Mircea Chindris

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.

Frequency control of isolated wind and diesel hybrid MicroGrid power system by using fuzzy logic controllers and PID controllers

This paper presents a new application of fuzzy logic (FL) to1 an isolated network with a High Penetration, no-storage wind-diesel (HPNSWD) system. As a result of a study referring the behavior of an isolated electric system facing frequency disturbances, a fuzzy logic controller (FLC) was developed to improve the system´s dynamic performance. The validity of the proposed controller is evaluated by computer analysis using MATLAB/SIMULINK. The simulation results demonstrated that a small-scale wind turbine generation unit can be freely operated in an isolated distribution network without creating violation in power balance and voltage profile. The effectiveness of the fuzzy logic controller is then compared with that of a proportional-integral-and differential (PID) controller.

Assess the impact of photovoltaic generation systems on low-voltage network: software analysis tool development

The integration of photovoltaic generation systems into power networks can cause both benefits and drawbacks. However, utilities have to control and operate their systems properly, in order to assure the availability and quality power supply to the users. Therefore, utilities should consider technical constraints and existing regulation in order to assess the impact of photovoltaic systems and limit their integration. On the other hand, regulation includes few operation constraints and they are not implemented in current software analysis tools. In the present paper a tool for assessing the impact of PV integration on low-voltage networks is described. The voltage fluctuation, the inversion of the power flow and the increase of short-circuit capacity are the problems considered in the proposed tool. Further work will focus on harmonics distortion.

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.

The load flow calculation in radial electric networks with distributed generation under unbalanced and harmonic polluted regime

The growth of distributed generation (DG) which is consumer driven and not centrally planned presents a number of challenges and opportunities specific mainly to low voltage (LV) distributed networks. The growth of DG in order to meet the UK governments environmental targets for 2010 will taken into consideration for example the following LV distribution network constraints: steady-state voltage limits, voltage unbalanced limits and harmonic limits. This paper presents an algorithm developed by the authors in order to perform load flow calculation in unbalanced and harmonic polluted low voltage radial networks with distributed generation. The authors propose to model the harmonic complex quantities (a set of complex quantities corresponding to each harmonic component) through abstract data types with complex parameters. For harmonic polluted and unbalanced radial electric networks that include distributed generators, it was considered the backward/forward sweep with some specifications and adaptations.

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.

Mitigation of Capacitor Bank Energization Harmonic Transients

In the present study, an overview of the capacitor bank energizing transients is presented and the means of harmonic mitigation technique are investigated. Capacitor bank connection is essential due to reactive compensation reasons. Transients produced upon of capacitor bank energizing, reach in harmonics, may be harmful for the capacitor and become an increasing concern for both electric utilities and customers since its switching usually occurs quite frequently. We know that power factor correction capacitors are sensitive to harmonics and one factor of the failures of power capacitors indicates the prime odd harmonics like 5th, 7th, 11th, 13th harmonics. A time domain simulation using PSCAD/EMTDC was performed to predict the harmonic spectrum of the distorted voltage waveform in a real 11 kV Romanian distribution subsystem

The Optimization of Microgrids Operation through a Heuristic Energy Management Algorithm

The concept of microgrid was first introduced in 2001 as a solution for reliable integration of distributed generation and for harnessing their multiple advantages. Specific control and energy management systems must be designed for the microgrid operation in order to ensure reliable, secure and economical operation; either in grid-connected or stand-alone operating mode. The problem of energy management in microgrids consists of finding the optimal or near optimal unit commitment and dispatch of the available sources and energy storage systems so that certain selected criteria are achieved. In most cases, energy management problem do not satisfy the Bellmans principle of optimality because of the energy storage systems. Consequently, in this paper, an original fast heuristic algorithm for the energy management on stand-alone microgrids, which avoids wastage of the existing renewable potential at each time interval, is presented. A typical test microgrid has been analysed in order to demonstrate the accuracy and the promptness of the proposed algorithm. The obtained cost of energy is low (the quality of the solution is high), the primary adjustment reserve is correspondingly assured by the energy storage system and the execution runtime is very short (a fast algorithm). Furthermore, the proposed algorithm can be used for real-time energy management systems.

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.