Working of Static Relay

 

• The primary of C.T. is connected to transmission line and secondary is connected to some filter circuit.
• This filtered signal is given to rectifier where it is converted from AC into DC (Electronics equipments works on DC supply)
• The DC output is given to level detector circuit, where level of current is compare with fixed value (or threshold value)

For Over current relay

I in < I set – No output

I in > I set – Output is present (and trip signal is send to CB.)

Output of level detector is very small hence, an amplifier is used to make this output large in magnitude and finally this large output is given to CB.

If it is require that relay should operate after some time then a timing circuit is used.

• The timing circuit gives desire time log i.e. relay operates after some time only.
• E.g. if time lag is set at 1 sec, then relay waits for 1 sec after the fault and when 1 sec is over, then only relay starts operating. (I.e. we stop the operation for some time).