We are going to discuss here in this article a clear idea about SOTF Protection.
Sometimes it becomes necessary to open a circuit breaker just after closing. This is due to the presence of a fault during the closing of the circuit breaker. This type of condition may arise due to different reasons. Such as if the maintenance people forget to remove the earthing rod from the transmission line after maintenance work. This may also happen when there is a permanent fault in the transmission line. However, without knowing that, an operator closes the circuit breaker after the tripping. This means the operator closes the circuit breaker without an auto-recloser or by bypassing the auto-recloser.
What is SOTF Protection or Switch On To Fault Protection?
If a fault is present during the closing of the circuit breaker, it can cause significant damage to the system. This is because, after closing the circuit breaker, if the fault persists, the protection scheme activates. Therefore, it should activate the circuit breaker to open and clear the fault. But this entire protection process may take some time. This delay is according to the time setting given to the main protection relay. Normally, a distance protection relay can actuate the opening operation or trip a circuit breaker after around 300 milliseconds. Therefore, we are allowing the fault current to pass through the system for 300 milliseconds even after closing the circuit breaker during a faulty condition.
To avoid this kind of situation, SOTF protection comes into the picture. SOTF means Switch On To Fault, which refers to switching on the circuit breaker when a fault already exists in the circuit.
Distinguishing between Persistence and Occurrence of Faults
Now, think about the situation when the circuit breaker is already closed, but a fault occurs in the transmission line. In that case, the distance protection relay and other associated relaying systems will operate to clear the fault. But the SOTF protection, if present, should not operate during this condition. SOTF must operate just after the remote or manual closing of the circuit breaker. If the same fault occurs during the normal condition of the line, SOTF must not actuate to trip the circuit breaker. This tripping must happen through the distance protection scheme or other schemes associated with the line.
So, the question may arise: how will this Switch On To Fault Protection distinguish whether the fault was already present in the circuit during the closing of the circuit breaker or if it occurred during normal operation? The main working principle of SOTF lies in this: distinguishing between the time of the fault.
Monitoring Conditions
- Breaker status (status change):
Not exactly the breaker’s current status, but the change of status. The SOTF relay should sense the change of breaker status from open to closed. If only the closed status is given to the relay, it cannot detect whether the breaker has just closed or not. So, instead of a closed status, we need to provide an open-to-close status change signal to the relay. This signal comes from the auxiliary contact of the circuit breaker, which changes its NO/NC contact during breaker operation. - Closing switch signal:
Another way is through the closing switch. The manual or remote closing switch of the breaker in the control room can send a signal to the SOTF relay, indicating that the breaker has been closed manually or remotely. This means whenever we close the breaker from the control room switch, that switch will send a signal to the SOTF relay to start its activation.
Thus, the SOTF relay starts its activation (or gets activated) either by the changing condition of the auxiliary contact, by the signal from the closing switch, or by both.
Operation of SOTF Protection
Once the SOTF relay is activated during the closing of the circuit breaker, we should note that it is activated regardless of whether a fault exists or not. Whenever the breaker physically closes, the SOTF relay gets activated. But this activation continues only for a limited time. Otherwise, malfunctions may occur, as SOTF protection might operate during real faults occurring in the line.
Therefore, it is general practice to keep the SOTF scheme activated for up to 500 milliseconds from the instant of breaker closing. After 500 milliseconds, the scheme automatically deactivates.
During this active time window, if a fault persists:
- The current transformer (CT) sends the fault current signal.
- Due to the high current, the voltage across the breaker reduces, and the SOTF relay also receives this signal from the line VT or bus VT.
If the current crosses a preset value, the SOTF initiates a trip signal to the circuit breaker. Normally, testing engineers set the SOTF actuation current above the normal load current but below the fault level. They also set the voltage threshold below 75% of the normal voltage to sense faulty conditions.
If condition (1) or (2) is true, the SOTF becomes active. Additionally, if condition (3), or (4), or both are true, then the SOTF issues a trip signal to the breaker to interrupt the fault current and isolate the faulty part of the system. On the other hand, if condition (1) or (2) is true but both conditions (3) and (4) are not true, then Switch On To Fault protection remains active without initiating any trip pulse for up to 500 milliseconds and then automatically deactivates.