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What is the purpose of SOTF being used inspite of distance
Protection given?
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Answer Posted / ofa khoirun niam
Dear Engineers,
SOTF also known as Switch On To Fault is one of the protection type feature. How it works? Let's see this. During closing or energization on the line in which fault is happening within 20 ms from CB closing, then SOTF feature will clear the fault by opening the CB as instantaneously manner, even if the fault is happen in the zone 1 or zone 2. When the faults happens after 10 or 20ms, CB will opened instantaneously by Line Distance Zone 1 feature when the fault location in Zone 1 or CB tripped with time delay 300 ms to 500 ms when the fault happens in Zone 2.
This type of protection schemes are mostly working while re energizing the line after maintenance if earth switch not opened at other end of line (or maybe another fault happens) . Normally this will be in Zone 2 of distance as Zone 1 will cover only for 80% of line so in this case SOTF will act faster than distance.
Why SOTF should be provided and why distance (21L) feature could not be used. Because there is no impedance could be detected by Distance (due to CB in opening position, no current is flowing). And also there is no line voltage when the fault is caused by earthing switch (ES) in closed position in the opposite Substation, maybe missing by operator forgot to open the ES.
Wthout SOTF feature, line protection could not operate to trip the CB, even breaker failure also will no work or not be initiated. It very possible to resulting in time delay around 20ms until Line distance working properly. massive unit instability maybe occur.
So, SOTF is one of the feature relay should be tested during commissioning the HV Substation Protection Relay. I hope my explanation could be very easy to be understood. CMIIW
Best Regards,
Ofa Khoirun Ni'am
Electrical Engineer
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