A $25$ kVA, $400$ V, $\Delta$-connected, $3$-phase, cylindrical rotor synchronous generator requires a field current of $5$ A to maintain the rated armature current under short-circuit condition. For the same field current, the open-circuit voltage is $360$ V. Neglecting the armature resistance and magnetic saturation, its voltage regulation (in % with respect to terminal voltage), when the generator delivers the rated load at $0.8$ pf leading, at rated terminal voltage is _________.
