A Circuit breaker is an essential component in electrical power system. It is designed to interrupt fault currents and hence protect equipment from damage. A crucial aspect of their performance is the duty cycle. The duty cycle defines the operational requirements and limitations of the breaker in terms of repeated opening and closing operations.
Whenever there is a fault in the network, the circuit breaker must trip to clear the fault. However, it has been found that 95% of the faults occurring in the system are transient faults. Transient faults are cleared instantly after they occur.
After the circuit breaker trips, if it is closed again, the system returns to its normal working condition. However, if this operation is performed manually, there will be a significant time lag, leading to an unnecessary power outage. To eliminate this unnecessary delay, an automatic reclosing scheme has been introduced.
In this scheme, a high-voltage circuit breaker is automatically closed after a certain time interval following its tripping due to a fault. If the fault is transient and has already been cleared, the breaker remains closed, restoring the power flow. If the fault is not cleared within that time gap, the breaker trips again immediately after closing.
After this second tripping, the stored potential energy of the circuit breaker is entirely exhausted. Therefore, it needs to be restored for the next closing attempt. One key principle to keep in mind is that whenever a breaker is ready for closing, it must also be capable of tripping instantly after being closed.
Suppose the circuit breaker is a spring-spring circuit breaker, meaning both closing and tripping operations are performed using the stored potential energy of charged (deformed) springs. In a high-voltage circuit breaker, the closing spring remains fully charged in normal conditions, regardless of whether the breaker is open or closed. However, there is another spring associated with the tripping mechanism, called the tripping spring. This tripping spring remains in a relaxed or uncharged state when the CB is in open position. The tripping spring is always smaller in size compared to the closing spring.
Now, let’s move on to the actual duty cycle of a circuit breaker.
O-0.3s-CO-3min-CO
High-Voltage Circuit Breakers typically have a duty cycle like O-0.3s-CO-3min-CO as per standards. This duty tells us,
- How fast it can close after an open operation.
- The time it needs between operations.
- The recovery period before it can operate again.
| Symbol | Meaning |
|---|---|
| O | Open (The breaker trips, interrupting the circuit) |
| 0.3s | 0.3 seconds pause (Time before the breaker can close again) |
| CO | Close-Open (Breaker closes the circuit and then opens it immediately if fault persists) |
| 3min | 3 minutes pause (Cooling and recovery time before the next operation including its charging time) |
| CO | Close-Open (Breaker becomes ready for next close-open operation) |
Step-by-Step Explanation of Duty Cycle of a Circuit Breaker
O (Open) – The circuit breaker trips (opens) due to a fault. The fault may be a transient fault of a permanent type fault. A transient fault may occur due to ionized air paths between conductors or to ground, flashing over of insulators due to pollution and moisture, temporary contact of trees or branches with high-voltage lines due to wind, direct or indirect lightning strikes on transmission lines, transient overvoltage due to sudden switching operations of large loads, birds or animals bridging conductors, oscillation of conductors due to strong winds or ice formation, many other reasons. The circuit breaker opens means the power flow is stopped.
0.3 Seconds Pause –The breaker remains open for 0.3 seconds before attempting to reclose. These 0.3 seconds represent the dead time. During dead time, the arc caused by the fault gets time to disappear, allowing the interrupting medium to deionize and hence regain its dielectric strength. If the fault is temporary, it will clear during dead time.
CO (Close-Open) – After 0.3 seconds, the breaker is closed by the action of auto reclosure. If, during dead time, the fault is cleared, the circuit breaker remains closed. However, if the fault still exists in the system, the breaker opens again immediately. There will be no time gap between the closing and opening operations. Here, it is important to note the spring condition at different stages. Before the initiation of the duty cycle, the circuit breaker remains in a closed condition. Both the tripping and closing springs are in their charged condition. During the first open operation, the tripping spring gets discharged. During dead time, this spring remains uncharged. During the first closing operation, just after dead time, the closing spring gets discharged, and during its discharge, it charges the tripping spring. In this way, during the closing operation, the tripping mechanism gets ready for tripping. During the tripping operation that follows the closing operation, the tripping spring gets discharged. Now, after this CO (Close-Open) operation, both springs have been discharged.
3 Minutes Pause –The breaker now waits for 3 minutes before attempting to close again. Circuit breakers operate using stored potential energy (e.g., spring-charged or hydraulic mechanisms). After two operations, meaning an open operation followed by a close and an immediate open operation (O-0.3s-CO), the stored energy is fully exhausted. As already explained, in a spring-spring mechanism, both springs are discharged. The 3-minute wait time allows the spring (or hydraulic) system to recharge, ensuring the breaker is ready for the next closing attempt. Every time a breaker interrupts current, it generates heat due to arcing. Breaker contacts and arc chutes need time to cool down to avoid overheating. If the breaker performs two consecutive faulty closings within a short period, excessive heat could damage the contacts or even weld them together.
After 3 minutes, the circuit breaker becomes ready for the next close-open operation. This time delay is provided by the auto reclosing mechanism. If anyone tries to close the CB manually during the 3-minute delay, the system will prevent the closing operation.