Directional overcurrent

Step 1: Establishing Direction

Operating region

S₁  =  Reference Direction  =  Polarising Signal  =  V_POL

S₂  =  Current Signal  =  I

OPERATION when S₂ is within ±90° of S₁  :-

Restrain region

RESTRAINT when S₂ lags S₁ by between 90° and 270° :-

Step 2 : Polarizing Voltage selection

OPERATE SIGNAL =          I_A

POLARISING SIGNAL :-     Which voltage to use ?

                                                            Selectable from

                                                                           V_A

                                                                           V_B

                                                                           V_C

                                                                           V_A-B

                                                                           V_B-C

                                                                           V_C-A

Case 1:

Applied Voltage        :           V_A

Applied Current        :           I_A

is this connection suitable for a typical power system ?

           No, the fault current at near to zero sensitive point and operation of relay will be insensitive. Polarity voltage will also change.

Case 2:

Applied Voltage   :    V_BC

Applied Current   :    I_A

is this connection suitable for a typical power system ?

         Yes, Because Polarising voltage remains healthy & Fault current in centre of characteristic

Step 3: Relay connection angle

          The angle between the current applied to the relay and the voltage applied to the relay at system unity power factor

e.g.  90° (Quadrature) Connection  :    I_A  and V_BC

Step 4: Relay Characteristic Angle (RCA)

     The angle by which the current applied to the relay must be displaced from the voltage applied to the relay to produce maximum operational sensitivity

(i) 90° Connection - 45° R.C.A.

Application :

1.    Plain feeder, zero sequence source behind relay

(ii) 90° Connection - 30° R.C.A

Application

1.    Plain or Transformer Feeder :-  Zero Sequence Source in Front of Relay

2.    Transformer Feeder :-  Delta/Star Transformer in Front of Relay