 Energised by two quantities(voltage and current)  Φ1 and Φ2 set by voltage and current  Eddy currents induced in the disc due to two fluxes  Interaction b/n Φ1 and Φ2 produce a torque  T proportional to VIcosΦ  Power factor increases by compensating winding and shading coil  θ between I₁ and I₂ is equal to (90-Φ) INDUCTION DISC TYPE DIRECTIONAL RELAY

T = I₁ I₂sin (90-Φ)α I₁ I₂ cos Φ α VI cos Φ  In induction cup two opposite poles energised by voltage, another two poles by current  Φ < 90 degrees, Torque is positive,Power flow in reverse direction, relay operates  Φ > 90 degrees(b/n ) Torque is negative, Power flow in positive direction, relay will not operate  For normal power flow the relay supplied by V and –I,Reverse power flow the relay supplied by V and I (a)Phasor diagram for directional relay (b)Connection of current coil for reverse power relay

 Which operates when current exceeds pick-up value in specific direction  Contains two relaying units  The secondary winding of the overcurrent unit is kept open  Directional unit operates secondary winding of over current unit  Overcurrent unit may be wattmetric or shaded pole type  Wound coil instead of shading coil with ordinary strip

 Sending end Non-directional relays(Symbol )  Receiving end Directional Relays (Symbol )  Faulty feeder is isolated and healthy feeder is maintained

 This type protection is costly and complex  Each feeder requires 2 relays  A non directional relay is required at one end and a directional relay is required at other end  Operating time is determined by the grading

 Earth Fault  A fault which involves ground  Ex: Single line- ground, LL-G  Phase Fault  Faults which do not involve ground  Ex: Double line fault (line-line) Earth Fault Protection : The protective scheme used for the Protection of power system element against earth faults Phase Fault Protection : The protective scheme used for the Protection of power system element against phase faults  Protective schemes are different

 Used for Protection of a section(or an element )  Phase fault relays are called overcurrent relays  Operating principle and construction is same  Plug setting of earth fault relay is 20% to 80% In steps of 10%  Earth fault relays are more sensitive than phase fault relays.  Plug setting for Phase fault relays is 50% to 200% in steps of 25%  Overcurrent relays used for phase faults

 The earth fault relay is energised by a residual current  ( ia+ ib+ ic) is called residual current  Normal condition residual current zero  Balanced condition current through earth fault relay zero.  Minimum plug setting is 20% or 30%  Magnitude of earth fault current depends on fault impedance and neutral earthing  Fault impedance of E. faults > phase faults  Due to Earth fault current< Phase fault current  Plug setting % is low for earth faults compared to phase

 Earth fault relay C.T Burden is More than phase fault relay Because of relay is set at low % values( Impedance is high)  Fig (b) and (c) used for protection of transformer and alternator  Zero sequence current flows through the neutral which actuates earth fault relay  Fig (d) specifies special C. T, Known as core-balance C.T

 Three overcurent relays  Scheme is against phase faults  This scheme will also sense earth faults  Less sensitive  Two overcurent relays and one earth relays  Over current relay and earth relay in series  C.T burden is high and cause saturation

 One directional overcurrent relay required  Operating principle is same  It contains two elements  Directional element  I.D.M.T element  Directional element has two coils  One coil Energised by current  Other by voltage  Current coil energised by residual current  Voltage coil energised by residulal voltage

 Neutral point is grounded through P.T  Potential coil is connected to secondary of P.T  I.D.M.T has plug setting 20% to 80% Directional Earth Fault Relay