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#1 klangwcng


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Posted 14 July 2005 - 03:56 PM

Can anyone help me to understand more about harmonic distortion, between current and voltage distortion?

What level of total harmonic distortion (THD) in percentage is considered "dangerous"?

#2 marke


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Posted 14 July 2005 - 06:48 PM

Hello klangwcng

Voltage distortion is the error between the voltage waveform and a pure sinewave. Likewise current distortion is the error etween the current waveform and a pure sinewave.

In a standard supply system, we have a generator which is producing a voltage which should be pretty clean in that the voltage distortion should be pretty low.
At the point where whe receive our supply, we will getvoltage from a finite impedance. This means that as we draw current from the supply, we will have a small voltage drop.
If we apply a purely resistive load, we will drop the voltage by a constant percentage over the whole waveform - essentially like a voltage divider. This will result in a sinusiodal current and no additional distortion on the voltage waveform.

If we connect a discontinuous load or a non linear load to the supply, the current drawn will be a function of the applied voltage and the connected load. For example if we connect a resistive load with a rectifier in series, we will have current flow in one half of the sinwave only. The current waveform is now very distorted. This will cause a voltage drop to occur only during that half of the waveform and so we will get distortion of the voltage at the point were we take our supply.

In other words, current distortion is causes by non linear loads. Anything that is electronic will probably draw a distorted current. The distorted current drawn from the supply causes a small voltage distortion. This will be reflected in a small way in other loads.

If we apply a distorted voltage waveform to a capacitive load, we will get an increase in current through the capacitors because the impedance of the capacitor reduces as the frequency increases. The effect of distortion on a waveform is to introduce high frequency harmonics and these flow through the capacitors.

Distortion figures are difficult to quote as it depends on the type of connectd load. The limits on current distortion are related t the supply impedance and the limits on voltage distortion are related to the actual harmonic number, not just the thd. For capacitors, harmonics are a major issue. They can also be an issue for motors. For resistive loads, they are not usually a problem at all.

Best regards,

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