I am reading a book about transformers and I am confused by the following paragraph about synchronization check:

When the transformer trips or is taken out of service, the primary and secondary voltages decay because the supply voltage has been disconnected. However, the voltage on the load side of the transformer decays more slowly because of the back EMF induced by induction motors located downstream. Thus, the two sets of windings become out of synch. This occurs at approximately 40% of rated voltage, causing the synchronization check relay to activate and the transformer breaker to trip if it is not already open. This protects the transformer from electrical system upsets or loss of supply that cause a loss of synchronization while the transformer breaker is still closed.

What is the point of the relay opening, if the transformer breaker has already tripped? Also, what do they mean by "40% of rated voltage". Primary side or secondary?

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    \$\begingroup\$ Maybe post a picture to avoid ambiguity and name the book. \$\endgroup\$
    – Andy aka
    Commented Mar 17, 2021 at 9:28
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    \$\begingroup\$ Also posted here : engineering.stackexchange.com/q/41013/10902 \$\endgroup\$
    – Solar Mike
    Commented Mar 17, 2021 at 9:40
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    \$\begingroup\$ Krack - Hi, Posting the same question to different parts of Stack Exchange is strongly discouraged, as it can waste readers' time by duplicating effort in multiple places. At least that duplication has now been identified by another member, but I strongly recommend you to choose one site and ask there. If you don't get the answer you want, then end the topic there, before you step to another site. Always disclose and link to any other places where you have asked the same question. Show your research. Properly reference / link any book or website that you are asking about. Thanks. \$\endgroup\$
    – SamGibson
    Commented Mar 17, 2021 at 10:11
  • \$\begingroup\$ If you search for synchroscope (which is the instrument used to do that) you'll find many fine explanations \$\endgroup\$ Commented Mar 17, 2021 at 15:08

1 Answer 1


Imagine what would happen if the transformer was reconnected while the loads on the secondary were still spinning down and no longer in phase with the grid supply.

Reconnection out of phase would be a Bad Thing : I hope you already understand (or the book explains) why.

So it would make sense for there to be a secondary means of disconnection to prevent that.

Also remember that 40% of rated voltage on one winding necessarily implies 40% (to a good approximation) on the other winding, so the answer to the last question is yes.


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