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A prototype of a fully electronic on-load semiconductor tap changer for power transformer has been
designed and built. With the emergence of high power semiconductor devices, problems associated with the mechanical on-load tap changer have been properly rectified. In this work, the prototype was constructed with triacs as the switching devices and microcontroller as the triggering circuit. The results obtained from this work show that the prototype has a faster time response of approximately 0.44s to react to any load changes. It also produces no arching problems as it has no mechanical contacts and requires no maintenance, and can be considered as one of the fast solutions of the voltage sag or voltage swell. The system has been tested for reliability and proven to be reliable in maintaining the output voltage of the system.
The main purpose of our project is to design a fully electronic tap changer.Triacs are used as a switching device to turn on selected tap of the power transformer.Microcontroller with its loaded software acts as the triggering element to the triac. Step down and the opto-coupler is connected between input and the out of the microcontroller respectively thus isolating the low voltage circuit of the microcontroller from the damaging high voltage circuit of the power transformer.
INTRODUCTION
One of the main concerns of any power utilities is the quality of the power supplied to the customers, as these customers demanded an uninterrupted supply with a minimum
case of disruption. By addressing these concerns, the power utilities can reduce the cost related in generating, transmitting,distributing and maintaining the power system.
There are several measures that have been taken to rectify these problems, such as by employing voltage regulator,capacitor and dc stored energy. In this paper, focus is being
given to the power transformer with tap changer; on-load and off-load. The former is preferable, as there is no disconnection of the power transformer when changing the tap setting, thus the operation of supplying the load demand is remained uninterrupted. Lately, online monitoring of power transformer has become of interest to the power utilities, as the power transformer is one of the most expensive single elements of the high voltage transmission system
On-load tap changer power transformers are an essential part of any modern power system, since they allow voltages to be maintained at desired levels despite the load changes.Although the first on-load tap changers were developed in the
early part of this century, modern versions still have not altered radically from these designs and in essence, they are complex mechanical device.
The application of semiconductor or solid state devices in designing the tap changer have the advantage of faster response, almost virtually maintenance free and better performance in term of power quality when compared to its conventional counterpart. The only setback of solid-state devices is cost efficiency and high conduction loss.Furthermore, as solid-state devices must be permanently
connected in the circuit, some sort of protection against high voltage surges travelling down the transformer winding is required.
In this project, the improvement is concentrated on maintaining the voltage supply by changing tap setting via microcontroller through triac assisted selector. The results
obtained from this experiment show that the proposed semiconductor tap changer is able to monitor the voltage supply and maintain it within the specified range. The system
takes approximately 0.44s to response to the load changes.
SEMICONDUCTOR TAP CHANGER
The main purpose is to design a fully electronic tap changer with a prototype constructed as the model of the operation as shown. Triacs are used as the switching device to turn on the selected tap of the power transformer.Microcontroller with its loaded software acts as the triggering element to the triac. Step-down transformer and opto-coupler isolator is connected between the input and output of the microcontroller, respectively, thus isolating the low voltage circuit of the microcontroller from the damaging high voltage
circuit of the power transformer.
The block diagram shows the detailed blocks diagram for the semiconductor tap changer used in this work. A few extra devices are inserted in the prototype to provide a better accuracy and safety for the system. A feedback loop circuit of 110V/6V step-down transformer, rectifier, peak detector, filter and opto-transistor, is incorporated into the prototype. Its function is to convert the 110V AC line voltage to an acceptable DC level voltage for the microcontroller operation and provide a protection from damaging the microcontroller.
The rectifier converts the AC voltage signal to DC voltage signal. As the output of the rectifier is not constant but with ripples, peak detector and filter is employed to get a better signals. Peak detector will detects the peak value of the rectifier’s output signal and gives a constant DC equivalent voltage and then the filter will filtered out any noise and further improve the signals so that it is free from any ripples and within a certain range of frequencies. While the optotransistor acts as an electric isolator to protect the input of the microcontroller.
NMIT-0020 F68HC11 microcontroller is used as the logical central process control to process the input signal and produce a suitable output signal according to the program loaded into the microprocessor. The microcontroller acts as a trigger by injecting pulses to the selected triac representing the appropriate taps. At any instant, only one triac will be in its ON state while the others are in OFF state.
The below fig shows the connection of the microcontroller,resistors, opto-couplers, triac circuits, load and power transformer. Opto-coupler protects the output of the microcontroller from the high voltage value of the power transformer. It also functions to maintain the ON-OFF switching operation of the triac. When the microcontroller has samples the DC voltage from the rectifier, and determines the appropriate tap setting to maintain the voltage, it will generate pulse signal to the designated opto-coupler. This opto-coupler will then activates the triac connected to it.
Once the triac is ON, it will stay ON until the gate terminal voltage of the triac falls below the holding current. The rest three triacs are at its OFF condition and will continues to be in this condition until the microcontroller decides to change its tap setting based on the output of the load. So, when the microcontroller senses changes in the load voltage, it will compute the new tap setting and gives an appropriate pulse to the selected opto-coupler. It will then turn on the triac and the load voltage will returns to normal.
The software loaded into the microcontroller is written using PROCOMM. It samples the input given to the microcontroller and compares the value with the determined value written in the program. The software has been given a set value of 100V. The signal is first converted to digital value by the internal analog-to-digital converter before the microcontroller could processes the information. If the value is 10% more or 10% less than the nominal value, the microcontroller will quickly change the tapping to a lower or a higher taps setting respectively. Microcontroller will continue changing the setting to maintain the voltage within the set value. If the tap setting is at its maximum or minimum, alarm signal will be generated and indicated by the flashing LEDs. Otherwise, the taps setting will remain unchanged.
Any variation of the output voltage of the power transformer will be detected by the microcontroller, which in turn computes and executes necessary command instruction to be pass on to the appropriate triac. The semiconductor tap changer will changes the tap position when the variation is out of the permissible range. Thus the voltage of the system could be maintained at nominal value.
TRANSFORMER
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:
By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP.
PRINCIPLE:
The transformer is based on two principles: firstly, that an electric current can produce a magnetic field, and, secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.