Electrical synchronous phase modular is basically a synchronous motor operating under no-load,by adjusting the excitation.Excitation is used for controlling electrical reactive power therefore power factor and voltage also controlled.
over excitation: If excitation of synchronous machine is increased up to over excitation the synchronous machine act as shunt capacitor(synchronous condenser). According to shunt capacitor operation the synchronous machine delivering lagging reactive power and absorbing leading reactive power from mains.
under excitation: If excitation of synchronous machine is decreased up to under excitation the synchronous machine act as shunt reactor. According to shunt reactor operation the synchronous machine absorbs lagging reactive power and delivering leading reactive power from mains.
"under excited synchronous machine is used for compensating ferranti effect in power lines"