One of the most common reasons for the non-selective tripping of the transformer differential protection are the incorrectly set up adapters.

In order to accommodate for different voltage levels, different CT turn ratios and possible phase shift due to transformer vector groups, all transformer differential protection relays in PowerFactory come with an appropriate Adapter.

- The nominal voltage corresponds to the nominal voltage of the terminal in which the CT is located at,

- Current transformer ratio corresponds to the measuring CT ratio and not the power transformer one,

- Current transformer connection corresponds to the connection of the measuring CT and not the power transformer one,

- Phase rotation corresponds to the vector group of the transformer and only the phase-shift in degrees needs to be entered, observed from the primary side.

If the adapters are not set or set incorrectly, then the relay will not be able to properly and accurately find differential and stabilising currents, leading to faulty operation and instability.

For the example on how to set up the transformer differential protection, please have a look at the following FAQ:

https://www.digsilent.de/en/faq-reader-powerfactory/do-you-have-an-example-on-transformer-differential-protection.html