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Power-User
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Effects of Harmonics on Current Transformer Measurement

05/06/2014 9:31 AM

Say we have a classical c.t and conventional ammeter using electrodynamics principles.

What would be the effects of high harmonics on the measurement?

Big deviations from the true value ? Then is it because of c.t or ammeter or both?

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Guru
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#1

Re: Effects of Harmonics on Current transformer measurement

05/06/2014 11:32 AM

Am I correct in assuming that your point is that an analog meter is not going to respond fast enough to higher frequency harmonics because of needle inertia and will therefore read too low? I hope I'm wrong and that you can demonstrate a better understanding of the art and science of metering. Awaiting your answer.

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Guru

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#2

Re: Effects of Harmonics on Current Transformer Measurement

05/06/2014 8:39 PM

If you full-wave rectify the signal from the current transformer, the galvanometer movement will indicate the average current. Normally, you are interested in rms current.

For a perfect sine wave (no harmonics) the rms value is .707 times the peak value whereas the average current is 2/pi times the peak current. The meter face will be calibrated to read rms.

If there are harmonics present, the waveform will not be a simple sine wave. Other waveforms will have a different relationship between average and rms current, and the meter will not read correctly.

http://www.learnabout-electronics.org/ac_theory/ac_waves02.php

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Guru

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#3

Re: Effects of Harmonics on Current Transformer Measurement

05/07/2014 4:47 AM

A correction to my post above:

In an electrodynamic meter movement, the measured current energizes both the field and the armature of the movement. In this case, the torque generated is in response to the average of the square of the current and corresponds to the rms value of the current regardless of the waveform.

http://en.wikipedia.org/wiki/Microammeter#Moving-coil_ammeters

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Power-User

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#4

Re: Effects of Harmonics on Current Transformer Measurement

05/07/2014 1:34 PM

CT itself is not a problem upto about 1000Hz frequency (IEC60044).

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