Purpose -the change in the way of active power filters (APF) location can lead to overall cost reduction due to less number or less power of APFs required. The paper goal was to minimize the APF currents what is equivalent to solution with less apparent power of installed devices. The next step consists in development of new methods of APF optimal location. Design/methodology/approach -some scripts integrating optimization and harmonic analysis methods in Matlab and PCFLO software environments have been developed in order to achieve the goal. Findings -solution to the minimization problem determines the current spectrum of an APF connected to a selected system bus in accordance with some optimization strategies which among others enable minimization of THDV coefficients. Research limitations/implications -the APF control algorithm defined in the frequency domain and based on given current spectrum could lead to some problems with synchronization between APF instantaneous current and compensated current waveforms. Originality/value -there are many papers on APFs but usually systems in which an APF is connected near a nonlinear load are analyzed. Some attempts to solve the more complex problems of synchronized multipoint compensation have been already made but there is still no generally accepted and commonly used solution.

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This paper presents the results of recently conducted research on Luenberger observers with non-proportional feedbacks. The observers are applied for the reconstruction of magnetic fluxes of an induction motor. Structures of the observers known from the control theory are presented. These are a proportional observer, a proportional-integral observer, a modified integral observer, and an observer with additional integrators. The practical application of some of these observers requires modifications to their structures. In the paper, the simulation results for all mentioned types of observers are presented. The simulations are performed with a Scilab-Xcos model which is attached to this paper. The problem of gains selection of the observers is discussed. Gains are selected with the described optimization method based on a genetic algorithm. A Scilab file launching the genetic algorithm also is attached to this paper.

Abstract. The significant problem of compensator placement and sizing in electrical networks has been analyzed in the paper. The compensation is usually realized by means of passive or active power filters. The former solution is widely used mainly because of the economical reasons, but the latter one becomes more and more popular as the number of nonlinear loads increases. Regardless of the compensator type the most important goal consists in voltage and current distortion drop below levels imposed by standards. Nevertheless, the desired effects should be achieved with the minimum cost. So far a few objective functions have been proposed for this optimization problem. It is claimed that minimization of the compensator currents leads also to the minimum costs. This paper shows that such simplified approach could lead to suboptimal solutions and in fact a function g(·) reflecting the relation between the compensator size and its price must be incorporated into objective functions. Moreover, in this case it is very easy to compare solutions obtained using compensators offered by different suppliers -it is enough to change the function g(·). Theoretical considerations have been illustrated by an example of active power filter allocation and sizing.

Abstract. The paper discusses the problem of stability of a proportional-integral Luenberger observer, designated for the state variables reconstruction of a linear, time-invariant dynamical system. It is proven, that there exists such a class of observed systems, for which the observer is always unstable, independently of its gains. Stability can be provided in every possible case after application of proposed modifications to the structure of the observer. It is proven, that stability of the modified observer depends only on its gains. It is shown, that an induction motor is the exemplary observed system, for which application of the unmodified observer is impossible due to its lack of stability, while the modified observer provides proper operation of the control system. Finally, some experimental results are presented, obtained in the multiscalar control system of the induction motor, equipped with the modified proportional-integral observer.