Directional Static Overcurrent Relay:
For obvious reasons of obtaining selectivity overcurrent relays are made directional. Directional static overcurrent 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 pick-up 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.
The induction cup type of electromagnetic relays, even though are very sensitive, have dead zone in their operation. In static directional relays this problem is less serious because static comparators are very sensitive and it is comparatively easier to make a static directional unit reliable down to 1% of system voltage which is well within the minimum fault voltage.
The choice of directional unit depends upon the type of comparator used. The comparator used in directional overcurrent relay may be Hall effect generator; Rectifier bridge comparator; Instantaneous coincidence comparator or Integrating coincidence phase comparator. The Hall crystal as phase comparator has gained popularity only in Russia.
Figure 10.39 represents the directional static overcurrent 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 (Φ – θ).
where
- Is is the magnitude of set current,
- Φ is the phase angle between V and I and
- θ is the relay characteristic angle.
For maximum sensitivity of relay Φ should be equal to θ.
The directional static overcurrent relay can be made either instantaneous or time lag type.