COMPREHENSIVE GUIDE FOR DIRECTIONAL RELAY ELEMENT 67

Directional Relay

A directional relay is one which is actuated by two electrical parameters namely current and voltage. The directional elements in the relay help the relay recognize the forward electrical faults and not the faults behind the relaying point.

This feature can be best explained with the help of an electromechanical directional relay. It may be noted that directional elements are now embedded in digital relays in which electrical parameters are computed by microprocessors. Electromechanical relays are nowadays obsolete but it is helpful for understanding and explanation purposes.

WORKING PRINCIPLE OF DIRECTIONAL RELAY

Let us take an example of a directional relay electro-mechanical type. A diagram of the construction of the relay along with the phasor diagram is shown.

Two fluxes F₁ and F₂ are being set up by voltage and current respectively. Because of the flux F₁, an eddy current is introduced in the disc which interacts with flux F2 and produces a torque. Similarly, flux F2 introduces an eddy current that interacts with the flux F₁ and produces a torque at the disc.

The resultant torque helps rotate the disc which is proportional to VI cos Φ. Where Φ is the phase angle. Therefore the torque is maximum when the voltage and current are in phase. To produce the maximum torque in fault conditions with a low power factor, compensating winding and shading are provided for the electromechanical relay.

The torque produced by an induction relay is given by T = F₁ F₂ sin θ which is proportional to   I₁ I₂ sin θ, where F₁ and F₂ are fluxes produced by I₁ and I₂, respectively. θ is the angle between the two fluxes F₁ and F₂ or the currents I₁ and I₂. Voltage if assumed as one among the actuators, the current flowing in the voltage coil lags behind voltage by approximately 90°. Assume this current to be I₂. The load current I lags the voltage V by Φ and the angle θ between I₁ and I₂ becomes equal to (90 – Φ).

Here the voltage is a polarising quantity. The polarising quantity produces one of the two fluxes and therefore, taken as a reference with respect to the current.

The torque produced is positive when the value of cos Φ is positive, i.e. Φ is less than 90°. When Φ is above 90° (between 90° and 180°), the torque at the disc is negative. When power flows in the normal direction at a particular relay location, the relay is connected to produce negative torque. The angle between the actuating quantities supplied to the relays is kept (180° – Φ) to produce negative torque. Because of any reason if the power flow reverses, the relay producing a positive torque will operate. In this scenario, the angle between the actuating quantities, Φ is kept below 90° for it to produce a positive torque.

For the normal flow of power, the relay is supplied with V and – I. For reverse power flow, the actuating quantities become V and I. Torque becomes VI cos Φ, i.e. positive. This is basically done by reversing the connection of the current coil.

Relaying units supplied with a single actuating quantity discussed earlier are non-directional overcurrent relays. Non-directional relays have a simple construction and are less expensive compared to the directional relays.

Directional Relay Connections

On the occurrence of a close in fault, the voltage drops to a low level, and the connected directional relay fails to develop sufficient torque for its operation. Under certain fault conditions, the power factor may also be very low due to which insufficient torque will develop. If the relay is connected in the normal way to develop a torque proportional to VI cos Φ, these types of problems cannot be overcome.

Therefore, for sufficient torque generation during all types of faults, irrespective of the fault locations with respect to the connected relays, the terminal connections of the relay are to be modified. Each relay is energized by current from the respective phase and voltage from the remaining two.

There are two methods of connection, one of them is known as the 30° connection and the other the 90° connection.

30° connection

In the 30° connection, the current coil of the relay of phase A draws phase current I_A and line voltage V_A-C. Similarly, the relay in phase B is energized by I_B and V_B-A, and the relay in phase C with I_C and V_C-B. The relay generates the maximum torque when the current and voltage are in phase. This condition with the present connection is satisfied when the system power factor is 0.866 lagging.

90° connection

 The 90° connection gives better performance under most circumstances. In this connection, the relay in phase A is energized by I_A and V_B-C, the B phase relay by I_B and V_C-A, and the C phase relay by I_C and V_A-B. The relays are designed to develop maximum torque when the relay current leads voltage by 45° and has internal compensation. For all types of faults, L-L, L-G, 2L-G, and 3-Φ, the phase angle seen by the relay is well below 90°. This connection also ensures adequate voltage polarization, except for a three-phase close-up fault when the voltages on all phases become very small.

The 90° connection is better than the 30° connection for symmetrical three-phase faults.

APPLICATIONS OF DIRECTIONAL RELAY

Directional relays are used in the protection of parallel feeders. Figure shows a directional overcurrent protection scheme. Here A and B are the sending end breakers using a nondirectional relay, and C&D are using a directional relay. For a fault at F, B will trip because it’s a nondirectional relay, and D will also trip because the direction of current at the relay element is reversed. However, breaker at C does not trip because the current is flowing in the normal direction in the relay. Thereby isolating the faulty section.

Similarly, directional relays can be used for protection in the ring main and radial feeders.

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