a = Φ1= Flux 1; b = Φ2 = Flux 2; c = α; d = θ
Fig. 2.15 Directional power relay
T α Φ1 x Φ2 x sin α
Since, Φ1 α V,
Φ2 α I and
α = 90 – θ
T α V I sin (90 – θ)
α V I cos θ
α power in the circuit.
Torque produced is positive when cos θ is positive, i.e. θ is less than 90o. When θ is more than 90o (between 90o and 180o), the torque is negative.
At a particular relay location, when power flows in the normal direction, the relay is connected to produce negative torque i.e. the driving torque and restraining torque (due to spring) help each other to turn away the moving contact from the fixed contacts. The angle between the actuating quantities supplied to the relays is kept (180o – θ) to produce negative torque. Consequently, the relay remains inoperative. If due to any reason like reverse power of Turbo Alternator (reverse power called non fault condition), the power flows in the reverse direction, the relay produces a positive torque and it operates. In this condition the angle between the actuating quantities θ is kept less than 90o to produce positive torque. This is shown in figure 2.16 (i). For normal flow of power, the relay is supplied with ‘V’ and ‘– I’. For reverse flow, the actuating quantities become ‘V’ and ‘I’. Torque becomes positive. This can be achieved easily by reversing the current coil, as shown in figure 2.16 (ii).
d = Φ
2.16 (i) Phasor diagram for power relay & (ii) Connection of current coil for power relay