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This paper presents a comparative analysis between results from applications of the p-q and the p-q-r theories in shunt active power filters for three-phase four-wire systems, discussing aspects related to the influence of the system voltage in the control methods that calculate the compensating currents. It is shown that in some cases, a preprocessing of the system voltage is required if the goal is to achieve sinusoidal compensated currents. On the other hand, when the goal is to compensate zero-sequence current, the need of energy storage elements in the active filter is discussed. In this case, if zero-sequence components are present simultaneously in the system voltage and load current, they produce zero-sequence power flow, and the control methods based on both theories must contain additional calculations to allow the elimination of energy storage elements in the active filter. A control strategy based on the p-q theory is proposed to eliminate the neutral current without the need of energy storage elements, with the advantage of avoiding the extra transformation from [alpha][beta]0 to pqr coordinates that is needed in the p-q-r theory. Simulation results are presented for the purpose of comparing the performance of both control methods. Index Terms--Active filters, p-q theory, p-q-r theory.

The thyristor-controlled series capacitor (TCSC) is a commercially available flexible ac transmission system (FACTS) device developed for controlled compensation of transmission-line impedance, which consequently leads to the control of transmission-line power flow. The gate-controlled series capacitor (GCSC), a FACTS device based on a pair of gate-commutated switches in parallel with a capacitor, can also control impedance and power flow of transmission lines. Here, it is shown that in many situations where a controllable series compensator must be installed, the GCSC may be used instead of the TCSC, possibly with some advantages. A comparison of the sizing of the TCSC and the GCSC components is presented. It is shown that the capacitor in the GCSC can have lower megavolt-amperes than in the TCSC, especially for power-flow control applications. Also, it is shown that the thyristor valve in the TCSC needs to have a higher current rating than the gate-commutated switch valve in the GCSC. Index Terms--Flexible ac transmission systems (FACTS), gate-controlled series capacitor (GCSC), series compensation, thyristor-controlled series capacitor (TCSC).

This work presents the development of analytical models for thyristor-controlled reactors (TCRs). A nonlinear model for the TCR was developed based on the use of Generalized Switching Functions and from this model, a detailed linear model was derived. The linear model allows for the analysis and precise understanding of the behavior of the TCR under small disturbances both in the time and frequency domains, for frequency ranges up to some tens of hertz. This model clearly shows that the TCR dynamics are operating point dependent. System parameter variations are also correctly considered in the model. With the proposed model, it is possible to design static var compensators (SVC) controllers in an integrated form, avoiding risks of instabilities and guaranteeing a good overall dynamic performance for the system. Validation of the models was done by comparing simulated results obtained with the proposed model with those obtained with a traditional electromagnetic transients program (EMTP). Index Terms--Generalized switching functions, reactive power control, static var compensators, SVC analytical models.

The subsynchronous resonance (SSR) phenomenon may occur when a steam turbine-generator is connected to a long transmission line with series compensation. The main purpose of this work is to verify the capability of the gate-controlled series capacitor (GCSC) to mitigate SSR. For this study the GCSC was tested in conjunction with the IEEE First Benchmark Model. The actual possibility of controlling SSR was verified by digital simulation using the Alternative Transients Program/Electromagnetic Transients Program. As a result it was shown that GCSC can damp SSR even without a specific control. It was also shown that, using a simple controller, SSR as well as electromechanical oscillation can be damped. Index Terms--Controlled series compensation, flexible ac transmission system (FACTS), GCSC, IEEE First Benchmark, subsynchronous resonance (SSR).

This paper describes the GTO Controlled Series Capacitor - G4CSC, an equipment for controlled series compensation of transmission systems. The principles of operation of the GCSC are reviewed and harmonic analysis is performed, showing how much of voltage harmonics are produced by the GCSC. Novel configurations using multi-module and multi-pulse GCSC's are proposed in order to reduce the voltage harmonics. Digital simulations using the EMTP are presented to validate the proposed configurations. Index Terms--FACTS, GTO, series compensation.

The basic theoretical analysis for series connection of semiconductor power switches (valves) for zero-voltage switching (ZVS) is presented. The influences of the switching device parameters on its voltage distribution during steady and transient-state conditions are analyzed. When ZVS technique is used, the design of the voltage equalization elements is easier than when hard switching is used. Simulated results are presented to validate the analysis and design methodology. This work is important for very high-power soft-switching valves and converter designs. As an example to validate the theory, a study based on the gate turn-off thyristor (GTO)-controlled series capacitor is presented. Index Terms--Controlled series capacitor, GTO valves, series connection of switches.

An effective method of eliminating numerical oscillations in switching functions due to Gibbs phenomenon in mathematical representations is introduced in this letter. Switching functions for the gate-controlled series capacitor (GCSC) are presented to illustrate the proposed technique. Index Terms--FACTS devices, Gate-Controlled Series Capacitor (GCSC), switching functions.

• The analytical model characterized the AC and DC sides of an MMC in frequency domain. • The analytical model showed the influence of control gains in impedances/admittances. • The analytical model can be used for time-domain simulations. • The analytical model predicted the stability of a power-electronics-based grid. This paper presents a linearized Thévenin-equivalent circuit for a grid-forming Modular Multilevel Converter (MMC) under double closed-loop control, comprehending an outer voltage control-loop together with an inner current control-loop. Both the AC-side Thévenin model and the equivalent admittance of the DC-side were derived in this work. The proposed MMC model takes into account the influence of passive elements and the control-loop effects on the impedances and gains. With this model it was possible to observe how the control system parameters changes resonant frequencies in both DC-side equivalent admittance and AC-side equivalent model. A comparison between the frequency response of the proposed analytical model and the frequency response obtained from the nonlinear time-domain model is presented to validate the proposed MMC model. The proposed model allows analyzing the dynamic response of the MMC as grid-forming converter in the frequency domain. Finally, as an application example, the stability of an ordinary power-electronic-based system (comprised of a grid-forming MMC supplying a current controlled MMC) was assessed and PSCAD/EMTDC simulations are presented to validate the results predicted by the proposed model. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of a new high voltage DC (HVDC) transmission system, which is based on current and voltage source converters (CSC and VSC) in the same circuit. This proposed topology is composed of one CSC (rectifier) and one or more VSCs (inverters) connected through an overhead transmission line in a multiterminal configuration. The main purpose of this Hybrid HVDC (H2VDC), as it was designed, is putting together the best benefits of both types of converters in the same circuit: no commutation failure and system’s black start capability in the VSC side, high power converter capability and low cost at the rectifier side, etc. A monopole of the H2VDC system with one CSC and two VSCs—here, the VSC is the Modular Multilevel Converter (MMC) considered with full-bridge submodules—in multiterminal configuration is studied. The study includes theoretical analyses, development of the CSC and VSCs control philosophies and simulations. The H2VDC system’s behavior is analyzed by computational simulations considering steady-state operation and short-circuit conditions at the AC and DC side. The obtained results and conclusions show a promising system for very high-power multiterminal HVDC transmission. [ABSTRACT FROM AUTHOR]

• Power injection variations due to Distributed Generators can cause voltage fluctuation. • Grids with resistive lines and unbalanced loads needs of voltage magnitude and phase compensation. • The Dynamic Direct Voltage Compensator (D2VC) can regulate voltage profile. • D2VC control is based on the estimation of the instantaneous symmetrical components of the voltage. This work proposes a voltage regulator to solve quality issues at the point of common coupling (PCC) of distribution grids affected by the intermittency of renewable energy sources. The proposed solution is a voltage compensator based on single-phase voltage source converter (VSC) units, called Dynamic Direct Voltage Controller (D2VC). The three-phase D2VC topology comprises three single-phase VSC back-to-back units. For each unit, one converter is connected in series with the grid at the PCC and the other converter is connected in shunt similar to an UPFC (Unified Power Flow Controller). To avoid coupling transformer, the series converter is directly connected in series to the existing distribution transformer thus no extra transformer is necessary. The D2VC can regulate the three-phase voltage profile at the PCC and balance all phases independently. The control strategy for each series converter is based on the estimation of the instantaneous symmetrical components of the voltage. Simulation results using PSCAD/EMTDC and an experimental platform of the D2VC are shown to confirm all the D2VC advantages in grids with resistive lines (R >> X). The proposed solution is much lighter and effective than the conventional indirect voltage control (reactive power control) and more robust than on load tap changer transformer. [ABSTRACT FROM AUTHOR]