Therefore there is the development of distance protection in which the action of relay depends upon the distance (or impedance) between the point where the relay is installed and the point of fault.
The relay operates when ratio V/I reduces below a set value, When a fault occurs on a transmission line the fault current increases and the voltage at the fault point reduces.
The voltage at voltage transformer location depends on the distance between the voltage transformer and the location of the fault.
Farther is the distance of fault from voltage transformer more will be the voltage available at voltage transformer and smaller is the distance legs will be the voltage.
Hence assuming constant fault resistance value of line measured from relay location corresponds to distance between the relaying point and the fault along the line. Hence such protection is called Impedance protection,
For a given setting, the distance relay picks up when impedance measured by it is less than the set value.
Hence it protects a certain length of the line. Hence it is also called distance relays.
Considering zero fault impedance the voltage at the fault point will be zero.
The voltage at relay location O will be equal to the voltage drop along the length of the transmission line (up to fault point F), where as same current I is flowing in the line at 'O’up to 'F'.
If a fault has occurred near ‘O'the voltage at 'O’ would be different, the current would be more because of the reduction in line impedance.
If the fault occurred away from 'O'the voltage at ‘O' would be lesser and current would also be lesser.
In distance relays the ratio V/I is measured. The current gives operating torque and voltage gives restraining torque.
Hence for values of 'Z' above certain settings, the relays do not operate. Hence it protects only a certain length of line equivalent to its impedance setting .
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