Electronics > Projects, Designs, and Technical Stuff
3 phase Power filtering with unbalanced loads?
max_torque:
I have a device that takes a 3phase 400VAC 16A input, and drives a range of "load" devices. The big loads primarily resistive (heating elements) but a few are inductive as they are small (250W) AC induction motors. Those loads are not all on at any given moment, and are controlled by logic, so any given load can be on at any given moment and viseversa. The Loads are between each line and neutral, and have been allocated to be as evenly spread as possible, ie the 3 big heaters are across L1->N, L2->N and L3->N. If all loads were on, then there would only be a small neutral current due to that balancing, but if the loads are unbalanced, then the neutral current will be around 16A.
Ok, so that's the setup, the question resides around what sort of AC power filter architecture i need to provide, primarily to prevent external noise coming into this unit and messing things up (loads are Thyristor controlled and therefore to some degree potentially noise sensitive). The unit is going to be used in a high noise environment with regard to the AC supply (there can be up to 2MW of high performance prototype inverters / motor controllers running in the same room....)
I think:
1) The loads are unbalanced so that rules out a common mode type filter arrangement for the 3 phases
2) Neutral must be considered a power carrying conductor in this application
Schaffner sell what looks to be a suitable power filter for 3Ph + N asymmetric loads
http://www.farnell.com/datasheets/2188999.pdf
Options include simply buying that (expensive) filter device and including it inside our unit, or desiging our own power filter (we can reverse engineer the Schaffner part...... :-DD ) to include on the power switching pcb that our unit already requires
Comments anyone? :-/O
MagicSmoker:
Most 3-phase filter chokes (aka "reactors") are wound on a single E-core so are coupled by default. This gives very good performance both for common mode noise current, as well as *balanced* normal mode load currents, but it will not work very well, and might even saturate, if asked to supply unbalanced load currents returned to neutral. So, you'll likely have to use individual chokes for your filters, and because of the phase-angle control of the heating elements these will need to be Tee filters, rather than Pi or LC.
EDIT - pretty much exactly like the Schaffner filter which I checked *after* writing the above...
David Hess:
That Schaffner filter only implements common and differential mode filtering and does nothing for load balancing.
Wouldn't two Scott-T transformers connected back to back and then in parallel serve to balance a 3 phase load?
NiHaoMike:
Wouldn't it be easier to add snubbers to the thyristors in order to increase immunity to spikes? Put in provision for chokes on the board but try putting in wire links at first.
T3sl4co1l:
How are the loads unbalanced, are you sinking them to neutral or to ground?
Even if you were, that just means using a 4 or 5 line CMC. ;)
If you need diff mode filtering as well, yeah, just use single inductors, of whatever size and rating you can get. Be sure to get saturation current higher than peak current draw -- which is 1.4 * Irms for sinusoidal loads only, and can be significantly higher for pulsed loads (rectifier + cap input filter).
Nice thing about 3ph is you can save a lot of ground leakage current by canceling it out. Typically, 'X' type caps are wired in a wye configuration, and the "neutral" node can be 'Y'-capped to ground with fairly aggressive values (10nF+). The ground current goes up considerably if one phase drops, of course.
Tim
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