Fluke 6105A - The Effect of Phase Uncertainty on Power Accuracy

Calibration
Best Calibration Accuracy 7
7-5
Note
Zero-detection phase meters may give erroneous results in the presence of
even harmonics because even harmonics can cause the zero crossing of a
composite waveform to differ from the notional fundamental frequency zero
crossing. Sampling techniques, on the other hand, can give phase
uncertainties as low as 0.0008 degrees and are not susceptible to other
harmonics affecting the measurement.
7-10. The Effect of Phase Uncertainty on Power Accuracy
As power = V.I.Cos(A), the contribution from phase angle accuracy can be shown with
the following example:
If phase accuracy is ±0.05 degrees, at nominal PF = 0.5, Cos(A) could vary between
Cos(59.95) and Cos(60.05) i.e., 0.5008 to 0.4992. This represents a range of
%3.0%100*
5.0
4992.05008.0
=
−
If
Φ
is the set phase angle and
)(
φ
u
is the phase accuracy, the general case of phase
accuracy contribution to power accuracy u(P) is given by:
%100)
)cos(
))(cos(
1()( ×
Φ
+Φ
−=
φ
u
Pu
Table 7-1 shows how phase uncertainty affects power accuracy at different power factors.
Table 7-1. The Contribution of Phase Uncertainty to Power Accuracy
Phase Uncertainty PF = 1.0 PF = 0.75 PF = 0.5 PF = 0.25
0.0008 ° ±0.000 % ±0.001 % ±0.002 % ±0.005 %
0.050 ° ±0.000 % ±0.077 % ±0.151 % ±0.338 %