What determines the phase difference between two voltages at an AC bus?
\$\delta\$ is called the power angle. It is the difference in phases between \$ V_1 \$ and \$ V_2 \$. You are correct that depending on its value, positive or negative, it determines the direction of power flow.

$$\delta = \phi_2 - \phi_1$$

Since \$ \phi_2 \$ and \$ \phi_1 \$ can vary, \$ \delta \$ is therefore variable.