CT saturation curve from secondary injection
CT saturation curve from secondary injection
(OP)
I used a Megger CTER-91, followed procedure and produced a nice saturation curve of 1200:5 CT's. With known sceondary voltage injected and known primary amps measured, how do I proceed to interpret the results? What computations do I do and what nameplate values from tested CT's are pertinent? I would greatly appreciate some guidance.






RE: CT saturation curve from secondary injection
RE: CT saturation curve from secondary injection
RE: CT saturation curve from secondary injection
You can plot this data and compare with published curves for your CT, or you can compare like items. As a rule of thumb, the saturation voltage will be a bit higher than the CT's accuracy class for C-class CT's.
Now, as to what happens when you fault? You can reconnect your secondary wiring and inject an amp at the CT terminals (or the closest terminal block to the CT for normal purposes)and again measure the voltage that your test set takes to push this amount of current. Here, the CT's winding is a high impedance path, so the vast majority of the current will flow through your protection scheme. With this voltage and current you can determine your current loop impedance and calculate your expected terminal voltage at the maximum calculated fault current. If you want to save yourself the calculation, take voltage it takes to push one amp and multiply that times 100. In either case, if the result is less than the saturation voltage you just determined, then your installation should not get into trouble with saturated CTs.
If you have too much circuit impedance, you can easily see what fault level will put the CT into saturation.
Now, since you're dealing with OCB's be sure that you check the saturation on all the CT's. they should be very close for equal ratings. I have seen cases where shifting hardware has shorted the primary path, causing those CTs to drastically drop in saturation due to the shorted primary path. In one case I investigated, this caused a misoperation of a protection scheme on and out-of-zone fault.
old field guy
RE: CT saturation curve from secondary injection
RE: CT saturation curve from secondary injection
But if you've shown the CT still has its C rating (assuming here), know the burden and the magnitude + X/R of the primary fault current then
(1 + X/R) * Ipu * (|RB + j * ω * LB| * 100) / VR < 20
tells you that you won't have saturation problems.
Ipu is the magnitude of the fault current in per units of the CT rated current.
RB and LB are the burden resistance and inductance.
VR is the C rating of the CT.
It can be a bit above 20 and the relays still won't have any trouble. Let it get into the hundreds and the CT response is really ugly.
RE: CT saturation curve from secondary injection
The nameplate of the CT would indicate if the CT is within specification or not, as all that has been presented is the turns ratio, whereas other information (e.g. 5P5F20 or 5P20 1VA for IEC areas) would indicate whether or not the CT is above or below specification.
The other main aspect is what the CT is connected to, as the burden of the CT secondary wiring and the protection device dictates whether or not saturation would occur and at what level.
The connected burden and the voltage for the knee point will give you the maximum amount of current that can flow in the secondary circuit before saturation. Apply the turns ratio to this current and you'll get the maximum amount of primary current that can flow before the CT saturates.
RE: CT saturation curve from secondary injection
FOR EXAMPLE IF I REQUIRE A C200 CT BUT IF I BUY c400 , WOULD IT
MAKE ANY DIFFERENCE IN TERMS OF PROTECTION APPLICATION; IS THERE ANY
DISADVANTAGE IN BUYING A BETTER CT?,