Nicola Chiesa

Bio: Nicola Chiesa is an academic researcher from Equinor. The author has contributed to research in topics: Transformer & Inrush current. The author has an hindex of 13, co-authored 23 publications receiving 585 citations. Previous affiliations of Nicola Chiesa include SINTEF & Norwegian University of Science and Technology.

Transformer Model for Inrush Current Calculations: Simulations, Measurements and Sensitivity Analysis

Abstract: The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation.

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.

Power Transformer Modeling for Inrush Current Calculation

Abstract: In this thesis a general purpose transformer model for low and medium frequency transient studies is developed and verified. The inrush current phenomena during no-load energization of power transf ...

Inverse Hysteresis Models for Transient Simulation

Abstract: History-dependent and history-independent models of magnetic hysteresis are proposed. Spline approximations to the magnetization curves make these models applicable to major loops of any shape. The H(B) representation of reversal curves permits the development of a method that generates reversal curves guaranteed to be free from nonphysical negative slopes and excursions outside the major loop. The applicability of the models to electrical steels and a powder core is illustrated.

Novel Approach for Reducing Transformer Inrush Currents: Laboratory Measurements, Analytical Interpretation and Simulation Studies

Abstract: Transformer inrush currents can lead to a reduction of transformer lifetime and inadvertent tripping of relays. This paper investigates a novel approach for minimizing the inrush current with a potential application in circuit-breaker (CB) control strategies without independent-pole operation and residual flux estimation. For the analyzed transformer, the worst-case inrush current is halved compared to the rapid-closing switching strategy. Measurements of inrush current transients are performed on an unloaded 11-kV distribution transformer varying disconnection and connection instants systematically. This reveals a characteristic pattern in the extremal value of the inrush current as a function of switching times. The pattern is reproduced with simulations and extended to alternative winding configurations. A condition for minimum inrush currents, consistent for all phases and winding configurations, is identified and explained physically. The impact of the current chopping capability of the CB is important and is discussed in this paper.

Hybrid Transformer Model for Transient Simulation—Part I: Development and Parameters

Abstract: A new topologically correct hybrid transformer model is developed for low and midfrequency transient simulations. Power transformers have a conceptually simple design, but behaviors can be very complex. The selection of the most suitable representation for a given behavior depends on the type of transformer to be simulated, the frequency range, and other factors, such as the internal design of the transformer and available parameters or design data. Here, a modular model suitable for frequencies up to 3-5 kHz is developed, utilizing a duality-based lumped-parameter saturable core, matrix descriptions of leakage and capacitive effects, and frequency-dependent coil resistance. Implementation and testing of this model was done for 15-kVA 208Delta-120Y 3-legged and 150-kVA 12,470Y-208Y 5-legged transformers. The basis and development of the model is presented, along with a discussion of necessary parameters and the approaches for obtaining them

DGs for Service Restoration to Critical Loads in a Secondary Network

Abstract: During a major outage in a secondary network distribution system, distributed generators (DGs) connected to the primary feeders as well as the secondary network can be used to serve critical loads. This paper proposed a resilience-oriented method to determine restoration strategies for secondary network distribution systems after a major disaster. Technical issues associated with the restoration process are analyzed, including the operation of network protectors, inrush currents caused by the energization of network transformers, synchronization of DGs to the network, and circulating currents among DGs. A look-ahead load restoration framework is proposed, incorporating technical issues associated with secondary networks, limits on DG capacity and generation resources, dynamic constraints, and operational limits. The entire outage duration is divided into a sequence of periods. Restoration strategies can be adjusted at the beginning of each period using the latest information. Numerical simulation of the modified IEEE 342-node low voltage networked test system is performed to validate the effectiveness of the proposed method.

Dynamic State Estimation-Based Protection: Status and Promise

Abstract: The introduction of the microprocessor-based numerical relay in the 1980s resulted in multifunctional, multidimensional, communications-enabled complex protection systems for zone and system protection. The increasing capabilities of this technology created new unintended challenges: 1) complexity has increased and selecting coordinated settings is a challenge leading to occasional miscoordination; 2) protection functions still rely on a small number of measurements (typically three voltages and three currents) limiting the ability of protection functions to dependably identify the type of fault conditions; and 3) present approaches are incapable of dealing with hidden failures in the protection system. Statistically, 10% of protection operations are misoperations. This paper presents a new approach to protection that promises to eliminate the majority of the problems that lead to misoperations. The approach is described, demonstrated in the laboratory, compared to traditional protection functions and its application to a substation coordinated protection system capable of detecting and dealing with hidden failures is described. This paper also discusses the planned field testing of the approach.

Transformer Model for Inrush Current Calculations: Simulations, Measurements and Sensitivity Analysis

Abstract: The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation.