Harmonic Suppression

An isolation transformer will have some self inductance, which will suppress some of the higher frequencies. At circuit-board-level, making the circuit work properly comes first and economics is not a primary consideration.

The question is very incomplete. What harmonics is he trying to suppress?

What in the nature of the circuit. For example if this was a 3 phase power circuit and you wanted to suppress the Third harmonic, then you could use a delta connected 3 phase transformation, since a delta connection is a natural short circuit the third harmonic. I'm sure that this does not apply, This is a vague answer to a very vague question.

Quite.

Reminds me of Monty Python's "I've come here for an argument routine"

"Quite" indicates agreement, though.

no it doesn't

Isolation transformers have little effect on lower order harmonics.

Well, harmonics are not a simple problem, and there is no cheap solution for it. However, the first thing you should do, is to measure the THD (total harmonic distortion), at several buses and feeders on the electric supply, just to locate the source of the harmonics, and to know the order and magnitude of them. Remember, you must use a TRUE RMS VALUE ammeter in order to measure currents with a harmonics content; and, if possible, use a frequency spectrum analyzer to know the frequency and value of every harmonic. Once this is done, let´s say, for instance, that you find a VFD (variable frequency drive for a squirrel cage AC motor) as the probable cause of harmonics; then, you can dedicate a single transformer and a line, from the substation to feed that VFD, and use series line reactors (with perhaps a 5 % impedance) between the transformer and the VFD, this will work fine, and is cheaper than the harmonic filter. Or, maybe the cause are 20 PC´s and their No-Breaks, distributed in all 3 phases, which are monophasic loads, but will produce that all 3 phase third harmonics sum at the neutral wire of the transformer; in this case, you can use a thicker cable. If you provide more details we can give you a more specific advise. Good luck.

Your comments are correct and appropriate. His question is not clear.

Are you aware that a 3 phase power transformer (all windings on a common core) when connected in Delta configuration, will short circuit the triplen harmonic voltages and cancel them out. .

Only the primary side of the transformer needs be connected.

When used as a supply to a third harmonic generating device, it will filter the harmonic currents from the system.

Many large power system transformers have an enclosed Delta winding on the core just for this purpose. Often the leads are not even brought out. It makes an excellent filter. Of course it will not do much for single phase.

I agree with you, but when you have non linear single phase loads, like PC's and the UPS supplying them, it´s almost certain the transformer has a Delta-Wye or a Wye-Wye connection, and in this case, on the neutral wire the triplen current harmonics, from the 3 phases will add, even though if there is a delta connection on the primary the phase currents won´t have triplen harmonics.

You are right about the triplen harmonic voltages cancellation, nevertheless in the wye connection, at the secondary there will be harmonics currents, and no matter the delta connection on the primary gives a closed loop for triplen current harmonics circulation, they will increase the losses (all: eddy currents, hysteresis, and copper), thereby heating the transformer. Things like these, sometimes make compulsory the use of a K rated transformer, and it´s funny that the american and the european methods to estimate this rating are very different.

Thanks for your comments.

Mister.. Sounds like we are on the same page.. You seem to have a good handle on it. I was not aware of the European standards being different.

I have seen large transformer banks burn up on triplen harmonic currents, the relays did not sense it but, I am also 80 yrs old and haven' t attended any IEEE functions in 15 years.

I am, also hearing many new terms like earthing pits etc. Totally different ball game than what I am use to playing.

Snakers

Not really. harmonics are usually a very complicated subject and can be caused by a vast variety of connected loads. VFD's, UPS.s, Computers, and the list goes on. You need to log your power for a few days with a good logger. Log V. A. kW kVA kVAr THDVolts THDAmps and the individual Harmonics If you can track the problem to a single source then you are in Luck. If not and the problem is serious then a harmonic filter is probably the only solution.