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Flexible ac Transmission Systems (FACTS) play an important role in improving the transfer capability and stability of a power system. In the application of voltage-sourced converter (VSC)-based FACTS controllers, it is important to study how a VSC impacts the flows in a power system. In this paper, we investigate this flow control problem using two sensitivity approaches, one using an injected voltage source formulation and the other an equivalent impedance formulation. The applications of sensitivity analysis for line active power redispatch and for new series VSC siting in a 1673-bus system are presented. Index Terms--Equivalent impedance, FACTS controllers, sensitivity analysis, voltage source model, voltage-sourced converter.

Due to their ability to provide reactive power support and control active power flow, voltage-sourced-converter (VSC)-based flexible ac transmission system controllers can be used to improve the power transfer capability of congested transmission lines. A converter-based controller structure has the inherent feature to be used to provide shunt or series compensations. This inherent feature is utilized in the convertible static compensator installed at the New York Power Authority's Marcy substation, which can operate in 11 configurations of shunt and series compensations. Moreover, each configuration can be operated in different control modes. The paper presents a novel approach to model converter-based transmission controllers for load-flow calculations. The paper focuses on modeling converter-based controllers when two or more VSCs are coupled to a dc link (e.g., unified power-flow controller (UPFC), interline power-flow controller, and a generalized unified power-flow controller). This approach also allows efficient implementation of various VSC operating limits, where one or more VSCs are loaded to their rated capacity. A computer program incorporating this approach is developed to illustrate the maximum dispatchability of UPFC and IPFC in a large power system. Index Terms--Flexible ac transmission systems (FACTS), interline power-flow controllers (IPFCs), Newton-Raphson algorithm, rated-capacity operation, unified power-flow controllers (UPFCs), voltage-sourced converters (VSCs).

This paper develops a measurement-based method for estimating a two-machine reduced model to represent the interarea dynamics of a radial, multimachine power system. The method uses synchronized bus voltage phasor measurements at two buses and the line current on the power transfer path. The innovation is the application of the interarea oscillation components in the voltage variables resulting from disturbances for extrapolating system impedances and inertias beyond the measured buses. Expressions for the amplitudes of the bus voltage and bus frequency oscillations as functions of the location on the transmission path are derived from a small-signal perturbation approach. The reduced model provides approximate response to disturbances on the transfer path and offers an alternative to model reduction techniques based on detailed system models and data. Index Terms--inertia extrapolation, interarea oscillations, phasor measurement, power system model reduction, power transfer interface, reactance extrapolation.

Many large interconnected power systems such as the U.S. eastern interconnection and the U.S. western power system are characterized by many power transfer paths or interfaces with high loading. Disruptions of these transfer paths frequently lead to increased loading on neighboring transfer paths, which themselves will become less secure and could cause further disruptions. State estimators have limited performance under large system disruptions, because of low sampling rates and potentially poor solution quality due to topology errors. Furthermore, disruptions in external power systems cannot be readily seen by control room operators because most state estimators only use reduced models for external systems. A system of well-placed phasor measurement units (PMUs) that provide voltage and current magnitude and phase at a high sampling rate can provide useful system dynamic security information. In this paper we apply energy function analysis using phasor data to monitor the dynamic status of power transfer paths. The ideas will be illustrated using actual data captured by several PMUs in the U.S. western power system. Index Terms--Energy function, power system stability, power transfer path, synchronized phasor measurement unit (PMU).

For a system with a unified power-flow controller (UPFC), maximum power transfer capability is often achieved when the UPFC is operated at its rated capacity and conventional voltage and line-flow setpoint regulation is no longer possible. This paper uses injected voltage sources to directly model a UPFC and impose the rating limits in a Newton-Raphson load-flow algorithm. A dispatch strategy is proposed for a UPFC operating at rated capacity in which the power circulation between the shunt and series converters is used as the parameter to optimize the power transfer. Voltage-stability curves for two test systems are shown to illustrate the effectiveness of this proposed strategy. Index Terms--Flexible ac transmission system (FACTS) controllers, Newton-Raphson algorithm, unified power-flow controllers (UPFCs), voltage-source converters (VSCs), voltage stability.