Optimal Control Schemes for Power System with Unified Power Flow Controller (UPFC)

FACTS are one aspect of power electronics revolution that is taking place in all areas of electrical energy. A variable of powerful semiconductor devices not only offer the advantage of high speed and reliability of switching but, more importantly, the opportunity offered by a variety of innovative circuit concepts based on these power devices enhance the value of electric energy.

In generation area, the potential application of power electronics is largely in renewable generation. Photovoltaic and fuel cells requires conversion of dc to ac. Generation with variable speed is necessary for economic viability of wind and small hydro generators. Variable-speed wind generators and small hydro generators requires conversion of variable frequency ac to power system frequency. These applications of power electronics in renewable generation area require converter sizes in the range of few kilowatts to few megawatts.

In coming decades, electrical energy storage is expected to be widely used in power systems as capacitor, battery and superconducting magnet technologies move forward. Batteries are widely used already for emergency power supplies. These require ac/dc/ac converters in the range of a few kilowatts to a few tens of megawatts. On the other hand, variable speed hydro storage requires converters of up to a few hundred megawatts.

In transmission area, application of power electronics consists of High Voltage Direct Current (HVDC) power transmission and FACTS. HVDC is often an economical way to interconnect certain power systems, which are suited in different regions separated by long distances or those have different frequencies or incompatible frequency control. HVDC involves conversion of ac to dc at one end and conversion of dc to ac at the other end.

What is most interesting for transmission planners is that FACTS opens up new opportunities for controlling power and enhancing the usable capacity of the lines. The possibility that current through a line can be controlled at reasonable cost enables a large potential of increasing the capacity of the existing lines with larger conductors, and use one of the FACTS controllers to enable corresponding power to flow through lines under normal and contingency conditions. These opportunities arise through the ability of FACTS controllers to control the interrelated parameters that govern the operation of transmission line including series impedance, shunt impedance, current, voltage, phase angle, and the damping of oscillations at various frequencies below the rated frequency.

In distribution area, an exciting opportunity called Custom Power. The custom power concept incorporates power electronics controllers and switching equipment, one or more of which can be used to provide a value-added service to the customers. In general, these custom service applications represent power electronics in the range of few tens of kilowatts to few ten of megawatts of conversion or switching equipment between the utility supply and customer.

On the end-user side, power electronics conversion and switching technology has been fast growing area. Complementing the Custom Power technology is the whole area of power conditioning technology used by customers, under the term Power Quality. Uninterruptible power supplies (UPS) and voltage regulators represent the major growth area in power electronics. In end use, the converter sizes range from a few watts to ten of megawatts.
The term active filter is a general one and is applied to a group of power electronic circuits incorporating power switching devices and passive energy storage circuit elements such as inductors and capacitors. The functions of these circuits vary depending on the applications. They are generally used for controlling current harmonics in supply networks at the low and medium voltage distribution level or for reactive power and/or voltage control at high voltage distribution level. The

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