Search results

The paper proposes to deal with the problems concerning the N‐phase (N+1)‐wire system with sinusoidal voltage sources and nonlinear loads. In the model of the N‐phase voltage source the inner impedance has been included. The problem of the optimization of working conditions of the system is a minimization of RMS value of its line currents as well as their distortions caused by nonlinear loads.

The solution of this problem is based on the frequency domain. It is obtained by means of Lagrange's multipliers and the suitable measurement experiment.

After optimization source currents are sinusoidal with minimized RMS values. After connection of the designed compensator to the system under research the phase currents are equal to determined active currents.

This method can be used for some classes of nonlinear loads, i.e. for systems with inertialess (non‐reactive) elements, which consume the active power of the basic harmonic of the voltage source and where the currents of the system are periodical. The mentioned power is an additional constraint of the presented minimization.

The working point of the system can be obtained by means of the compensator LC, RLC or (RLC,‐R). It will always be a linear one and its structure consists of two components: elements with parameters determined for the basic harmonic and the filter for elimination of the higher harmonics caused by nonlinear loads.

The presented method has been generalized for N‐phase (N+1)‐wire systems.

The problems presented in the paper concern a three‐phase four‐wire system with periodical non‐sinusoidal voltage sources with inner impedances and asymmetrical linear three‐phase loads. Generally, the line currents of the system are asymmetrical. The purpose of the paper is to improve the working conditions of the system by means of symmetrization.

A method of symmetrization of these systems has been proposed. In this paper, the symmetrization problem has been solved by using the symmetrical components theory and compensation of reactive power for each of voltage harmonics under consideration.

After symmetrization currents become symmetrical and their RMS values and active power on source impedances become lower. The realization of symmetrization makes possible: reduction of RMS values of source currents, an assurance of equal loads for individual phases of the system supplied from sources with inner impedances, after symmetrization the voltage source generates and load consumes greater useful active power.

The simplified structure of the compensator has been proposed in the paper. The symmetrization has been presented with reference to new structure of the compensator.

The purpose of this paper is to deal with the problem of symmetrization of asymmetrical three‐phase delta connected nonlinear load. In the model of the three‐phase sinusoidal voltage source have also been included inner impedances. The purpose is to obtain symmetrical line currents of the voltage source, to minimize RMS values of currents and to minimize higher harmonics generated by nonlinear loads.

This symmetrization of the system is realized by means of a symmetrizing system, which is composed of LC one‐ports. In order to solve the problem the symmetrical component theory is applied. The structure of symmetrizing system is consisted of two components: parameters determined for the basic harmonic and the filter for elimination of the higher harmonics generated by nonlinear loads.

After symmetrization line currents of the source will be symmetrical with lower RMS values than before symmetrization, and the source will generate the greater active power than beforehand.

This approach can be used for inertialess (non‐reactive) elements in systems, where currents are periodical.

The results of symmetrization can be useful for high‐power systems where LC one‐ports can be used, e.g. for arc furnaces.

Application of presented methods makes possible to improve the working point of the system, i.e. voltage source can generate greater active power than before symmetrization and line currents can be symmetrical.

The problems presented in the paper deal with the three‐phase three‐wire system with sinusoidal voltage sources and nonlinear loads. In the model of the three‐phase voltage source the inner impedance has been included. In contrast to earlier proposed methods, the solution of this problem is based in the frequency domain. This method can be used for systems with some classes of nonlinear loads, i.e. for inertialess elements, which consume the active power of the basic harmonic of the voltage source and where the currents of the system are periodic. The above‐mentioned power is an additional condition of the presented minimisation. The solution (the active currents) is obtained by means of Lagrange factors and a suitable measurement experiment. The last stage of this method is the determination of the parameters of the compensator, which connected to the system under research evoke the active currents of the voltage sources. This working point for the three‐phase three‐wire system can be obtained by means of LC, RLC or (RLC,‐R) compensators.

In the paper an application of fuzzy logic controller (FLC) for control of thyristor DC drive is presented. During synthesis of the FLC a robustness against variation of structure of the current control plant was taken into account. Comparison between the fuzzy control system and an ordinary digital control system, carried out by simulation method, proved that FLC as a robust controller gives better performance in the range where non‐linearity and parameter variation is observed. The simulation results were confirmed by the laboratory experiment.