From my undergraduate classes I know that active power flows from the bus with the higher power angle \$\delta\$ to the bus with a lower power angle. If I want to reverse the direction of power flow I would now have to increase the power angle of the bus with the lower power angle.
How would I achieve this reversal in power flow? How do I increase the power angle at a bus?
In general power system operators do not have any way to control the angle of voltages of the buses in the grid. What are controllable are the power injections and voltage setpoints of generators. There are also switched shunt reactive power devices (capacitors & reactors) and some transformers' on-load tap changers, all of which mainly affect voltage magnitude and reactive power flows rather than voltage angles and active power flows.
For addressing phase angles, sometimes there are phase-angle transformers or more complicated power-electronic-based devices called FACTS, but for the most part the phase angles just are what they are based on the net power injections on the nodes of the system and the branches that are in service at the time. Active power will flow from buses with net generation toward buses with net load.
As a simple example for how you might reverse power flow on a line, suppose you have two areas connected by a single transmission line: If there is more generation than load in Area A and more load than generation in Area B (where total generation minus load and losses across both areas is 0), then you will see power flowing on the transmission line from Area A to Area B as shown below.
Area A -->-->-->--> Area B
(P_gen > P_load) (P_gen < P_load)
In order to reverse the direction of power flow on the transmission line, Area A generation must be decreased and/or load increased and Area B generation must be increased and/or load decreased so that now Area B has more generation than load and Area A has more load than generation. Now active power will flow from Area B toward Area A.
Area A <--<--<--<-- Area B
(P_gen < P_load) (P_gen > P_load)
It may help to think of the two bus systems as mechanical engine systems with a slightly flexible shaft connecting the two together. As with most engines, increasing the load on the motor will tend to slow it down.
If we load System A locally it will tend to slow down. System B will then start to contribute some mechanical input to A and there will be some torsion (twisting) on the shaft with A leading B. This would be equivelant to your power angle δ.
If B becomes more loaded than A the situation will reverse and the torsion will be the opposite direction.
The electrical connection works in a similar manner. If one system slows down it will be "dragged" along by the other. The frequency has to say the same but the phase may be out a bit and the driving system will lead the system being fed.
