Loading of Transmission line

With that much information it is difficult to say.

Since a 400 kV line is usually built to carry about 500 MW, 20 MW is not very much!

The capacitance of 12 km of line would draw about 10 amps charging current (7 MVA). 12 km is too short to see any transmission line effects [like receiving end volts higher than source end!] N.B. wavelength at 50 Hz is about 1500 km. But I wonder about power factor effects of transformer magnetising current on a generator or resonance at even smaller loads - if this is not yet Grid connected.

How much lagging transformer magnetizing current do you see before connecting to this line? What is Transfo rating and ratio from generator?

calculate the surge impedance loading and then compare. u can calculate it using

surge impedance loading = (V*V)/Zc; where Zc = sqrt(L/C); where L is the inductance of the transmission line and C is the capacitance.

For a British 400 kV line with 2 x 258 mm² conductors, the surge impedance is 296 ohm and the corresponding matched load 540 MW [(400/√3)²/296] . Hence my "about 500 MW". Since we do not know how many conductors/phase, size or spacing or inductance/capacitance values one cannot be more accurate. But the actual question was "is 20 MW too small a loading". A 500 MW line is not stressed by 20 MW, so it is a question of effect on connected items and we do not know anything about them.