Jozef Ritonja

Decentralized Control for Active Magnetic Bearings

In the paper a decentralized control for active magnetic bearings is discussed. For the numerical evaluation of different control algorithms a mathematical model of the controlled plant is determined. For this reason basic characteristics and fundamental operation principle of magnetic levitation as well mathematical model of the rigid shaft are presented. Modern control systems for magnetic levitations are discussed. The majority of industrial applications of active magnetic bearings were still based on conventional PID control system. The control system design is presented as rule based procedure. The sensitivity of control system on supplementary dynamics, parameter variations and measurement noise is analyzed. The active magnetic bearing laboratory system is shown. The obtained simulation and experimental results are compared

Effects of saturation and hysteresis on magnetisation dynamics: Analysis of different material models

Purpose – The purpose of this paper is to provide a comprehensive analysis of different material models when observing the magnetisation dynamics and power losses in non-oriented soft magnetic steel sheets (SMSSs). Design/methodology/approach – During the analysis four different magnetic material models were used for describing the static material characteristics, which characterised the materials’ magnetisation behaviour with increasing accuracies: linear material model, piecewise linear material model, non-linear H(B) characteristic and the static hysteresis material model proposed by Tellinen. The described material models were implemented within a parametric magneto-dynamic model (PMD) of SMSSs, where the dynamic responses as well as power loss calculations from the obtained models were analysed. Findings – The momentous influences of various levels of detail on the calculation of dynamic variables and power losses inside SMSS with non-uniform magnetic fields were elaborated, where various static mate...

Estimation of Area's Frequency Response Characteristic During Large Frequency Changes Using Local Correlation

This paper discusses methods for estimation of areas frequency response characteristic (AFRC) during large frequency changes. The operating point changes significantly during such transient events; consequently the estimation method should adapt to the time-varying nature of the AFRC. Moreover, available measurements have a poor time resolution and are considerably contaminated by errors. Therefore, new estimation methods are proposed that are based on local correlation between the frequency deviation and interchange power variation measured on all areas tie-lines. The presented estimation methods were compared with a classical approach that is based on the ratio of mean changes as well as with conventional least squares parameter estimators. Extensive numerical simulations and field measurements were applied for performance evaluation, where the obtained results show robust and satisfactorily accurate responses of the proposed estimation methods.

Adaptive control for damping of power system oscillations

The paper treats the simple adaptive control for stability improvement of a synchronous generator connected to an infinite bus. Presented are: the influence of the operating point on the eigenvalues of the synchronous generator simplified linear model, the simple adaptive power system stabilizer derived from the model reference adaptive control based on command generator tracker theory, the effects of changing the design parameters and the bursting phenomenon. The control system design was carried out for the simplified linear model of a synchronous generator but for the evaluation of the control system performance the full order non-linear model of the synchronous generator was used. Simulation and experimental results show the applicability of the presented approach.

Optimal operating point of medium frequency resistance spot welding systems

Resistance spot welding manuals provide a range of recommended Welding Parameters (WP) which produce a strong weld. The welds strength is tested out with, e.g. the tensile-shear strength test. Because different sets of WP can produce a weld with the same strength, the operator can select different sets of WP. However, the welding systems efficiency might be different when using different sets of WP. Therefore, this paper investigates the efficiency of the Medium-Frequency Direct-Current Resistance-Spot-Welding (MFDC RSW) system when operating with different sets of WP. Because the released heating energy in the welding spot is determined by the welding current value and the welding time, only these two WP are analyzed. The aim of this paper is to determine the set of WP at which the MFDC RSW system operates at the highest possible efficiency.

Evaluation of Load Frequency Control Performance Based on Standard Deviational Ellipses

This paper discusses new criterion for the evaluation of load frequency control (LFC) performance. The proposed LFC performance evaluation is based on a standard deviational ellipse that characterizes a bivariate relationship between the frequency deviation and the areas interchange power variation. The standard deviational ellipse is evaluated using Tylers maximum-likelihood estimator, which is distribution-free within the family of elliptical distributions. In addition, a target standard deviational ellipse is proposed, using predefined values for a target frequency bound and the areas frequency bias. The presented approach is based on the accordance of the estimated ellipse with the target ellipse, resulting in several grades for the LFC performance. A comparison of the proposed criterion is made with other LFC performance criteria known from the literature. Extensive measurements were applied, and the obtained results show the correctness of the proposed approach.

Impact of the Switching Frequency on the Welding Current of a Spot-Welding System

This paper deals with an analysis of a middle-frequency resistance spot-welding (RSW) system with a dc welding current controlled by a pulsewidth modulated inverter, which supplies a welding transformer with a full-wave rectifier mounted at the secondary. Welding transformers in the automotive industry are usually mounted on the arm of a moving robot, so the weight is important. To achieve the same welding power with a large welding current, the transformers weight can be reduced with a higher PWM switching frequency. This higher frequency allows a reduction in the transformers iron-core cross section with shorter primary and secondary windings. Unfortunately, the leakage inductances prevent the RSW system from achieving the same nominal welding current at higher frequencies. The frequency-dependent maximum welding current is a characteristic behavior of the RSW system, which can be determined by sophisticated and time-consuming simulations or with the expensive measurements. The third option presented in this paper is determination of an analytical solution, which allows calculation of the maximum welding current as function of frequency or any parameter of the RSW system circuit model. The analytically calculated frequency-dependent function of the maximum welding current was completely confirmed by measurements on an industrial RSW system and by numerical simulations.

Analysis of ACE target level for evaluation of load frequency control performance

This paper discusses the determination of a target level for an area control error ACE that is used for the evaluation of a load frequency control (LFC) performance. According to the new ENTSO-Es network code on LFC a statistic model should be used which assumes random and normally distributed averaged ACE and frequency deviation. However this is not experienced in practice. Modifications are proposed based on the performed analysis of field measurements within an anonymous control block in a synchronous area Continental Europe (CE), whereas applying the standard frequency range. Moreover, the optimistic and pessimistic dependency models were also tested, whereas the optimistic dependency model was found to be more realistic. The obtained results show the correctness of the modified ENTSO-Es model for the discussed CE control block.

Correlation-based estimation of area's frequency response characteristic during large disturbances

This paper presents different correlation-based estimators for an areas frequency response characteristic during large disturbances. All the proposed estimators are based either on the overall or the local Pearson correlation between the frequency deviation and the interchange power variation. Extensive numerical simulations and field measurements were applied for the performance evaluation, while the obtained results were also compared with the classic ENTSO-E approach. Considering the requirements for the measurement error, the obtained results showed satisfactorily accurate responses for the proposed estimators.

Direct and indirect adaptive control for synchronous generator semiconductor's excitation system

The paper discusses the stability problem of a synchronous generator connected to an infinite bus. From the comprehensive and systematic eigenvalue analysis of the simplified linearized mathematical model of the synchronous generator it is evident that the variations in the dynamics of the synchronous generators are considerable. Therefor the conventional power system stabilizers with fixed parameters do not ensure damping of the contemporary synchronous generators operating in modern power systems at peak performances. It was found out that the adaptive control represents the most logic choice for high performance power system stabilizers. In the paper a detailed survey of the available adaptive control concepts is presented. Featured is the possibility of application of (-) direct adaptive control, where a signal from controller is calculated directly without intermediate identification of the controlled plant, as well as (-) indirect adaptive control with separate identification of the controlled plant and the controller with variable parameters calculated from identified model.