Protection Relays - Part 5

Anti-pumping relay

The anti-pumping relay is a device in circuit-breaker whose function is to prevent multiple breaker closures. For instance, if the operator gives the closing command to the breaker by pressing the close button and the breaker closes. However, a fault in the system causes the breaker to trip. Since the close command is still in the pressed condition, there is a chance of the breaker closing again and being tripped by the relay multiple times. This can damage the closing mechanism of the breaker. The anti-pumping relay prevents this by ensuring that the breaker closes only once for one close command from the control panel.


Directional Power Relay

The directional power relay is used to protect a synchronous generator, running in parallel, from motoring. Motoring occurs due to the failure of the prime mover such as a turbine or an engine driving a generator that is connected to the grid. The generator which is running at the synchronous speed will continue to run at the same speed. However, the power required to keep the generator running along with the prime mover will be drawn from the mains. Hence, power flows in the reverse direction i.e. bus to generator. This condition is called reverse power.

Directional power operation may cause damage to the prime mover. Hence, reverse power protection is a vital part of the generator protection scheme.

The reverse power relay operates by measuring the active component of the load current, I x cos φ. When the generator is supplying power, the I x cos φ is positive, in a reverse power situation it turns negative. If the negative value exceeds the set point of the relay, the relay trips the generator breaker after the preset time delay.

The typical setting for reverse power is 4% in case of turbines and 8% in case of diesel engines. The time delay can be set from 2 to 20 seconds.


The Vector Surge Relay

The vector surge relay is used to decouple synchronous generators from the grid utility in case of grid failure.

Synchronous generators are generally operated in parallel with the grid utility. This ensures greater reliability and enables the generator to export power to the grid. In this condition, there is a chance, of a momentary interruption of the grid supply which may result for a few milliseconds. Such temporary interruptions can be caused to mal-operation of the circuit breakers on the grid transformer side.

For a synchronous generator, running in parallel with the grid utility, such a temporary interruption and restoration of the supply can be dangerous. As the restoration of the supply can be asynchronous i.e. the generator and the grid are now not in a synchronised condition. The can lead to the consequences of wrong synchronization such as damage to the generator or the prime mover.

The vector surge relay prevents this condition by decoupling the generator from the grid as soon as the grid supply fails. This is an extremely fast acting relay with an operating time of less than 300ms from relay operation to breaker opening.

Principle:

The vector surge relay functions by monitoring the rate of change of the rotor displacement angle of the generator. During parallel operation there is an angular difference between the terminal phase voltage (Up) and the internal synchronous voltage of the generator (Ui). This is due to the fact that the generator rotor is magnetically coupled to the generator stator and is forced to rotate at the grid frequency. The angle between the vector of the mains voltage Up and synchronous electro-motive force is known as the rotor displacement angle.

This angle is constantly varying and is dependent on the torque produced by the generator rotor. In the case of the grid failure, there is sudden change in the rotor displacement angle.

This causes a surge in the generator voltage shown in the figure. The relay works by monitoring the time taken between the zero-crossings in the waveform. Under normal operation, the time interval between two consecutive zero-crossings is almost constant. During the grid failure, the vector surge which occurs causes a delay in the zero-crossing. This delay is detected by a highly sensitive timer inside the relay and the relay operates.

The relays are usually set to operate for a change in the rotor displacement angle of 0 to 20 degrees