Equations (1-1) through (1-5) indicate that the active and reactive power/current flow can be regulated by controlling the voltages, phase angles and line impedance of the transmission system. From the power angle curve shown in Figure 1.1(c), the active power flow will reach the maximum when the phase angle δ is 90º. In practice, a small angle is used to keep the system stable from the transient and dynamic oscillations [4]. Generally, the compensation of transmission systems can be divided into two main groups: shunt and series compensation.