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 construction and phasor diagram is given below.

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 F₁ and produces a torque at the disc.

The resultant torque helps rotate the disc. The torque is proportional to VI cos Φ. Where Φ is the phase angle between voltage V and current I. 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. The angle between F₁ and F₂ or I₁ and I₂ is θ. If one of the actuating quantities is voltage, the current flowing in the voltage coil lags behind voltage by approximately 90°. Assume this current to be I₂. The load current I (say I₁) lags V by Φ. Then the angle θ between I₁ and I₂ is equal to (90 – Φ).

Here voltage is a polarising quantity. The polarising quantity is one which produces one of the two fluxes. The polarising quantity is taken as a reference with respect to the other quantity which is current in this case.

Torque produced is positive when cos Φ is positive, i.e. Φ is less than 90°. When Φ is more than 90° (between 90° and 180°), the torque is negative. At a particular relay location, when power flows in the normal direction, 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. If due to any reason, 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 Φ is kept less than 90° to produce a positive torque.

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

 Relaying units supplied with a single actuating quantity discussed earlier are non-directional overcurrent relays. Non-directional relays are simple and less expensive than directional relays.

Directional Relay Connections

When a close-up fault occurs, the voltage becomes low and the directional relay may not develop sufficient torque for its operation. Under certain fault conditions, the power factor may be very low due to which insufficient torque is developed. 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.

To get sufficient torque during all types of faults, irrespective of their locations with respect to the relays, the relay connections are to be modified. Each relay is energized by current from its respective phase and voltage from the other two phases.

 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 is energized by 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 is designed to develop maximum torque when its 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.

For three-phase symmetrical faults, the 90° connection is better than the 30° connection.

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 in the protection of ring mains and radial feeders.

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