snubber design guide for 36kv transformers

[rajbhanu]

I am looking for a procedure to calculate the values of R and C in a RC snubber circuit for 38kV transformer. I don't have the appropriate textbooks to refer to at the moment and would highly appreciate any help/suggestion regarding the design procedure.

 

[7anoter4]

What is the transformer rated power [S-kVA]and the rated
short-cicuit impedance[%]?

 

[rajbhanu]

Thanks 7anoter4.

The transformer ratings are:
38kV insulation/26.4kV-587V/150kV BIL
Impedance: 7.2% on 1/2 KVA base

It has been provided with a station class Surge arrestor rated for 36kV.

I am looking to calculate the values of R and C for the snubber.

Thanks.

 

[7anoter4]

If I well understood your query you want to build a BIL Tester that means a PD [Partial Discharge] testing equipment. To meet a given BIL rating a device must be able to withstand a simulated lightning wave that has a wave shape of 1.2 by 50 µsec and in both positive and negative polarities. The wave reaches its peak in 1.2 µsec and decays to ½ of the peak value in 50 µsec.
The problem is not only the snubber but also the measurement of PD.
You may follow IEC 60270 proceeding .See:

http://webstore.iec.ch/preview/info_iec60270{ed3.0}en_d.pdf

or "Snubber circuit calculation"

http://www.hagtech.com/pdf/snubber.pdf

or "DESIGN OF SNUBBERS FOR POWER CIRCUITS"

http://www.cde.com/tech/design.pdf

But I think what do you need it is a PD detector like this [for instance]

http://www.cde.com/tech/design.pdf

 

[rajbhanu]

Thanks for your help and document references.
I am actually looking to design a RC snubber circuit as shown in figure 2 of the attachment file here. The transformer in my case is rated for 2MW.

I am really uncertain about the design criteria as to the variables I should consider for the design.

Thanks

 

[rajbhanu]

I am sorry...the attachment file is here

 

[prc]

rajbhanu,Why do you want to invent wheel?This problem with VCB is well known to power engineers from 70's.Proven solutions are alreday there. You may contact VCB makers like ABB, Siemens for guidance. They will give you all parameters.

 

[7anoter4]

First of all I agree with prc. My experience with ABB Switchgear [with VCB] where no other surge circuit was provided for a dry type transformer [up to 7.2 kV] except a ZnO arrester. For a motor, a surge capacitor was accommodated in the switchgear. If the motor terminal box dimensions are as per NEMA MG-1 ch.20 then the arrester and parallel connected capacitor may be introduced in there.
See, for instance, Toshiba recommendations for Vacuum Circuit Breaker (VCB) Application for Surge.

http://www3.toshiba.co.jp/sic/english/swgr/info/tech/vcb/02.htm

or Cutler-Hammer:

http://www.farah.com.jo/up_to_date_files/2.PDF

A combination of arrester and RC circuit named ZORC is recommended for dry transformer[less then 2 MVA] at voltage of 36 kV. [See TABLE VII: Transformer Protection on page 23]
Also this link may be useful:

http://www.simotech.co.za/downloads/strike/zorc.pdf

 

[rajbhanu]

I very much appreciate your suggestions. Thank you guys.

But my position in this snubber circuit is such that I need to find some documents which explicitly states some design techniques, some formula, expression etc for calculating the resistor and capacitor values of a RC snubber which my boss can point out in the bidding document for contractors.

I went through with the documentation in abb website but havenot been able to find out something discrete as my boss desires.

Thank you prc and thank you 7anoter4.