Failed Bushing Current Transformer

[111R]

I've seen a few test results for CTs that were operated with an open secondary circuit. The CTs are relaying-type, bushing CTs with an accuracy rating of C400. The excitation curve is distorted and the CT saturates below 100V when using secondary excitation test equipment. An identical, properly-operating CT has an IEEE 45 knee point around 280-300 VAC.

The strange thing is that the ratio and winding resistance is accurate and comparable with good CTs on the same breaker. There is less than 0.5% ratio error and the winding resistance is in the 500 mohm range.

With the bad excitation curve following operation of an open circuited CT, I would assume that the CT has shorted secondary windings due to insulation flashover. Why would this not skew the ratio and winding resistance readings?

Do CTs normally short turn-to-turn or turn-to-core?

Thanks!

 

[collies99]

You CT is likely partially saturated or magnetized. You need to go through a demagnetization process, that's why your ratio and resisatnce is still OK.

A high power CT tester can probably due the trick as is you case 100V at 10A is 1KVA,if you can source a varible power supply with that power capability then it can be desaturated/demagnetized easily.

Alternatively if no variable power supply is available and since the CT is hooped anyways, you can flash back and forth with a 9V battery, thne perform demagnetization process with a regular EZCT-10 tester that can at least feed 10A on the low voltage scale. Pump 10 Amps then back-off slowly. And if the voltage rise back towards 400V on the second attemp you are good to go on the CT. Flashing the CT with a battery is the hardest part to get to the right point on the hysteresis curve since it is all trial and error and mostly good luck when it does work.

I had a brand new Eaton gear with C200 and it saturated at 40V and its ratio and resistance was OK. The above was tried at it did finally worked, took about 1 hour of trial and error and I was ready to send it back.

 

[prc]

I think the failure mode will be turn to earth/core and not turn to turn.

 

[scottf]

A couple of points:

- Magnetization caused by being open-circuited is likely the reason for the lower Vkp. As suggested above, you need to demagnetize the CT and then retest. Most modern CT test sets can do this.

- If the CT was operated with an open secondary for a significant amount of time and with an appreciable primary current, excessive heat was likely generated by the core. The excessive heat could have caused a short to from winding to core or from turn-to-turn of winding. Depending on the type of ratio test you did, it very well may not pick up a single shorted turn or a short to the core.

Bottom line...if you can demag the core and the Vkp tests as expected, then likely no problem. If you can't get the Vkp to test correctly after demaging the core, then you likely have damage to the CT.

It should also be noted that if the CT was operated open-circuit for a continued period of time with significant primary current, even if it tests good, the life of the insulation was likely shortened. You could consider performing an insulation resistance test and inducted voltage test (800V applied to the secondary winding, since the CT is rated C400).

 

[prc]

scottf, we are talking about bushing CT where there is no integral primary winding for CT. Hence should we be concerned about the turn to turn insulation damage?

 

[oldfieldguy]

Is your CT still in place? If so, look for a shorted PRIMARY path, usually due to shifting of support components. A shorted primary path, even one with a bit of impedance to it, will seriously skew the saturation curve.

Ratio tests will typically be well within expected values and insulation resistance tests on the secondary will be normal.

old field guy

 

[collies99]

Are these CTs around the circuit breaker bushing ?

If the primaty did not fault to ground before you took it off line it should be ok.

Some bushing CTs are easily accesible for inspection, especially older style CB where top CT cover plate can be removed and CT inspected. Housing on newer type are not so easily removable for an inpection.

Hard to imagine a primary to secondary fault path since the CT is around the bushing assembly. But i guess you csn meggar between primary to secondary. Bus bar CT is a different story as per free dtsnding type-oil filled.

 

[111R]

Yes, these are window-type, bushing CTs on a 27kV outdoor distribution vacuum breaker. Once I can access the breaker, I will try the demag process. I have a power supply to do this.

With that said, is there any point in checking insulation resistance or power factor between secondary winding and ground? Since the core is left floating and I can't access the actual core to check for core-turn shorts since it's encapsulated, are these tests going to tell me anything?

I'm confused as to how we could have a primary fault to ground. The bushing lead is encapsulated in a bushing rated for at least the line-ground voltage (14.4kV) and the CT is installed over this. I don't see how a 2000:5 CT open circuited on the secondary would cause any sort of drastic voltage rise on the primary. I might be misunderstanding.

Thanks for all of the help.

 

[collies99]

There are no primary fault to ground on that CT set.

Once you have demag it and get back to you normal sat voltage level, plese a perform a secondary wiring insulation test, megger at 100Vdvc.

If the open circuited secondary did occured with primary current flowing then the peak voltages generated may well break down the insulation system in the secondary circuit.

Example. The broken circuit via a test switchor other have a .5" gap, and to satisfy the secondary circuit loading requirement the CT would try to prodcue the current proportional to its voltage output if it could or take more primary current for its excitation until it saturates. So we get a peak voltages at 3500V for arguement sake. Can that break down 600V insulation class between X1-X2 wiring ? You bet it can. So do a wiring to wiring insulation test and the each conductor to ground for that particular CT set.

Good luck and let us know of your progress.

 

[collies99]

Make that megger at 1000 vdc.

 

[scottf]

I wasn't speaking to primary winding insulation failure...I'm talking about secondary winding turn-to-turn insulation failure. The enamel coating on the magnet wire used for secondary winding can be damaged by excessive heat, such as the heat generated by a deeply saturated core (i.e. during secondary open-circuit). Also, the voltage developed across the secondary winding during an open circuit could cause turn-to-turn insulation failure, in conjunction with the excessive heat.

 

[collies99]

Hi Scottf

How would you test for turn-to-turn insulation failure in this CT ? and have you found the test to be effective for epoxy/resin encapsulated CT?
CT.

 

[scottf]

The test for a turn-to-turn failure is the excitation test.

To recap:
- You performed an excitation test on a CT that had been operated open-circuited. You got a lower than expect Vkp.
- Possible reasons for that could be magnetization of the core, secondary turn-to-turn short, or inadvertent primary loop.
- You need to rule out inadvertent primary loop and demagnetize the core.
- If you do those 2 things and still get a low Vkp, then you likely have a problem with the CT.
- If the Vkp returns to the expected value after demagnetizing the core or identifying the primary loop, then it is recommended that you check the wiring insulation by performing an insulation resistance check from the secondary wiring to ground at 1 kV. If the bushing CT is a case epoxy type with no ground shield or external ground connection, then you can wrap a conductive material around the unit to test against.

 

[oldfieldguy]

I was once commissioning a new GCB and found a pole with TWO CT's exhibiting unusually low saturation. A visual examination found that a tiny bit of flashing from the manufacturing process was contacting the aluminum shell of the CT weather cover, completing a conductive loop through those CT's. A couple of judicious scrapes with a screwdriver blade to remove the offensive metal and the CT saturation problem went away.

old field guy