CT SATURATION QUESTION 2

[tszpun]

Hello experts,

Could you tell me, what will happen to CT if the secondary is open circuited for 1min and for 1 hour? Theory says it will be saturated. However, can this CT still be used ? or should i replace this CT ?

 

[wcaseyharman]

If it were me I’d have find someone to meggar the CT and perform a saturation test to verify it’s condition.

 

[bacon4life]

If it had only steady state current, the open circuit voltage could have been low enough to have avoided permanent damage. Step changes in currents produce can produce extremely high voltage spikes. Have you found any evidence of arcing/flashovers? In some cases I have had open circuit CTs that were fine since the flashovers occurred at the terminal blocks in the control cabinet rather than at the actual CT. So in addition to testing the CT, Meggar all the wiring that could have been exposed to voltage spikes.

 

[scottf]

The first question would be - what type of CT was it? Bushing-type/LV switchgear type? Medium-voltage dry-type? HV oil-filled type?

When a CT is operated open-circuit, the core is saturated. Depending on the design and function of the CT (meaning how many secondary turns, cross-sectional area of the core, and primary current level) very high voltages can also be present on the secondary winding.

Therefore, damage can occur in 2 ways:

1 - when the core saturates, a lot of heat if produced. The heat generated is a function of the core size and primary current level. This excessive heat can damage the magnet wire insulation, the lead-wire insulation, the insulation between winding layers, etc... This could lead-time a turn-to-turn or layer-to-layer short.

2- when high voltages are present across the secondary winding, insulation can be damaged internally and/or externally. Normally, there is arcing across the secondary terminals and the damage occurs outside of the core/coil, but not always.

If the unit is a low-voltage CT, then insulation resistance (Megger) and excitation tests would be recommended.

If the unit is a dry-type MV CT, same tests, plus possibly a partial discharge test if you have access to that kind of test equipment.

If the unit is a HV oil-filled unit, add in DGA test on the oil.


If the unit was a MV or HV CT and the core had protection accuracy ratings and the unit was open-circuited in the range of 1 hour, I would be very careful about re-energizing the unit without performing off-line tests like partial discharge. And if you do continue to use the unit, understand that the useful life may have been shortened.