| With the fast development of the worldwide economy, to meet the increasing demand of the power energy, modern power systems are developed rapidly characterized by the high-voltage, the large quantitative capacity, the large-scale spreading of area and the long-distance transmission. In our country, most of the power recourse is located in the north-west area, but the main power energy consumer is massed in the south-east area. As a result, it is necessary to transmit large amount of electrical power over long distance from the north-west to the south-east. How to ensure the stability and to enhance the reliability of the power systems is the main challenging problem.FACTS(Flexible AC transmission systems) which combines the techniques of the power electronics, computer, modern control theory provides powerful capability of regulating the active and reactive power flow, depressing the power losses and generation costs of the transmission line, improving the stability of the AC transmission systems of power system by controlling the bus voltage, transmission reactance and power phases. Therefore, it attracts great attention in the recent years.UPFC (the unified power flow controller) as one of the most powerful FACTS devices, can regulate the power flow, maintain the bus voltage and damp the oscillation of the power systems becomes the focus of the current research. The first practically used UPFC was realized in the Inez power station of America in 1998. Its successful operation demonstrated the effectiveness of the UPFC. However, very few information can be found from the publications about the details of the UPFC.By reviewing the published literatures about the UPFC, this thesis aims to investigate the performance of the UPFC by both the digital simulation as well as the experimental tests. The main work done by this thesis is as follows:Firstly, based on the analysis of UPFC, the capacity of the UPFC is determined by use of the multiple restriction including the thermal limits of the transmission line and the limits of the voltage injection by the series converter, Also, details about the operation parameters of the UPFC are determined with the consideration of the limits of the bus voltage and the power transmissions. Using the PSCAD/EMTDC software and the above mentioned results, the operation limits of the UPFC are investigated and a detail dynamic model for the UPFC is obtained which includes IGBT based converters, SPWM based converter control systems, a traditional PI based controller and a single-machine infinite-bus power system By use of this model, power flow and voltage regulation characteristics as well as the transient stability recovering ability of the UPFC on the power system in the steady state and transient state are investigated by the simulations.For the purpose of the promising efficiency, this thesis taking the controllable voltage sources as the series and shunt converters, this model includes all of the UPFC operating limits and can describe in detail of the static and dynamic properties for the UPFC. With the consideration the power exchanging between the AC power system and the converters and the balances of the active power among the two converters and the DC capacitor, this paper deduces a dynamic equivalent model for the DC capacitor of the UPFC. In order to guarantee the accuracy of the equivalent model, the losses of the device circuits and the IGBT is modeled by a resistance connected to the DC capacitor in shunt equivalent circuit. This simplified dynamic equivalent model of UPFC will be used in the study of the control system in the following chapter.Fuzzy control algorithm has an advantage of less depending to the accuracy of the mathematical model for the controlled system. It is used together with the conventional PID in this thesis. Therefore, a self-adaptive fuzzy PI controller is obtained and used for the UPFC. In the proposed control, two kinds of feedback inputs are used. One of the inputs is the errors of the line active and reactive power and the other one is the variation of the errors. The outputs of the control are feed forward to the control system.. A set of the fuzzy rules are obtained using the simulation results given in chapter 2, which is used to determine the corresponding coefficients of the fuzzy-logic controller. The simulations of both the steady state and transient state are carried out to investigate the characteristics of UPFC with the proposed controller. Comparison is made by the simulation results between the UPFC controlled by proposed and the conventional PI controller.Based on the principle of PSS, a supplement controller, which uses the variation of the power flow in the transmission line as the input and the output is used to control the bus voltage and the active power transmission to damp out the low frequency oscillation of the power system and to enhance the power system transient stability after large disturbances. Satisfy results are obtained and given in the thesis.For the purpose of demonstrating the effectiveness of the UPFC and the proposed control, a 15kVa experiment prototype UPFC is developed and tested in a laboratory physical test environment. Theoretical result obtained in the above mentioned chapters are verified. Details of the prototype UPFC and the controller are all given in the thesis together with the test results. Also, the performance of the two kinds of PI control is compared.
|
PDF Full Text Request