M. Papadopoulos

Harmonic analysis in a power system with wind generation

Variable-speed wind turbines inject harmonic currents in the network, which may potentially create voltage distortion problems. In this paper, a case study is presented for a 10-MW wind farm, intended to be connected to a network with extended high-voltage submarine cable lines. First, the system modeling approach and the harmonic load-flow calculation is described. Then, the harmonic impedance of the system is calculated for a variety of configurations and operating conditions, and its main characteristics are discussed. Harmonic load-flow calculations are provided to indicate potential voltage distortion problems. A simplified methodology, suggested in relevant IEC publications, is applied to the system and its results are compared to those of the harmonic load flow. A discussion on the summation of harmonic currents within a wind farm is also included

Dynamic behavior of variable speed wind turbines under stochastic wind

It is recognized that the most important advantage of the variable speed wind turbines (VS WTs) over the conventional constant speed (CS) machines are the improved dynamic characteristics, resulting in the reduction of the drive train mechanical stresses and output power fluctuations. In this paper alternative configurations of the electrical part of a VS WT are considered, using a squirrel cage induction generator and voltage or current source converters, as well as a double output induction generator with a rotor converter cascade. The WT operation is simulated for typical wind speed time series and the examined schemes are comparatively assessed. It is shown that, using suitable converters and controls, a great reduction of the mechanical stresses and output power fluctuations can be achieved, compared to the CS mode of operation of the WT.

Wind turbine effect on the voltage profile of distribution networks

The operation of wind turbines in the distribution networks may affect the power quality offered to the consumers. One of the most important considerations is the effect on the voltage profile, i.e. the induced slow voltage variations, which are the subject of this paper. Two alternative approaches are presented for their evaluation. The first, adopted by many utility guides and recommendations, is deterministic, seeking to ensure that the voltage deviations always remain within certain limits. The other recognises the statistical nature of the voltage variations and conforms to latest European Norm, EN 50160. Rather than assessing the maximum deviations that can possibly appear, the probability distribution of the voltage is calculated and then the conformity to the standards is assessed. In applying the statistical method, either time series, or directly probability distributions can be used. As a study case, the methods are applied to an existing MV distribution feeder, where significant wind power is installed. Measurement data are provided for the same feeder.

Graphics Aided Interactive Analysis of Radial Distribution Networks

This paper presents an interactive graphics package for distribution system analysis. The radial operation of the distribution networks is exploited in order to effect a fast and accurate calculation of the power flows, the voltage drops and the power losses at the various segments of a feeder for a given set of typical load curves. The algorithms developed are based on an efficient method to store the network topology and can easily accommodate changes in the infeeding nodes and variations in the network structure due to switching operations. The interactive environment allows the user to obtain the results of his studies against the one-line diagram of the network and effect changes in it in basically diagrammatic terms. The package requires very modest computer resources and provides planning and operating engineers with a powerful tool to conduct analytical distribution system studies with ease and flexibility.

An advanced control system for the optimal operation and management of medium size power systems with a large penetration from renewable power sources

An advanced control system for the optimal operation and management of autonomous wind-diesel systems is presented. This system minimises the production costs through an on-line optimal scheduling of the power units, which takes into account the technical constraints of the diesel units, as well as short-term forecasts of the load and renewable resources. The power system security is maximised through on-line security assessment modules, which enable the power system to withstand sudden changes in the production of the renewable sources. The control system was evaluated using data from the island of Lemnos, where it has been installed and operated since January 1995.

Operation and control of island systems-the Crete case

In this paper, an advanced control system for the optimal operation and management of isolated power systems with increased renewable power integration is presented. The control system minimises the production costs through on-line optimal scheduling of the power units, taking into account short-term forecasts of the load and the renewable resources. The power system security is supervised via on-line security assessment modules, which emulate the power system frequency changes caused by pre-selected disturbances. For each of the above functions, a number of techniques have been applied, both conventional and AI based. The system has been installed in the dispatch center of Crete since June 1999, and is under evaluation.

On the harmonics of the slip energy recovery drive

In this letter an investigation of the harmonics of the slip energy recovery drive is presented. The analysis takes into consideration not only the rotor-side rectifier, as is customary, but also accounts for the harmonics introduced by the recovery inverter. Simple relations are provided that predict all harmonic frequencies of the drive currents and torque as a function of its supply frequency and operating slip.

Transient analysis of extended distribution networks

An analytical method is presented for the analysis of the transient behavior of extended distribution networks during short-circuits or start-up using interactive, graphical techniques. A fast algorithm is used which exploits the radical nature of the network to calculate the nodal voltages. Two models are available for the transient analysis of the induction motors, which can be used according to the degree of detail desired. Typical load types for each distribution substation are considered, and the parameters of the loads at each node are determined by the relevant composition of the aggregate nodal load. Disturbances provoked by any type of short circuit are considered, and the user can obtain the currents and voltages at any point of the radial network selected. The program is applied to the analysis of the transient behavior of a realistic distribution network during faults on the network. Comparison of the results with those obtained from the standard practice of considering the loads concentrated on the HV busbars of the HV/MV substations has shown differences of more than 150%. >

Control Issues of a Permanent-Magnet Generator, Variable-Speed, Wind Turbine

A prototype of the electrical part of a variable speed wind turbine is considered, equipped with a permanent-magnet synchronous generator. The modelling of the generator and power electronics interface is checked with measurements realized in the prototype, under both steady-state and dynamic conditions. Measurements and control functions are performed by using a microprocessor. The outcome of the simulation and experimental work are actually utilized in the development of a 25 kW wind turbine, in the frame of a research project.

Wind Turbine Flicker Calculation Using Neural Networks

The connection of wind turbines to the distribution networks may affect the voltage quality offered to the consumers. One of the factors contributing to this effect are the rapid variations of the wind turbine output power, which cause respective fluctuations in the supply voltage, referred to as flicker. This paper presents a neural network based model for wind turbine flicker emission calculations. Neural network training patterns are developed using a simulation model of a typical 500 kW stall-controlled wind turbine, by varying all wind and network parameters that might affect the expected flicker levels. The proposed neural network model predicts flicker emissions with sufficient accuracy under any normal operating conditions (wind speed mean value and turbulence intensity) and network characteristics (short circuit capacity, angle of Thevenin impedance and local load). The paper also includes an extensive discussion on the dependence of the flicker severity on the wind and network parameters considered.