Grid Interconnection & Tie-Line Synchronization (2D)

Two independent AC power areas, each governed by droop control, sit on either side of a tie-line breaker. With the breaker open the areas are free to drift to different frequencies, and a synchroscope needle spins to show the live phase slip — the exact instrument operators watch before closing a real interconnection. Close the breaker while synchronized and the two systems lock onto one shared frequency, with the tie-line carrying real power between them according to the AC power-angle equation; close it badly out of step and you get a visible synchronizing transient instead. Adjust each area's generation and the tie-line's reactance to see how interconnection lets two grids lean on each other's reserves.