Electrical Interview Questions such as SOTF, Stubbus, Breaker Fail

Switch On To Fault (SOTF):

• Switch On To Fault Schemes are protection functions intended to trip a transmission line breaker when closed on to a faulted line.
• If you are switching on to fault,Voltage is not going to rise as it should and fault current as decided by the fault MVA is going to flow.Many Line protections include this SOTF protection as a built inlogic.The closing of CB enables the logic for a set time say 200ms.If the Voltage does not rise to 50-60 % of the rated voltage in onecycle & if the current is more than the set current it is sensed as a SOTF and a three phase tripping impulse is given and recloser isblocked.
• With the offset-zone time delay bypassed, there can be provision on SOTF protection. This is required where there are Line-side Voltage Transformers, to provide fast tripping in the event of accidental line energization with maintenance earthing clamps left in position. (from chap 11 of NPAG, pg 175)
• To detect SOTF, configure CB Open/Close Switch to the IED as its Binary Input for the closing pulse.

Stubbus Breaker:

• Sensing of the state of the line isolator through auxiliary contacts enables the current values transmitted to and received from remote relays to be set to zero when the isolator is open. Hence, stub-bus protection for the energized part of the bus is then possible, with any fault result in tripping of the relevant CB.

Breaker Failure Stages:

• When the outgoing feeder say, Transformer or Line or any Bay trips its CB using master trip relay, the contacts of the relay also initiates Breaker fail of the bus.
• BF stage 1 – retrip its own breaker
• BF stage 2 – initiate Intertrip and all those breakers which feed to the fault, in order to isolate the system.

Reference:

1. Switch On To Fault Schemes in the context of Line Relay Loadability by North American Electric Reliability Council, Draft 2006. http://www.nerc.com/docs/pc/Itm_3b_att%20C_SOTFWhitepaperDraft5-31-06.pdf