DESIGN, IMPLEMENTATION AND PERFORMANCE STUDY OF PROGRAMMABLE AUTOMATIC VOLTAGE REGULATOR

DESIGN, IMPLEMENTATION AND PERFORMANCE STUDY OF PROGRAMMABLE AUTOMATIC VOLTAGE REGULATOR

ABSTRACT

This paper proposes the design and implementation of a Programmable Automatic Voltage Regulator (PAVR) with higher precision, appropriate hysteresis and defense of anomaly. Current systems available locally lack precision and suffer the problem of oscillating between two output voltage and hence creating surge at the output which can damage valuable electronics. To avoid these, the stabilization of power voltage, minimization of output wave rate and unchangeable power-voltage to the instruments are needed while the load changes. That requires the maintenance of stable voltage and rapid reaction against the sudden change of input voltage and load. This paper defines the shortcomings and introduces a new system in the tolerable and substantial stable of 220V with '4.5% output accuracy for any deviation of input supply voltage within 100V-340V. To control the whole system automatically a microcontroller is used with some protection devices where to detect fault and the circuit implementation in this system are simple and flexible than conventional analog control circuitry. A simulation for both circuit and program has been accomplished for establishing better performance.

Keywords: Programmable Automatic Voltage Regulator (PAVR), Hysteresis, Microcontroller, Protection, Simulation.

INTRODUCTION

AC power supply by Power Development Board (PDB) in Bangladesh is subjected to variation from time to time. Moreover in rural areas supply voltage remains lower than specified most of the times. This poses a considerable threat to the sophisticated electronic devices. For that reason, many important electric machine or electric equipments may destroy. Power quality related problems, in particular voltage sags, surge and brownouts have a major negative impact on industrial productivity. This appears to be true for both industrialized as well as developing nations. So ensuring the input voltage to remain in a tolerable pre-specified limit has become a necessity in rural as well as some urban areas. In order to save these we need to use the Automatic Voltage Regulator.

An AVR is an electronic device that automatically regulates a variation of input voltage at a certain desired level to load. The voltage of main power supply may be affected by various troubling physical factors, so that special regulating equipment is required to keep the voltage steady. In replace of AVR, Programmable AVR is more flexible, easy to modify and the best for good precision and hysteresis. The existing systems like servo-stabilizers, CVTs, Ferro resonant regulators, thyristor ac regulators, tap changers and the electronic regulators are the available means for voltage regulator. Here the performances of existing commercially available technologies are compared by this system regarding response, faults handling, precision, efficiency and other important parameters. It is essential that the supply ac voltage is needed to operate automatically due to the interconnections among the systems in modern age. The electronic control circuit may be utilized to get the desired output which is very simple, flexible, reliable and cost effective.

AVR mainly functions to measure and regulate the input voltage for producing the stable output and to provide the Protection against sag, surge, spike, impulse, notch, brown out, over voltage, under voltage, over current and hysteresis to the sophisticated equipments and machineries. In this system, the whole operations are implemented by a microcontroller. Microcontroller here performs all actions in accordance with the program maintaining proper precision and hysteresis as we desire and the undesired input transitions are handled by the AC protection devices. The whole proposed design is illustrated in the following block diagram (Figure-1).

 

Figure-1: Block diagram of PAVR

A multi-tapped transformer with input supply and switches connected in primary and in secondary respectively to obtain regulated and stabilized output voltage at the load side is used. Here microcontroller plays the important role to decide and hence to control the switches through which secondary tap carries the power from input to load with a steady voltage. This system also provides the protection against surge, spike, lightning, overvoltage and excess current by adding AC breaker (MCB), Automatic Voltage Switcher (AVS) [25] in the Input line. In the upcoming sections the designing procedure with simulations, programming, the control operation of microcontroller and performance analysis will be discussed.

DESIGNING PROCEDURE AND DISCUSSION

Description of Circuit Design

As the designing commitment which is to stabilize automatically a large range (100V- 340V) variation of input voltage at a normal prescribed level output voltage (Table-1) with a great precision. For this the voltage regulation of input supply is designed which is regulated automatically in such a way that when the input voltage varies, the output voltage will remain stable at a constant value. To achieve this, a microcontroller has been programmed is such a way that when the dc input to microcontroller varies in accordance with the variation of input voltage, microcontroller will operate the suitable switch to get a regulated output from desired transformer tap. Table-1 represents the designing configuration of the multi-tapped transformer.

Firstly, the microcontroller compares a converted variable DC voltage which is found by stepped down the ac supply voltage with the range, which is shown in Table-2 and is set in the memory of the microcontroller. The voltage range for the program has been chosen in such different manner for letting switches functioned to select the different taps for getting desired output of around 220V AC (Table-2). Thus the stable output voltage occurs at the output of this designed regulation system for any short of variation of input voltage automatically. The whole schematic of the PAVR is shown in Figure-2 and Figure-3.

 

Figure-2: Schematic diagram of PAVR (Part-1)

 

Figure-3: Schematic diagram of PAVR (Part-2)

Simulation Procedure

To determine the accurate level of voltage for designing of switching and making the microcontroller program this circuit has been simulated using PSPICE Simulator. The operation of switching of PAVR at different level of input voltage is observed as well. Here, for input supply voltage of 130V, the switching operation is shown graphically in

Figure-4 where the switch under PB0 and Q0 is “ON”. At any input voltage within 100V- 340V, one and only one switch is activated at a time as defined in the program.

 

Figure-4: Simulation for 130V Input

Flowchart description

The microcontroller program of the PAVR is designed and functioned, as the following flowchart (Figure-5), to measure the received DC voltage and then to compare with the prescribed range stored in the microcontroller memory and finally to obtain a decision for the output. In the flowchart, it is also shown that when the received DC input is out of the range (i.e. above 5V and below 1V), microcontroller will operate “No Operation (NOP)”. That means, the PAVR will remain “OFF” during excess high and low voltage AC supply to protect the devices and equipments from damaging. Here the increasing and decreasing of DC input voltage are given orderly to maintain the hysteresis properly.

 

Figure-5: Flowchart for the program of microcontroller operation

Description of Program Operation

The program that was designed for the PAVR has been simulated using PIC Simulator IDE software [28]. PIC Simulator IDE is a basic complier, assembler, disassembler and debugger for microchip family PICmicro. Firstly, the microcontroller has to be selected to load the program. Then the simulation has to be run. The sequences of operation have to be executed as follows.

Option>Select Microcontroller

Tools>Assembler

File>open/PAVR.asm

Tools>Assemble & Load

Simulation>Start

Tools>Microcontroller View

Tools>8×LED Board

The overview of simulation is presented by Figure-6 with PIC Simulator IDE interface. In this figure, the PORTB0 is displayed as ON when the input voltage is in the range of 1.0V-1.5V (2Ch-0Fh).

 

Figure-6: The simulation interface of PIC Simulator IDE with output Display

Description of Microcontroller Operation

A PIC16F876A CMOS FLASH-based 8-bit microcontroller (Figure-7) is used to control the whole system which is more reliable, simple and flexible than conventional analog control circuitry. It features especially Self-reprogrammable under software control, 2 I/O pins & 3 I/O ports, 256 bytes of EEPROM data memory, 2 Comparators, 5 channels of 10-bit Analog-to-Digital (A/D) converter, Programmable code protection, Power-saving Sleep mode, Selectable oscillator options and In-Circuit Debug via two pins.

 

Figure-7: Pin diagram of PIC16F876A Microcontroller with program placing.

An assembly language program is loaded in the microcontroller memory (Figure-7) that configures the A/D module, comparator module and I/O ports, and sets different registers of the memory with the different range of digital code (Table-3). In the program the registers are assigned by any arbitrary names to make the program more readable.

Here port-A is used as input port and port-B as output port. Firstly, microcontroller receives the DC input corresponding to supply AC input from stepped-down transformer at RA1 of PORTA. And then it is compared with the ranges that are set in the memory of microcontroller in the form of code via an A/D converter and a comparator. After that it makes anyone of the output pins (RB0-RB7) of PORTB HIGH corresponding to the range, as mentioned in Table-2, which in turn activates a respective switch to get the regulated output from the transformer. Here a constant voltage level is maintained although the input voltage level goes high than the constant desired level (Table-4). To sense this change, the microcontroller periodically checks terminal voltage and compares this with defined reference voltage levels. For designing purpose the reference voltage and code as a range has been calculated to corresponding pins of PORTB. This is shown in Table-2 and Table-3.

Table-1: (For the Design of Multi-tapped Transformer) Output Voltage Range corresponding to Input Range and Transformation Ratio at different Taps at Normal Input Voltage of 220V

Table-2: Selection of output pins, Switches and Transformer Taps for the prescribed input DC voltage range corresponding to input AC supply voltage range.

Table-3: Hexadecimal Value of digital code equivalent to prescribed DC voltage range corresponding to AC input and PAVR output voltage.

Table-4: Experimented Analog DC input for ADC of Microcontroller and Regulated AC Output Voltage for typical AC Input Supply

PERFORMANCE ANALYSIS

This system has been experimented practically after designing. Here for some typical AC input voltage supply, the regulated output approximate to desired level is found that is shown in Table-4. From this table, it is clearly revealed that this PAVR is able to regulate any variation of input voltage of a system within 100V-340V at a stable range of 210V- 230V and also to defend the system from damage due to extreme decrease and increase in AC supply voltage which is out of the range of 100V-340V. In Figures below present some performance curves of PAVR. Figure-8 shows the desired response curve of PAVR which is produced from the desired output voltage (210V-230V) for any input supply voltage within range of 100V-340V. In Figure-9, a typical input-output voltage characteristics curve of PAVR is put on view of the regulated output voltage that is near about 220V AC (Data in Table-4).

The Figure-10 exhibits a practical input-output voltage characteristics of PAVR for random supply. In this response curve, it confirms the design requisition of a stable output in range of 210V-230V as the regulation of any change in AC supply voltage within 100V- 340V, and the protection any system from spoiling due to low voltage i.e. below 100V and high voltage i.e. above 340V. A hysteresis curve, given in Figure-11, shows the hysteresis behavior of PAVR during the operation of switches at the transition of different ranges of input supply voltage. It is needed to maintain properly to prevent the frequent vacillation of switching between “ON” and “OFF”.

 

Figure-8: Desired Response Curve of PAVR

 

Figure-9: Typical Input-Output Voltage Charactesistics of PAVR

 

Figure-10: Practical Input-Output Voltage Characteristics of PAVR for Random Supply

 

 Figure-11: Hyteresis Curve

CONCLUSION

It is clear that, from above design and discussion, and the comparison shown in Table-5 with some other common existing AVR systems, my proposed proprietary PAVR performs better than any other existing systems. Because it is mainly programmable that can be programmed as the demand maintaining proper precision and sufficient hysteresis over a wide range of input variation. Here the protection against the excessive high and low voltage and current is confirmed which is crucial for the sophisticated electrical and electronic equipments. It is mentionable that the PAVR is very much cheap than other systems because of having microcontroller in place of discrete electronic components and simple protection units. Therefore, the circuit design and implementation are very much easy, flexible and the efficiency of this system is good enough as well. According to market comparison study, the commercially available AVR has a three to four step stabilization of the input variable voltage where the output becomes a big changing stable value within a prescribed range that is not an absolute design to get an output precised. For this reason in my research the way has been adopt to make the system for getting the precision output within in a large variation of input is the design of the main transformer having a many number of taps in the secondary winding side of the transformer maintaining a small turn difference between two adjacent taps. PAVR is applied to all electrical and electronics equipments especially in communications and precision instruments of manufactories.

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