DC Circuit breaker using SCR

INTRODUCTION

                  The project deals with the design of the DC circuit breaker using SCR (Silicon Controlled rectifier). A circuit breaker is a tool which breaks the circuit when any fault occurs in the circuit and after normal condition, it brings the circuit in ON condition and the circuit again to conduct. The circuit breaker is the advanced form of the fuse. The circuit is disconnected by fuse when the rated current flows in the circuit and it can’t be changed in ON position after coming in normal position and external agent is required to do it. But the circuit breaker does it itself. The SCR is semi-controlled device and act as switch which can be changed in on position by applying gate pulse but changing it into off position is not in our hand and to do so a technique is required which is called commutation of SCR. In this project we are using complementary-commutation.

There will be two SCR and the cathode terminal of both scr is connected to ground (-ve terminal of source) and one terminal of capacitor is connected to anode of one of the scr and other terminal to that of other scr. The load is connected to source voltage and one of the terminals of capacitor and one resistor is connected to another terminal of the capacitor to bypass the current whenever fault is occurred. The this is done by microcontroller i.e. Arduino. We are designing the DC CB for 20V and 1A rating.

CIRCUIT DIAGRAM

CIRCUIT DESCRIPTION AND PRINCIPLE OF OPERATION

               The basic circuit for DC circuit breaker is shown in the figure A. The SCR 1 is main SCR. When it is necessary to break load current, the SCR 1 is turned off.  There is no control circuit given in this figure for simplification. When the SCR 1 is turned on, the load voltage is equal to supply voltage and charging of the capacitor is done through path (+) V_dc – R1 – (+) C – C (-) – SCR 1 – V_dc (-). The polarity of capacitor charging voltage is shown in the figure A. When it is require breaking load current, the SCR 2 is turned on. As soon as the SCR 2 is turned on, the SCR 1 is turned off due to reverse voltage across it. The capacitor once again charges with reverse polarity through path (+) V_dc– LOAD – (+) C – C (-) – SCR 2 due to turned off the SCR 1. When voltage across capacitor reaches up to supply voltage, the load current becomes zero resulting SCR 2 is turned off. The current passes through load resistance R1 becomes less than the holding current of the SCR 2.

The value of R1, R2 and C can be calculated as follows:

Supply voltage, Vs =20V

Rated current, I = 0.5A

∴  Vs/I=20/0.5=40 Ohms

Holding current of SCR=20mA, so

R2 = 20/20*10^3=1kOhm

If safety factor=2 and turn of time of SCR = 130us, then

C=130*10^(-6)/(40*ln(2))= 4.7uF

SOURCE CODE

double v = 0;

double c = 0;

void setup(){

Serial.begin(9600);

pinMode(8,OUTPUT);

pinMode(9,OUTPUT);

}

void loop()

{

for(int i = 0; i < 1000; i++){

v = (v + (0.0048828* analogRead(A0))); // (5 V / 1024 (Analog) = 0.0049) which converter Measured analog input voltage to 5 V Range

delay(100);

}

v = v /1000;

c = (v -2.515)/ 0.1; // Sensed voltage is converter to current

Serial.print("\n Voltage Sensed (V) = ");

Serial.print(v,4);

Serial.print("\t Current (A) = ");

Serial.print(c,4);

if(c>0.13){

  digitalWrite(8,HIGH);

  digitalWrite(9,LOW);

}

  else{

  digitalWrite(9,HIGH);

  digitalWrite(8,LOW);

  }

  delay(500);

}

OUTPUT

                            

PROTOTYPE MODEL