What if CT secondary is open circuit?

CT secondary shall always be kept short circuited. What happens if CT is open circuit?

Current Transformer (CT) is an instrument transformer used to reduce or mutiply current by a pre-defined factor. 

It has small number of primary turns and large number of secondary turns. Generally primary winding has only one turn, as primary conductor simply pass through CT core.  Secondary winding has large number of turns (may be 1000, depending on ratio). In the image below, only few turns are shown in CT secondary for simplicity . Primary winding and secondary winding are wound on magnetic core.

From the above, Primary current Ip flows through primary winding. Due to this current magnetic field is developed and magnetic flux φ flows in the core. This alternating flux produces voltage Vs in secondary winding. Normally, Zs is very low (or may be short circuit),  this voltage develops current Is in secondary winding. 

According to Lenz's law:

The current induced in a circuit due to a change in a magnetic field is directed to oppose the change in flux and to exert a mechanical force which opposes the motion.

Therefore, flux produced by secondary current is opposite to flux produced by primary winding. Theroritically, both flux should be same but in oppsite direction. However, in actual, there will be a very small difference due to leakage flux. The resultant flux, difference of flux produced by primary current and flux produced by secondary current, is very small for normal load current. It magnifies in case of higher fault current. CT core is designed to carry resultant flux upto a certain fault level. 

For low value of Zs, CT can be considered as short circuited transformer. Reactance of primary winding will be very small, hence very low voltage drop across primary terminals.

If CT is open circuit, there will be no secondary current and no flux produced by secondary current. Magnetic core will get saturated by high amount of flux produced by primary current. 

In saturated part of waveform there will be no change in flux and due this there will be no e.m.f. induced in secondary. This will result in no current in secondary.

As CT is already staurated in this case, voltgae developed will not be sinusoidal. It will contain harmonics and wave shape will be spiky. The peak value of voltage will be very high in the range of 2-3kV. This higher voltage is dangerous for persons working in panels and equipment. This high voltage can lead to failure of insulation in CT, Cables, relays, terminal blocks etc.

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