harmonics

[mshields]

I have a project where the facility has several 4160 to 480V substations. Down stream of each of these are a multitude of existing 6 pulse drives.

As part of our project we adding drives. Obviously we need to conduct a harmonic study which is inclusive of the old and the new to know whether or not be need filters, active filters, etc. But the question came up:

Will the harmonics on one of the multiple system. i.e. off of one of the 4160V substations effect the other 480V systems which are via a separate substation.

All of the 4160V to 480V transformers are delta wye. Given that this is the case, I believe the answer is NO. We need to look at each separate 480V system relative to the impedance of the transforer(s) feeding into that system but that is all.

Sanity check here. Do you agree?

Thanks,

 

[kingpb]

Mike,

You are correct in that the delta-wye transformer will not pass third harmonic current to the 4.16kV system. However, don't limit your study to the 480V "system". Look at each individual drive and make sure it is not providing harmonics onto the system due to malfunctioning or faulty components. If the harmonics are high, you could also have degradation of the sub-transformers going on.

You also want to try and mitigate the harmonics at the source, not at the bus. Otherwise, as the loads change on the bus, the harmonics will also change. Your Point of Common Coupling (PCC) should be the input side, or line side, to the drive itself. Drive manufacturers are good at using other loads on the bus to help reduce the amount of filters and so forth that they need to supply in order to meet IEEE requirements. But if the loads change, so do your harmonics.

 

[ramsy]

Will the harmonics on one of the multiple system. i.e. off of one of the 4160V substations effect the other 480V systems which are via a separate substation.

If that falls under seperately derived systems (SDS), according to NEC 250.104(A)(4), it seems all wye xfmr neutrals, and harmonic frequencies, would be joined thru the electrode bonding system, piping, building steel, etc..

So, some power conditioning may be needed at each SDS. Liebert makes an interesting case for power conditioners with its Datawave magnetic synthesizer.

 

[boater bill]

Do any of the existing drives, I am assuming VFD's, have input line reactors? My experience is an input line reactor is needed to protect the drive from nusance spikes on the incoming power and keep the harmonics down on the incoming line. Kingpb is correct that as the bus has additional loads the harmonics would change.
It's amazing how much harmonics a UPS adds to the line.

 

[mshields]

3rd harmonics

3rd harmonics

Aren't 3rd harmonics a non-issue with 3 phase drives?

Isn't that why when a trap is used, it's a 5th harmonic trap?

Mike

 

[kingpb]

Aren't 3rd harmonics a non-issue with 3 phase drives?

Isn't that why when a trap is used, it's a 5th harmonic trap?

Mike

The short answer: You are correct!

This is a good little document that sums it all up. http://www.mirusinternational.com/downloads/hmt_faq02.pdf

Not an endorsement, but Mirus makes very good equipment.

 

[ramsy]

Don't forget shaftgrounding.com type systems that dump motor-shaft frequencies directly on the electrode system, bonded to all SDS neutrals.

As I understand a 2002 Rockwell study "Shft_Brngs.pdf", discharge currents up to 30 amps pass directly to case housings from motor shafts @ 5-10 MHz, or without shaft grounding, bearings to shaft @ 50 MHz.

This bearing fluting study measured some of this current discharge as common-mode frequencies (N-G) traveling from shaft to cable shieldings.