CT Saturation

How do you know these were saturated?

Possible cause of saturation are usually

- Secondary Open circuit.

- Improper burden (including its X/R ratio)

- Primary current much higher than the rated maximum current.

In short anything that can create an overfluxing of the core.

The "knee point" which was achived in the just starting ( as per manufacturer it is nearly at 100v while,it was appaired at 20vol appx.??

At starting No inrush ?

Any sort of symmetry in the phases ?

Did you check-up the burden X/R?

As a post mentioned, hopefully there is no DC offset?

I am not able to visualise any other reason at the moment.

Following are the reasons for CT saturation:

1) Presence of DC offset in the fault current. This will be more pronounced if the X/R ratio of the system uptil the fault location is considerably high. CT saturation can also occur if the fault current is by and large symmetrical in nature (with negligible DC offset).

2) Connected burden of the CT can also cause CT saturation, if the secondary current flowing in the CT is of such a magnitude that the voltage across the CT secondary has crossed its knee point voltage value.

3) Improper turns ratio.

4) Open circuit of CT secondary. This will almost lead to damage of the CT.

CT with reasonable Fs (Factor of safety) /ALF (Accuracy limit factor) has to be used for measurement/protection to avoid saturation.

The causes may be due to-

1. improper burden

2. Wrong core material

3. If it get Hi-pot or IR tested by using DC Voltage.

Another probable reason for the failure could be: the brand new CTs are measuring CT but saturation test had been done for protection CT. The saturation level of protection CT is higher than the measuring CT.

See the previous thread for additional help:

http://cr4.globalspec.com/thread/43440

- MS

The Knee voltage of measuring CT is lesser than Protection CT. For both cases, sturation come just after the knee point. If those CT's are belong to metering category then their knee point voltage will be less and saturation will start much earlier than the protection CT. So there is no scope that those CT's may get tested as protection CT.

Knee point voltage is such a value that after this point, 10% increment in applied voltage will cause 50% increment in exciting current.

SIDHU16:

your point # 1 & 3 are doubble checked & found ok, While could be the reason of early saturation point due to the wrong core material.

MSAMAD:

as you mentioned, it could be the one of reason also.

Thanks both of you

CR44

Dear Forum member,

Please check this website..

1)www.carrel.co.nz. click to CT notes.

2) www.arwtransformers.co.uk

3) http://www.itl-uk.com

Maybe it was due to poor manufacture, this would not be the first time that this has happened. They may have branded the unit with the wrong label.

Otherwise, check for possible loose connections.

Cheers

joe sparky:

Yes,It could be a reason also.

This is similar to as said "ASAMAD"

Thanks

CR44

If they are from a reputable manufacturer, and are marked as the type you ordered, the most likely reasion is that the excitation tests are not being implemented correctly.

Most Current Transformers relay on the current in the secondary cancelling the flux due to current in the primary. There are two reasons this might not happen within the nominal current rating of the CT: the secondary is incorrectly loaded or left open circuit, or the current pulse is so long that the secondary current decays.

The actual "physical" limit on a transformer is the Voltage-Time product. For a CT this is usually defined in terms of a Current-Time product when a specified load is connected across the secondary.

There will also be at least one maximum DC current rating. They can be:
Damage current (self explanatory)
Bias current - the DC current at which the CT will continue to operate correctly
Saturation current: the maximum DC current at which the CT is guaranteed not to saturate.
Warning: when I last looked, different jurisdictions used different terms, and some even used the same terms for different measures.

P.S. These transformers typically have many more turns on the secondary than on the primary. Very small current drive into the secondary can upset the operation, so you need to be very careful if the secondary load is in any way active. (Also chack that the test department has the orientation of the terminals correct).