Voltage problems of Eza(BGA) to the medium voltage grid

the following facts:

It is a bio-gas plant Cable jointing Which one feeds via a transformer 20kV/400V in the medium voltage network.
The "normal" power now as usual, is increased.(approx. 21 kV L-L). Now it comes to tripping of the EZA-NS-
Circuit breaker by the Spannungssteigerungsschutz(U>Schutz). The U > protection measures MS side.

The setting values of the protection relays (U > protection) 1.1 UC are-60 s and 1.15 UC 0, 5 s.
The transformer to stufbare Get up position, i.e. 20kV/400V. There are 5 levels to select +/-2 x 2, 5%, so
21/20.5/20/19.5/19kV / / 400 V.

What are the ways to counteract the increase in voltage?

My ideas:

1 transformer to promote? You could switch the transformer on 21kV/400V.This would result in, but then that when feed-in operation
the "generated 400V" via the transformer on 21kV be transformed, which in turn the mains voltage higher
increases? If they are deemed the transformer on E.g. 19kV/400V, the NS-side tension when one would be vice versa
Mains voltage of 21 kV too high. As a result the EZA protection would then trigger that would bzw.der generator in order to
Surge not boot up.

2. reactive power feed? The network is inductive. If you the generator now "inductive"(=induktiver wie momentan)
feed is the mains voltage should now actually decrease or?

Can someone help me?

About advice, comments, tips, and also corrections of my "ideas", I would Very much appreciate you.

I wish you kind regards and Happy Easter