VAR Control on Siemens SGT5-4000F

In my current plant is with 3xSGT5-4000F. Generator is: SGen5-2000H, 22kV, 431MVA, 0.9pf.

The -VAR limit mentioned on Reactive Capability Curve is -205MVAR. We were trying to show client this value and keeping -205MVAR for 1 hour and observe rotor, stator & cold gas temperature.

Since exporting and importing VAR is depended on Grid capacity
we followed these steps

1.we put AVR control in Manual and manually adjusting the voltage rise and lower(22kV+/- 7.5%), which in turn increases and decreases VAR.

2.First we tried to decrease the voltage, step by step and we could only reach -160VAR. We conclude this is because of the Gird limitation.

3. Then we run our second Generator with same capability curve. Now 2xSGT5-4000F is running and connected to the same HV bus. (practically paralleled)

4. Then we step wise increased the voltage in one generator and step wise decreased in other. Now one generator is supplying +260VAR and the another Generator is absorbing -186MAR.

5. Our intention was to produce all +VAR required for the other machine to touch -205 (as in RCC).

6. But even one generator is producing all the VAR required (> +260MVAR) to absorb by another; the other generator could not reach -205MVAR. We got stopped by 'under excitation limit' block form excitation controller. and we could not reach the -7.5% of voltage also. The lower voltage limit is 20.3kV, but we only reached 20.9kV.

7. The excess +VAR produced was gone out to Grid (around 40 MVAR), not absorbed by the another generator as -VAR.

Please advice, what is wrong with our trying

Tony Tomson