Eduardo Campero-Littlewood

Synchronous Machine Parameters from Frequency-Response Finite-Element Simulations and Genetic Algorithms

This paper presents a novel way to obtain parameters of synchronous machine equivalent circuits from standstill frequency response data using a hybrid genetic algorithm. The genetic algorithm is capable of finding a global minimum within a search interval of the fitness function used to match the equivalent circuit and the measured machine transfer functions, notwithstanding the initial guess of the identification process. Therefore, methods such as the maximum likelihood estimation technique could be substantially enhanced. Results obtained in the identification procedure show that good matching can be obtained with either negative or positive leakage inductance values. These results cast some light on the possible physical meaning that circuit parameters may have. Finite-element modeling is used here to determine the transfer functions of a turbine generator. This approach is consistent with the general aim of obviating the requirement of field testing.

Applications of coupled field formulations to electrical machinery

Purpose – This paper aims to give a perspective about the variety of techniques which are available and are being further developed in the area of coupled field formulations, with selective bibliography and practical examples, to help postgraduate students, researchers and designers working in design or analysis of electrical machinery.Design/methodology/approach – This paper reviews the recent trends in coupled field formulations. The use of these formulations for designing and non‐destructive testing of electrical machinery is described, followed by their classifications, solutions and applications. Their advantages and shortcomings are discussed.Findings – The paper gives an overview of research, development and applications of coupled field formulations for electrical machinery based on more than 160 references. All landmark papers are classified. Practical engineering case studies are given which illustrate wide applicability of coupled field formulations.Research limitations/implications – Problems ...

Hybrid genetic algorithm for the identification of high-order synchronous machine two-axis equivalent circuits

Abstract A hybrid genetic algorithm is used to find high-order equivalent circuits (ECs) of synchronous machines using standstill frequency response (SSFR) data. The algorithm performs satisfactorily despite the great deal of local minima surrounding the optimal solution of high-order ECs. It gives circuit parameters that simultaneously fit the three independent transfer functions given by the d-axis two-port network of the synchronous machine. It is found that as the order of the EC is increased, the optimization index used in the identification procedure is enhanced in a clear fashion. This leads to a new way for determining the right number of rotor branches required to correctly reproduce the SSFR data. The q-axis network is also analyzed with the hybrid algorithm. The so-called Canays inductances are included in this one-port network to test if the fitting properties of the q-axis EC can be improved. The SSFR data used in this work is generated by a finite element model of a turbine generator, but actual data can also be readily handled.

A comprehensive finite-element model of a turbine-generator infinite-busbar system

This paper shows the development of a two-dimensional finite-element magnetic model of a turbine generator coupled to an infinite busbar through a transmission line and a delta-star connected transformer. The finite-element equations of the machine are strongly coupled to the circuit equations of the transmission line and transformer. The circuit equations developed for the delta-star connection enable the simulation of several short circuit conditions at the transformer terminals with the aid of fault matrices. The calculation of electromagnetic torque is performed by surface integration of the Lorentz force, which leads to a unique result for the finite-element model used in this work. The coupling with the external controls of the machine (automatic voltage regulator and governor) is performed in a weak form. Nevertheless, any control model can be easily connected to the finite-element model. Since the use of this finite element model is meant for transient simulations, it is desirable to obtain some simplifications to speed up calculations. Hence, a three-phase current sheet is proposed in this work to avoid the explicit modelling of the stator coreback. This approach also leads to a simple approach to model motion, where remeshing is not necessary.

New Analytical Formulas for Electromagnetic Field and Eddy Current Losses in Bushing Regions of Transformers

This paper presents a new and rigorous analytical calculation of electromagnetic field and eddy current losses in the zones of transformer tanks where bushings are mounted. This is done by solving Maxwells equations in the regions surrounding bushings, with the corresponding boundary conditions and considering linear permeability. Then, by solving the modified Bessels equation, the analytical formulas to calculate the magnetic field and eddy current losses in these regions are obtained and several cases are studied. The results are compared with 3-D finite element simulations and show very close correspondence. The obtained formulas allow straightforward calculations that can help designers to select proper parameters to optimize the design of transformers. This paper can be taken as the basis for the analysis of the nonlinear permeability case.

Techno-economic Evaluation of Reduction of Low-voltage Bushings Diameter in Single-phase Distribution Transformers

Abstract The contributions of this article are the analysis and economic evaluation of the impact on tank wall losses of a diameter reduction of low-voltage bushings of pole-mounted single-phase distribution transformers. Finite element simulations of 5- to 167-kVA transformers were performed. The study was motivated when bushing manufacturers reduced diameter from 4.6 to 3.6 cm. Results show that when the diameter of low-voltage bushings is reduced, (i) load losses increase and (ii) total owning cost decreases for transformers up to 15 kVA and increases for transformers of 25–167 kVA. The insertion of non-magnetic material between bushing holes is also evaluated.

Efficient finite-element computation of synchronous machine transfer functions

An efficient two-dimensional finite-element (FE) model is developed for the calculation of synchronous machine transfer functions. The numerical model uses two equivalent sinusoidally distributed stator windings substituting the actual three-phase machine by an equivalent two-phase one, leading to simplified FE simulations. Just two complex solutions per frequency are needed to obtain the three transfer functions that completely describe the two-port nature of the d axis network. A new FE-based method is proposed to accurately establish the rotor base quantities, allowing the calculated transfer functions to be in per-unit. Results are validated by comparing the performance of a two-axis equivalent circuit, derived from the FE transfer functions, with a reference transient FE program.

Separation of core losses in distribution transformers using experimental methods

The separation of eddy current and hysteresis losses in transformer cores is obtained using the two-temperatures and the two-frequency methods. Loss calculations for six ratings using the voltage test waveform ratio are included to compare and analyze results. A brief description of the test methodologies, that are easy to apply, is given. An example of the application of the methodologies, the obtained measurements and obtained results are included. In some cases, the results show that eddy current losses for the analyzed ratings are greater than 60% of no-load losses. Results include the impact of no-load losses in total owning cost of transformers.

Testing robustness and performance of PSS–AVR schemes for synchronous generators using finite-element models

The design and tuning of power system stabilizers (PSS) and automatic voltage regulators (AVR) of synchronous generators is usually performed using low-order two-axis models. Remarkable robust characteristics and systematic construction can be obtained with the recent developments on linear and nonlinear control theory. However, the properties and performance of the control designs are also evaluated using low-order equivalent-circuit models because large synchronous machines are not readily available to designers and researches. This may lead to doubts about the controller behavior under true operating conditions. The main objective of this work is to show that robust control designs, obtained with small order models, keep their good performance characteristics when applied to the actual machine. In order to circumvent the lack of the real machine, a very detailed finite-element model is developed to represent turbine generators connected to large power systems. The numerical model incorporates simulation of rotor motion, iron magnetic nonlinearity and eddy currents in the solid rotors of turbine generators. The control design is performed using constructive nonlinear control, which gives a systematic controller construction coupled to a simple tuning scheme.

Calculation of Nonlinear Electromagnetic Fields in the Steel Wall Vicinity of Transformer Bushings

Successful analytical formulas have been previously proposed to calculate the losses in tank regions of transformers assuming linear permeabilities in the analyzed boundary-valued problem. This has resulted in easy-to-implement and low-cost computational design procedures from a transformer factory economical point of view. However, designers and analysts of transformers are constantly seeking new ways of reducing transformer losses in actual power networks with thousands of transformers. As a result, this paper has focused on proposing new analytical formulas to determine the electromagnetic field in bushing regions of transformers, taking account of the true nature of the nonlinear permeability behavior of the tank wall. This way, the nonlinear Maxwells equations in the regions surrounding the bushings are solved using an integral equation formulation that properly includes boundary conditions. A practical iterative procedure is thus proposed to solve the resulting nonlinear equation. The iterative scheme shows excellent numerical convergence properties with a very low computational demand as compared with finite-element (FE) nonlinear models. A comparison between our analytical results and those of 3-D FE simulations reveals a close match for a wide range of conductor currents. Hence, our new formulas can be used to improve the design of transformers, increasing their efficiency.