A method for synchronization of power electronic converters in polluted and variable-frequency environments

This paper presents a new synchronization method which employs an enhanced phase-locked loop (EPLL) system. The operational concept of the EPLL is novel and based on a nonlinear dynamical system. As compared with the existing synchronization methods, the introduced EPLL-based synchronization method provides higher degree of immunity and insensitivity to noise, harmonics and other types of pollutions that exist in the signal used as the basis of synchronization. The salient feature of the EPLL-based synchronization method over conventional synchronization methods is its frequency adaptivity which permits satisfactory operation when the centre frequency of the base signal varies. The proposed EPLL-based method of synchronization is also capable of coping with the unbalanced system scenarios. Structural simplicity of the EPLL-based method greatly simplifies its implementation in digital software and/or hardware environments as an integral part of a digital control platform for power electronic converters. The primary application of the proposed synchronization method is for the distributed generation units, e.g., wind generation systems, which utilize power electronic converters as an integral part of their systems.     

Methodology and practice of in situ differential scanning electron microscopy

Contrast plays a crucial role both in qualitative and quantitative imaging in scanning microscopy. Usual methods of obtaining high contrast images in the scanning electron microscope (SEM) involve performing specific operations on the video signal already produced by the SEM. In this article, the concept of in situ differential imaging in the SEM is discussed. In this imaging modality, a true differential image of the sample is generated simultaneously with the normal video. The signal can be obtained at low and high video band-widths, thus allowing low contrast objects to be readily imaged. Various methodologies developed to perform in situ differential imaging are reviewed. A characteristic of all these techniques is their sensitivity to edges, a feature which is extensively used in a number of applications. The ability to obtain feature enhancement in any desired direction is another important attribute of this approach. Examples are given on the use of the method in general imaging as well as in the metrology of critical dimensions.     

Analysis of Ferroresonance Modes in Power Transformers Using Preisach-Type Hysteretic Magnetizing Inductance

The magnetic hysteresis phenomenon plays an important role in the ferroresonance behavior of a transformer. However, most existing transformer models for the analysis of the ferroresonance phenomenon either ignore the hysteresis effects or represent it based on the hysteresis major loop and scale it for lower current levels. Such a modeling approach does not reflect the actual physical behavior of the magnetic core with respect to the ferroresonance phenomenon. This paper, based on Preisach theory, introduces an accurate model of hysteretic inductor to represent a single-phase transformer for the investigation of the ferroresonance phenomenon. Based on the developed model, time-domain simulation studies show that even in the case of a single-phase transformer where no magnetic coupling exists between the three-phases, all known types of ferroresonance, including fundamental, sub-harmonic, quasi-periodic and chaotic modes, may occur. The effects of hysteresis and its initial conditions, grading capacitors and transformer winding capacitance on occurrence of ferroresonance and its change of mode are also discussed     

Experimental performance evaluation of a wavelet-based on-linevoltage detection method for power quality applications

This paper evaluates the performance of a wavelet-based, online (real-time) voltage detection scheme for power quality applications. The objectives are: (1) to demonstrate suitability of the proposed method in detecting faults/disturbances in a power system; and (2) to compare its performance with that of a conventional scheme. Two (STS) systems are chosen as frameworks for comparison; a low-voltage laboratory STS setup for which measured results are provided, and a medium-voltage STS system for which detection times are derived based on simulation, using the EMTDC/PSCAD     

Small-signal dynamic model of a micro-grid including conventional and electronically interfaced distributed resources

A systematic approach to small-signal modelling of a micro-grid system that includes conventional (rotating machine) and electronically interfaced distributed resource (DR) units is presented here. The proposed approach incorporates fundamental frequency deviations in the overall system model and provides a methodology for the analysis of autonomous micro-grid, which inherently is more prone to frequency changes than the conventional utility grid. The model represents (i) electro-mechanical dynamics of the synchronous machine including the exciter and the governor systems, (ii) dynamics of the voltage-sourced converter and its real/reactive power controllers and (iii) the network dynamics. The model is intended for the controller design/optimisation, evaluation of angle/voltage stability, investigation of torsional dynamics, controller interactions of electronically interfaced DR units and low-frequency power quality issues. Typical results from application of the proposed modelling approach to a study system are presented. The results are qualitatively verified on the basis of the comparison with those obtained from time-domain simulation in the PSCAD/EMTDC environment     

An accurate model for the DC-side voltage control of the neutral point diode clamped converter

This paper presents an in depth analysis to show that the conventional power balance model for the dc-side control of the Neutral Point Diode Clamped (NPC) converter is not adequately accurate for all operating conditions. The inaccuracy is due to the fact that the instantaneous power of the ac-side inductors is assumed to be negligible in the power balance equation. This paper shows that this assumption obscures (i) the impact of operating point which is a direct consequence of the inherent nonlinear nature of the problem and (ii) the nonminimum-phase property of the plant in the rectifying mode of operation. Thus, the conventional dc-side control model may lead to unsatisfactory performance or even instability. To overcome the shortcoming, this paper also develops accurate nonlinear and linearized models for the dc-side controller design, including the impact of the ac-side interface reactors. The analytical results and the conclusions are validated by digital time-domain simulation of a study system in the PSCAD/EMTDC environment. The mathematical developments and conclusions of the paper are equally applicable to the conventional two-level VSC.     

Fast ferroresonance suppression of coupling capacitor voltage transformers

This paper describes a procedure for fast suppression of the phenomenon of ferroresonance in coupling capacitor voltage transformers (CCVT) without major change in the CCVT design. It is shown that it is possible to adjust parameters of the secondary overvoltage protection and the filter circuit so that the ferroresonance can be cleared in a very short time interval. The study cases reported in this paper show that ferroresonance is effectively cleared within two cycles. An implementation of metal oxide varistors (MOV) as part of passive ferroresonance protection is also addressed. The Electromagnetic Transients Program (EMTP) is used for modeling transients and fine-tuning the ferroresonance suppressing circuit. The studies are conducted on the Trench TEHMP161A CCVT.     

Order reduction of the dynamic model of a linear weakly periodic system-part II: frequency-dependent lines

The methodology for the order reduction of the dynamic model of a linear weakly periodic system presented in the first part of this paper is applied here to the case when the power network includes frequency dependent transmission lines. The technique for obtaining the external equivalent of the network is described. A comparison is given between systems having lines with constant or frequency-dependent parameters. Time-domain simulations have shown that the reduced-order equivalent leads to fast and accurate results.     

Passivity Enforcement Using an Infeasible-Interior-Point Primal-Dual Method

Application of the network equivalent concept for external system representation in electromagnetic transient studies is well known. However, the challenge in application of an equivalent model, approximated by rational functions, is to guarantee passivity of the corresponding model. Therefore, there is a need to enforce passivity of the equivalent model. In this paper, the passivity enforcement problem is first formulated as a quadratic programming (QP) problem, and then solved based on an efficient implementation, using an infeasible-interior-point primal-dual method. An application of this method for passivity enforcement of a six-port admittance model (with large passivity violations) demonstrates its effectiveness and computational efficiency.      

On the application of the complex torque coefficients method to the analysis of torsional dynamics

The complex torque coefficients method has been widely accepted for the analysis of the phenomenon of torsional interaction of turbine-generator units in power systems. This paper shows that, depending on the system parameters and the operating point, the complex torque coefficients method may exhibit limitations and not accurately and fully predict the system behavior in the frequency range of interest. These shortcomings consist of inability to i) predict monotonic instability due to real poles, ii) identify all electromechanical oscillatory modes, and iii)accurately predict damping (and consequently stability) of the oscillatory modes. This paper develops mathematical expressions to highlight the limitations of the complex torque coefficients method. Quantitative results based on three case studies, including a study on the first IEEE Benchmark System, are reported and results from eigenvalue analysis method, complex torque coefficients method, and time-domain simulation are presented and compared. This paper concludes that the complex torque coefficients method can be used only as a preliminary method for the investigation of torsional interactions and the results must be verified by other methods.