Modelling and locating unified power-flow controllers for static voltage stability enhancements

This paper presents a screening technique for greatly reducing the computation involved in determining the optimal location of a unified power flow controller (UPFC) in a power system. The first-order sensitivities of the generation cost with respect to UPFC control parameters are derived. This technique requires running only one optimal power flow (OPF) to obtain UPFC sensitivities for all possible transmission lines. To implement a sensitivity-based screening technique for guidance in optimally locating a single UPFC in a power system, we propose a new UPFC model, which consists of an ideal transformer with a complex turns ratio and a variable shunt admittance. In this model, the UPFC control variables do not depend explicitly on UPFC input and output currents and voltages. Accordingly, this model does not require adding extra buses for UPFC input and output terminals. IEEE five-, 14- and 30-bus systems were used to illustrate the technique

Due to ever-increasing power demand, the power system is facing issues such as congestion, line outages, system reliability, and stability challenges. The impetus to revise the traditional power transmission theory and practice, and to introduce new concepts for improving the performance of the existing AC transmission system caused the development of flexible AC transmission system controllers (FACTs). Due to their capability of controlling transmission system parameters, the utilization of FACTs controllers has become essential to enhance the power system capability. To investigate the performance of FACTs controllers, this study incorporates the Unified Power Flow Controller (UPFC) in the power system. UPFC is used to mitigate transmission line congestion by controlling the transmission line power flow. In this paper, a modified control circuit for UPFC is implemented. Moreover, the selection of a suitable location for UPFC is necessary because of its techno-economic impacts. Therefore, this work uses a sensitivity-based approach for the selection of a suitable place of UPFC. To decide the suitable location, control parameters of UPFC along with system loss sensitivity indices are used. The model of UPFC along with the control circuit is simulated in PSCAD/EMTDC. To investigate the performance of UPFC, a five bus seven-line transmission system is used. After installing the UPFC at an appropriate location, the power flow results of transmission lines are compared with and without the usage of UPFC.

Unified power flow controller (UPFC) gains growing application in power systems due to its power flow regulation ability. However, there is still a dilemma in the incorporation of UPFC into security-constrained economic dispatch (SCED). The modeling of UPFC involves nonlinear and nonconvex operational constraints, which is difficult to solve efficiently and accurately. Conventional methods adopted Taylor expansion or sensitivity analysis to realize the convexification of UPFC models, which leads to a loop structure and is subject to initial point choice. In this paper, a novel three-terminal injected UPFC model is proposed. In the proposed model, the voltage source variables are eliminated completely. Instead, only power injection variables are introduced to equivalently describe physical characteristics and constraints of UPFC. The nonlinear and nonconvex operational constraints of UPFC are transformed into mixed integer second-order cone form. Moreover, compared with conventional UPFC models, the control characteristic of UPFC is considered in a more accurate and practical manner in the proposed SCED, which helps to prevent potential risks of power flow over-limit under <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -1 contingencies. Simulation results show the effectiveness and efficiency of the established solution.

UPFC Power Frequency Model for Power System Stability Analysis

This paper proposes a power system oscillation damping scheme via a supplementary signal applied to a Unified Power Flow Controller (UPFC). The supplementary signal is based on the active power flow along the transmission line. The controller designed is based on fuzzy logic. The UPFC model and its controls are implemented in the Power System Toolbox (PST). To demonstrate the effectiveness of the controller under dynamic conditions a two area power system is used. When a large disturbance is applied, simulation results show that the UPFC can significantly enhance power system operation and performance.

Unified Power Flow Controller (UPFC) combines function of voltage control, power flow control and oscillation damping. Before installation to a system, analysis for interactions between UPFC and system are needed. Building up steady, transient and dynamic models by present software to realize accurate simulations of UPFC under different system situation is a key step. In this paper, an UPFC model under dq axis including dynamic characteristics on DC side is built by switching function method. After control scheme analysis, effects of different damping strategies are compared. Simulation results show the correctness of the model and the validity of UPFC in controlling power flow and damping system oscillation.

This paper proposes a novel model of unified power flow controller (UPFC) for enhancing voltage stability margin of the power systems. This model is based upon an innovative approach where, real power exchange of series and shunt elements of UPFC are decoupled with each other. The UPFC model is incorporated into an existing Newton-Raphson load flow algorithm and Jacobian matrix is modified to include the decoupled modes of power exchange of series and shunt elements of UPFC. The modeling approach presented in this paper is tested on standard IEEE 30-bus and IEEE 118-bus systems to validate its feasibility.

UPFC (Unified power flow controller) is one of the most powerful and complex FACTS (Flexible AC transmission system) devices in a power system today. It is primarily used for simultaneous or independent control of real and reactive power in transmission lines for flexible, reliable and economic operation and loading of power systems. In this paper, the effectiveness of UPFC has been explored in transmission system network when subjected to three phase to ground fault condition. The comparison has been made between VSI (Voltage Source Inverter) based UPFC and ZSI (Z-Source Inverter) based UPFC. From the results so obtained, it has been established that the performance of ZSI based UPFC is superior as compared to that of VSI based UPFC.

Voltage instability and voltage collapse have been considered as a major threat to present power system networks due to their stressed operation. It is very important to do the power system analysis with respect to voltage stability. Flexible AC Transmission System (FACTS) is an alternating current transmission system incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability. A FACTS device in a power system improves the voltage stability, reduces the power loss and also improves the load ability of the system. FACTS have made the power systems operation more flexible and secure. Amongst the several FACTS controllers, the Unified Power Flow Controller (UPFC) is most effective to improve the enhancement of voltage stability and reduces the power loss. This study investigates the application of Particle Swarm Optimization (PSO) to find sizing of Unified Power Flow Controller (UPFC) device to minimize the voltage stability index, total power loss, load voltage deviation, and cost of FACTS devices to improve voltage stability in the power system. A new model is proposed in this thesis to improve existing power-based model by using the Norton Equivalent Theorem. The proposed model can be integrated with the Equivalent Current Injection (ECI) power flow model easily. By ECI algorithm, it is much quickly and precisely to implement power flow calculations. Finally It is observed from the results that the voltage stability margin is improved, the voltage profile of the power system is increased and real power losses also reduced by optimally sizing UPFC device in the power system. Index Terms: Voltage stability, Unified Power Flow Controller (UPFC), Equivalent -Current Injection (ECI), Modified particle swarm optimization (MPSO)

The Unified Power Flow Controller (UPFC) is the most complex and powerful FACTS device, which can provide full or independent control of line transmission parameters, such as voltage, line impedance and phase angle. In this paper, steady-state model of unified power flow controller (UPFC) and power flow algorithm of power system with UPFC were studied. The parallel and series sides of UPFC were equivalent to separate power injections, and an alternating iteration algorithm of power flow for power system with UPFC was presented. In order to improve the convergence of the algorithm, this paper analyzes the impact of UPFC’s various control strategies on the convergence of power flow calculations, and proroses a control strategy for converting constant power control to constant variable control during the iterative process. Based on the actual project of Nanjing Western UPFC, the correctness and effectiveness of the model and algorithm are verified.

The application of unified power flow controller (UPFC) has become an inevitable trend to deal with challenges faced by modern power system such as optimization of power supply and system operation. However, review researches on UPFC are less while the researches on specific technologies mostly focus on theoretical fields, which are difficult to guide actual engineering. To this end, this paper discusses several key issues of UPFC in its actual application. First of all, technical process of UPFC in actual application was proposed and its six key issues were pointed out. Then, for three issues in the process of “scheme-obtaining of UPFC application”, the principles of necessity to configure UPFC, the method of determining UPFC location and capacity, the method of determining UPFC control strategy are discussed. Finally, for three issues in the process of “influence analysis and optimization for UPFC”, simulation calculation method of power system with UPFC, influences of UPFC on power system operation, changes strategy of operating parameter for UPFC are discussed. The work done in this article aims to provide a reference for the actual application of UPFC.

FACTS technologies can have major positive impacts on power system reliability performance and the actual benefits obtained can be assessed using suitable models and practice. Emerging techniques for composite power system reliability evaluation mainly focus on conventional generation and transmission facilities. In this paper, improvement of Reliability in composite electric power system is examined by incorporating Unified Power Flow Controller (UPFC). Two test systems, Reliability Test System (RTS) and Roy Billinton Test System (RBTS) are considered to show major improvement in reliability. A state space reliability model of multi-module UPFC has been developed and incorporated in the system. Load point & System indices performances are presented to examine the impact of UPFC on the composite electric power system reliability. Investigation results show a significant improvement in the Load point & system indices when utilizing UPFC.

The Unified Power Flow Controller (UPFC) integrates properties of shunt and series compensations and can effectively alters power system parameters in a way that increases power transfer capability and stabilizes system. Simple Proportional-Integral (PI) controllers are applied to UPFC as usual controllers, but parameters of PI controllers are usually tuned based on the trial-and-error approaches and they are incapable to obtain a good dynamic performance under a wide range of operating conditions. To solve this problem, robust control methods are proposed to design UPFC controllers. Also a supplementary damping controller to increase damping of power system oscillations is developed. A Single-Machine Infinite Bus (SMIB) power system installed with a UPFC with system parametric uncertainties is considered as case study. The system parametric uncertainties are obtained by 40% changing parameters from their typical values. Proposed methods are compared with each other to show their effectiveness and robustness. The objectives are to study and comparison robust control methods performance for UPFC control problem.

A new and comprehensive load flow model for the unified power flow controller (UPFC) is presented in this paper. The model is incorporated into an existing Newton-Raphson load flow algorithm. Unlike existing UPFC models available in open literature, it can be set to control active and reactive powers and voltage magnitude in any combination or to control none of them. A set of analytical equations has been derived to provide good UPFC initial conditions. Hence, the algorithm exhibits quadratic or near-quadratic convergence characteristics. Suitable guidelines are suggested for an effective control coordination of two or more UPFCs operating in series or parallel arrangements. Test results are presented which demonstrate the effectiveness of the new model.

New approach for optimal UPFC placement using hybrid immune algorithm in electric power systems