CT Saturation calculation
CT Saturation calculation
(OP)
I have a CT with following data
Ratio : 300:5
Class : C100
Burden : 0.1 ohm
I don't have magnetisation curve for this CT. I want to calculate whether CT will saturate at fault current 3000A.
Total Burden (Including CT secondary, CT leads and relay) is 0.45 ohm
Excitation voltage (V) = (Fault current/CT ratio)* Total burden
V = (3000/60)*0.45
V = 22.5V
I don't have magnetion curve to compare this voltage.
Is there any other method so that I can be assured the CT won't be saturate.
V =
Ratio : 300:5
Class : C100
Burden : 0.1 ohm
I don't have magnetisation curve for this CT. I want to calculate whether CT will saturate at fault current 3000A.
Total Burden (Including CT secondary, CT leads and relay) is 0.45 ohm
Excitation voltage (V) = (Fault current/CT ratio)* Total burden
V = (3000/60)*0.45
V = 22.5V
I don't have magnetion curve to compare this voltage.
Is there any other method so that I can be assured the CT won't be saturate.
V =






RE: CT Saturation calculation
This is based on 20 times rated secondary current of 5A into a 1 ohm burden (20 x 5A x 1 ohm = 100V)
Now, you use the term "saturate" and if you really mean staying in the linear region below the knee-point, then you would need to check the curve to be sure. However, from a practical perspective, you should be no where near the knee-point with 3000A primary current and 0.45 ohms total burden, which is 22.5 A, as you calculated.
A good rule of thumb is the actual knee-point voltage is approx. 70-75% of the accuracy limiting voltage (the "100" in C100). So the knee-point of this CT is in the area of 70-75V using the IEEE definition of knee-point (tangent at 45deg).
RE: CT Saturation calculation
The secondary winding resistance, remanence and X/R ratio of the system should be also evaluated.
RE: CT Saturation calculation
I read that the secondary resistance was included in the 0.45 ohms he listed.
The system x/r ratio and the remanence factor have no bearing on at what voltage the CT will saturate at.
They can play a part in determining what voltage might be present on the CT secondary during a fault, especially if considering a reclosing operation.
How is the answer misleading?
RE: CT Saturation calculation
When selecting a CT, the calculated saturation voltage should be > Is*Z.s*(1+ X/R). X/R ration also have effect on time-to-saturate
If you have some residual flux permanently left in a CT, say a C100 class CT, do you think you can drive the CT to the saturation point by build up to same level of voltage or in other words pass same level of the primary current?
RE: CT Saturation calculation
RE: CT Saturation calculation
A few points:
- I was giving a practical answer. Normal ranges of X/R ratio would not cause this CT to saturate at 3000A primary.
- % remanence is really only a factor for reclosing actions, i.e. breaker open and then close back into a fault after xxx ms. I assumed since he didn't mention anything complicated that it was a simple over-current protection question.
CTs that become magnetized (remenance) will tend to "work off" the remenance over time as a result of varying current levels.