# How can power factor be reliably estimated given the phase error in current sense transformers?

The majority of mains power sensing devices I've seen (both commercial products and ICs) utilise a current sensing transformer. Many of them also calculate or estimate power factor. However, the phase error specified in the datasheets of almost every current transformer is in the order of ±15° when operated with the recommended burden resistor.

If the magnitude of phase error between the real current and the sensed current is ±15°, and if my calculations are correct, that implies that a load with a power factor of 0.86 could be reported as anywhere between 0.71 and 0.97, which is hardly a useful level of accuracy.

My guess is that in reality the error typically only manifests as additional lag on the current waveform when measuring 50-60Hz mains, rather than lead, which would mean a range of 0.71-0.86 for a power factor of 0.86. This is better, but still pretty bad.

Is this just accepted as a limitation of the approach, or am I missing something here?

• How do you know that the phase error isn't compensated for in the code? Oct 3, 2022 at 16:03
• @Andyaka Is the phase error constant, or otherwise calculable? My guess is there's some proportionality to the dI/dt, which seems like a catch-22 in terms of being able to correct for it. Oct 3, 2022 at 16:07

Mind, I don't know about the particulars of mains-frequency CT design.

I suspect they just don't want to measure it. Consider what ±15% means: it could be inductive or capacitive. Now, either case is plausible -- there's a LOT of wire on there to make up the some thousands of turns needed for low frequency use -- but it beggars belief that the phase could be, not only inductive or capacitive, but that far out, at mains frequency.

I suspect they just don't want to characterize it for every part manufactured.