For obvious reasons of obtaining selectivity over-current relays are made directional. Directional relay senses direction of power flow by means of phase angle between V and I, When the phase angle between V and I exceeds certain predetermined value, the directional relay operates with a condition that the current is above the pickup value. Thus directional relay is a double actuating quantity relay with one input as current I from CT and the other input V from PT.
Fig 23 represents the static directional relay with two inputs V and I. The inputs are supplied to the phase comparator. A phase shifter is included in the voltage input circuit, whose output is fed to the phase comparator so that the output from phase comparator under phase faults/earth fault condition is maximum. The output of phase comparator is fed to the level detector. The output of the level detector is amplified and in case a timer is necessary, the output is applied to the output device through the timer.
Relay will operate when Is < = I cos (Ф – θ)
Where Is is the magnitude of set current, Ф is the phase angle between V and I and θ is the relay characteristic angle. For maximum sensitivity of relay Ф should be equal to θ
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