Auto Reclosers vs. Circuit Breakers: Key Differences for Power System Protection

While circuit breakers provide essential protection, they are designed to trip and remain open, requiring manual intervention to restore power. This can lead to prolonged outages, especially in extensive distribution networks, where many faults are transient or temporary in nature. This is where auto reclosers come into play, offering a smarter and more efficient approach to power system protection, particularly in overhead distribution systems.

An auto recloser, also known as an automatic circuit recloser or recloser, is a specialized type of circuit breaker designed to automatically re-energize a power line after a temporary fault. It is essentially a sophisticated circuit breaker with the added intelligence to automatically attempt to restore power a pre-set number of times before locking out permanently. The core function of an auto recloser is to enhance grid resilience and minimize customer interruptions by automatically clearing temporary faults, which constitute a significant portion of faults in overhead distribution networks.

The working principle of an auto recloser builds upon the fundamental principles of circuit breakers but incorporates an automatic reclosing sequence. When an auto recloser detects a fault (overcurrent or short circuit), it initially operates like a standard circuit breaker, tripping and interrupting the fault current. However, instead of remaining open, the auto recloser initiates a pre-programmed sequence of operations:

Fault Detection and Tripping: Similar to a circuit breaker, the recloser detects fault conditions using current transformers and voltage sensors, triggering a trip mechanism.

Automatic Reclosing Sequence: After tripping, the recloser waits for a pre-determined dead time. This dead time is crucial as it allows time for temporary faults, such as lightning strikes or insulator flashovers, to self-clear. For example, ionized air from a lightning strike will dissipate, or a tree branch might fall clear of the lines during this dead time.

Reclosing Attempts: After the dead time, the auto recloser automatically recloses the circuit, attempting to restore power. If the fault was temporary and has cleared, the recloser will successfully re-energize the line, and normal operation resumes, often without customers even noticing a brief interruption.

Multiple Reclosures: Auto reclosers are typically programmed for multiple reclosing attempts, usually ranging from one to four. This allows the recloser to handle multiple temporary faults that might occur in quick succession or to attempt to clear more persistent temporary faults.

Reclaim Time: If the fault persists after a pre-set number of reclosing attempts, the auto recloser will assume it’s a permanent fault and will lockout, remaining open. A reclaim time is then initiated, which is a longer time delay before the recloser can be manually or remotely reset. This lockout prevents repeated reclosing attempts on a persistent fault, which could cause damage or be unsafe.