Why does current lead voltage when measured by a CT?

Radian · 2022-07-01T17:39:43.703Z

I don’t understand why the current waveform I get from CT’s is always leading the voltage waveform. I always thought inductors lag and capacitors lead? I tried googling it and have come up with barely any mention except the occasional statement that this is simply the case and that it’s influenced by the value of the burden resistor. Lower R, less lead.

Robert.Wall · 2022-07-01T19:59:43.897Z

It’s called phase error, it is the output current (usually) leads the input current. It’s not a matter of current leading voltage, voltage in relation to c.t’s is largely meaningless. The cause is down to the way the magnetics in transformers work. I did write the theory up a few years ago, but it was never published.

CT Theory 3.pdf (54.8 KB)

Radian · 2022-07-02T09:23:21.267Z

I very much appreciate your unpublished paper. Been a while since I poured over a phasor diagram. I get what you mean by saying that voltage in relation to CT’s is largely meaningless… it only really relates in terms of timing. What I think is lacking is an intuitive understanding of the phase error. We expect current to lag behind applied EMF in an inductor as it is being counteracted by the back EMF caused by the developing magnetic field per Lenz’s law. This seems, to me at least, to be being flipped by the inverted roles of the CT’s primary and secondary.

Robert.Wall · 2022-07-02T12:07:01.998Z

Radian:

This seems, to me at least, to be being flipped by the inverted roles of the CT’s primary and secondary.

Maybe you should think in terms of everything (almost) that you know about voltage transformers being flipped on its head for current transformers. The secondary operates as a current source, so current becomes your reference. Voltages happen as a result of the c.t. secondary enforcing, or trying to enforce, the secondary current.