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MSP430F6733: Question related with SLAA122 Current-Transformer Phase-Shift Compensation and Calibration Application Report

Volkan Celik
Prodigy 30 points
Part Number: MSP430F6733

Hi,

On page 2 of Current-Transformer Phase-Shift Compensation and Calibration Application Report - SLAA122 (February 2001), Figure 1 shows mains voltage V, mains current I(in) and the output current I(out).

I(out) is the current that we measure after Current Transformer (CT).

According to the Figure 1, it seems like I(out) leads I(in) by φ.

And the following paragraph confirms that.

Also in the next page, in Chapter 2.2, first paragraph confirms that too by saying "To overcome the problem of phase shift across the CT, the output current could be delayed by the same amount as the inherent phase shift so that both the actual and measured power factors were equal..."

What I think is there is somethink wrong here.

As I know, CT lags the current.

So the graph in Figure 1 should be like I(out) lags I(in) by φ.

And the paragraph in Chapher 2.2 should be like "To overcome the problem of phase shift across the CT, the output voltage could be delayed by the same amount as the inherent phase shift ..."

Am I right?

Thank You.

  • 0
    TI__Genius 10980 points
    Hi,

    I believe your confusion here is due to the fact that Figure 1 only shows one possible case of CT phase response, as an example. Whether the output current leads or lags the input current is dependent on whether the loading has a net positive or negative reactance (whether it is inductive or capacitive).

    Regards,
    Nathan
  • 0 in reply to Nathan Siegel
    Prodigy 30 points

    Hi Nathan,

    Thank you for your interest.

    NathanS said:
    Whether the output current leads or lags the input current is dependent on whether the loading has a net positive or negative reactance (whether it is inductive or capacitive).

    As I know, if the load is inductive or capacitive, it affects the relation between the current and the voltage.

    It won't affect the input current and the output current relation.

    But the effect of CT is different and just in one way.

    It causes the output current lagging the input current.

    What do you think?

  • 0 in reply to Volkan Celik
    TI__Genius 10980 points
    Hi,

    No, whether the load is inductive or capacitive can affect the phase shift between the input and output currents, in addition to the phase shift between current and voltage. The output current lagging the input current (due to an inductive load) is the more common case, but capacitive loads can be present with CTs as well, and this will cause the output current to lead the input current.

    Regards,
    Nathan
  • 0 in reply to Nathan Siegel
    Prodigy 30 points
    Hi,

    So then, according to what you said, assume that I have a capacitive load.
    And I measure the current that flows to that load over a CT.
    At one instant of a time, for example I am measuring the positive peak point of the current (which is actually I(out)) with my microcontroller.
    But the real current that flows to the load (which is I(in) here) has a little time to reach its positive peak.
    So it means, what I am measuring is the near future of the real current?
    Is it possible?
    Maybe in this small period of time, somebody else will cut the cable and the real current, I(in), won't be able to reach its positive peak.
    Is this approach wrong?

    Thank you.
  • 0 in reply to Volkan Celik
    TI__Genius 10980 points
    Hi,

    You shouldn't really think of it as future current. It is the same signal (current), just with a phase shift. This is not quite equivalent to a time delay.

    Regards,
    Nathan

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