NEPTUNE CONNECTION 4

[elenapredescu]

I have to make a project for 150 MVA 220/115 kV autotransformer and the customer ask forrotection of the tertiary windings from overvoltage stresses by means of "NEPTUNE connection" of suitably design surge arresters.-What does it means? How I can solve the requirements?

 

[ScottyUK]

I am going to guess that this is a reference to the shape of the trident carried by the mythological god Neptune, which would suggest that the arrestors are connected in star with the common point connected to earth. But that is only a guess - it is not a term I have encountered before. What country does the phrase originate from?


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If we learn from our mistakes I'm getting a great education!

 

[elenapredescu]

Thank you.
The project is for Kosovo.

 

[slavag]

Hi elenapredescu
I don't know what is Neptune connection.
It's standard practic for not-used tertiary windings of AT or trafo.
two possibilites.
1. Surge arresters on every phase, as Scotty wrote.
2. Grounding one of phase via reactor.

In first, don't forgot add VT with broken delta connection for 59N protecion of not-used tertiary windings of AT.

Best Regards.
Slava

 

[elenapredescu]

Hi,
thank you for helping me. The tertiary windings, in this project, is required only for suppression of harmonics. The two terminals are brought out. The single protection that I used is CT for 50N (technical spec ask for).I hope I'll solve the problem after I'll disscus with the customer in case they will be more specific.

 

[kalbur]

For Neptun sheme you can check following link:

http://library.abb.com/global/scot/...BB_Applicationguide PTHASA3021EN_01.05.09.pdf

page 40

I can suggest you to ask the transformer's produser or to read the full trafo's documentation about the protection scheme. Some transformers don't need of neptun scheme, and can be used phase to GND arestors. Also keep in mind that there is some old transformers (prodused 20-25 years ago and over)whitch needs a capacitors connected to the tertiary windinig.

 

[prc]

Normally arresters are provided on stabilising tertiary terminals of transformers only when the transferred surge from HV or MV exceeds the BIL of the tertiary bushings.In such a case, if arresters are not provided, the bushings will flash over causing a LLG fault on tertiary windings.To avoid this, best way is to use bushings of higher BIL than winding.In India for EHV auto transformers 36 KV bushings are provided for 11 KV windings and 52 KV bushings for 33 KV windings.

Another alternative is to provide surge capacitors between tertiary terminals and earth which will bring down the transferred surge level. In Europe and sometimes in India too,4 arresters are provided in Neptune connection.( 3 arreseters in star with the fourth bridging the star point to earth)This way phase to phase and phase to earth protection is achieved.Each arrester rating shall be more than 0.667 times the maximum phase to phase system voltage as explained in ABB manual.

But in the case proposed, there is no need for any arreseter as one corner of delta only is brought out and normally this will be kept earthed.But in case this terminal is kept floating then arrester between terminal and earth will be required to avoid flash over.

There is one more scenario.Some utility may keep the corner in open condition with one of the terminal earthed and other terminal floating.Then stabilising effect will be no more.Such a case arresterw ill be required on floating terminal if higher BIL bushing is not provided.