| Thyristor Controlled Series Compensation (TCSC) is one of the FACTS (Flexible AC Transmission System) device most widely and successfully used in practice. It can regulate the impedance smoothly in large-scale by changing the firing angle of thyristor. Because of this flexibility of line impedance, TCSC is expected to control power flow over the line, improve system stability, damp power system oscillation, mitigate sub-synchronous resonance (SSR) and enhance power transfer capability of particular transmission corridors.Because of the non-linear nature of the device, digital simulations are often used to study the dynamic characteristics of TCSC. Since the reliability and accuracy of digital simulation is heavily affected by the numerical models and algorithms used, there is a limit to digital simulation when investigating the TCSC dynamic performance.Experimental setup has physical characteristic similar to real TCSC to a large extent, and it is not restricted by system operation condition. Furthermore, it can be used to investigate complex phenomena such as the breakdown of MOV. Such phenomena are very difficult, if not impossible, to investigate using digital simulation methods. As the control system of the TCSC experimental setup could be the same as those used in the practical power system, controllers based on various theories can be tested and the performance of them can be verified before their practical application. Consequently, it is of great importance to develop experimental setup of TCSC for the investigation of its basic operation characteristics and control strategy. This dissertation is devoted to implementation of TCSC experimental device and the study on control method of TCSC.Firstly, the design and application of a set of experimental device for dynamic simulation of TCSC is described in this dissertation, which includes the main TCSC circuit, the simulated power system, the protection system and the control system. As a platform for dynamic simulation study, the components of TCSC are modularized and can be flexibly combined. In addition, the structural parameters of TCSC can beregulated according to the requirement of the simulation experiment. This device contains a capacitor bank that is divided into four sections in parallel, a metal oxide varistor (MOV) and a thyristor controlled reactor with three taps. By the combination mode of parallel capacitors in-group and reactor with different of parallel capacitors in-group and reactor with different taps, the TCSC device can keep the same series compensation level with different line length of 6ookm, 400km and 300km.Hierarchical structure is applied to the controller of this TCSC experimental device and the controllers in different layers perform different control objectives, so the research on various control strategies can be conveniently conducted. This experimental device also has general interface to investigate TCSC performance and relative problems, such as impact on relaying protection, transient stability, and sub-synchronous oscillation. The experiment results show that the physical simulation platform is an effective tool to investigate FACTS devices. It has the capability to carry out measurement, protection and control schemes in real-time. By means of appropriate control strategy the impedance can be rapidly adjusted by the presented experimental device, the switching over among different impedance modes, e.g., the inductive and capacitive modes, can be flexibly implemented.Whether the objective of the TCSC device can be implemented or not depends on its impedance control. There are many control modes for TCSC to fulfill its benefits to power system, such as transient stability control, power control, current control and so on, however, all of these control modes will be translated into impedance control mode directly or indirectly to achieve their purpose. So, the relationship between frequency impedance and firing angle is very important and the key factor of TCSC device. Some experimental studies show that the resistance of reactor and thyristor valves can influence the effective impedance characteristics of TCSC.In this dissertation, the expression between firing angle and conduction angle is ascertained considering the resistance effects of reactor and thyristor valves firstly.The result shows that the condition (3 = 71-2a is not satisfied when the resistance effects are considered and dual-solution for one firing angle are also obtained in amajority of steady operation area of TCSC. The digital simulation and the result of dynamic simulation experiment all support the existence of dual solution phenomenon This dissertation also points out that TCSC can just move along one branch of frequency impedance curve and will not step to the other one if only the firing angle is changed. Through the analysis of phasor diagram, the conduction current of thyristor is limited by quality factor and current value of the reactor branch, which is the reason causing dual-solution phenomena.TCSC can not only adjust impedance smoothly but also switch between capacitive state and inductive state quickly. The mode-switching control of TCSC is important to power system stability control and is also one of the key problems of TCSC. On the basis of modeling and simulation, a practicable mode-switching control strategy was proposed in this dissertation. By means of forced synchronization of current in thyristor branch with the line current, the mode switching from capacitive veriner mode or block mode to bypass mode can be implemented. In the switching strategy from capacitive veriner mode or Block mode to inductive veriner mode a method of allowed-section triggering was put forward. To provide synchronization signal of line current for switching control in time, a method to forecast the crossover point of the current was given. Digital simulation and experiment results show that the proposed switching strategy possesses good dynamic performance the mode switching from capacitive to inductive mode could be completed fluently and swiftly.Whether the performance of TCSC device can be implemented or not depends on its impedance control. The reason of the phenomenon is thoroughly analyzed that the robustness of traditional PID control is bad when different impedance levels are ordered to step to a same level by traditional PID control. A new nonlinear PID control method for impedance control is presented in this paper by combing the immune feedback law with conventional PID control. Based on traditional PID control structure and its immune response can be regulated according to the parameters of the controller, the IMF-PID control method is practical. The comparison of the simulation results from the proposed approach with that from traditional PID control shows that the proposed approach can quickly drive theoutput impedance to the desired level with less overshoot and possess good dynamic and static performances, so it is available.In sum, physical simulation platform is capable to support the investigation of the operation, protection and control strategy of TCSC. With more and more FACTS devices applied into power system, the experimental device of TCSC will play a important role in power system research. This dissertation discusses, analyses and resolves problems in the exploitation of TCSC experiment device, and use simulation to prove and experimental results to test the theories put forward in this dissertation. Man>' conclusions and experimental results are of practical significance, and provide reference to TCSC operating and control. |
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