Power generation and transmission is a complex process, wherever power is to be transferred, the two main components are active and reactive power. In a three phase ac power system active and reactive power flows from the generating station to the load through different transmission lines and network buses. The active and reactive power flow in transmission line is called power flow or load flow. Power flow studies provide a systematic mathematical approach for determination of various bus voltages,three phase angle, active and reactive flows through different lines, generators and loads at steady state condition. Power flow analysis is also used to determinethe steady state operating condition of a power system. For the planning and operation of power distribution system, power flow analysis is used. It is very important to control the power flow along the transmission line. Thus to control and improve the performance of ac power systems, we need the various different types compensators.
The continuing rapid development of high-power semiconductor technology now makes it possible to control electrical power systems by means of power electonic devices. These devices constitute an emerging technology called FACTS(flexible alternating current transmission systems). FACTS technology has a lots of benefits,such as greater power flow control ability, increased the loading of existing transmission circuits and has the less cost than other alternative techniques of transmission system is used. The UPFC (unified power flow controller) is one of the most versatile devices. The main function of the UPFC is to control the flow of real and reactive power by injection of a voltage in series with the transmission line. Both the magnitude as well as the phase angle of the voltage can be varied independently. Real and reactive power flow control can allow for power flow in prescribed routes, transmission lines loading is closer to their thermal limits and can be utilized for improving transient and small signal stability of the power system.
1.2 LITERATURE SURVEY
The demand of electric power is increasing day by day. This situation has necessitated a review of the traditional power system concepts and practices to achieve greater operating flexibility and better utilization of existing power systems. During the last two decades, various high-power semiconductor device and control technologies have been introduced [1, 2]. These technologies have been instrumental in the broad application of high voltage DC and AC transmissions. The UPFC is the one of most powerful Facts device, introduced by Gyugyi, . For the Power system load flow studies the UPFC current based model is used, which improve the steady state and dynamic performance of the system [4, 5]. In this model the shunt compensation of UPFC is controlled to maintain the system bus voltage and the two components of UPFC series voltage, which are in phase voltage and quadrature voltage, are coordinated to respond to the power variations of the line. In case of static performance the power injection model is used. The sending and receiving ends of the UPFC are decoupled. The active and reactive power loads in the PQ bus and the voltage magnitude at the PV bus are set at the values to be controlled by the UPFC. The active power injected into the PV bus has the same value as the active power extracted in the PQ bus since the UPFC and coupling transformers are assumed to be lossless .
The UPFC current based model has implemented into a full Newton-Raphson program by adding the UPFC injected powers and by derivatives the elements of Jacobian matrix with respect to the AC network state variables, i.e. nodal voltage magnitude and angles, at the appropriate locations in the mismatch vector and Jacobian matrix. The UPFC minimize power losses and maintain stability limits, without generation re-scheduling, is shown by numeric examples. UPFC have the capability to regulate the power flow and minimizing the power losses simultaneously . The power injection model of (UPFC) the operational losses also taken into account and the effects of UPFC location on different power system parameters are entirely investigated . A general sequential power flow algorithm based on current based model of FACTS devices has been presented in [7, 8]. The algorithm is compatible with Newton-Raphson and decoupled algorithms. It is important to ascertain the location for placement of UPFC which is suitable for various contingencies. An effective placement strategy for UPFC is proposed .
1.3 OBJECTIVES OF THESIS
The main objective of this project is to reduce the active power losses and maintain the voltage stability limits by optimal location of UPFC in power system.
In this project we follow the following procedure:
‘ Currents as a variable and performs the load flow studies with unified power flow controller.
‘ To introduce an alternative proposition for steady state modeling of unified power flow controller.
‘ To develop MATLAB program for power injection model.
‘ To develop MATLAB program for current based model.
‘ To compare the results between these methods using Newton-Raphson method.
1.4 ORGANIZATION OF THE THESIS
‘ In Chapter-2, power injection model of UPFC and modification of Jacobian matrix due to injected powers explained.
‘ In Chapter-3, current based model of UPFC and modification of Jacobian matrix due to currents explained.
‘ In Chapter-4, proposed model algorithm and results of proposed and existing models are presented and comparison of results is made.
‘ In Chapter-5, conclusion of the thesis and scope for future work is presented.
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