Consider the two power systems shown in Figure A below which are initially not interconnected, and are operating in steady state at the same frequency. Separate load flow solutions are computed individually for the two systems, corresponding to this scenario. The bus voltage phasors so obtained are indicated in figure A. These two isolated systems are now interconnected by a short transmission line as shown in figure B, and it is found that $P_1=P_2=Q_1=Q_2=0$.



Figure A

Figure B

The bus voltage phase angular difference between generator bus X and generator bus Y after the interconnection is