Delta Wye transformer X0 connections

Status
Not open for further replies.
M

merc1136

Member
Location
Detroit, Michigan, USA
Controls engineer in automation. Long time lurker, first time poster. Working with delta wye isolation transformers, size doesn't matter for my question but I will tell you any way to paint a picture, 480V primary 240V secondary, 51kVA. I am making all my ground and bonding connections inside the transformer. I place a ground bar inside the transformer enclosure, firmly attached to the side wall. Incoming ground (EGC) goes to ground bar. Outgoing ground (SSBJ) goes to ground bar. System bonding jumper goes from X0 to ground bar. So far so good. Last connection, GEC, is supposed to go from X0 to closest (in my case) grounded plant column.

Now, it would be easy to connect the GEC to the ground bar, but the code reads that the connection is to be made to X0. That means X0 needs two connections on it, yet transformer only comes with a single attachment point. I am not an electrician so what I ask is this - do you make the GEC connection at X0 exactly as the code says, or would making it at the common ground bar which is way easier acceptable? Forgive me if this was already covered, I did search, and I'm sure it is so easy it will give any electrician a chuckle, but I don't mind. I'd rather be correct than liable.
 
FYI::: From 250.30, in part:
This (grounding electrode conductor) connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in 250.30(A)(1) is a wire or busbar, it shall be permitted to connect the grounding electrode conductor to the equipment grounding terminal, bar, or bus if the equipment grounding terminal, bar, or bus is of sufcient size for the separately derived system.
 
augie47 said:
FYI::: From 250.30, in part:
This (grounding electrode conductor) connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in 250.30(A)(1) is a wire or busbar, it shall be permitted to connect the grounding electrode conductor to the equipment grounding terminal, bar, or bus if the equipment grounding terminal, bar, or bus is of sufcient size for the separately derived system.
Click to expand...
Thank you! I greatly appreciate your reply!
 
merc1136 said:
Yes, 480 to 240 exactly, it feeds VFD panels that rectify the 240 to 340V DC. It is imperative for noise that X0 be grounded / bonded.
Click to expand...
I seriously doubt that having a ground at the wye point changes your noise situation one bit.

It does however, protect your filtering capacitors and MOVs from exploding.
 
VFD fed motors produce huge common mode currents which are capacitively coupled to the frame of the motor and are trying to get back to the transformer neutral. Bonding the neutral of the wye to all the metal surrounding the motor is essential. The connection to soil, not so much.'
'
-Jon
 
winnie said:
VFD fed motors produce huge common mode currents which are capacitively coupled to the frame of the motor and are trying to get back to the transformer neutral. Bonding the neutral of the wye to all the metal surrounding the motor is essential. The connection to soil, not so much.'
'
-Jon
Click to expand...
We do make sure the PDP/transformer panel, the drive panels, and the motors are all brought to a common ground block. The VFDs are only one part of the system. The panels also include 24V controls, 10-15V serial communications, etc. You need that solidly grounded wye or else problems will start to pop up somewhere in the system.
 
merc1136 said:
You need that solidly grounded wye or else problems will start to pop up somewhere in the system.
Click to expand...

Agreed. I work on instrumented test systems where we have strain gauges on structures with VFD fed motors.

The point I was making was that the critical aspect for such systems is the 'grounding ' in the sense of connection of the wye point to all of the unintentional conductors surrounding the system. Things like the motor frame, building steel, etc.

The connection to soil is pretty irrelevant, but since things like building steel and slab rebar are pretty much in contact with the soil, we end up making the connection to soil anyway.

Imagine you put your equipment in a large enough aircraft. You would absolutely need to bond the wye neutral to the frame of the aircraft, but wouldn't need to drag a ground rod out the back.

Jon
 
merc1136 said:
Yes sir. Custom made to our specifications. We do not use the line to neutral for anything though, just the phase to phase voltage.

View attachment 2558363
Click to expand...
I figured so, when I think of 240 three phase the most common in the US is delta connected, but I got to thinking, you probably had a custom transformer. May be common overseas, and looks to be getting more common over here.
 
Status
Not open for further replies.
Top