Cost effective second generation AC-modules: Development and testing aspects

In the framework of the European research project PV2GO, a new AC-module inverter was developed, taking into account all relevant aspects from a European market's point of view (standards, market, application, and research and development goals). The project goal was to achieve the overall system costs of 3 Euro per Wp for a modular plug-and-play photovoltaic system. For the photovoltaic-module, a standard 130-Wp Eurosolare module was chosen. The research and development (R&D) goal was to develop an advanced DC-control system consisting of a state-of-the-art programmable digital device and an Application Specific Integrated Circuit (ASIC) for the AC-control of the inverter. According to the topology concept, thermal and magnetic designs were optimized with regard to production technology and packaging for large-scale production. The new AC-modules were tested in a number of field-test sites in various parts of Europe and their reliability was assessed through Highly Accelerated Stress Tests. Efficiency and power quality have been tested in the laboratory. Further in the PV2GO project an optimization study of the manufacturing process of the new generation of AC-modules for high volume output was done. Another task was the pre-certification procedure to assure compliance with the European guidelines and standards.     

Bag the sags

This article describes the vulnerability of variable speed drives (VSD) to voltage sags on a theoretical basis. Then, three embedded mitigation methods are addressed in theory which protect textile processes against voltage sags, and practical measurements with a sag generator are shown. The use of embedded solutions such as kinetic buffering and boost convertor or an active front end will increase the voltage sag immunity of the process. Finally, two processes in the textile industry are described, and a brief cost-benefit analysis of the solutions is made. Experiments with a sag generator show that the voltage sag behaviour can be well described by theoretical models.     

Towards an international tradable green certificate system—The challenging example of Belgium

In Europe, a common framework for renewable energy sources (RES) is aspired. Tradable green certificates (TGCs) are a market-based cost-efficient means to stimulate electricity production from RES. Since TGCs are the most widespread support scheme in Europe together with feed-in tariffs, chances are that a common European framework could well be based on TGCs. However, while integrating currently existing different national TGC systems, any remaining differences should be carefully considered. Just how difficult the creation of an international TGC market would be is illustrated in this paper by the case of Belgium, where no less than 4 different TGC systems exist nowadays. The example of Belgium illustrates that harmonizing different TGC systems is easier said than done and represents a serious challenge. This clearly illustrates that a single European support scheme for RES, however desirable, is still far in the future.     

Electro-mechanical analysis of the audible noise of an inverter-fedsquirrel-cage induction motor

The audible noise emitted by a three-phase squirrel cage induction motor fed by a frequency inverter is determined by the electromagnetic and mechanical behavior of the components of the drive. A computerized analysis method is described that uses theoretical and experimental data for predicting the audible noise spectrum components. Starting from the inverter voltage and the motor data, the electromagnetic forces are predicted and analyzed. The mechanical natural frequencies are considered, either in a theoretical (finite element calculation) or in an experimental (modal analysis) methodology, to predict the vibrational behavior of the machine. The scheme contains the following steps: analysis of the frequency spectrum of the inverter used; synthesis of the magnetic force spectrum accounting for the motor data: calculation or measurement of the resonant data of the stator assembly; and comparison of the force spectrum and the resonant data. These various steps are discussed     

A Voltage and Frequency Droop Control Method for Parallel Inverters

In this paper, a new control method for the parallel operation of inverters operating in an island grid or connected to an infinite bus is described. Frequency and voltage control, including mitigation of voltage harmonics, are achieved without the need for any common control circuitry or communication between inverters. Each inverter supplies a current that is the result of the voltage difference between a reference ac voltage source and the grid voltage across a virtual complex impedance. The reference ac voltage source is synchronized with the grid, with a phase shift, depending on the difference between rated and actual grid frequency. A detailed analysis shows that this approach has a superior behavior compared to existing methods, regarding the mitigation of voltage harmonics, short-circuit behavior and the effectiveness of the frequency and voltage control, as it takes the R to X line impedance ratio into account. Experiments show the behavior of the method for an inverter feeding a highly nonlinear load and during the connection of two parallel inverters in operation.     

Analytical Approach to Grid Operation With Phase Shifting Transformers

Analytical expressions are derived to gain insight in the operating principles of phase shifting transformers (PSTs) in a highly meshed grid. To this extent, the dc load flow algorithm is adapted to account for such devices. This leads to a linear expression for the relation between PST settings and the active power flow in the lines. Based on these equations, the total transfer capacity (TTC) can be described mathematically, which allows for optimization. Furthermore, the linear least squares method is used to distribute a cross-border transport evenly over the interconnectors.     

Modifying the feature-selective validation method to validate noisy data sets

Objective validation and ranking of measurements and simulations may be done by methods such as feature selective validation (FSV). FSV is used to compare two EMC-measurement results. Owing to the noisy nature of these type of data, the FSV results are corrupted. The reasons are discussed and solutions are proposed to make FSV feasible in a broader area of applications. The final solution is a combination of denoising the data and changing the weight of the data to be in accordance with our visual interpretation.     

Finite-element modeling of thermal contact resistances andinsulation layers in electrical machines

Small transition layers with a significant temperature drop, such as thermal contact resistances and thin insulation layers, are difficult to consider in coupled finite-element thermal/magnetic models of electrical machines. A novel approach using mesh cuts and duplicated nodes at the thin-layer position is derived and discussed here. Results are compared to classical methods involving the use of nonscaled and scaled finite elements. Practical examples demonstrate the features and implementation aspects of the methods used     

A vibrational model using modal shapes and magnetic forces: experimental results for a permanent magnet machine

Contents The frequency inverter current spectrum generates a complex magnetic field in the airgap producing attracting forces having a wide spectrum and leads to audible noise. A mathematical model is presented, predicting vibrations of a permanent magnet machine, based on the modal analysis technique including mechanical damping. Modal forces produced by the magnetic field, are related to the supply currents using a finite element calculation. Experimental results are presented.

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Ein Schwingungsmodell mit Hilfe der Modalen Analyse und der magnetischen Kräfte: experimentelle Ergebnisse für eine Permanentmagnetmaschine

Übersicht Das Stromspektrum eines Frequenzumrichters generiert ein komplexes magnetisches Feld im Luftspalt, welches Anziehungskräfte mit einem breiten Frequenzgang hervorruft, die wiederum zu Geräuschen führen. Ein mathematisches Modell wird vorgestellt, welches es ermöglicht, die Schwingungen einer permanent erregten Maschine vorauszusagen. Es basiert auf der Modalen Analyse und enthält die mechanische Dämpfung. Vom Magnetfeld hervorgerufene modale Kräfte werden mit den eingespeisten Strömen mittels der Finite-Elemente-Methode in Verbindung gebracht. Experimentelle Ergebnisse werden dargestellt.

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The authors wish to thank the Council of the Belgian National Science Foundation for granting a project sponsoring the research.This paper presents results of the Belgian programme on Interuniversity Poles of Attraction initiated by the Belgian State, Prime Minister's Office, Science Policy Programming. The scientific responsibility is assumed by the authors.     

Development of a flickermeter for grid-connected wind turbines using a DSP-based prototyping system

The measurement of one of the power-quality problems (flicker), which may arise due to the presence of wind-turbine generators, is presented. This paper focuses on the design of a complete fully digital flickermeter relying on an in-house-developed measurement system, which features a data-acquisition system, using a C6711 DSP-based starter kit (DSK) and a field programmable gate array (FPGA). To study flicker produced during the wind turbine's continuous operation, special attention is given to the simulation of the voltage fluctuations on a fictitious grid.