Re: Harmonic filter for better power quality

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Hi, K.R.,

It sounds like you are trying to correct for harmonics employing 5th and 7th harmonic tuned filters. This can be difficult and result in system issues if the total harmonic loads are more than 50% of the total loads.

In order to meet the cancellation capacity needed for 5th and 7th harmonic filters, it is very likely that the total displacement PF is corrected to leading. If the leading PF is substantial, the voltage will be increased to unacceptable levels for the connected loads (>10% of actual). This will be harmful to your loads – some may fault due to over voltage or fail completely.

Additionally, it is possible that safety devices and contactors begin to fail to operate. Leading PF causes contacts to become pitted as they are actuated. One result is that circuit breakers may fail to open or fail to close.

Another issue is that PF capacitor systems are designed to switch according to the displacement PF at the point of monitoring (where the CT is located). This does not assure that the proper amount of filtering steps are activated at the right times. It is possible that the 5th and 7th tuned filters are over loaded and suffer premature failure.

Finally, if additional loads are added to the network, the whole filtering/PF correction system must be re-examined for suitability of use.

We suggest that active harmonic filters be used when more than 50% of the total loads are harmonic producing. This may be more expensive but the system will not encounter any of the above issues. [Do remember to insure that 3-5% impedance reactance is installed at each harmonc rectifier.]

Regards, Jim