Current transformers- over voltage on open circuiting secondary

[prc]

It is well known that open-circuiting the secondary circuit of CT while carrying primary current will create overvoltage on secondary terminals. But will this lead to violent failure of CT with explosion? Has anyone noticed CTs in service with open-circuited secondary terminals?

 

[sushilksk]

violent failure of CT with explosion: It depends, if it is dead tank type CT with oil filled and porcelain housing, pressure developed can explode CT.
In some cases over voltage can create insulation failure of cables, or terminal blocks, causing short circuit of CT secondary. In this case CT is saved.

 

[cranky108]

Or the insulation can take place in the CT, and the CT is toast.

 

[thermionic1]

We had an open CT on our system a few years back. It was a ring bus, dead tank breakers, with the CT's paralleled in the control house, then feeding a digital distance relay. It took about a month for the problem to show. The overvoltage caused the relay magnetics board transformer to flash over and trip on one end only, which is how the problem was discovered. The line carries about 150MW @ 345kV, normally.

Megger of each separated CT circuit was inconclusive from the relay panel. C800, 3000:5 CT's were isolated at the breakers and tested. During excitation tests, the bad CT was identified. Removing the CT shrouds revealed some arcing damage, but nothing spectacular.

 

[System Protection]

Only seen one open circuited for a brief moment.

C800s are pretty good at letting you know they are open. (Terminal blocks inside control house melted together)

No known permanent damage to the CTs. Tested okay afterwards.

I've heard of smaller CTs being left open for a long period of time with no violent failure but I've never seen it firsthand.

 

[waross]

I've heard of smaller CTs being left open for a long period of time with no violent failure but I've never seen it firsthand.

I was trying to identify a couple of CTs.
My thought was that if they were driving different burdens, I may be able to see different terminal voltages.
I put my digital multi-meter on the terminals of one CT and the meter made a funny buzzing sound.
(Auto ranging to 1000 Volts.)
The multi-meter never worked again.
To this day I am thankful that all that I lost was a multi-meter.
It turned out that a wire had been cut in a wiring bundle and part of the wire removed.
This was the same plant where the synchro-scope wiring had a 30 degree connection error, so that when the 'scope showed "In phase" there was actually a 30 degree difference.
It had been that way for years.
Breaker trips when trying to synchronize were common.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

CT's fail all the time of differing causes. But it is hard to predict, and hard to convince some people that they have failed.
But as for open CT's that I know of, a melted terminal block in a transformer cabinet with lots of smoke is what I have seen.

 

[scottf]

This question very much depends on the type of CT.

LV, MV, or HV
Bushing CT
Dry-type MV
Oil-filled HV

Bottom line is that a CT left with an open secondary will fail in some manner given enough time.

Often times, the initial failure will be at the secondary terminals, depending on the open-circuit voltage of the CT and level of primary current.

The open-circuit voltage of larger ratio protection class CTs can be in the range of 20-50 kVp and can be quite dangerous.

Also, as previously mentioned above, when a CT has an open secondary, the core is sent into deep saturation and a lot of heat is developed at the core. Given enough time, this can lead to catastrophic failure of MV and HV CTs.

As a general statement, I would not recommend using a CT that has been operated with the secondary open-circuit.

 

[prc]

Thank you, Scott. I was waiting for your response. What I meant was an oil-filled dead tank ( hairpin ) CT. When such CTs fail, there is normally an explosion with porcelain shattering. Many power engineers interpret that such failure can happen from open circuiting of the CT secondary. Post | Feed | LinkedIn.

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I am not convinced. The secondary voltage will indeed go up. If you have taken a CT core flux density of 0.1 T for the protection core, the voltage may go up a maximum of 20 times the burden voltage(assuming 2 T as saturation flux density). This will not be sufficient to puncture the thick primary insulation and cause a power flow from the primary. Any arcing from the secondary high voltage will not have arc energy and may not cause an explosion. However, a primary insulation failure will cause the grid to collapse inside CT, and the high energy will result in pressure wave shattering porcelain.

Core flux density will not exceed 2.0 T, and balance flux will overflow from the core. If you calculate the core loss, you will find the losses are in a range that can be dissipated from the core.

Some field engineers have noted that they only discovered secondary openings in CTs during the planned maintenance period. Till then, nothing happened.
In Jenkins's textbook on CTs (1967 UK), it is mentioned that CTs shall be designed to withstand secondary open circuit voltage continuously. (page 73)

 

[scottf]

I have certainly seen oil-filled CTs fail catastrophically due to prolonged secondary open-circuit.

I think your statement about the secondary voltage going up about 20 times the burden voltage is not correct...or not totally correct. For an rms standpoint, yes, but the peak voltages developed on the heavily distorted secondary wave form can be very high. I just pulled one of our design sheets for a core design for a HV oil-filled CT with a 3000:5A MR / C800 design. The open-circuit voltage of this design is 47kVp. And the heat generated by the core in deep saturation is quite high and would cause a total temp rise of over 125C once stabilized. That is more than enough to damage primary insulation leading to line-to-ground fault. Now...all that assumes rated current through the primary.

 

[sushilksk]

Practically, I have seen one CT failure due to CT open circuit in metering core at 132kV level. In all other cases, there was another current path developed by melting of cables / terminal blocks etc.