons, LEDs, bolts and screws. The LM35 temperature sensor is fixed firmly to the body of the transformer. This temperature sensor produces an analog voltage signal at its output terminal which is directly proportional to the temperature of its environment. The heat dissipated by the transformer is detected by the temperature sensor through conduction and as such the temperature sensor produces an increasing or decreasing voltage signal at its output when the transformer heats up or cools down respectively. This voltage signal is sent through wires to the analog to digital converter (ADC) register of the PIC16877A microcontroller. The ADC converts the analog signals from the temperature sensor to digital signals and stores it in the designated registers of the microcontroller's memory.
Pre-set temperature values are also inputted into designated registers of the microcontroller's memory with aid of three contact switches. These pre-set values are reference values that govern the operation and decisions of the microcontroller. The temperature of the transformer which is required to trigger the fan relay and turn on the fan depicts an example of a pre-set temperature value. If the temperature of transformer exceeds the pre-set value in the microcontroller's memory, the microcontroller triggers the fan relays, thus the fan comes on and blows cool air into the cooling chambers and fins of the transformer. The fan is only turned off when the temperature of the transformer drops below the pre-set value. The LCD screen displays the temperature of the transformer to ensure that the temperature is properly monitored. The LCD also acts as a visual interface between the device operator and the microcontroller by displaying pre-set temperature values that are being entered and stored in the microcontroller's memory. Lastly the LCD shows changes in the fan status whenever it comes on or goes off.

CONCLUSIONS
The device is finally tested and proven to work optimally but as always there always is room for modification. Such recommendation and modification would be stated and explained in the section below. This deviceautomatic temperature control of distribution transformers and can be applied in various aspect of our society where a steady temperature is required such as industries, bakeries, incubators, homes, halls and offices, etc.
• For higher efficiency and longevity, a three-phase induction motor should be used because of their rugged nature and ability to withstand heat and other environmental factors.
• An SMS notification system should also be put in place so as to alert the DSOs whenever temperature values rise above the allowable degree so that he can be notified