Oskars Petersons
= Oskars Petersons
{c}
{tag=Metrology author}
\b[Oskars Petersons] worked on electrical <metrology> at the National Institute of Standards and Technology. With <John D. Ramboz>, Petersons coauthored the 1991 report describing NIST's laboratory <current transformer> calibration service. The report treats current ratio and <current-transformer phase displacement> as distinct measurement results.<A Calibration Service for Current Transformers by John D. Ramboz and Oskars Petersons>[Title page; Abstract; §1.3, Terms Related to Transformer Calibration]{ref}
A <current transformer> allows a secondary <electric current> to represent a larger or smaller primary current. Its marked ratio is a nominal conversion, whereas the measured ratio can differ. The report expresses the measured ratio as the marked ratio multiplied by the <current-transformer ratio correction factor>, and uses a secondary-leading sign convention for the reported phase displacement.<A Calibration Service for Current Transformers by John D. Ramboz and Oskars Petersons>[§1.3, printed pp.5–6; Figure 3]{ref}
An original calculation illustrates the distinction. A marked ratio of $200$ A:$5$ A gives $K_n=40$. If the <current-transformer ratio correction factor> is $1.0002$, the measured magnitude ratio is $K=40.008$. A secondary reading of 5 A then represents 200.04 A under those stated conditions, rather than exactly 200 A.
Magnitude alone is not a full alternating-current description. If the secondary <phasor> leads the primary by $\beta$ and their magnitude ratio is $K$, a consistent convention is
$$
\underline I_p=K\,\underline I_s e^{-i\beta}.
$$
The magnitude correction and phase rotation address different errors. A measurement system can reproduce one accurately while requiring correction of the other. The joint report's calibration circuits, including compensated current comparators, give these distinctions an apparatus-level setting rather than leaving them as ideal-transformer identities.<A Calibration Service for Current Transformers by John D. Ramboz and Oskars Petersons>[§2.1, Calibration Circuits, Figures 8–9]{ref}