Wind Power - Understanding Renewable Energy Systems - page 215

Figure 5.24 Simple Equivalent Circuit ( R 1 = 0) of a Cylindrical Rotor
Machine for One Phase
A synchronous machine can generate reactive power if desired. This is the most
significant advantage of the synchronous machine compared to the
asynchronous machine described later. The stator current of a synchronous
machine can run through all phase angles. This is also called four-quadrant
operation . Figure 5.25 shows the vector diagrams of a cylindrical rotor
machine for four different operation conditions.
Depending on the phase angle, states are described as underexcitation and
overexcitation . If a synchronous machine is underexcited, it behaves like a coil
and absorbs reactive current. An overexcited synchronous machine has the
same behaviour as a capacitor and generates lagging reactive current.
Figure 5.25 Vector Diagrams of a Synchronous Machine with
Cylindrical Rotor