In a differential protection scheme, electrical relays are used to detect and protect against faults that occur within a protected zone, such as a motor, transformer, or generator. The primary purpose of differential protection is to monitor the difference between the currents entering and leaving the protected zone. When a fault occurs within the zone, the currents will no longer be equal, triggering the relay to operate and isolate the faulty section from the rest of the system.
Here's how an electrical relay operates in a differential protection scheme:
Current Sensing: Current transformers (CTs) are installed at the entry and exit points of the protected zone. These CTs measure the currents flowing into and out of the equipment and convert them into proportional secondary currents.
Current Comparison: The secondary currents from the CTs are fed into the relay. The relay then compares the currents to determine if there is any significant difference between them. In a healthy system, the currents entering and leaving the protected zone should be balanced, and the relay will not act.
Operational Criteria: The relay is typically set with specific operational criteria, which determines the allowable range of difference in currents. This setting depends on the type and size of the protected equipment, as well as the level of sensitivity required.
Fault Detection: If a fault occurs within the protected zone, the current balance will be disrupted, and the relay will detect an abnormal difference between the currents.
Operation and Tripping: When the current difference exceeds the set threshold or criteria, the relay operates and issues a tripping command to the circuit breaker connected to the protected zone. This opens the circuit and isolates the faulty section, preventing the fault from affecting the rest of the system.
Time Grading: In more complex protection schemes, differential relays may have time grading to ensure that the relay closest to the fault operates first. This coordination prevents unnecessary tripping of healthy parts of the system.
Backup Protection: Differential protection is usually used in conjunction with other protective relays, such as overcurrent relays or distance relays, to provide backup protection and improve the overall reliability of the system.
Differential protection is highly effective in quickly detecting and isolating internal faults within electrical equipment, ensuring minimal disruption to the power system and reducing the risk of severe damage or hazards.