# What is the best approach for reducing harmonic distortion?

I have a power system where on some buses, large converter driven machines result in high harmonic distortion. As far as I know, there are mainly two ways to reduce the harmonics.

1. A filter to remove (reduce) unwanted harmonics within a range of frequencies
2. Reactors to create a smoother wave form.

I'm wondering what are the pros and cons for the two alternatives, and are there any other (better) alternatives? I cannot change any of the existing equipment, but I can add equipment (criteria: smallest size then lowest cost). The converters are 6-pulse rectifiers without filtering, and cannot be substituted (12-pulse would typically give a smoother wave).

The filter will only reduce some harmonics, and I have problem with the 5th, 7th and odd harmonics between 25-37th. I could design a filter to reduce the higher order harmonics, but then I would be stuck with the high 5th and 7th components. (Or have a filter to remove the 5th and 7th, leaving me with the higher order components).

Reactors seems like the best approach, but almost all similar systems I've seen have filters. The size and weight must be as low as possible (even if that might increase the price of the equipment).

The equipment rating is in the range 800-1500 kVA and I have THD above 10%. There are no resonance frequencies in the network (the lowest is around the 180th harmonic).

Is one of the approaches generally better than the other. Will there be any significant difference in size for the two?

I hope someone has any views on this. Thanks!

• My experience is purely analytical, so I can't give any advice as to the space, weight, cost trade-offs, sorry. May I suggest reading some of Schneider's Cahier Techniques white papers? ECT #199 "Power Quality" and ECT #152 "Harmonic disturbances in networks, and their treatment" seem relevant. – Li-aung Yip Dec 15 '13 at 10:41
• @Li-aungYip, thanks! Those were very good references. Read and stored in the "Useful references"-folder =) Still hoping for something more practical =) – Stewie Griffin Dec 15 '13 at 10:58
• Robert: the Cahier Techniques are all very good reading. I would download the entire collection for your files. – Li-aung Yip Dec 15 '13 at 11:02
• Where are you having issues with Harmonic Distortion in relation to your converter driven machines? Upstream in your power distribution system or after the converter? It sounds as if your 6 pulse converter is some type of VFD. Is this a 3 phase 480 Volt system? If this is the case, common practice is to add an inductive coil (also known as a filter or choke coil) before the converter - to keep Spikes and Harmonics from getting repeated back into the rest of your distribution system. – Tinkerer Dec 15 '13 at 14:30
• @gerrishp2, You're right, they are VFDs, and yes, it's towards the distribution system (3 phase, 690V). The increased reactance at 50Hz will be 20% of the increase at 250Hz (5th comp). Since we don't want a significant increase in reactance at 50Hz, it can't be significant at 250 or 350Hz either. It will probably help some at the +25th harmonics, but I would think the main purpose would be to avoid transients/spikes and not harmonics. Am I wrong? I would think capacitors would be better fit for this? – Stewie Griffin Dec 15 '13 at 19:32