From the above figure, it can be seen that the primary current IP of the current transformer is divided into two parts after being transformed into the secondary current. A small portion is used for internal loss excitation, magnetic loss, and eddy current loss. The other part flows into the secondary circuit, which is the secondary current IS measured externally. Therefore, the excitation current is the reason for the error in CT.
The excitation impedance Ze is not a constant, but it is determined to be constant under the same induced electromotive force. Therefore, we can obtain the relationship between Es and Ie by measuring the excitation characteristic curve. The composite error of CT under various operating states can be obtained through calculation.
The relationship curve between the accurate limit coefficient and the allowable secondary load is commonly referred to as the error curve.
10% Error Curve: The relationship curve between KALF (the ratio of primary current to rated primary current) and the allowable secondary load when the current error is set to 10%.
5% Error Curve: The relationship curve between KALF (the ratio of primary current to rated primary current) and allowable secondary load when the current error is set to 5%.
The steps to calculate a 10% error:
1. Measure the DC resistance Rct of the secondary winding and calculate the impedance Z2=Rct+j Xct of the secondary winding. Xct can generally be ignored or take values between 0.1 and 0.2.
2. Draw the excitation characteristic curve to obtain the U=f (Ie) relationship curve
3. Since the error has been set to 10%, i.e. I2=10Ie, the following relationship exists:
According to the equivalent circuit diagram, I2=Ie+IS
And I2=10 Ie
Therefore, 10 Ie=Ie+IS
So we can obtain: IS=9 Ie
4. Multiple of primary current Ip and rated primary current Ipn:
KALF= Ip/ Ipn=Kn I2/ Kn Isn = I2/ Isn=10 Ie/ Isn
Isn: Rated secondary current
When Isn=5A, KALF=2Ie
When Isn=1A, KALF=10Ie
6. Take any value, according to KALF=10 Ie/Isn, the Ie value can be obtained.
7. Find the voltage value U corresponding to Ie on the excitation curve
According to the formula Es=U - Ie Z2, the relationship curve between Ie and Es can be obtained, from which the value of Es can be calculated.
9. Calculate the allowable load Zb=Es/Is Z2=Es/9Ie-Z2 according to the formula
According to the above calculation steps, a 10% error curve can be drawn.
Similarly, a 5% error curve can be drawn.
