24-07-2012, 10:53 AM
POWER FLOW CONTROL THROUGH UPFC
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SYNOPSIS
The power electronic based flexible AC transmission systems (FACTS) have been developed and used as economical and efficient means to control the power transfer in the interconnected AC transmission systems. This allows power transfer through the lines with higher transmission capacity. Among the FACTS components, Unified Power Flow Controller (UPFC), is unique and the most versatile. It can provide simultaneous and independent control of voltage magnitude, active and reactive power flow. The simulation is done in Matlab-Simulink to investigate the effect of UPFC on the voltage of the related bus, also considering the effect on theamount of active and reactive power flowing through the transmission system.
INTRODUCTION
As the power systems are becoming more complex, it requires careful design of the new devices for theoperation of controlling the power flow in transmission system, which should be flexible enough to adapt to any momentary systems conditions. The operation of an ac power transmission line, are generally constrained by limitation of one or more network parameters and operating variables
[1].Flexible Alternating Current Transmission System or FACTS is a technology introduced by Electrical Power Research Institute (EPRI) in the 80s.
[2]. Its principle role is to increase the transmission capacity of the ac lines and to control power flow over designated transmission lines.
FACTS technologies involve conversion and switching of power electronics in the range of a few tens to few hundred megawatts.
REAL POWER (P) AND REACTIVE POWER (Q) CONTROL
Fig 2. represents the schematic diagram of two machine system indicating Vs and Vr as sendingendreceiving end voltage respectively .Neglecting reactance of the transmission line, X is the inductive reactancΔeofline or tie.
UPFC construction
The UPFC consists of two voltage source converters; series and shunt converter, which are connected to each other with a common dc link. Series converter or Static Synchronous Series Compensator (SSSC) is used to add controlled voltage magnitude and phase angle in series with the line, while shunt converter or Static Synchronous Compensator (STATCOM) is used to provide reactive power to the ac system, beside that, it will provide the dc power required for both inverter. Each of the branches consists of a transformer and power electronic converter. These two voltage source converters shared a common dc capacitor .The energy storing capacity of this dc capacitor is generally small. Therefore, active power drawn by the shunt converter should be equal to the active power generated by the series converter.The reactive power in the shunt or series converter can be chosen independently, giving greaterflexibility to the power flow control. The coupling transformer is used to connect the device to the system. Figure 6.shows the schematic diagram of the three phase UPFC connected to the transmission line.