Analysis Of Operation Characteristic And Experiments On Power Electronics Devices In Flexible Alternating Current Transmission System

FACTS technology brings the users of high voltage transmission systems fresh opportunities as well as challenges. It appears to be one the most important alternatives to overcome both the inflexible condition of most of the power sytems and the continuously growing demand of power. It is an effective method to realize security, the economical, integrated control of power systems. It offers a good way to improve the stability operation of complex and interconnected power systems. This paper gives an overview of the emerging FACTS technology with emphasis on the STATCOM, SSSC and UPFC. Of the FACTS controllers of interest here, the STATCOM has the ability to increase/decrease the terminal voltage magnitude and, consequently, to increase/decrease power flows in the transmission line. The SSSC controls power flow by changing ehe series reactance of the line and is immune to classical network resonances. The UPFC is the most comprehensive multivariable FACTS controller.It can not only work separately as a STATCOM or a SSSC but also control the transmission line real and reactive power in the transmission line, improve the transient stability margins, damp power oscillations and provide voltage support. So it is necessary to further research the performances of FACTS controllers. To solve the problems existed in the research field of FACTS controllers above, this paper works in the four areas as listed below.The successful incorporation of FACTS controllers in the power systems requires a clear understanding of all possible approaches and their operating characteristics. In this paper, first, a systematic study is performed to derive operating areas of the shunt current injection and power injection of STATCOM. By the analysis of mathematical model, DC bus voltage and voltage balance of the STATCOM system, the corresponding control system is designed. STATCOM installed in the long transmission line can support key nodes voltage and improve transmission capacity of the power system. So this paper discusses how STATCOM for controlling the transmittable power in the transmission line depends on the location of STATCOM in the network. The analysis indicates that STATCOM installed in the middle line can control furthest the transmittable power in the transmission line and enhance transient and steady state stability of power systems. Considering the control scheme being the key to achieve better compensation, this paper discusses the reactive power compensation mode and automatic control mode of STATCOM. It can be concluded by analysis that the automatic voltage control mode is a better choice for the distribution systems. The automatic voltage control mode includes three types: the constant voltage control mode, the direct voltage droop control mode, the voltage droop control mode with the compensating current. By comparing three control modes above, it is shown that the linear operating range of a compensator with given maximum capacitive and inductive ratings can be extended if a regulation droop is allowed. On the other hand, it is reasonable to expect by analysis that, with suitable and fast controls, STATCOM can change the power flow in the system during and flowing dynamic disturbances so as to increase the transient stability limit and provide effective power oscillation damping.Second, a systematic study is performed to derive operating areas of the series voltage injection and power injection of SSSC. By the analysis of mathematical model, DC bus voltage, this paper proposes ans describes the self-startup technique of SSSC without energy storage. In this paper, two different compensation control modes of SSSC, the voltage compensation mode and the impendance compensation mode, are discussed. By analyzing compensation areas and power-angle characteristics of SSSC based on two above compensation modes, it can be concluded that the voltage compensation mode is better suitable for SSSC and the corresponding control system is designed. Under certain conditions, the certain magnitudes and phases of sending-end voltage and receiving-end voltage, this paper presents power operation plane plots of SSSC in a two-dimensional space. It provides an intuitionistic measure for understanding how SSSC impacts on the system power. This also compares the power ratings of STATCOM and SSSC when performing the same reactive power compensation job. As a result, SSSC is superior to STATCOM in actual operation. Moreover, the power oscillation damping performance of SSSC is analyzed in a simple single machine infinite bus system.Third, this paper discovers that the phases of the sending-end voltage and the receiving-end voltage change with the power flow in the transmission. Considering the factor, this paper accurately analyzes the power flow control performance of UPFC. Furthermore, the real/reactive power absorbed/injected by the components of the UPFC system during power flow changes is plotted in 2D and 3D space respectively. That provides an effective method to determine the power rating of the power conversion parts of the UPFC and judge the availability of the control scheme for UPFC. By observing the power map, it can be found that every point of the transmission real & reactive power is corresponding to a certain voltage injected by the series part of the UPFC. However, for the power map of the injected power by the series part of the UPFC one power operating point may corresponding to more than one voltage injected by the series part of the UPFC. Therefore, it is necessary for effectively controlling power flow to directly control the transmission line current or real/reactive power in control system. By analyzing the effect of placement of UPFC to power flow control, it is conclude that placement of UPFC should be considered based on actual demand. Considering the internal relations among the variables of UPFC, a relatively complete control system is designed: a coordinated control scheme is used in shunt converter, cross-coupling control scheme and cross-decoupling control scheme are used in series converter. Simulation results show the effectiveness of the proposed control system.At last, a 15kVA FACTS experimental system is established in this paper. Combining office technology and the dynamic simulation laboratory conditions, this paper studies the performance of STATCOM, SSSC, UPFC in two infinite bus systems at 380V rating and a simple single machine infinite bus system at 800V rating. And abundant effective experimental results are gotten in this paper. These works lay a sound foundation of the powerful FACTS controllers for future application.