Harmonic Suppression
Harmonic Suppression
(OP)
Which would be better choice to suppress harmonics generated by variable speed drives (VSD): phase shift transformers, or traditional 5% line reactors? It looks that two phase shift transformers (one at phase angle 0 and the other at -30) feeding two VSD’s will eliminate the higher order harmonics; however both drives have to be running at the same time. What if only one drive is running? Thanks






RE: Harmonic Suppression
The reactor would certainly be the better way to go. As you've already surmised, using a phase shift will only work if both drives are operating, but also both drives would have to contain equal input currents for them to operate ay their best. Also, even at best case, it won't be eliminating the higher order harmonics.
What a phase shift transformer will do is make the two drives appear, to the upstream source, as a single twelve pulse VFD. Under perfect conditions you would be then only having harmonic currrents of the order n*12 +-1; so that your predomninant harmonics will be the 11th, 13th, 23rd, 25 etc. So it has gotten rid of your lower end harmonics, the 5th, 7th etc.
RE: Harmonic Suppression
RE: Harmonic Suppression
You can run the two phase shifted transformers into the same VSD by using two different rectifier front ends. That way you get the benefits of 12-pulse operation with having to load balance the two VSDs. For 18-pulse operation you need 3 phase shifts and 3 rectifiers.
RE: Harmonic Suppression
timm, if one drive is off then the only thing a phase shifting transformer can do to the harmoic content of the other drive is to phase shift it. It won't effect the amount of harmonics, nor their order.
RE: Harmonic Suppression
If you add another transformer, much of the remainder become divisible by 3. So an 18 pulse drive has 2 (more) transformers to create the two phase shifts, and it has 3 separate bridge rectifiers, but all three are feeding the exact same load, the inverter, so all of the bridges therefore share the load equally.
If you do not have the load being shared EXACTLY in either of these configurations, it doesn't work as well, so doing it with multiple separate drives with transformers can only work if the drives are sharing the load evenly. Doing it without the other drive does nothing at all.
A simple 5% line reactor does absorb some harmonics, but can almost never be enough to mitigate a single drive for meeting IEEE 519 limits. But if the drive is small and the system is large, it may be fine in terms of the effect that drive has on the total harmonics at the PCC. So we cannot tell you if it is better or not, each and every installation must be evaluated on its own merit. The main reason that you see engineers require 18 pulse drives however is because if all of your individual devices within a facility will mitigate themselves to acceptable levels, the cumulative effect is that the levels at the PCC will carry through to being acceptable. So it's kind of a "cheat code" way of engineering a system that you don't know anything about in advance. That cannot be said of just slapping a line reactor ahead of a drive. Might be ok, might not.
"Will work for (the memory of) salami"
RE: Harmonic Suppression
You won't have any reduction if only one of two drives are operating, but maybe the distortion of a single drive is within acceptable limits and you only need suppression if you have two drives running.
RE: Harmonic Suppression
RE: Harmonic Suppression
If you have two separate drives, and use a phase shifting transformer upstream of one of them, to shift the voltage by 30 degrees, it will not effect the current at the input of either of the drives one jot. What it will do is mitigate the harmonic content at the point of common coupling of both the drives; ie at the upstream point.
jraef
The reason you have the zero shift transformer is just to match impedance, otherwise all of the current would try to go through the lower impedance half.
On a single drive with a 12 pulse bridge, sure; but with two individual drives I wouldn't be putting a 0 phase shift transformer in. timmm was talking about the two individual drive scenario, so that's what I responded to.
RE: Harmonic Suppression
RE: Harmonic Suppression
Generally speaking, I find 5% reactors get you down to about 30% current THD and most plants find this acceptable.
At about 20x the cost of a reactor, the MTE Matrix AP filter will get you to 5% current THD. For similar costs, the Mirus Lineator will get you to about 8% current THD. My testing found the TCI H7G filter didn't perform any better than a line reactor.
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
RE: Harmonic Suppression
"Will work for (the memory of) salami"