Switch on to fault (SOTF)

 Switch on to fault (SOTF) feature is used to instantaneously isolate the fault in case a line is charged with persisting fault / remote end earth switch closed. Main requirement is to avoid time delayed tripping in this condition.

Normal case when line is charged from both ends: Consider line is protected by distance protection, during fault following happens:

1. Fault in Zone-1: Instantaneous tripping (phase selective is implemented).
2. Fault in Zone-2: Instantaneous tripping with carrier receive from remote end (phase selective is implemented).
3. 3-phase Fault near to bus: Relay will decide direction based on memory voltage.

In case line is charged with persisting fault (remote end still open): Consider line is protected by distance protection, during fault following happens:

1. Fault in Zone-1: Instantaneous tripping (phase selective is implemented). Auto reclose will close the breaker, if implemented.
2. Fault in Zone-2: Delayed tripping with Zone-2 time as no protection will operate from remote end and no carrier will be sent from remote end. Auto reclose will not operate for delayed tripping.
3. 3-phase Fault near to bus: Relay cannot decide direction based on memory voltage as there is no memory voltage.

From above, we need a protection which can instantly clear faults in Zone-2 and 3-phase fault near to bus. Line closure is detected either from binary input wired from CB close switch or from dead line detect based on voltage and currents (Typical setting Voltage <40% and Current <20%). SOTF feature remain activated for ~0.2 seconds after this detection.

Different methods for detecting SOTF conditions are as below, distance relay may use combination of these:

1. Non-directional Zone-2 impedance based SOTF protection: In this case fault impedance is measured regardless of its direction. If it is less than Zone-2 impedance instantaneous 3-pole trip will be issued and auto reclose is blocked.
2. Over current based SOTF protection: In this case current through all phases is monitored. If current in any phase is higher than set value, which is kept ~ 200% of maximum full load current of line, 3-pole trip is issued and auto reclose is blocked

 

CT saturation

CT saturation is one of the phenomena which is important for differential protections. During saturation, core of CT gets saturated due to higher level of flux density than designed value. After this no more change of flux occurs even the primary current is still changing. 

Due to no change in flux secondary current vanishes. We may get secondary current having distorted waveform during CT saturation.

For differential protections, this may lead to differential current for out of Zone faults and relay may give unwanted trip command. Most of the manufacturers compensate CT saturation with different techniques.

 Let us see two cases with and without CT stauration

For case-1 External fault without CT saturation:

Differential current = 1 + 1 + 2 – 4 = 0 kA
Bias current = 1 + 1 + 2 + 4 = 8 kA
Idiff / IBias = 0/8 = 0.0

For case-2 External fault with CT saturation, CT for fourth feeder is staurated and measuring only 1kA current instead of actual 4kA current: 

Differential current = 1 + 1 + 2 – 1 = 3 kA
Bias current = 1 + 1 + 2 + 1 = 5 kA
Idiff / IBias = 3/5 = 0.6

For case-2 we can avoid unwanted relay operation if we keep differential slope setting higher than 0.6

* For simplicity current is considered without any DC component.