Solid Earthed Neutral of Delta Wye(Star) Transformer

[Ntneric]

I am working in SE Asia to provide 1500kVA 11kV/380V step down delta wye(star) transformers as part of the electrical distribution.
Earthed neutral in the secondary side of Tx is required as a code requirement.
There is a debate about whether it is necessary to provide PVC sheath conductor or just bare conductor for the earthed neutral (i.e. from the star point to the earth electrode)
My view is that the star point should be zero current and voltage under normal condition. The use of Bare conductor (for example tinned cu. tape)should have no problem.
However, when there is say earth fault, how can we provide proper earth fault setting so that automatic disconnection can be provided and the person who get in touch with the bare conductor will not get fatal electric shock.
Any advice? or a PVC sheath conductor should be adopted?

 

[cranky108]

By zero current on the neutral, you are assuming there would be no single phase load.

What about physical protection of the ground lead? What would happen if it were to become disconnected?

Ground leads can become damaged, or stolen, and if that is a possibility, it needs to be protected.
Another thought is to paint or cover it so it does not look like copper.

 

[waross]

the person who get in touch with the bare conductor will not get fatal electric shock.

That is a good reason for an insulated conductor.

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

 

[prc]

In case of large transformers in substations, this neutral is formed by copper or steel strips supported from tank by post insulators before being connected to station earth grid. It is never insulated and if safety is the issue, this can be an issue in substations also.

 

[cranky108]

'never insulated' I would say, almost never insulated. There are a very few exceptions.

We still have a ungrounded Y transformer feeding a delta system in a location where we can't get a good ground (rock).
The ungrounded X0 bushing is used for neutral shift detection.

So for the few applications that are ungrounded, they are special, And I don't recommend it unless it is well thought out.

In fact in some countries there 240V to homes is ungrounded, but that has some interesting problems.

 

[Ntneric]

I have seen both bare and insulated. How to justify?
The netural point will be directly earthed via cu. tape to the earth electrode to from a TT earthing system in my case.
Earth Fault Protection will be provided. Is it correct to assume that if prospective earth fault can trigger the earth fault protection in a very short period then it can be justified?

 

[jghrist]

The voltage drop across the ground conductor during a heavy fault current would not be fatal. The impedance of 10 feet of #4/0 cable is 0.00062 ohms. If the fault current was 20 kA, the voltage drop would only be 12 volts. Where does the "fatal electric shock" come from?

 

[Ntneric]

Good and very convincing point. Thanks jghrist.

 

[waross]

As I understand the OP this is not a substation but a distribution installation.
In my part of the country, the ground wire on a pole mounted distribution transformer may be as small as #6 AWG.
It is generally insulated.


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

 

[cranky108]

From my understanding, the electrical panel at the service entrance is also to be grounded.

It is doubtful that the two will be in reach of each other.

 

[TestMonkey]

Neutral to ground bonding can effect the proper functioning of ground fault protection schemes. Improper placement can create objectionable current or bypass the protection CT's.

Also you may require an additional neutral to case bond for the transformer.

A good review of NEC Article 250 could yield you the proper answers.

 

[cranky108]

I don't think SE Asia follows NEC, and utilities also don't follow NEC. Most utilities in the US follow NESC.

 

[MatthewDB]

If the fault current was 20 kA, the voltage drop would only be 12 volts. Where does the "fatal electric shock" come from?

You're ignoring ground electrode resistance. You can easily get nearly full line voltage on the ground electrode conductor in open neutral conditions with TN-C (combined earth/nuetral).