Critical Issues-EMI Filters

For EMI filters, the only real critical issue is whether the filter needs to protect against common mode noise, normal mode noise, or both. It must be remembered that at these high frequencies, the ground is just another path, it is not a mystical dumping site where noise, once placed, is never seen again.

There are several issues to consider with passive harmonic filters. First, the specific harmonic and its worst case magnitude must be known in order to provide the correct filter. If the harmonic current increases, the filter may be damaged.

Second, once a filter is installed, it may alter the power system's overall frequency response. This can create a situation known as resonance, which may dramatically increase voltage distortion, damage other loads, or damage the power system's components.

Consulting a harmonics specialist with prior experience in using passive filters to reduce harmonic content is highly recommended.

Active filters are fairly new to the industry. Some designs are for series applications, which means they are installed in series with the loads. They can filter both voltage distortion and current distortion. While this helps with certain filtering tasks, it also means the active filter must be sized for the total load, not just the harmonic portion. This makes handling load growth difficult.

The newest designs are for parallel applications. They only filter the current, but they are sized only for the level of harmonic current, not the total load. They can expand to grow with the load.