An electrical power system often suffers from different abnormal conditions. For examples, it may suffer from short circuit faults. The system may go through the overloading conditions. There may be insulation failures. Lightning surges may strike on the system. We call these abnormal conditions together as electrical faults. Any these faults can damage electrical equipment if not we clear the fault in time.
These damages may interrupt power supply for prolong time. To minimize damage and to maintain system stability, electrical protection systems come in to the picture. We install protection devices at different point throughout the power network. We can divide the protection system mainly in two sections. Actually, an electrical protection system consists of primary protection and backup protection.
Primary Protection
Primary protection is the first line of defence. The primary protection confines for a particular equipment or for a particular zone. In other words, we design the primary protection to detect faults occurring within an equipment or a specified zone. Also, the primary protection actuates the isolation of the faulty equipment or section very fast. The primary protection disconnects only the faulty part, keeping the other portions of the system in normal service condition. So, we can say, a primary protection scheme is selective, and fast.
Examples of primary protection include differential protection for transformers, distance protection for transmission lines, and bus differential protection etc.
When a fault occurs, current transformers and voltage transformers send abnormal secondary current and voltage to the relay circuit, respectively. The protection relays sense these currents or voltages and act to send DC power to the tripping coils of the circuit breakers. As a result, the circuit breaker trips the faulty equipment or zone from the system.
Backup Protection
Although a primary protection system is highly reliable, but still may fail. The failure of primary protections may occur due to relay malfunction, circuit breaker failure, CT or VT problems, wiring defects, or auxiliary supply failure. Therefore, the system needs to have backup protection. Obviously, it clears the fault if the primary protection fails to operate.
Backup protection acts as the second line of defence in the power system. The philosophy of backup protection is that it initiates it operation along with the primary protection. However, its operating time is greater than the primary protection. So, if primary operating duration overs, but fails to clear the fault, backup protection system sends the trip signal to the CB. However, if the primary protection succeeds to clear the fault, then the backup protection stops and returns to its initial condition. Therefore, the backup protection is slower than primary protection. Obviously, it covers a larger portion of the system.
Types of Backup Protection
We can classify the backup protection into local and remote backup protection.
We install the local backup protection in the same substation. Remote backup protection is located at a remote substation. Remote backup clears the fault by tripping remote-end breakers in the event of failure of local backup protection.
A special form of backup protection is breaker failure protection, also known as Local Breaker Backup (LBB) protection. This scheme operates when a circuit breaker fails to trip after receiving a trip command from primary protection. In such cases, all the CBs associated with the part of bus section trip to isolate the fault.