SOTF Protection stands for Switch On to Fault. This protection is provided for high speed clearance of detected fault immediately after Manual Closure of Circuit Breaker. Let us suppose that a fault is existing and we gave a closing command to Breaker, then in that case SOTF protection will immediately trip the Breaker.
SOTF Protection is helpful in the sense that while taking maintenance on a Line, it may happen so that Earth Switch of a particular Circuit Breaker is close. In that case if we give closing command to the Circuit Breaker, it will immediately trip on SOTF Protection. But it is normal to strike in your smart brain that how can we close a Breaker when a fault is already existing? Thing is that you can close the Breaker by bypassing all the required logics, so in that case SOTF will come into picture. Suppose while maintenance Earthing Rod was used for discharging a particular section but after competition of maintenance the Earth Rod is still connected to that section. In such case there is nothing or no logic to prevent closing of Breaker and as soon as one closes the Circuit Breaker, it will trip on SOTF.
SOTF Protection gets enabled whenever all the three poles of a Circuit Breaker is open for more than a settable time say 110s (This time can be changed and configured as per the design.)
SOTF is enabled in two cases. They are mentioned below.
Case1: If no Closing command to the Breaker is Present.
When all the three poles of Circuit Breaker is open for more than a particular time say 110 s, as soon as we give a closing command to the Circuit Breaker, the SOTF is enabled for 500 ms and then gets reset.
Case2: When an external closing command is present.
Here external closing means closing the Circuit Breaker either manually or remotely. The SOTF logic is activated immediately. As soon as all the poles are closed SOTF is enable for 500 ms and then is reset.
During this SOTF time frame of 500 ms, individual distance protection can be enabled or disabled. When a particular distance zone say Zone-2 is enabled, Zone-2 will actuate immediately without waiting for Zone-2 time delay if there exists any fault in this zone.
How SOTF Relay Senses a Fault?
Basically there are two elements in relay providing SOTF feature. They are Voltage Level Detector and Current Level Detector. The purpose of Voltage Level Detector is to ensure Dead Pole situation and Current Level Detector ensures that a fault occurred i.e. Current Level Detector is used for detection of fault. Therefore SOTF Relay element must have two settings, one for Voltage Level Detector and another for Current Level Detector. Typical setting for both the elements are as follows:
Vph < 75% VN
Iph > 5% IN