CT Shorting 2

[111R]

When testing bushing CTs on a breaker (3-pole, dead-tank GCBs with 2 CTs per bushing), I've been in the habit of shorting the secondaries of all 12 CTs mounted on the breaker since the burden is isolated and the CTs would be open circuited otherwise. As each CT is tested, I lift the shorting screws from the secondary terminal and connect it to the piece of test equipment.

I've talked to a few people who claim that this is unnecessary and time-consuming during testing and I wanted to get additional input. This does not seem like a good idea to me, but I can't fully explain it for all test sets.

On turns ratio tests, some test sets inject up to 800A of current through the individual poles of the closed breaker by connecting to the bushing terminals on both sides of the pole. If any of the CTs mounted on the phase with the current applied are open circuited, they will build up a dangerous voltage on the secondary. This is no different than an open circuit during normal operation.

Secondary injection test sets explain their turns ratio test as applying a voltage through the secondary and measuring the voltage on the primary by connected leads on the bushings terminals across the pole. The secondary applied voltage can be up to the saturation voltage of the core. I'm having a harder time understanding exactly what's going on with the primary side in this type of test set:

The voltage is dropped across the inductance of the primary bus through the bushing CT, correct? If there are two CTs stacked per bushing, I would think that the flux through the core of the 1st CT could have an effect on the CT directly below or above it. If this nearby CT is open circuited, could it cause issues with high voltages on the secondary? Could the excitation of this core interfere with the readings on the CT being tested?

Thank you.

 

[oldfieldguy]

I've tests a lot of circuit breakers (GCB and OCB)over the years. For the most part, I used a secondary injection set that operated exactly as you state in your fourth paragraph. I never shorted the terminals intentionally, but in retrospect, most breakers come from the factory with all CT's shorted.

If you use primary current, as you state in your third paragraph, then you MUST short the CT's not under test and the CT under test should be connected to a low-impedance measuring instrument, other wise, as you said, high voltages will be generated.

I have not experienced coupling of the flux between adjacent stacked CT's in either bushing-mounted installations or the CT's stuck on the jugs behind indoor circuit breakers. The stray flux is well-controlled.

In the secondary injection test you describe, the CT secondary winding receives a voltage of known magnitude. The current in this winding is compared to the excitation curve of the CT. The voltage (high impedance measurement only) of a conductor through the window, either the current path of the breaker, or a conductor supplied by the test technician, is measured and the ratio of the CT is calculated and if your instrumentation allows, the polarity of the CT is verified. Since the current through the window of the CT is in the milliampere or less range to make this voltage measurement, the voltage impressed on other CT's should be negligible.

old field guy

 

[111R]

Thank you!