A. Cziker

Propagation of unbalance in electric power systems

Power quality has become an important issue for electric power engineering. Nowadays, the distribution electric networks have unbalanced operating regimes, mainly produced by the great number of single-phase loads. Unbalanced line currents produce unbalanced voltage drops on the three phases of the supply system. Consequently, the voltage system within the supply network will become unbalanced. Voltage unbalance influences different components of the electric networks: the effects on the motors are the growths of losses, supplementary heating, and finally, the motors life is shorted. In the distribution and transmission electric networks the main effect of the unbalanced currents is the existence of additional power losses. Taking into account the above mentioned aspects, it is necessary to study the propagation of unbalance through the electric power system, upstream from the LV distribution level. By using the symmetrical components theory, transformers with different types of connections were studied, and their influence on the unbalance propagation was analyzed. A numerical example is presented for the propagation of unbalance from the secondary side of a transformer with Yzn connection to its primary side.

Fuzzy controller for a shaded daylighting system

Daylighting has become a more important feature of recent constructions due to the sustainable design movement, and suitable lighting controls provide building operators with the means to manage the way lighting energy is used in buildings more efficiently. Unfortunately, classic control systems, based on continuous dimming present some difficulties to adjust their performances to the rapid changes in daylight. Therefore, fuzzy control could be a better solution. The paper analyzes the possibility to implement this new technique in the closed-loop daylighting control including shading control and presents the structure of a fuzzy controller proposed by authors; its operation rules and the influence on the imposed value of the illuminance level are also studied.

Fuzzy logic controller for Steinmetz symmetrizing circuitry with variable reactor

The presence of high power single-phase loads produces certain disturbances in low voltage distribution networks: supplementary negative and zero sequence currents will cause excessive heating of electrical machines, saturation of transformers, ripple in rectifiers, protection and interference problems etc. Load compensation at the load bus is an effective method to eliminate those undesired sequence currents. The Steinmetz circuitry, consisting of a reactor, a capacitor and the receiver, suitable connected with each other, is widely used for this purpose. Unfortunately, as usually the load characteristics fluctuate, the parameters of the supplementary reactive elements should also vary. The paper deals with the implementation of fuzzy controlled reactive elements in the Steinmetz scheme in order to compensate the load variation.

Analysis of the Steinmetz compensation circuit with distorted waveforms through symmetrical component-based indicators

This paper deals with the use of a set of indicators defined within a symmetrical component-based framework to study the characteristics of the Steinmetz compensation circuit in the presence of waveform distortion. The Steinmetz circuit is applied to obtain balanced currents in a three-phase system supplying a single-phase load. The circuit is analyzed without and with harmonic distortion of the supply voltages. The compensation effect is represented by the classical unbalance factor and by the Total Phase Unbalance (TPU) indicator defined in the symmetrical component-based framework. Comparing the two indicators, it is shown that the classical unbalance factor is insufficient to represent the effect of voltage distortion and fails to detect the lack of total unbalance compensation occurring with distorted waveforms. Correct information is provided by calculating the TPU indicator.

Voltage unbalance mitigation using a distributed generator

The paper presents a new technique to mitigate the voltage unbalance in LV distribution grids using distributed generation (DG). Usually, public LV networks supply many single phase consumers; in this case, the unbalanced line currents produce the unbalance of standard symmetrical supply voltage system. This voltage system negatively influences other three-phase loads operating in the same system, i.e. motors, transformers, capacitor banks etc. Consequently, the utility must guarantee to these consumers a balanced voltage system that; this condition can be obtained by using distributed generation. The paper contains aspects regarding the unbalance in LV distribution grids, distributed generation and analyzes the opportunity of involving DG in voltage unbalance mitigation.

Interharmonics analysis using Fourier Transform and virtual instrumentation

Interharmonics represent those electrical signals whose frequencies are not integer multiples of the supply fundamental frequency. They correspond to electrical components that are not synchronized with the systems fundamental frequency; due to this fact, they have a more destructive impact than harmonics on the elements of the power system. International organizations like IEC and IEEE International Force Task Committee have defined the terminology, limits and measurement guidelines for interharmonics; even though, there still are difficulties in their detection and measurement with an acceptable accuracy. The paper presents the mathematical basis of Fourier transformation and some general aspects regarding a new algorithm that authors propose for interharmonics analysis. The classic Discrete Fourier Transform is adjusted using windows with different widths accordingly to the interharmonics spectrum. The paper also contains a numerical example and the description of the virtual instrument developed specifically for interharmonics detection and evaluation.

Impact of unbalance in harmonic polluted power networks

The paper tackles an issue that concerns both the suppliers and consumers of electric energy: power losses in distribution power networks. On the other hand, the harmonic pollution in power networks has become a permanent issue, and in many situations along with the unbalance determines a steady state that is unbalanced and non-sinusoidal. In the design and exploitation stage the power engineers calculate the power losses considering the ideal case (sinusoidal and balanced), approach that brings many disadvantages. Thus, the authors propose a methodology for power losses calculus that take in consideration the existence of unbalance and harmonic pollution. The suggested method implies the use of the power quality indices that characterize the real steady state.

Implementation of a lighting control system based on fuzzy logic

Abstract Classic control systems based on continuous dimming present some difficulties to adjust their performances to the rapid changes in daylight. Therefore, fuzzy control could be a better solution. The paper analyzes the possibility to implement this new technique in daylighting control and presents the structure of a fuzzy controller proposed by authors; its operation rules and the influence on the imposed value of the illuminance level are also studied.

UPQC — The best solution to improve power quality in low voltage weak distribution networks

The answer to global problems challenging energy sector was to use more efficiently local alternative (non-conventional fossil fuel or renewable) energy resources. In power systems, the use of renewable energy (RE) based distributed generation (DG) has increased considerably all over the world; the presence of DG units at distribution voltage level determines the transition from traditional passive to new active distribution networks. Concurrently, the modern distribution networks supply a large variety of non-linear loads based on power electronics contributing to increased current and voltage harmonics issues; dynamic loads and unbalance in power systems cause other power quality problems. The Romanian power system faces some challenges in order to ensure the reliability and quality of the power supply, especially in low voltage (LV) rural and sub-urban grids; thus, the power quality issues must be appropriately identified in every network while their assessment is the key element in the design of measures aiming to diminish all existing disturbances. A research project aims to identify the power quality issues and the impact of DG or other non-linear loads on LV distribution networks in Romania; the final goal is to develop an equipment able to remove or, at least, to mitigate all electromagnetic disturbances considering the characteristics and sensitivity of end use equipment within customer facilities.

Harmonics and interharmonics analysis of power signals using Gaussian filter banks

The paper presents a method based on the Fast Fourier Transform and Gauss Window combined with a recursive algorithm for the estimation of the voltages and currents harmonics and interharmonics. The proposed method uses pairs of band-pass filters to obtain the frequency domain components of the power signals. The band-pass filters emerge from the grouping made between the Gauss Window and the Fourier Transform. The outputs of these filters are attenuated copies of the input distorted signals and they are utilized to calculate the characteristics of the harmonic and interharmonic components present in the spectrum of the power signals. The analysis is performed on a time interval equivalent with 10 cycles of the fundamental frequency as the standard literature recommends. The proposed algorithm was implemented in a virtual instrument used to process synthetic and field data. The obtained results showed high precision in harmonics and interharmonics detection, with no leakage effect even in the case of fundamental frequency deviation.