HV Delta/Star Transformers (Earthing) 3

[Sanman1966UK]

I am working on a project which has a 5MVA 11000V/6600V Delta Star Transformer. Even although both sides are HV and there is no neutral would i be expected to earth the Star point. If a neutral is required then there will be a further step down transformer shall be provided to a suitable voltage.

If so/not what are the technical reasons behind the decision. Economics? legal?

Does anybody have any simple schematic ar guidance notes?
Regards
Sandy

 

[GamezBeCJ]

Sandy:
If the transformer is furnished with an X0 bushing, then it means it is designed to operate as a grounded star or wye. If the neutral bushing is not provided, then you have a transformer designed for a floating neutral, and you don't have to ground it.
Anyway, you should always ground the transformer tank.
Hope this helps.

Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist

 

[rbulsara]

Refer to following link..

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

 

[Sanman1966UK]

All good posts guys and helpful, but not realy a post for the discussion of professional libabilty or experiance.

As for the record, I am a Building Services Engineer and I am the link between all the component parts of the Electical design team. However I am not a PE specialist and do listen to specialist parts of the design team. However, I do like to learn and provide useful and interesting questions to the PE designer (my director in our head office) and the manufacturers (schnieder). These guys dont always agree. Schnieder say dont bond the star point, and PE designer says bond the star point.

Rather sit as piggy in the middle i thought with a bit of an overview i could provide some useful input. I also front the client meetings and explain the design decisions, sometime decisions which i struggle to understand.

So basicly, i have 2 PE specialists who dont agree, should I hire a 3rd and take a vote on the best design, or should I try to learn a little bit of the basics and try to help two opposites work as a team to reach a common decision.

 

[CJFlatters]

Typically, you could earth the star point through an earthing transformer or earting resistor to limit the fault current to FLA or less.

This is normal practice in the UK on MV systems

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[alehman]

There are technically sound reasons for any of several grounding methods. In the U.S., no code would require any particular solution based strictly on the information you provided. Typically the seconday connections would be insulated for L-L voltage. If so, you could leave it ungrounded. The risk with ungounded systems is high transient voltages if there is any sort of fault. Search this forum for threads on that topic.

You can ground it via an impedance to eliminate the transient voltage risk and have minimal ground fault current. Or you could solidly ground it, which would be necessary if you have L-N loads. Most industrial applications would ground X0, either solidly or via impedance. If there is equipment served at 6.6kV, you should consult suppliers of this equipment for grounding requirements as well. If your downstream transf is Y-Y and is required to supply unbalanced loads or supply ground fault current, you will need to ground X0. This is not an issue if the downstream transf is D-Y.

 

[Bung]

A lot depends on whether or not there are other earthed star points connected to the same voltage level as the star winding on this new transformer. If the protection in other parts of the system is designed assuming only one star point (neutral earthing point) then adding another (effectively a zero phase sequence injection point) can cause problems. Multiple earths can also cause problems with copper telecomms circuits and was at one stage just plain not alloweed in the UK - don't know if that is still the case.

Have both the PE's on this job got all of the same information - they may disagree because each has been told only what he needs to know for hsi part of the job? Has anybody asked the protection engineer?

Bung
Life is non-linear...

 

[rbulsara]

I re-read the question..specalilly the latter part..I seek furhter clarification.

Legal/tehcnical/econinic reason for what?

For grounding the star point as alehman said there are many good reasons which in turn avoids potential financial losses and therefore aviods probable legal implications, depending upon what happens if you do not ground a sytem.

Technically, you can have a ungrouded, grouded or impedance grounded system. Impedance grounded system have costs and some benefits and needs to be evaluated accordingly.

As for not carrying neutral and providing a down stream xfmr, if a need be for a load requiring a neutral..that in fact would be a primarily economic and somewhat technical reason. Why carry neutral if not required...plus most of the loads on MV system will be 3 wire loads such as transformers or motors..If a small loads requires a neutral.(most likely at a different voltage) it is economincal to provide a smaller transforemr to meet that odd requirement..
I can't think of any legal implications with that.

My thinking is that I would have much better chance to defend my design if it is in line with what most people do and if it is in compliance with published (and least deabatable) standards, rather than having one of a kind system.