ECN Forum Forums General Topics of the Trade General Discussion Area Passive Harmonic filters

Hello,
I would like advice from anyone who has sold / installed a small passive harmonic filter. I am proposing this to be installed to a 100A PDU feed sub-sub-panel. This is the only "noisy" sub-panel for a MGE Epsilon PDU. I've specified and installed AHF filters in front of UPS's and MCC's, but cost and harmonic values rule out bigger units. One device I am looking at is a Mini-Z zero sequence filter made by PQI (Power Quality International). Any experience with these?

Have you looked over here.. http://www.eng-tips.com/
..they are not keen on talking to non-engineer types, but they may be of help

How noisy is the feeder to the PDU? How much current THD and voltage THD? How much load relative to the rating is connected?
Sometimes its not worth the hassle. More new power supplies are provided as time goes on, with less impact on harmonic influence.

The panels I am considering this filter for are sub-fed from a PDU. Harmonic content is about 80% current THD, Voltage THD at 1.5%. Amps are 7.5A / phase, 13A on the neutral (this is on the feeders). These are a pair of feed-thru 100A 42 space panel boards. At the rate PC's are added to these panels, I'm concerned about cooking the neutral and ground.
I have found an RMS value in excess of 35 amps flowing on the ground back to the UPS, on a #4 AWG. There are many frequencies riding on this wire. (up to 10kHz).
I believe I have two issues here; fundemental voltage distortion will increase above 3% soon from additional PC's. And 2nd, stray oscillary transients run along the grounds. If left unchecked, something is going to fail.

Definitely keep your eye on it with a true RMS meter and your THD meter. I wouldn't do anything yet until the panel is a bit more loaded.
You'll find as more load is added, they begin to cancel each other out. Non-linear power supplies draw power when they want. With enough load eventually they start to draw at such "randomness", that the effective THD reduces.
Also be sure that you are using a clamp-on that is somewhat close in size to your load, so as to increase accuracy of the measurement. Some clamp-on's that are designed to measure 1000A, don't measure 14A very accurately.

[This message has been edited by Ron (edited 10-11-2005).]

[This message has been edited by Ron (edited 10-11-2005).]

Thanks Ron. One more final reply and I'll let this rest. These panels are supplying a small army of PC's in the "buisness continuity room" (code speak for disaster recovery department)of a major insurance co. I appreciate your hesitation to add hardware until a larger draw is pulled from the panels. This company buys electronic hardware (PC's, servers, routers ect. in bulk at year's end, ussually devising a plan to connect it all together at once with little engineering). I say this knowing there is a definite probability that 25 more identical workstations will arive this year and will pull similar current patterns from the SMP's. I was introduced to these folks after a 150KVa UPS blew-out this summer. Admittedly I'm trying prevent another business operations failure in guessing worst-case senarios. The collection of PC's has never really been tested under real world conditions.

Assuming 25 new computers, your loads per phase will increase from 7.5A per phase to 17.5A per phase. Still significantly less than the 80A per phase (100A x 80%) that the PDU can tolerate.
Although the load per phase may almost double, if you can keep the loads balanced per phase, I would be interested in how the neutral current would be effected. I would expect the neutral current to remain pretty constant or reduce.

You could spread the harmonic pulses around some more (in terms of phase angle timing) if you operate some of the PCs off of 208V (phase to phase). PC power supplies usually have 120/240 selector switches. What this can do: Say most of the PCs are operating off of 3 phases of 120V. At phase angle 0, 1/3 of the PCs draw a spike of current off phase A, and return it on the neutral. Then at 120 degree phase angle PCs on phase B draw a spike of current, and return it on the neutral. Now if you connect a PC (after setting the power supply to 240V setting) across phase A and B for 208V it will draw, at phase angle 150, a current spike current and return it from phases A and B, and the neutral sees nothing. This is in between times of PCs on phase A and neutral, B and neutral and C and neutral are drawing their current spikes. This off loads the neutral some, and also should make the power company a little happier.

[This message has been edited by wa2ise (edited 10-12-2005).]

Thank you for the insight; abeite un-othordox for I.T. departments to follow. Your concept is electrically valid but would cause a sizable increase and confusion in cost to re-wire and re-label exsiting 120v 5-15 repts to 6-20's, not to mention loading some portion of the panels with 2pole 20's. I can't imagine this working in the long run. (too many crossed human wires). Thanks, I DO follow your reasoning
Eric

Mr. Wise, that's not a bad idea, but the power cords and the receptacles on the circuits feeding them must be changed!