3 phase autotransformer neutral 4

[Comaqc]

Our company sells and installs low power (max (12 kW induction heating systems to heat various elements in the industrial environment. For instance, there are more than 50000 of our power supplies reliably heating paper machines rolls.
Our systems run on 3 phase power. Normal voltage is 208 Volts. A new customer in Europe is using a 400 to 220 volts auto transformer to power our systems. In his case, the transformer feeds six 8 kW power supplies. The reason for this introduction is that our power supplies regularly destroy themselves when power is interrupted, either by throwing the main disconnect or in one case, by a power failure in the plant.
Because a large number of identical units are running reliably when using a delta/Wye secondary transformer, we suspect the auto transformer is generating abnormally high voltages on its secondary, either at turn ON or turn OFF, The neutral of the auto transformer is not used as a common nor is it grounded. The front end of all our power supplies is a rectifier bridge with 120 µfd capacitor bank on the DC rail. Does someone have expertise with auto transformer (or isolated secondary XFMR) causing large voltage transients? Should the neutral point of the auto transformer be earthed?

 

[NAZ55]

Here are some suggestions

1- may be use different size capacitors

2- add resistive load during the startup.

3- If not already use three phase disconnects.

4- Ground auto-neutral (biggest culprits of ferroresonance are floating wye and delta)

5- As Bill mentioned if you can connect the neutrals of auto and supply, you can then install surge protector between the phase and the neutral.

 

[itsmoked]

Bill I think you are way over the top making this more complex. (Maybe I am also ignorant.)

The OP is talking about your run-of-the-mill heating of rollers in printing presses. Aren't printing press places where you are probably talking under 100kW? These places likely don't have quasi-exotic high impedance grounding systems do they?

I have never in my life come across a facility with a high impedance ground scheme. I certainly understand the use and functionality of them but most places seem to not embrace that unless they are over some large load amount 1MW(??) or have some other exotic requirement.

I think you may be sending the OP into the Valley of Shadows.


Comaqc; The wye point of the autotransformer should be hooked to the system neutral. Not grounded.

That is your only 'general' option.

If you have systems with no available neutrals then you are dorked. You cannot tie the center point to ground. That would be very bad for several reasons.

The alternatives would be:
You would need to come up with additional protection devices to absorb/block/shunt the harmful spikes to prevent them from frying your controls.

Alternatively you can stop using auto transformers and use regular isolation transformers. These can have their secondaries tied in a wye and be legally grounded. (In most cases.) But probably you could just use the secondaries in delta and be done with it.

Third you could make everyone happier and design your system to run on 400V.

Fourth understand waross's open delta scheme using 1/3 less transformers and see if it avoids letting the smoke out of your controllers.


But wait! There's more! You are asking about where to hook the wye point on your test setup. I thought the point of the test setup was to learn how the controllers were being fried, not preventing them from frying.. If you can duplicate a fry job you can then actually devise a prophylactic to prevent further issues in your field units. Don't hook your center point to neutral as you may never see a problem then.






Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

I think we can agree that the Wye point of the Auto Transformer must not be grounded as this would result in significant current through the ground conductor and surely be a code violation.
As Waross pointed out "many people who work in the field on these systems use the terms ground and neutral almost interchangeably" They should receive a slap over the head for that, Ground and Neutral may be at the same potential but if we allow people to confuse the two sooner or later someone will take it literally.
Roy

 

[NAZ55]

roydm,

significant current through the ground conductor? Please explain what you mean by this.

It is my understanding that neutral should be grounded, but only at one point, this is because if there's a fault we do want it to return to the source. Since the supply transformer neutral is connected to the ground, connecting auto's neutral to ground would create multiple paths for neutral current and is therefore a code violation.

 

[roydm]

If you ground the Wye point any out of balance current will pass through the ground. Anyway it's a moot point because grounding the wye point is against the code. If its to be connected to the source Wye point it must be done through the neutral conductor.
Roy

 

[Comaqc]

You experts have lost me on electrical grounding and neutral a while back. However, one thing that I realized last night was that I created my own confusion by calling my High Frequency Inverters "power supplies". It took me all this time to realize that when you were talking about power supply, you were referring to the main plant transformer, which is feeding our auto transformer.

Now, all your messages make a lot more sense.

My plan for testing with an auto transformer totally backfired when the 30 k VA transformer I purchased ended up being Delta connected; no neutral center. I called the supplier who acknowledged his mistake. Then, your comments made me realize that there was no useful testing I could perform here anyway, even with a Wye auto transformer. This is because the standard power distribution (in Canada at least) is from a 550-600 Volts Delta primary to 208 volts Wye secondary. I got no neutral on the power supply transformer to connect the Wye secondary to.

Because I am told that Europe use supply transformers with a Wye secondary, I have now requested that our partner in Germany perform the neutral to neutral connections testing by themselves.

I am quite hopeful that connecting the neutral of the auto transformer which is driving my inverters to the neutral of the power supply transformer will alleviate all our problems.
Thanks to all, René

 

[itsmoked]

There was no reason you couldn't run the tests..

You just get a WYE transformer and NOT hook up the wye point.
Run tests.
See if same problem presents.
Hook the wye point to your plant's neutral.
See if problem stops.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PHovnanian]

Two things: A Wye autotransformer configuration with a floating neutral may produce odd voltages if the input is not switched simultaneously.

Thing #2: You are feeding a nonlinear load from this transformer. As such, it will draw odd harmonic currents. Harmonics in multiples of 3 (3, 9, 15, etc.) behave much like zero sequence currents in that they sum in the neutral. Or circulate in a delta winding. You don't have your neutral connected and, unlike other installations, you don't have a delta. So the harmonics will see a high impedance and produce harmonic voltages.

I'd suggest connecting the autotransformer neutral point back to the system neutral (not ground!)

 

[waross]

I am quite hopeful that connecting the neutral of the auto transformer which is driving my inverters to the neutral of the power supply transformer will alleviate all our problems.
Thanks to all, René

I believe you are correct.

Bill
--------------------
"Why not the best?"
Jimmy Carter