Effect of CT Phase Shift on Metering Accuracy
The phase shift of a current transformer (CT) is important to the overall system measurement accuracy. However, it is difficult to quantify the phase shift rating of a CT to a percentage error, since it depends on the power factor of the system being measured.
Since the phase shift of a CT adds displacement between the input current and the output of the CT, the result is that the meter will see the resulting waveform slightly offset from the actual current. Since power factor, and subsequently measurement of power (kW and kVAR) depends on the phase relationship between voltage and current, the induced displacement angle plays a factor in overall measurement accuracy.
Additionally, due to the sinusoidal nature of the relationship, the effect of this shift depends on the starting angle (the system power factor).
That is: at unity power factor, the phase shift of the CT will contribute very little to the error. However, at lower power factors, the error is much larger.
For example, assuming a 1-degree phase shift, and no other errors:
|
CT Phase Shift (degrees) |
System PF |
System VA |
System kW |
Measured PF |
Measured kW |
kW Error |
|
1.0 |
1.000 |
1000 |
1000 |
1.000 |
999.85 |
0.02% |
|
1.0 |
0.900 |
1000 |
900 |
0.907 |
907.47 |
0.83% |
|
1.0 |
0.800 |
1000 |
800 |
0.810 |
810.35 |
1.29% |
|
1.0 |
0.700 |
1000 |
700 |
0.712 |
712.36 |
1.77% |
|
1.0 |
0.600 |
1000 |
600 |
0.614 |
613.87 |
2.31% |
|
1.0 |
0.500 |
1000 |
500 |
0.515 |
515.04 |
3.01% |
The table above demonstrates how the same CT can generate wildly different errors based on the power factor of the system.
Additionally, the phase shift of the CT is usually not linear. The displacement angle usually varies based on CT input current. The burden (load) placed on the CT also serves to affect not only the linearity, but the phase shift characteristics.
It is ideal to source CT that have the best CT accuracy possible, which includes not only the linearity of the CT, but the phase shift also. When using 5A CTs, it is recommend to use “ANSI” grade CTs if possible.
Elkor's mA output CTs are all quantified to measure the effective turns ration and average phase shift characteristics. These parameters are labelled on the CT, and can be input into the meter to provide a correction factor. As a result, Elkor's metering system including meter and CT can achieve industry leading accuracy using low costs system components.