Load Angle

The correct sentence is "when the active load increases the load angle increases", to reach a balance between the torque on the shaft and the resistant electrical torque on the generator terminals.

The load angle or power angle, δ, is the angle between the induced EMF and the voltage at generator terminals.
It represents, in each moment, the angular position of the polar wheel between the no-load and load positions. For a stable system, a variation Δδ of the load angle must match a variation ΔP of the same sign.

The static stability limit is defined as the maximum power that the machine can exchange with the network without losing the synchronism in response to a slow load variation and without excitation current variation.
For a cylindrical rotor machine, the static stability limit is reached when δ = ± π/2, thus the stable working area corresponds to -π/2 < δ < + π/2.

For more detailed information, please refer to synchronous machines theory.