Reactive Power

The possible reason is that the lecturer is testing - to see if you can apply what you have been taught.

However, supposing this is a real situation....

a) The generator is connected to a line, cable or compensator which is contributing 30 -40 = -10 MVAr and there is no metering of this connection or you have not read that metering. Usually, negative values are capacitance loads.

b) The generator is 800 MVA, so 10 MVAr error is due to its metering error of 1% full scale or more. In comparison, the unit transfo metering error is insignificant and the actual reactive power is 30 MVAr (output from generator and input to auxiliary transformer). In practice, generator's of this size are connected direct to main transformers, so one would expect 30MVAr magnetising current.

c) The auxiliary load is about is about 0.8 power factor with 50 MW power, so its reactive part is 50 x 0.8 = 40 MVAr.

d) Maybe someone got the meter connections or scalings wrong for auxiliary transfo. This is possible if they did not have the experience to give the MW readings (or any other available relevant readings, like the MW, kV or secondary (low voltage) metering readings from auxiliary transfo (which would give an indication of primary input).