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Topic Title: Transformer Auto-Close Scheme
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Created On: 10 November 2012 01:54 PM
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 10 November 2012 01:54 PM
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Posts: 14
Joined: 13 February 2003

Greetings All,
In our 220/33kV substation, the 33kV bus bars is fed from 2 No. 220/33kV transformers of different ratings/parameters, running independent with 33kV bus section breakers between the transformers in open condition. It is required by our regulator to design an automatic scheme such that in case of a transformer fault, the respective transformer breaker opens while the bus section breaker closes to allow continuously supply to the bus bar fed previously through the faulted transformer. However we have a concern that as the loads connected is mainly inductive type, closing bus section would cause the load feeders to trip on account of inrush. We think that it may be required to trip all load feeders (on the bus bar where transformer is faulted) before closing the bus section and close them one by one to limit the amount of inrush, after closing the bus section.
Have you used such scheme in any of your utilities? What is the period/time considered for the total scheme? Please advise any other technical issues to be considered in designing and installing such scheme.
 10 November 2012 04:06 PM
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Joined: 09 September 2002

Before embarking on a complex switching scheme, it may be worth reviewing the possibility of running the transformers with the bus section closed. "Smart" ACV schemes such as can be achieved using SuperTAPP & MicroTAPP AVC relays can allow mismatched transformers to operate satisfactorily in parallel at realtively modest cost.

If you wish to pursue this approach, I'd suggest you contact Fundamentals Ltd for thei assessment. They were the original developers of the ***TAPP range (even though some models are sold through Siemens).

Otherwise, it would be useful to assess the inrush on each of the load feeder circuits by either (or both) calculation or measurement to determine both the likliehood of trip and also extent of voltage dip that would be experienced on the healthy bus. As the loads connected to the faulted side will have been interrupted anyway, then delaying the re-energisation of these for a few seconds during a staggered reclosure is unlikely to cause significant upset to those customers.

 17 November 2012 06:42 PM
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I have seen schemes where a timer is incorporated in the closing circuit so that the breakers close with slight time delays.
Also on many wind farms if the 33kV bar gets shut down all the string feeders trip and are then brought back on in a controlled manner by the control room via SCADA.
 23 November 2012 09:36 PM
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In my experience a problem with such auto sequential closing schemes is that the re-closing scheme fails to work or stops mid sequence for all sorts of unanticipated reasons.

Your design must take account that you will be asking the re-closing scheme and all its sensors and software to operate without fault during arduos conditions where a serious fault has just occurred, also you must take every precaution to prevent the scheme "thinking" it detected a fault where non exists.

If you can avoid do so.

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