Research On Nonlinear Control Of Unified Power Flow Controller And Unified Power Flow Controller With Fault Current Limiting

Modern power systems are becoming increasingly huger and more complex through wide interconnections. Many problems such as improving transmission capacity, controlling power flow and limiting short circuit fault current are becoming more and more highlighted. In recent years the rapid development of flexible AC transmission system (FACTS) provides a new way to solve the foregoing problems. The unified power flow controller (UPFC) is a versatile FACTS device and is the emphasis of the research work in the field of power systems due to its powerful functions. An UPFC is able to control, concurrently or selectively, the transmission voltage, impedance and angle, and then, the real and reactive power flow in the line. Furthermore, an UPFC can also improve various steady-state and dynamic performances of the power system. This dissertation focuses on those problems concerning UPFC nonlinear control and the security of UPFC under short circuit fault current.This dissertation is divided into two parts. The first part refers to nonlinear control of UPFC. Considering the nonlinearity of UPFC, a nonlinear optimal control strategy based on exact feedback linearization theory and a nonlinear robust control strategy based on feedback linearization H∞theory are proposed. Moreover, a nonlinear stochastic optimal control strategy is presented to suppress random disturbances on the load side voltage of the UPFC. Details are as follows:1. The performance of the traditional linear controller for UPFC may deteriorate due to a large-scale change of the operating point. In order to solve this problem, a new nonlinear optimal control strategy for UPFC based on exact feedback linearization theory is presented. Firstly, the5th-order UPFC nonlinear model is proved to meet the conditions for exact feedback linearization. Then the nonlinear model is converted into a Brunovsky linear system via selection of a Lyapunov-like output function, an appropriate nonlinear coordinate transformation and state variable feedback. At last the pole placement method is applied on the transformed linear system to design the UPFC internal power flow controller. The simulation results demonstrate that the proposed control overcomes the inherent drawback of traditional PI control and provides better performance on power system transient stability improvement than PI control.2. A nonlinear robust model of UPFC is established and a new nonlinear robust control strategy for UPFC based on feedback linearization H∞theory is presented to solve the uncertain disturbance rejection problem faced by an UPFC in a real power system. The simulation results demonstrate that the given controller shows excellent performances and robustness under various kinds of disturbances.3. In the second research point above, all the disturbances which an UPFC may suffer in a real power system are formulated to a common disturbance vector. This method is generally practical but lacks the specificity and accuracy. Aimed at suppress the random disturbance on the load side voltage of UPFC, a new nonlinear stochastic optimal control strategy for UPFC is proposed. Firstly, a nonlinear stochastic model of UPFC with additive noises is established. Secondly, the model is exactly linearized to a linear stochastic system with multiplicative noises according to Ito’s Lemma. Thirdly, the stochastic Riccati matrix differential equation and its assistant equation are derived from stochastic HJB equation, and then a numerical algorithm of the solutions is given. At last, the nonlinear stochastic control strategy of the original UPFC system is gained through the control vector transformation. The simulation results demonstrate that the proposed control strategy can suppress random disturbances more effectively and maintain better control performance under many complicated disturbances than that of traditional LQG control method.The design methods of the three control strategies mentioned above can also be applied to other voltage source converter (VSC)-based FACTS devices, such as STATCOM, SSSC, GUPFC, IPFC, VSC-HVDC and so on.In the second part of this dissertation, a novel FACTS device called Unified Power Flow Controller with Fault Current Limiting (UPFC-FCL) is presented and developed in order to deal with the security problem of UPFC in case a short circuit fault occurs in the power system. Details are as follows:1. When a short circuit fault occurs in the power system, the series converter of UPFC may be damaged because of bearing high system voltage and large fault current. A novel FACTS device called Unified Power Flow Controller with Fault Current Limiting (UPFC-FCL) is designed to solve this problem. The topology of UPFC-FCL is given. The working principles in normal condition, dynamic process in fault condition, and corresponding control strategies of UPFC-FCL are analyzed. The mathematical model of UPFC-FCL is set up. The calculation and selection methods of DC reactor and DC capacitor are illustrated. The simulation results verify the feasibility and validity of UPFC-FCL. 2. An UPFC-FCL prototype is developed. The hardware structure and software structure of the control system of the prototype are designed. Various experiments on the prototype are conducted and the results demonstrate that:in normal condition, the UPFC-FCL is equivalent to a regular UPFC; when a short circuit fault occurs near the device in the power system, the FCL module will transform from "zero impedance" into "large impedance" immediately and automatically to limit the fault current. The UPFC-FCL can not only protect the UPFC series converter from being destroyed by the high system voltage and large fault current but also lower the fault current level in the power system, and thereby enhance the reliability and economy of the power system.The design methodology of the UPFC-FCL can also be applied to other VSC-based serial FACTS devices, and then giving birth to SSSC-FCL, IPFC-FCL, GUPFC-FCL, etc.