Afshin Rezaei-Zare

Bio: Afshin Rezaei-Zare is an academic researcher from York University. The author has contributed to research in topics: Transformer & Ferroresonance in electricity networks. The author has an hindex of 20, co-authored 56 publications receiving 1007 citations. Previous affiliations of Afshin Rezaei-Zare include Hydro One & École Polytechnique de Montréal.

Compensation of the Current-Transformer Saturation Effects for Digital Relays

Abstract: A current transformer (CT) is accurately modeled for representation of the CT saturation effects on digital protective relays. Simulation studies performed in the PSCAD/EMTDC platform are used to investigate the impacts of CT saturation on the current phasor estimation. A new algorithm is also proposed for detection and compensation of CT saturation effects, based on: a least error squares (LES) filter which estimates the phasor parameters of the CT secondary current; a novel saturation detection method which uses the output of the LES filter for saturation detection; and a new minimum estimation error tracking approach which enhances the precision of the phasor estimation. The proposed saturation detection/compensation algorithm is independent of the parameters of the CT, the burden, and the power system. The study results show that the proposed algorithm: 1) reconstructs the distorted current waveform, under dc and ac saturation conditions, with the required precision and speed and 2) performs satisfactorily under inductive burden and under deep and slight saturation conditions.

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

Abstract: 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

Duality Derived Transformer Models for Low-Frequency Electromagnetic Transients—Part I: Topological Models

Abstract: The objective of this two-part paper is to provide clarity to physical concepts used in the field of transformer modeling, to dispel common misconceptions regarding numerical instabilities, and to present unified modeling techniques for low-frequency transients. This paper focuses on proper modeling of nonlinearities (magnetizing branches) since these components are critical to determine the low-frequency behavior. A good low-frequency model should properly represent: normal operation, inrush currents, open and short circuit, out-of-phase synchronization transient of step-up transformers, geomagnetic-induced currents, ferroresonance, and harmonics. This paper discusses the derivation of electrical dual models from the equivalent (magnetic) reluctance networks and the direct application of the principle of duality. It is shown that different dual models need to be derived for different transformer geometries and the advantages and disadvantages of each method are discussed. This paper also compares double-sided versus single-sided dual models, and shows that the double-sided model is a more general approach. The mathematical equivalency of several leakage models (negative inductance, mutual coupling, and BCTRAN) is demonstrated for three-winding transformers. It is also shown that contrary to common belief, a negative inductance is not the source of numerical oscillations, but they occur due to the use of noncorrect topological models for representing the core.

Simulation of Transformer Hotspot Heating due to Geomagnetically Induced Currents

Abstract: This paper proposes a method to estimate transformer hotspot heating due to half-cycle saturation caused by geomagnetically induced currents (GICs). The method is based on fitting a closed-form analytical function to the calculated or measured thermal response of a particular transformer design, typically provided by the manufacturer, at its hotspot locations to a step dc current excitation. This fitted function can then be used to simulate the hotspot temperature profile for transformers of that particular design when subjected to GIC having an arbitrary profile over time. Using this approach, examples are presented of the winding and metallic hotspot thermal responses of the two transformers to recorded GIC time sequences, to illustrate how hotspots may be impacted on during a geomagnetic disturbance event.

Optimal Design and Tradeoff Analysis of Planar Transformer in High-Power DC–DC Converters

Abstract: The trend toward high power density, high operating frequency, and low profile in power converters has exposed a number of limitations in the use of conventional wire-wound magnetic component structures. A planar magnetic is a low-profile transformer or inductor utilizing planar windings, instead of the traditional windings made of Cu wires. In this paper, the most important factors for planar transformer (PT) design including winding loss, core loss, leakage inductance, and stray capacitance have individually been investigated. The tradeoffs among these factors have to be analyzed in order to achieve optimal parameters. Combined with an application, four typical winding arrangements have been compared to illustrate their advantages and disadvantages. An improved interleaving structure with optimal behaviors is proposed, which constructs the top layer paralleling with the bottom layer and then in series with the other turns of the primary, so that a lower magnetomotive force ratio m can be obtained, as well as minimized ac resistance, leakage inductance, and even stray capacitance. A 1.2-kW full-bridge dc-dc converter prototype employing the improved PT structure has been constructed, over 96% efficiency is achieved, and a 2.7% improvement, compared with the noninterleaving structure, is obtained.

Synchrophasor Measurement Technology in Power Systems: Panorama and State-of-the-Art

Abstract: Phasor measurement units (PMUs) are rapidly being deployed in electric power networks across the globe. Wide-area measurement system (WAMS), which builds upon PMUs and fast communication links, is consequently emerging as an advanced monitoring and control infrastructure. Rapid adaptation of such devices and technologies has led the researchers to investigate multitude of challenges and pursue opportunities in synchrophasor measurement technology, PMU structural design, PMU placement, miscellaneous applications of PMU from local perspectives, and various WAMS functionalities from the system perspective. Relevant research articles appeared in the IEEE and IET publications from 1983 through 2014 are rigorously surveyed in this paper to represent a panorama of research progress lines. This bibliography will aid academic researchers and practicing engineers in adopting appropriate topics and will stimulate utilities toward development and implementation of software packages.

Intelligent DC Microgrid With Smart Grid Communications: Control Strategy Consideration and Design

Abstract: Aiming at photovoltaic (PV)-storage urban building integrated system, this paper proposes a DC microgrid with multi-layer control and smart grid communications. The paper focuses on power balancing, with load shedding and PV constrained production, and takes into account the grid availability and grid vulnerability by smart grid messages. The system behavior modeling by MATLAB Stateflow leads to the whole control strategy design, which concerns the power balancing and imposed power limits by the utility grid, while providing interface for energy management. Experimental results evaluate the feasibility of the proposed control strategy. As further development of this control design, an intelligent multi-layer supervision is suggested. This supervision, able to exchange data with the smart grid, deals with the end-user demand, forecast of photovoltaic production, prediction of load consumption, and energy management. The major technical contribution of this paper is linked to the proposed control design that permits better DC microgrid integration (avoids undesired injection, mitigates fluctuations in grid power, reduces grid peak consumption) and provides possibility to reduce the negative impact on the utility grid thanks to the supervision interface. The power balancing control interface provides possibility for advanced energy management with low speed communication.

Transformer Design and Optimization: A Literature Survey

Abstract: With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.