Active Harmonic Filtration 1

[yorgi]

We are looking at installing Active Filters to mitigate enhanced harmonic propagation due to the addition of non linear loads to our network. We are experiencing about 2.8% Voltage THD at the 11kV point of common coupling which can peak at around 7% when differnet switching configurations are requireded (maintenance load sharing etc). As a result we are looking to add Active filters on 2 Large 2.7MW 12 pulse drives. We have calculated that we will need about 200amps per bridge. Has anyone experience of active filtration as this would be a first for us, and any information on manufacturers and or suppliers

 

[Skogsgurra]

There are quite a few systems available and Google is your usual friend.

But, at those levels, will it really be necessary? Is your 11 kV used internally? Or are there other consumers connected to it?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[sibeen]

But, at those levels, will it really be necessary?


Skogs, my initial thought.

yorgi, perhaps you should investigate a phase shifting transformer to feed one of the drives. Turn the site, or at least the drives part, into a 24 pulse system. I haven't looked at pricing of any active harmonic filters at 11 kV, so don't know what the price difference there will be between an active harmonic filter, and a phase shifting transformer; but I do know which of the two will be more reliable and need far less maintenance.

 

[yorgi]

Good points all round, We should not be where we are the kit should have been speced correctly, But we are where we are. What we are trying to achieve is a reduction without the disruption of TX change outs etc. We adhere strictly to G5-4/1 and are penalised by our DNO if we transgress. We also generate on site and as such Excessive harmonics cause issues at our power plant and have caused trips and PLC trip issues. Effectivly we are looking for a "quick fix" and a active filter solution was suggested by one of our major electrical suppliers. I was just wondering what experience posters on this site may have had. I have used smaller broadband passive filters in the recent past and have found them to be very effective. However at the powers we are looking to mitigate, the cabling required and the room available rules this out.

 

[jraef]

I have been using Active Harmonic Filters now for a couple of years, maybe not enough for a long term evaluation, but so far, so good. We have been putting in TCI H5 Series, not a lick of trouble (beyond initial engineering issues at our end). What I like about them is that they are adaptive. On two projects now, our gear would have been fine with passive solutions, but the site conditions changed immediately after we commissioned and the users have been very happy that by us having used the active solution, it maintained the correction. Luckily in both cases so far they have not had to bump up the sizes yet...


"Will work for salami"

 

[X49]

In my experience (albeit for low voltage installations), passive filters can be significantly more cost effective than active filters, especially if you only require filtering for two drives. I would definitely examine both options.

 

[Marke]

One issue to be aware of with active filters, is that they are like the output of a VFD in that there is a PWM bridge connected to the supply. Because it is a switching waveform, there is switching current in the output waveform.
A switching filter is connected between the output bridge and the supply to attenuate the switching frequency curents, but depending on the attenuation of the switching filter and the impedance of the supply, there can be significant levels of the switching voltage appearing on the supply.
The switching frequency is above the 31st harmonic, so these voltages do not appear when checked with a power quality meter, but we hve many experiences where power factor correction capacitors and lighting circuits are badly affected by these switching voltages.
Where the supply impedance is very low, this is not an issue, but if the reactive impedance is high, then it can become a problem.

Phase shifting transformers are useful at reducing 5th and 7th harmonics by cancellation, but some higher order harmonics are actually additive rather than subtractive, so not a perfect solution. Additionally, because it is a subtractive method, it requires identical harmonic loads on each transformer to give best results.

A good broad band filter such as the Lineator is an efficient solution.

Mark Empson
Advanced Motor Control Ltd