Wye - Wye Transformer grounding

[SectorZed]

I have a project that requires replacing an AC drive system, which is powered by a 3MVA 11kv//690V wye-wye transformer that is not grounded. The secondary has the neutral point available but it is not connected. I am not comfortable in leaving the system floating and the equipment manufacturer does not recommend this also. I wish to ground the secondary of this transformer using HRG but am concerned about triplen harmonics and any other issues that could arise by doing this with this type of transformer. Also will my ground fault detection circuit work with this arrangement?
Another alternative I am considering is to use a zig-zag earthing transformer with HRG to create my own neutral point, and leave the main transformer neutral unconnected. is this a better option?
Replacing the main transformer with a Dyn11 style is out of the question...
thanks for any thoughts on this issue.....

 

[Power0020]

How the existing system performed with wye-wye ungrounded? The main issue is the overvoltage during phase-to-ground faults on the secondary.

Using an NER or HRG on an auto-transformer is very dangerous as the primary voltages will appear on the secondary terminals during faults occurring on primary side with back feed on secondary, this is a very weird situation but phasor diagrams easily show that.

Solid grounding is a solution, check the harmonics issues with the drive manufacturer. I think you can put a third harmonic filter in the neutral point (this is a bit unusual solution, but I think it works)

See the link:

http://www.mirusinternational.com/downloads/hmt_faq11.pdf

Drive manufacturer should help with that too.

Again, groundng the seconrady with the primary neutral mostly internally connected to the secondary one means that the primary is grounded solidly as well. Check the 11 kV network configuration and relay setting accordingly.

 

[SectorZed]

I am unaware of the transformer primary neutral configuration as it is all internal inside the transformer. only the secondary neutral is brought out to terminals and is currently not connected. there is no tertiary delta winding
the existing system has a 6 pulse thyristor front end powering a DC bus for the AC drives. the new system will be IGBT front end.
all reference material for incoming power and grounding recomendations have a delta/wye transformer configuration. there is no reference to grounding recommendations for a wye-wye transformer....
as this is an unusual situation, I would like to get the grounding right, and understand the implications.

 

[jghrist]

Grounding the secondary of a wye-wye transformer only provides a ground source if the primary system is effectively grounded and the primary wye winding is grounded. What is the source of the transformer primary?

 

[HamburgerHelper]

I think that it depends on the construction of the transformer. If you ground one side of the transformer and it is a core type, the tank will act as a delta tertiary and give you that side grounding through some amount of impedance. Shell type won't act like this.

 

[SectorZed]

I am not aware of the transformer primary source, but it appears to be the 11kV bus for the whole plant. I note that on the single line diagram for the 11kv there is a core balance CT that detects earth fault. This is on the 11kv feed to the transformer.
it is difficult to know the type of the transformer, (Shell or core) but it does look fairly standard, (oil cooled)
So if the primary is not grounded, then grounding the secondary will be ineffective?

 

[waross]

You are concerned with the drive connected to the secondary. Grounding the secondary wye point will give you a grounded system on the secondary.
In some areas it is common to see wye:wye transformer banks of three transformers. Both the primary wye point and the secondary wye point are grounded.
The secondary may be High Resistance Grounded.
The phantom delta effect is a result of three legged cores. Most of the issues with a wye:delta connection may be avoided by floating the primary wye point.

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

 

[prc]

Some time back, I had posted in this forum that neutral of transformer star winding connected to solar inverters is always left floating. This seems identical case,any thing to do with the converter-inverter working ?