I Have A Doubt About Using Detuned Reactors With Capacitors Banks

Power Factor correction utilizing capacitors and inductors will affect system induced harmonics but does not fully address your issues.

I would suggest you first identify the source of all harmonics then eliminate them by installing tuned harmonic filters wherever necessary and as local to the culprit VFD(s) as possible.

Once you eliminate the harmonics it should be relatively easy to address any PF correction needed to the supply power.

I'd have a go at eliminating those naughty harmonics at source rather than at the downstream equipment, Mildred, then I'm funny that way.

This is a very interesting question in that methods of %Z are being used. But I am confused by the term "detuned reactor" since line reactors are simply inductive chokes in series with your VFD (variable frequency drive).

I suppose there are several methods by which the analysis could be performed, but the ratings I've seen in the catalogs have been based on a particular sized load with the choice of 3% or 5% reactance. All of this assumes that the fundamental frequency is 50 or 60 Hz at which frequency, the impedance will be smallest. At multiples of the fundamental, the impedance then increases proportionally so that harmonic current is suppressed more and more with higher octaves, right at the load.

If these harmonics are not suppressed, the capacitor bank may experience higher voltages than they are rated for by having the current in phase with and adding to the fundamental current. In some cases, the capacitors may ignite and burn up. I've seen this where I work. Consequently, I have been adding line reactors where there were none installed, and it seems to be improving (reducing) the harmonic content.

I'd like to know more about this myself. It feels like circular reasoning when trying to come to terms with the details of a particular solution. So far, line reactors seems to be the least costly of the options for treatment.

A

passive harmonic Filter" for VFDs is essentially exactly what you are describing doing on your own, it is a set of reactors and capacitors with RC snubbers that are tuned to the 5th and 7th harmonics, which make up the majority (but not all) of the THD coming from a 6 pulse inverter drive.

The problem with PHF systems is that they work today, then if something changes, they no longer work. I never recommend them for anything but the most rock solidly unchanging systems. In addition, the cost for PHFs tends to make them uncomfortably expensive once you get above 100kW drives. You never say what size drives you have or how many.

Lately I have been using Active Harmonic Filters instead, especially if there are multiple VFDs, and especially if they are not all running at the same time. The AHFs will track the harmonics and inject corrective current AS NEEDED into the system. As a general rule, an AHF can actively trach and correct your ITHD to less than 4%. The best I have ever seen with a PHF system is correcting to 7% ITHD.

If you have just one or two large VFDs, you might also consider going to an 18 pulse front end. But if the VFDs already exist, that can be challenging.

12 pulse front ends are good enough to meet the usual specifications and don't have the problems 18 pulse do with regard to unbalanced input line voltage. if you can build a 12 pulse system by adding a single transformer and 6 more diodes to your existing VFDs then it will be significantly cheaper compared to an 18 pulse, which will require a transformer rated to 1.1 kva requirement rather than the 0.57 pu required for a 12pulse configuration (augmenting the straight through connection to the line) The tapped inductor required should not be a significant cost either.

far as i know most 18 pulse configurations run the three rectifiers in series to avoid using a tapped inductor, so you have to buy a new transformer.