In conventional protection system each equipment is covered by a protective zone. The boundary overlap so that no dead zones are left in-between adjacent protective zones.
Each equipment/machine is covered by several protective systems
When abnormal condition occurs, the protection system senses the same and initiate the tripping for alarm or tripping of circuit breakers. The trip circuit of circuit breakers are closed. Circuit breaker opens and clears the abnormally operating part from the rest of the system.
The sequence of fault clearing process is:
1. Fault Occurs : Zero
2. Relays sense the faulty condition and act, to trip circuit of CBs. Few cycles to few seconds.
3. The circuit breakers open and clear the faulty part from rest of the system 60 milliseconds.
In conventional Protection System Protection and Control Functions are not combined.
**Principle of Automatic Protection:** The protective relays are mounted on protection panels housed in the control room. The protective relays include over-current and earth fault relays, differential relay, impedance distance relay etc. The relay coils or measuring circuits are connected to the secondary circuits of current transformers and voltage transformers in the main switchyard. The control cables are run between the control room to the outdoor switchyard. The abnormal conditions such as short-circuit in the protected zone results in increase in current in the primary circuit of the CT. Thereby the secondary current also increases and the actuating current of the relay increases.
If the actuating current is above the threshold value, the relay acts and the trip circuit of the circuit breakers is closed. The current from d.c. battery system flows through the trip coil of the circuit-breaker. The latch is pulled and the circuit-breaker operates and performs the opening operation.
Electtechknownews 
Leave a comment