Use of Neutral in a Auto_Transformer

[22574]

Any one can explain to me what is the use of Neutral in a autotransfomer? I have an Example: 200MVA,Single-phase Autotransformer with tertiary, Y-connected. 500KV-primary,230kV-secondary and 13.8kV.(a),(b).
My another question is: if the neutral-wire going to diverter switch will cut off, happen to the system if
this is energize?

Hope your reply.

 

[ScottyUK]

The neutral is common to both primary and secondary circuits. The primary tap is at 500kV: call this 100%. The 230kV tap on the same winding as the primary tap, but at (230/500) = 46% of the winding. Both taps are relative to the neutral point. On this voltage class there is a strong possiblity that graded insulation is used which allows lighter insulation to be used at the earthy end of the winding, reducing the unproductive space occupied by the insulation and reducing the size of the core needed. If the neutral is disconnected then there is a good chance of insulation failure if graded insulation has been used.

Why the question? This sounds a strange condition to even consider running a transformer in.


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I don't suffer from insanity. I enjoy it...

 

[cuky2000]

Probably you are referring to three single-phase autotransformers connected in Y-Y-D grounded configuration. The ground for the Y connection is shared by both the primary and secondary windings and externally is connected to the neutral bushing that sometime is labeled as Ho/Xo.

I am not sure if I understood your second questions. In general, neutral connection helps to stabilize the system voltage to the reference ground. During transient condition including inrush during the energization of the transformer, the difference in zero sequence impedance of the grounded versus ungrounded system may play some effect in dampening the transient particularly in the event of energizing a faulty autotransformer.

From the protection standpoint of an autotransformer, both the zero- and negative-sequence ground relay sensitivity needs to be set appropriately to clear the inrush during the energization process. Some protection philosophy suggests the setting of the ground relay well above the sequence current expected from unfaulted operations.

 

[22574]

Thank you very much for your reply.
Actually this scenario was actually happen.

3-sets of 200MVA single-phase in a bank to form a three phase, one of the 3-units suffer this kind of problem.

Almost three weeks of monitoring, 2-3 alarms of Buchols is happening everyday. DGA activities specially ethane, methane slowly
rise-up until last week they have finally look up inside.
They found out that the neutral wire strand are alomst cut-off but the outer-insulation was intact only it change color to like burnt.
If I can sent pictures its better so you can view it.

 

[RalphChristie]

Pictures will be very helpful, but also instructive, as a lot of us can learn from it.

See faq238-1161 (This how to post pictures in your post) written by itsmoked.

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Read the Site Policies at FAQ731-376

 

[22574]

Here is the link to view the picture.

eduardo

 

[waross]

Hi 22574
In answer to your original question, in this context, the neutral is one of the main connections to the transformer winding. One end of the transformer winding may be called the "Hot" and is connected to the high voltage. The other end of the winding is connected to the neutral point of the circuit and may be referred to as the neutral. In this case it will carry the full load current of the transformer, and given the amount of visible damage I am surprised that it hasn't gone completely open with major arcing and damage.
"Just lucky I guess"
respectfully

 

[22574]

Dear WARoss,
Thank you for your explanation too.
If you look at the picture, that connection was the neutral bushing and the two leads going down are from 230KV and from diverter switch. That wire going to the diverter switch was almost cutoff, only the tape insulation carried it, so you can see it still connected.
..