Answer / karna

Distance Protection is for Zone Faults and it will trip on
time delay. But SOTF means Switch On to Fault i.e when
fault is there without knowing that fault, we can close the
breaker means it will trip on SOTF, then we know that
already fault in the line. SOTF helpful for us like when we
taking LC for some maintenance in the line, at that time we
will provide Earth switch for safety. Without removing the
switch we can close the breaker means it will trip on SOTF.

Answer / p anbarasu

During energisation of the line vt input is near to zero. at that time any fault happens in the line distance protection is not able to operate because vt is near to zero. So we are enabling the SOTF protection in the relay.

SOTF function: The relay will check if any fault happens during energisation with in the 20ms, if fault is presents the relay will operate as a simple O/C relay and its isolate the healthy section from faulty dection.

Current detection(A,B&C) and No Voltage detection(A,B&C)--> 20ms Delay ---> Trip

Answer / deepak.vin208

SOTF is known as Switch On T Fault as the name suggest it will act when there is fault on the line when closing. Here you asked the question of why we use even Distance protection is there. In distance if the fault is in Zone 2 as per distance protection it will operate only after predetrmined time normally 0.3 or 0.5 Seconds for Zone 2 but this type of faults has to be removed instantaneously but in this condition as Distance Zone Fails SOTF which clear the fault instantaneously. This fault mostly occur while restoring the line after maintenance if earth switch not open at other end of line 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.

Answer / 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

Answer / ganesh

line fault or earth switch close that time we close the breaker SOTF function should trip but how ? fault time no voltage and impedance also zero that we know and No voltage condition VT fuse fail will be detected at the time main protection should block.so sotf function optd.

Answer / ofa khoirun niam

When an impedance protection scheme uses line-side voltage transformers, SOTF current fault detectors may operate significantly faster than impedance units when a breaker is closed into a fault anywhere on the line. The dynamic characteristics of typical impedance units are such that their speed of operation is impaired if polarizing voltages are not available prior to the fault.

Answer / ram

SOTF:When the line is being charged the earth isolaters are being opened so that the charging current wont get leaked r loss and the breakers will also be in open position when we close we may not be knowing whether the fault will occur from other Bus side or any other side so once we close the breaker the SOTF function will get enabled and it protects for 500ms once the fault occurs it trips the master trip relay and the master relay trips the breaker

Normally while charging the approximate current flow will be 0.2A

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