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Thoughts on Temporarily Paralleling Two 1600A ACBs with a Bypass Castell Key?

AMK

Hi everyone,

Looking for some insights on a proposed electrical setup and potential risks. We have a single LV switchboard with two 1600 Amp ACBs, each fed by separate transformers but connected to the same HV ring main. Both ACBs are equipped with Castell keys for interlocking. Normally, if one transformer fails, the procedure is to switch off its ACB, remove the Castell key, and close a bus coupler to maintain supply.

We need to perform some HV maintenance, which requires isolating each transformer one at a time. To avoid any supply interruption, there’s a suggestion to use a bypass Castell key to temporarily parallel the two supplies for a few seconds. The idea is to parallel the transformers, then switch off one ACB, keeping the load live throughout.

Has anyone done something similar or seen issues with this approach? Keen to hear your thoughts and experiences!

 Thanks -Andy

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    Paralleling identical transformers is done all the time, as is the related problem synchronizing generation to feed back into the supply. However, normally only when designed with this in mind from the outset, and the makers have supplied the instructions,

    Any difference in the two voltages will lead to serious circulating current -potentially both on the LV and HV side. Accurate measurements of the difference in secondary voltage and knowledge of the geometry of interconnecting cables and busbars, and so their resistance, will allow you to estimate how much one transformer will push back into the other.

    During the parallel period, what is the PSSC, and if a fault occurred, would anything be subjected to stresses out of spec?

    NE bonds, where are they and could switching like this introduce a floating condition where a transformer secondary loses its earth reference, while the primary remains alive? This is not good as the secondary tends to drift up to a good fraction of the HV voltage..

    I'd be more than a bit nervous without some top cover from someone who knows the design intimately.

    Mike.

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  • 0

    Paralleling identical transformers is done all the time, as is the related problem synchronizing generation to feed back into the supply. However, normally only when designed with this in mind from the outset, and the makers have supplied the instructions,

    Any difference in the two voltages will lead to serious circulating current -potentially both on the LV and HV side. Accurate measurements of the difference in secondary voltage and knowledge of the geometry of interconnecting cables and busbars, and so their resistance, will allow you to estimate how much one transformer will push back into the other.

    During the parallel period, what is the PSSC, and if a fault occurred, would anything be subjected to stresses out of spec?

    NE bonds, where are they and could switching like this introduce a floating condition where a transformer secondary loses its earth reference, while the primary remains alive? This is not good as the secondary tends to drift up to a good fraction of the HV voltage..

    I'd be more than a bit nervous without some top cover from someone who knows the design intimately.

    Mike.

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