Problem with Stamford generator too high voltage

Hello

I have problem with Stamford Generator Type:A07D786842, maker: Cummins generator technologies Norway AS.

During normal operation Voltage on this generator suddenly rising up from 440 V to aprox. 620 V, frequency is the same what was before problem is 60 Hz. Voltage regulator type is Stamford MX 321 AVR

We start test procedure of voltage regulator as per manual instruction

Cable connections on AVR and on generators was checked and found everything in order. Also we checked resistance and voltage between terminals on AVR and we found as follows:

Resistance between P2, P3, P4 terminals:

P2 – P3 3.07 Ohm

P3 – P4 3.05 Ohm

P2 – P4 3.02 Ohm

Voltage between this terminals:

P2-P3 201 V

P3-P4 202 V

P2-P4 202 V

And according to manual instruction should be 200 to 216 VAC and this is in range.

Exciter stator resistance, between terminals:

X – XX 19.45 Ohm - according instructions should be in range 18 – 30 Ohm and this is OK.

Resistance and voltage between6, 7, 8 terminals on AVR:

6 – 7 9.08 Ohm and 0.9 to 1.2 VAC this is not correct.

6 – 8 105 Ohm and 320 VAC this is not correct.

8 – 7 96 Ohm and 320 VAC this is not correct.

According instruction should be 170 to 250 VAC

We disconnected the cables between Isolating transformer and AVR and the same value of resistance is directly on transformer terminals.

Resistance between the others terminals:

E0 – E1 96 Ohm

K2 – K1 180 M Ohm

I think that isolating transformer is out off order but I'm not sure for 100%, Could You help me solve this problem?

Thanks in advance

XWS2