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The transfer capacity of the existing transmission lines is an important operational constraint on interconnected A.C. transmission network. To improve this capacity of the transmission line, one has to use the FACTS controllers. These controllers are known for their applications in improving power transfer capacity and power flow control using the existing infrastructure of a transmission utility as well as improving transient stability. In addition to these controls, FACTS controllers advantageously employed for transient stability improvement, power oscillation damping and voltage stability. With the FACTS controller, the transmission line capacity can be improved by 40 to 50% as compared to conventional mechanically-driven devices. Lower maintenance required for FACTS controllers improves effi-ciency in operation [1].

The FACTS controllers connected to the transmission line have following basic types depending on connection:

1 Series controllers:It has variable impedance and has a function to inject series voltage.

2 Shunt controllers:It has variable impedance and has a function to inject current in the system.

3 Combined series–series controller:It has separate but coordinated unified (common DC bus) controllers and can compensate reactive power as well as interline transfer active power.

4 Combined series–shunt controller:It can inject voltage and current and has unified and coordinated real power exchange between series and shunt controllers.

The UPFC, as shown in Figure 2.5, invented in 1991, is a real-time multi-functional dynamic compensator for the AC transmission line. It can control active and reactive power as well as voltage and VAR compensation. It also can improve the power quality of the associated system. Thus UPFC can control output voltage V_o and its angle ρ (0 ≤ ρ ≤ 2π) [1]. The operation of UPFC depends on types of voltage source converters. The shunt converter is Static Compensator (STATCOM) and series compensator is Static Synchronous Series Compensator (SSSC) [1]. The STATCOM is a static, shunt connected compensator which can control current independent of system voltage with the help of capacitor or inductor. In UPFC, capacitor-based current compensation considered. Hence depending on the system requirement, leading current can be injected in the system with the help of a capacitor connected to the common DC bus. It absorbs active power for charging the capacitor. So it has the role of Static Synchronous Generator (SSG) and Static VAR Generator (SVG). The SSSC is a series-connected compensator which can control series injected voltage. The magnitude of the series injected voltage is low. Still, the converter can change the voltage angle regarding sending end voltage, resulting in a change in receiving end voltage magnitude and angle depending on the system requirements.

Figure 2.5 Basic UPFC.

If the compensator connected with the energy storage capacitor, then it can absorb active power too. The transmitted power P and reactive power −jQ can be expressed by: