Current transformer compensation idea

From: analog (analog_at_ieee.org)
Date: 07/27/04 

Date: Tue, 27 Jul 2004 18:25:49 GMT

A typical current transformer may have a one turn primary and
a hundred or more secondary turns. The secondary is normally
terminated into a small resistor (possibly through diodes)
such that the core must support a small ac flux excursion.
With secondary signals in the volt range, the primary voltage
burden is minimal, usually a few millivolts.

Although dc drift may be a problem for some configurations,
a typical current transformer rarely comes anywhere close to
saturation during normal operation. In spite of this,
inductive signal droop may be a problem in high fidelity
applications (magnetizing current is typically very non
linear).

I have been toying with the idea of using active circuitry to
minimize magnetizing current. My first idea was to arrange
the current transformer to drive the summing junction of an
opamp rather than terminating it into a small resistor. This
would tend to keep the voltage across the CT's secondary at
zero, which would be a noticeable improvement over the
standard arrangement.

However, this would still leave the voltage burden from the
sense current flowing through the CT's winding resistance.
Even this could be largely nulled out by actively driving the
"grounded" end of the current transformer with a feed forward
signal proportional to current appropriately scaled just to
equal the drop developed on the internal winding resistance.

Okay, I have never built this circuit and don't have a real
application for it, but the simulator says all works great.
What I am wondering is whether anyone has used or seen such
a technique before or could imagine a situation where such a
circuit might prove useful. Note that this technique does not
eliminate the dc saturation problem (although it does make the
CT's core "look" much bigger). Comments or further ideas?

analog

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