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Differences of Voltage & Current THD


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#1 Guest__*

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Posted 26 September 2005 - 03:06 PM

Hi,

Referring to IEEE519, there are differents requirement limits for voltage and current THD.

As we carried out measurement in different sites, some THD I is exceeded limit but not in THD V. And some sites are both THD I and V are exceeded limit.

But the nos. of site its THD V & I both are exceeded limit is far less to the first case. What are the differences between THD V & I ? I only know THD V is induced from THD I.

Also, which THDs (I or V) when exceeded limits will affects electrical system more seriously ?

Thanks for your help.

Jack

#2 marke

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Posted 26 September 2005 - 07:04 PM

Hello Jack

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Solid state equipment and equipment including discharge paths draw non sinusoidal current from the supply. The distortion of the current generates current harmonics.
The current harmonics will in turn create a distorted voltage drop on the supply which can be measured as voltage distortion on the supply. The amount of voltage distortion caused by an individual piece of equipment is proportional to the current harmonics and the impedance of the supply at all the harmonic frequencies.

There are two problem areas to consider with harmonics.
1) problems caused by the current harmonics and 2) problems caused by the voltage harmonics.
The current harmonics alter the shape factor of the current waveform and reduce the effective power factor on the supply. This increases losses in the supply distribution system. Even order current harmonics increase the hysteresis losses in transformers more than odd order harmonics. This is usually reflected in allowable limits.
Voltage harmonics cause current harmonics to flow in equipment that would normally be harmonic free. Capacitors in particular are very sensitive to harmonic voltages and can suffer reduced life due to increased heating from harmonic currents. Motors can also be significantly affected by harmonic voltages.

Current Harmonics increase losses in the supply. Current harmonics cause voltage harmonics which can affect the operation of other equipment connected to the supply.

Best regards,

#3 Guest__*

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Posted 27 September 2005 - 03:18 PM

Hello Marke,

Thank you for your advise.

That means, current harmonic increases the supply loss by power factor reduction and will not causes any equipments damages. Voltage harmonic causes equipment damage and misoperation.

But voltage harmonic is induced by current harmonic. Is it mean that the place have current harmonic should have voltage harmonic ? Since they are co-related.

If the voltage harmonic is dropped on the supply voltage, will all the eqipments with the same power source also will be suffered by this distorted supply voltage?

In viewing in system stability, voltage harmonic affects system more seriously since it will cause equipments damage and misoperation. Do you agree ?

Also, if we add an active harmonic filter to improve the current harmonic THD, is it mean that we can have energy saving ? Since current harmonic causes supply losses.

Jack




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