Primary protection is the essential protection provided for protecting equipment.
As a precautionary measure, an additional protection is generally provided and is called "Back up" protection.
If primary protection fails, the back up protection comes into action and removes the faulty part from the healthy system.
Back up protection is provided for the following reasons :
1. If due to some reasons; the main protection fails, backup protection serves the purpose of protection.
2. Main protection can fail due to the failure of one of the components in the protective system such as relay, auxiliary relay, CT, PT, CB etc.
If the primary protection fails, there must be additional protection; otherwise, the fault may remain uncleared, resulting in a disaster.
As a measure of the economy, backup protection is given against short circuit protection and generally not for other abnormal conditions.
The extent to which backup protection is provided depends upon economic and technical considerations.
(1) Relay back up:
The same breaker is used by both main and backup protection, but the protective systems are different. Separate trip coils may be provided for the same breaker.
(2) Breaker back up:
Different breakers are provided for main and back protection, both the breakers being in the same station.
(3) Remote back up:
The main and backup protections are provided at different stations and are completely independent.
(4) Centrally co-ordinated back up :
The system having central control can be provided with centrally controlled backup. Central control continuously supervises the load flow and frequency in the system.
The information about load flow and frequency is assessed continuously. If one of the components in any part of the system fails, the load flow in the system is affected.
The central coordinating station receives information about the abnormal condition through high frequency carrier signals.
The stored program in the digital computer determines the correct switching operation, as regards the severity of the fault, system stability, etc.
The main protection is at various stations and backup protection for all stations is a the central control center.
The centrally coordinated backup is a teamwork of protective relaying equipment, high-frequency carrier current equipment, and digital computer.
(5) Back up protection by time - grading principle :
The current is measured at various points along the current path, e.g. at source, intermediate locations, consumer end.
The tripping time of these locations are graded in such a way that the circuit breaker, nearest the faulty part operate first, giving primary protection.
The circuit breakers at the previous station operate only as back up. In the Fig. if the fault occurs on any feeder, then circuit breaker B on that feeder will give primary protection and circuit breaker A will
give backup protection.
When the fault occurs, the relays at ‘A’ also start operating but are provided with enough time lag so that the circuit breaker ‘A’ operates only if the circuit breaker 'B' does not.
Most Asked Electrical Interview Questions:
What is Auto-Recloser In Transmission System ? How its Works ?
What is Aviation Light ? Why it is used ?