CASCADE TRIPPING

Definition:

A cascading failure is a failure in a system of interconnected parts in which the failure of a part can trigger the failure of successive parts.

Extensive interconnections also mean that a disturbance in one part of the system may quickly spread to the entire system, leading to tripping of loads/generators.

•In case of interconnected system operation becomes unviable, the system must “gracefully” split in to smaller systems(islands).

However, if the control and protection systems are inadequate to face this eventuality, a complete blackout may also occur, leading to loss of service of millions of consumers.

Role of Relays in cascade Tripping:

•The task given to the transmission network:

→Transport electrical energy from generators to customers.

•The task given to a relay:

→Protect a given piece of equipment from physical damage.

•A cascade Tripping occurs when these two tasks conflict and the relays win.

• A failure of the transmission system as a whole to fulfill its task.

Cascade Tripping steps

1. When a component in the grid gets overstressed, it will tend to disconnect itself from the grid through an automated process.

2. If a generator’s automatic control system causes it to begin to rotate too fast, the generator will shut down.

3. If a transmission line has too much current flowing on it, it will heat up, causing the metal conductors to expand and sag.

4. If the underlying tree is too close, electric current from the line will arc to the tree and a circuit breaker will disconnect the transmission line.

5. When this happens, the stress that was carried by the original component redistributes itself instantly throughout the remaining part of the network.

6. The redistribution of stress may cause another component to fail. This process is known as cascade tripping. It iterate rapidly and result in very large blackouts.