This is also a special type of over current relay with a directional features. This directional over current relay employs the principle of actuation of the relay, when the fault current flows into the relay in a particular direction. If the power flow is in the opposite direction, the relay will not operate. Normally, the conventional over current relay (non-direction) will act for fault current in any direction.
The directional over current relay recognizes the direction in which fault occurs, relative to the location of the relay. The principle of directional protection is as under:
Consider a feeder XY, passing through station A. The circuit breaker in feeder AY is provided with a directional relay R, which will trip the breaker CBy, if the fault power flow is in the direction AY alone. Therefore, for faults in feeder AX, the circuit breaker CBy, does not trip unnecessarily. However, for faults in feeder AY, the circuit Breaker CBy trips, due to direction feature of the relays, set to act in the direction AY. This type of relay is also called reverse power relay, So far as the direction of fault current (power) flow is concerned.
Reverse power flow relays with directional features, not only senses the direction flow, but also measures magnitude of power flow.
Directional Relay Connections
Whenever a near or close-up fault occurs, the voltage becomes low and the directional relay may not develop sufficient torque for its operation. To get sufficient torque during all types faults, irrespective of locations with respect to relays, the relays connections are to be modified. Each relay is energized by current from its respective phase and voltage. One of the methods of such connections is 30o connection and other is 90o connection.
30o Connection phasor Diagram
In this type of 30o connections, the current coil of the current coil of the relay of phase A is energized by phasor current IA and the line voltage VAC. Similarly, the relay in phase B by IB and VBA and in phase C by IC and VCB. The relay will develop maximum torque when its current and voltage are in phase.
90o Connection Phasor Diagram
In the above 90o connection, relay in phase A is energized by IA and VBC, relay in phase B, by IB and VCA and the relay in phase C by IC and VAB. The relay is designated to develop maximum torque when the relay current leads the voltage by 45o.
Constructional Details and Operation of Non Directional over Current Relay (Wattmeter Type)
It has a metallic disc free to rotate between the poles of two electromagnets (EM).
The spindle of this disc carries a moving contact which bridges two fixed contacts when the disc rotates through an angle, which is adjustable between 0o to 360o. By adjusting this angle, the travel of moving contact can be adjusted so that the relay can be given any desired time setting which is indicated by a pointer, The dial is calibrated from 0-1. The relay time from name plate curve is to be multiplied by time multiplier setting.
The upper magnet has two windings. The primary coil is connected to the secondary of CT through tapping in it. Theses tapings are connected to plug setting bridge. The secondary is connected to the lower electro magnet; the torque exerted on the disc is due to the interaction of eddy currents produced therein by the flux from the upper EM and the lower EM. The relay setting is 50% to 200% in steps of 25%.
Constructional Details and Operation of Non Directional Over Current Relay (Wattmeter Type)
A directional over current relay operates when the current exceeds a specified value in a specified direction. It contains two relaying units, over current units and the other a directional unit. For directional unit, the secondary winding of the over current (relay) unit is kept open (AB). When the directional unit operates, it closes the open contacts of the secondary winding of the relay may be either wattmeter or shaded pole type.
Under normal operating conditions, power flows in the normal direction in the circuit, protected by the relay and therefore, the directional unit does not operate. When a short circuit occurs, there is a tendency for the current or power to flow in the reverse direction. In such a case, the disc of the directional unit rotates to bridge he fixed contacts A and B, completing the circuit for the over current unit. The disc of the over current unit rotates consequently and the moving contacts attached to it closes the trip circuit. This operates the circuit breaker which isolates the faulty section.
The directional unit is made very sensitive so that with the lowest value of voltage which may be anticipated under severe fault conditions, sufficient torque is produced by the current to complete the operation and allow its contacts to close.
Shaded Pole Type Directional Over Current Relay
A directional relay responds to fault current flowing in a particular direction; the directional feature is achieved by incorporating a directional unit as shown in the figure.
The main flux is split into two fluxes displaced in time and space with the help of a shaded ring. The air gap flux if shaded pole lags behind the non-shaded pole flux. The fig shows how an induction disc type over current relay with split pole i.e., shaded pole magnet having in addition a directional unit consisting of a capacitance C or resistance capacitance RC circuit works as a directional relay.
The above figure shows the wattmeter type induction relay. Here a directional unit controls the angle between the two fluxes by varying the R-X parameters of the lower electromagnet, Another method of control in wattmeter type is to supply the lower winding from a separate voltage source, When the voltage of this source is equal and opposite to the output of the upper magnet secondary winding, there is no current in lower coil. So, no torque is produced. If it opposes and less than the secondary output or if it assists the secondary output, there is an operating torque. Alternatively, if this source voltage opposes and exceeds the output of secondary output, the current in the lower coil is reversed, giving torque. This latter method is used in translay relay.