Calculating transformer size for zig-zag configurations (artificial neutral for HRG)

[MechEdetour]

I don't trust my simulation to give you a solid answer. I don't have proper numbers for 480V source impedance, zig-zag transformer coil inductance or leakage inductance, or transformer iron saturation. If you want to do a full engineering analysis, I'm happy to share what I did to give you a starting point.

In my model, if I have a 56 ohm connection between 1 phase and the derived neutral, I get about 4.9A RMS through the 56 ohm resistance. If I change the transformers to 4:1 turns ratio, I get 3A of current through the same 56 ohm resistance. A smaller change than I expected, but the 56 ohm grounding resistance is pretty big.

If I use a 5.6 ohm grounding resistance, the corresponding numbers are 48.2A and 3.9A. So the impedance of the balanced zig-zag is below 1 ohm, and the impedance of the one built with 4:1 transformers is in the realm of 50 ohms. I've not calculated the values precisely, because as I said the parameters of the transformers themselves are all wrong. This is just to give you an idea of the magnitude of the effect.

Thanks for doing this - it is helpful. I think I have enough info to pester some people on my end and get a more detailed analysis. I'll circle back around if I come up with anything worth sharing.

 

[jim dungar]

CPT is Control Power Transformer in this case.

While not ideal, I think you'd be surprised to learn how often this is done (using qty 3 single phase CPTs and wiring them into a zig-zag config).

It depends on the transformer and where it is being employed.

The small control power transformers (CPT), typically used in motor control/industrial panels, have different characteristics than do the voltage/potential transformers (VT/PT) used in switchgear and metering.

 

[MechEdetour]

It depends on the transformer and where it is being employed.

The small control power transformers (CPT), typically used in motor control/industrial panels, have different characteristics than do the voltage/potential transformers (VT/PT) used in switchgear and metering.

In this case they would be Eaton Type MTK 2kVA units aka "Industrial Control Transformers".

 

[jim dungar]

Depending on the size, industrial CPTs often incorporate some compensating windings on their primary side. These transformers were originally designed to handle the high inrush current resulting from magnetic contactors opening and closing. Their voltage regulation characteristics are base on the requirements of magnetic starter coils for pickup and low voltage dropout.

For decades they have successfully been applied as open style transformers for general purpose loads. They have been known to not perform satisfactorily in metering applications, particularly when short circuits are involved.

 

[MechEdetour]

@jim dungar The sole premise here is to create a neutral point for HRG applications. No metering of any kind. I could be wrong, but I think in most cases it's easier to defer to 3 individual CPTs instead of a single 3-phase zig-zag transformer due to availability and size. CPTs can be had in many sizes at the lower end. So long as they can carry the low fault current of HRG applications, they're likely "good enough".

 

[jim dungar]

@jim dungar The sole premise here is to create a neutral point for HRG applications. No metering of any kind. I could be wrong, but I think in most cases it's easier to defer to 3 individual CPTs instead of a single 3-phase zig-zag transformer due to availability and size. CPTs can be had in many sizes at the lower end. So long as they can carry the low fault current of HRG applications, they're likely "good enough".

You could also use (3) individual power transformers.

A single transformer often reduces field wiring costs and installation errors for those installers not familiar with zig-zag. As a consulting engineer i never had control over the skill set of the installer. Specifying a single 3-phase unit was cheap insurance versus fixing an error during commissioning.

 

[MechEdetour]

You could also use (3) individual power transformers.

A single transformer often reduces field wiring costs and installation errors for those installers not familiar with zig-zag. As a consulting engineer i never had control over the skill set of the installer. Specifying a single 3-phase unit was cheap insurance versus fixing an error during commissioning.

Good point. Wonder how availability stacks up against individual units though. Definitely something worth looking into.