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Active Filters

Active Filters Common Mode Currents Active Front End VFDs

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#1 marke

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Posted 08 November 2014 - 08:47 PM

Active filters, series (active front end drives) and shunt comprise IGBT switching elements operating in much the same manner as the output of a VFD. These switches operate at a frequency above the line frequency and are pulse width modulated in a manner to reduce the harmonic current flow from the load into the supply.
The action of these switching devices generates high frequency currents that are fed back into the supply. The frequency of these currents is typically above the 31st harmonic and less than 150KHz. As such, they usually slip through any PQ testing and standards, so do not cause any compliance issues, but they do cause interference and reliability issues, particularly with any equipment containing capacitive filters such as EMC line filters and power factor correction.
Over the years I have seen many instances of lighting circuits overloaded when such active filters are operating, power factor correction circuits overloaded and at times, ballasts and SMPs failing. - Current standards compliance allows for this, as the currents fall outside the normal regulated frequency ranges.
Recently, I was called in to assist with problems at a site where there were perceived issues with the supply transformer and supply metering.

Before I arrived on site, two smart electronic electricity meters had failed catestrophically, (let the smoke out), and the supply transformer had been replaced with a larger one.
A Power Quality meter installed to help trace the problem had also failed.

On site analysis found 128Vpk-pk of high frequency voltage superimposed on the 230V phase-neutral voltage. This was common mode voltage from the series active filter due to the current from the filter and the HF transient impedance of the supply.

The supply comprised a 500KVA 400V transformer fed by an 11KV overhead line, approximately 15KM long. This was supplied by a typical small rural substation with a long HV supply line, probably at 33KV.
Not an unusual situation in a rural situation.

Moral of the story, although the active filters may comply with the local standards, the common mode voltages can cause major problems in the field. Often, the issues are not recognized, or are denied on the grounds of standards compliance.
Perhaps the standards need to be enhanced to cover the whole spectrum. Other circuits and components do not understand that high voltages outside the standards coverage are deemed to be OK by the authorities!!



#2 mariomaggi

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Posted 08 November 2014 - 09:04 PM

Excellent notes, as usual, Marke!
Regards, 

Mario


Mario Maggi - Italy - http://www.evlist.ithttps://www.axu.it





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