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This paper presents the control system based on the so-called repetitive control for a five-level flying-capacitor dynamic voltage restorer (DVR). This DVR multilevel topology is suitable for medium-voltage applications and operated by the control scheme developed in this paper. It is able to mitigate power-quality disturbances, such as voltage sags, harmonic voltages, and voltage imbalances simultaneously within a bandwidth. The control structure has been divided into three subsystems; the first one improves the transient response of the filter used to eliminate the modulation high-frequency harmonics, the second one deals with the load voltage; and the third is charged with maintaining balanced voltages in the flying capacitors. The well-developed graphical facilities available in PSCAD/EMTDC are used to carry out all modelling aspects of the repetitive controller and test system. Simulation results show that the control approach performs very effectively and yields excellent voltage regulation. Index Terms--Dynamic voltage restorer (DVR), flying capacitor multilevel converter, harmonic distortion, power quality (PQ), repetitive control, voltage sag.

An internal fault in a synchronous generator produces an effect similar to increasing the number of grouped coils in the stator winding, making it necessary to use additional time-variant inductances to represent the condition, with the ensuing increase in modeling complexity and computation time required for its solution. In this paper, a new model for the simulation of internal faults in synchronous generators is presented. The model is based on the so-called voltage-behind-reactance (VBR) representation, a contemporary reference frame, which has proved to be numerically more efficient than the classical phase-domain model used to study internal faults in synchronous generators; making it a better fit for large-scale, multimachine power systems applications, the long-term objective of this research work. An implementation for electromagnetic transients program (EMTP) type solutions is presented together with a test case where internal faults are applied, producing results that are in close agreement with results available in the open literature. Furthermore, an external perturbation is also carried out and results match exactly those produced by an equivalent VBR implementation. Index Terms--Internal fault, phase domain (PD), synchronous generator, voltage-behind-reactance (VBR).

This paper presents a new open-loop architecture for three-phase grid synchronization based on moving average and predictive filters, where accurate measurements of phase, frequency, and amplitude are carried out in real time. Previous works establish that the fundamental positive sequence vector of a set of utility voltage/current vectors can be decoupled using Park's transformation and low-pass filters. However, the filtering process introduces delays that impair the system performance. More specifically, when the input signal frequency is shifted above the nominal, a nonzero average steady-state phase error appears in the measurements. To overcome such limitations, a suitable combination of predictive and moving average finite impulse response (FIR) filters is used by the authors to achieve a robust synchronization system for all input frequencies. Moving average filters are linear phase FIR filters that have a constant time delay at low frequencies, a characteristic that is exploited to good effect to design a predictive filter that compensates such time delays, enabling zero steady-state phase errors for shifted input frequencies. In summary, the main attributes of the new system are its good frequency adaptation, good filtering/transient response tradeoff, and the fact that its dynamics is independent of the input vector amplitude. Comprehensive experimental results validate the theoretical approach and the high performance of the proposed synchronization algorithm. Index Terms--Grid-connected converters, phase-locked loop (PLL), predictive control, synchronization.

This paper presents a control system based on a repetitive controller to compensate for key power-quality disturbances, namely voltage sags, harmonic voltages, and voltage imbalances, using a dynamic voltage restorer (DVR). The control scheme deals with all three disturbances simultaneously within a bandwidth. The control structure is quite simple and yet very robust; it contains a feedforward term to improve the transient response and a feedback term to enable zero error in steady state. The well-developed graphical facilities available in PSCAD/EMTDC are used to carry out all modeling aspects of the repetitive controller and test system. Simulation results show that the control approach performs very effectively and yields excellent voltage regulation. Index Terms--Dynamic voltage restorer (DVR), harmonic distortion, power quality (PQ), repetitive control, voltage sag.

This paper presents new models of voltage source converters suitable for power system state estimation comprising STATCOM and VSC-HVDC applications. Both the back-to-back and the point-to-point VSC-HVDC variants are addressed. The state estimation models include dc measurements and converter control inputs resulting in an overall, seamless combination of the ac and dc devices or networks, in addition to providing for a higher degree of redundancy. Following a detailed description of all the relevant variables involved in the problem, taking into account equality constraints associated with both equipment and operation, a recommended set of state variables is proposed. These new models allow internal power losses of voltage source converters to be considered, leading to increased accuracy. The IEEE 14-bus system is used to illustrate and compare the main features of the proposed models. Index Terms--FACTS devices, STATCOM, state estimation, VSC-HVDC.

The paper presents the model of a voltage source converter--high voltage direct current (VSC-HVDC) suitable for optimal power flow (OPF) solutions using Newton's algorithm. The VSC-HVDC's ability to provide independent control of the converters ac voltage magnitudes and phase angles relative to the system voltage, which allows the use of separate active and reactive power control loops for system regulation, is well represented by the model. In this new development in Newton OPF, the VSC-HVDC system equations are incorporated directly into the matrix 14z for a unified optimal solution in a single frame-of-reference. The multipliers method is used to handle all inequality constraints of variables, leading to highly efficient OPF solutions of constrained power networks. The solution approach does not require structural changes in the linearised system of equations during the iterative process using Newton's method. The effectiveness of the VSC-HVDC model and its proposed implementation in Newton OPF is demonstrated by means of two sample systems. Index Terms--Newton method, optimal power flow, voltage-source converter (VSC), VSC-HVDC system.

An advanced time-domain representation suitable for accelerated periodic, steady-state solutions of large-scale electric power systems is introduced in this paper. The power system, which is three-phase in nature, is modeled by a set of ordinary differential equations. In addition to the all-important transmission lines' geometric imbalances, frequency-dependency and long-line effects, the transformers' saturation characteristics are also incorporated. In order to provide realistic initial conditions, the data needed for the modeling of the electric system is obtained from power-flow studies. The time domain solution of the overall set of differential equations is computed very reliably using a powerful blend of Newton methods and parallel processing techniques. Whereas the computation of the periodic steady-state solution is obtained with an acceleration procedure based on Newton methods and the Poincare map, the application of parallel processing techniques using multithread programming speeds up further the time taken by the acceleration process to get to the periodic, steady-state solution. To show the effectiveness and versatility of the newly developed environment, transient and steady-state analysis are carried-out for a three-phase version of the IEEE 118-node system, where nonlinearities are incorporated in the form of the transformers' saturation characteristics and a static var compensator. Index Terms--Harmonics, limit cycle, multithread programming, Newton methods, parallel processing, periodic steady-state, Poincare map.

A novel methodology is presented in this paper for conducting transient and steady-state analysis of electric networks containing nonlinearities and switching plants components. The method is termed the extended harmonic domain (EHD) method as may be seen as an extension of the harmonic domain (HD) method used in steady-state analysis. It is shown in the paper that EHD is a natural approach for conducting dynamic and steady-state studies of the evolution of harmonics in power circuits containing nonlinear components and flexible ac transmission systems (FACTS) devices. It is also shown in the paper that EHD provides a suitable basis for extending the well-known steady-state power quality indices to the transient range. To illustrate the use of the theory, a three-phase linear circuit and a three-phase PWM-STATCOM are modeled using EHD. The results are compared against those provided by industry standard tools as PSCAD/EMTDC and Matlab. Index Terms--Extended harmonic domain, Fourier series, harmonic domain, linear time periodic systems, switching functions, transient and harmonic analysis.

When a vacuum breaker is used to disconnect an unleaded transformer, high overvoltages may result on the secondary side. The phenomena of current chopping and multiple reignitions after contact opening, shortly before current zero, play a significant role in generating these overvoltages. Furthermore, the transient recovery voltage (TRV) is also influenced by virtual current chopping which may lead to fast interruptions and reignitions. The paper reports on a vacuum breaker model implemented in the ATP-EMTP program. Simulations have been performed to determine overvoltages generated by the vacuum breaker, before and after suppression. Network elements were simulated by simplified models of limited complexity. Key words: Vacuum circuit breaker, TRV, ATP-EMTP

The paper presents a new methodology for calculating magnetizing inrush currents. Similarly to the seminal work of Rajakovic and Semlyen, all the calculations are conducted in the frequency domain, hence, circumventing the many problems encountered when time domain simulations are used to solve this problem. Unlike the work of Rajakovic and Semlyen [1], where a pre-determined sequence of steady-state 'images' are obtained for a sufficiently long number of cycles to capture the complete picture of the inrush phenomenon, our method produces the full solution, with no loss of fidelity, of the overall inrush phenomenon. The solution is achieved by resorting to operational matrices and it is fully equivalent to that produced by time domain simulations but carried out entirely in the frequency domain. In principle, any orthogonal series expansion could be used with this method but in this paper Hartley series have been selected. The usefulness of the methodology is illustrated with simple but practical circuits. Comparisons against time domain simulations are given. Index Terms--Harmonic domain, Hartley series, inrush currents, operational matrices.

This paper presents a new power systems simulation environment suitable for the swift calculation of the periodic steady-state response of large power networks with large wind parks. In order to achieve the greatest computational efficiency and solution reliability, the wind generator model uses the state-of-the-art voltage-behind-reactance model of the induction generator; in this first part of the paper, the fixed-speed wind generator model is presented. The overall solving environment for the nonlinear system of differential equations representing the power system is the so-called Poincaré map method. A rather useful feature to reduce further the calculation times is the availability of an equivalent wind farm model based on multiple wakes. The Poincaré acceleration yields a vastly improved numerical solution compared to existing models of fixed-speed wind parks. The application of equivalent wind farm models yields a staggering reduction in the elapsed time to only 1.16% of the time demanded by the detailed model, while retaining a good approximation of the response of the wind farm. Full comparisons of the detailed and equivalent wind park models with PSCAD/EMTDC, are carried out and the agreement of the results is excellent. In this paper the emphasis is on harmonic generation. [ABSTRACT FROM PUBLISHER]