A large number of field FRT tests are already carried out with the test setup "Transformer cut in". As part of the OptiTransient research project, experiments have been carried out as an example on a grid forming inverter and compared with other possible test facilities. In addition to the usual amplitude changes or vector angles jump by coupling a length impedance the angle jump were also generated by the triangular switching of the coils. At no load, the voltage have a clean characteristics. Under load, the impedances influence the test subject.

Especially with simulation of transformer switching groups B and D without a supplementary Dy transformer, asymmetrical impedances and thus asymmetrical feed -in occur. Converters are particularly sensitive to unsymmetrical impedances.

Until now, steps-tests for "Transformer Cut In FRT-Tester" can not be feasible as a limit. Analyzes from the OptiTransient research project show that real grid drops often followed by an over voltage event after grid recovery. Step tests can realized with additional impedances which limited circular current.

The tests will be challenging if no further additional impedances are to be used from financial and practical considerations. Such tests can be carried out in the limited area with previously excluded turning closure.

Results will be shown to the various test facilities. The respective limits of the test facilities will also be discussed.

For the "Auto Transformer Cut-in", the integrated coil requires a special equivalent circuit diagram of the test device.

The mathematical relationships should be discussed and compared with the measurements.