Neutral to Ground Connections

[VEnjoy]

I have the following system looking for preference on neutral to ground connections and would appreciate your comments.
- 13.8kV system is fed by utility connected to Dyg step down transformer feeding the 480V 3Ph and 1Ph loads.
- The same 13.8 system is connected to a local generator through YgD step down transformer (D on generator side) with 480V generator terminal voltage. No Load on generator.

Preference:
1. Generator neutral (star point LV) connected to unit transformer neutral bus (HV side)then connected to ground bus and ground grid.
2. Separate neutral to ground connection for generator and unit transformer high voltage side.

Thanks.

 

[RRaghunath]

It is practice to have separate and distinct ground rods (in duplicate as a minimum) for generator neutral and the transformer neutral. These ground rods are part of a bigger ground grid that includes horizontal earth conductors and other ground rods (vertical).

 

[rbulsara]

The generator and the transformer are two separate sources and should be grounded individually at the sources. You can certainly bond the two ground buses (at earth potential) together. But removing the transformer should not render the generator ungrounded.

 

[waross]

It is important that both ground systems be connected together. This may also be a code requirement depending on site issues.

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

 

[PHovnanian]

If I understand the system configuration, you have three 'separately derived' systems here. I'm not sure where you are located (for code purposes), but each system in a single occupancy (building, etc.) must be bonded to the grounding system. This includes the neutral of the incomming 13.8kV system, the 480V distribution system and the 480V generator system.

Best practices (and in many locations the code) dictate that this should be a direct connection from each systems' grounded leg to the building grounding system. That would rule out Preference #1, which bonds the generator system to the system ground through the 13.8kV neutral bus.

 

[VEnjoy]

Thank you all for input but it looks there is a conflict between comments made by Waross and others.

The system is in US and NEC requirement for grounding of derived source at single location can be met by separate grounding of 13.8/0.48kV Dyg transformer feeding the load.
The question is related to 13.8kV feeder coming from the generator unit transformer. The 480V generator and step up Dyg unit transformer are located side by side so if they can not have a common ground bus then their ground grid should be isolated.

Additional clarifications is much appreciated.

 

[rbulsara]

There is no conflict between what waross said and others.

All non-current carrying metal surfaces at "ground" (earth) potential has to be bonded together per IEEE recommendations and NEC code as well. You still need to ground each system separately.

 

[rbulsara]

Let me rephrase it, you do not have to ground it separately, but you can have a non-separately derived system (per NEC definition) and you need to treat it accordingly.

 

[a10jp]

Isn't preference 1 and 2 talking about non-seperately and seperately derived system? This is a separate issue from sharing the ground grid between the system.

 

[rbulsara]

I don't think Preference 1 exactly meets the intent of a non-separately derived system per NEC, for one thing he is connecting neutrals of systems of two different voltages and they do not serve the common load at the same voltage.

A non-seprately derived system would the share the neutral of two systems at the same voltage and grounded at one point only, serving some common loads.

Why go to a neutral of 13.8KV system for grounding a 480V generator?

 

[a10jp]

Your're right. I did not read carefully. But a little confusing on Preference 1. Isn't it wye-delta connection for the transformer? Where would you get the neutral on HV?

 

[rbulsara]

a10jp:

OP says Delta is on the gen side and transformer HV side has the neutral (wye). In fact it should be called a step-up transformer.

 

[VEnjoy]

Thanks again for the comments.