How to Size Kva for auto zig zag transformer to create a neutral

[kowan7]

Hello I am a Electrical designer in HoustonTx. I purchased 5 Bard units model (WA70A2-F07BPXXXJ) @ 50Hz, 400Vac andthese units require a neutral wire. My current power scheme, I only have 3phase 400vac without a neutral. Original intention was to run a ground wirefrom the ground bar loop in the enclosure to the neutral in the HVAC, and thisworks but Mike pointed out that this violates NEC 250.6, after reading thearticle I see his point. Spoke with a bard sales representative thatrecommended using auto Zig Zag grounding transformers to create a neutral. Howdo I size these three single phase transformers? Is it by the size of the largestconductor or something else? Each HVAC will have operating current of 21A, canI use the neutral created by the auto zig-zag grounding transformer on all 5units, and if so how do I size the transformers.

 

[winnie]

How is your existing 50Hz 400V being supplied? This is important because you need to determine if your existing supply equipment has a neutral which is simply not brought out, or if you have a true separately derived system that does not have a neutral, or if you have a _non-isolated_ system where you already have a ground-neutral reference from the upstream supply.

In general, you would have a _single_ grounding autotransformer on a separately derived distribution system, to supply _all_ loads on that system. My (weak) understanding is that this autotransformer only has to be sized to supply the maximum possible neutral load. Protection requirements make this more complicated: if you have a small grounding autotransformer then you need some method to trip the entire system out if the autotransformer is overloaded. So it might be simpler/cheaper to have a 'full size' grounding autotransformer.

-Jon

 

[kowan7]

incoming power is coming from a 2kv transformer with 3 phase and neutral coming into 480v Switchgear, with a Neutral grounding resistor tied to ground. There is a breaker associated to low voltage switchgear that feed a 400vac panel Board that is only 3 phase 400vac.

kowan

 

[kowan7]

this panel board is then used to feed the 5 HVAc units in the enclosure.

 

[winnie]

480V with a grounding resistor means a resistance grounded system, which is not permitted to serve line to neutral loads.

But going from the 480V switchgear to the 400V 50Hz panel means that somewhere there is apparatus which is changing both voltage and frequency. The specifications of this conversion apparatus will determine if you can even use a grounding transformer.

-Jon

 

[GoldDigger]

It is also possible that the OP intends to drive the 400V 50Hz motors directly from 480V 60Hz, preserving the V/f ratio. (Trusting/hoping that the load in the compressor motor will not increase with speed.)