Explain the generator capability curve and operating limits.

The capability curve (P-Q diagram) shows the operating region of a synchronous generator. Limits include: armature current limit (circular arc centered at origin, set by conductor heating), field current limit (another arc, set by field winding heating), steady-state stability limit (minimum excitation for stable operation), prime mover limit (maximum power), and end-region heating limit (in under-excited region). The usable region is bounded by all these curves. Operating point determines megawatts (from prime mover) and megavars (from excitation).