Seppo Valkealahti

Power Losses in Long String and Parallel-Connected Short Strings of Series-Connected Silicon-Based Photovoltaic Modules Due to Partial Shading Conditions

Configuration of a photovoltaic (PV) power generator has influence on the operation of the generator, especially if it is prone to partial shading. In this paper, the mismatch losses and the power losses due to failure in tracking of the global maximum power point of a long string of 18 series-connected PV modules and three short strings of six series-connected PV modules connected in parallel are investigated under different partial shading conditions by using a MATLAB Simulink simulation model. The generators with parallel-connected short strings are studied in case when they have the same operating voltage and when they operate as separate strings. The results show that long series connection of modules and parallel connections of strings via a single inverter to the electrical grid should be minimized to avoid losses in case of partial shading conditions. Under partial shading conditions, short strings operating separately have the lowest power losses.

Photovoltaic Generator as an Input Source for Power Electronic Converters

A photovoltaic (PV) generator is internally a power-limited nonlinear current source having both constant-current- and constant-voltage-like properties depending on the operating point. This paper investigates the dynamic properties of a PV generator and demonstrates that it has a profound effect on the operation of the interfacing converter. The most important properties an input source should have in order to emulate a real PV generator are defined. These properties are important, since a power electronic substitute is often used in the validation process instead of a real PV generator. This paper also qualifies two commercial solar array simulators as an example in terms of the defined properties. Investigations are based on extensive practical measurements of real PV generators and the two commercial solar array simulators interfaced with dc-dc as well as three- and single-phase dc-ac converters.

Effect of Photovoltaic Generator Components on the Number of MPPs Under Partial Shading Conditions

Partial shading of photovoltaic (PV) power generators causes typically multiple maximum power points (MPPs) on the electrical characteristics of the generators. In this paper, the effect of partial shading on the number of MPPs has been studied by using MATLAB Simulink simulation models of a real PV generator composed of 18 series-connected PV modules. In most cases, partial shading conditions lead to the occurrence of multiple MPPs, but also only one MPP can be present despite of the partial shading. Conditions, which lead to one or multiple MPPs in electrical characteristics of PV generators, have been studied systematically. It was found out that reasons for having one or more MPPs originated directly from physical properties of PV modules: the ratio of MPP current to short-circuit current, power losses in bypass diodes, and the value of the parasitic shunt resistance of PV cells. These phenomena are thoroughly explained in this paper.

Dynamic terminal characteristics of a photovoltaic generator

Photovoltaic (PV) cells have non-linear electrical characteristics with only one operating point at which the maximum available power can be extracted. This is called the maximum power point (MPP). In order to maximally utilize the energy of solar radiation by using PV generators, they must be forced to operate at their MPP. This is done by using interfacing devices, which are capable of changing the operating point of PV generators and feeding the maximum electrical power to the loads. Although the static electrical characteristics are important when designing a PV generator, the operation of the interfacing devices is also highly affected by the dynamic characteristics of the PV generator. This paper investigates the dynamic properties of PV generators especially from the point of view of the interfacing devices used between the PV generators and the loads. These loads can be e.g. an electrical grid in case of grid-connected generators or batteries in stand-alone applications.

Operation of TUT Solar PV Power Station Research Plant under Partial Shading Caused by Snow and Buildings

A grid connected solar photovoltaic (PV) research facility equipped with comprehensive climatic and electric measuring systems has been designed and built in the Department of Electrical Engineering of the Tampere University of Technology (TUT). The climatic measuring system is composed of an accurate weather station, solar radiation measurements, and a mesh of irradiance and PV module temperature measurements located throughout the solar PV facility. Furthermore, electrical measurements can be taken from single PV modules and strings of modules synchronized with the climatic data. All measured parameters are sampled continuously at 10 Hz with a data-acquisition system based on swappable I/O card technology and stored in a database for later analysis. The used sampling frequency was defined by thorough analyses of the PV system time dependence. Climatic and electrical measurements of the first operation year of the research facility are analyzed in this paper. Moreover, operation of PV systems under partial shading conditions caused by snow and building structures is studied by means of the measured current and power characteristics of PV modules and strings.

Exposure to magnetic field harmonics in the vicinity of indoor distribution substations

Due to possible health effects of electric and magnetic fields, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) published in 1998 guidelines in which upper limits for public and occupational exposure were established. The guidelines are widely referenced by the EU directive of 2004. Thus, in Europe we have passed the time for discussion about the possible biological effects of magnetic fields and the duration of exposure to be allowed. Furthermore, in 2003, ICNIRP expanded their guidelines by taking into account the harmonic components of the magnetic field with phase angles. This statement has not been referenced widely. The aim of this study was to investigate magnetic fields, especially the harmonic components, in electric distribution (20/0.4 kV) substations from the public and occupational exposure point of view. The main finding is that the harmonic content of the magnetic field can dominate the exposure. The harmonic contents analyzed following the original ICNIRP guidelines varied between 49 and 74%. When taking into account the magnetic field phase angles, the harmonic content was only between 16 and 62%. By taking into account only the amplitudes of the harmonic components, ignoring their phase angles, the content of harmonic components is overestimated, on the average, by 30%.

Mismatch losses in PV power generators caused by partial shading due to clouds

Clouds cause spatial and temporal variability of solar radiation which is the main cause of fluctuating photovoltaic power fed into the grid. TUT solar PV power station research plant has been designed to enable systematic and thorough analysis of the dynamic phenomena in energy conversion processes of grid-connected solar PV power systems. Its climatic and electric measuring systems comprise an accurate weather station as well as an extensive mesh of solar radiation and module temperature measurements located through the PV facility. In this paper, we propose a method to generate a spatial irradiance map from an array of solar radiation measurements. Additionally, we utilize it as a tool to evaluate the mismatch losses in the operation of different PV generators due to partial shading caused by moving clouds.

Public magnetic field exposure based on internal current density for electric low voltage systems.

A measurement concept utilizing a new magnetic field exposure metering system has been developed for indoor substations where voltage is transformed from a medium voltage of 10 or 20 kV to a low voltage of 400 V. The new metering system follows the guidelines published by the International Commission on Non-Ionizing Radiation Protection. It can be used to measure magnetic field values, total harmonic distortion of the magnetic field, magnetic field exposure ratios for public and workers, load current values, and total harmonic distortion of the load current. This paper demonstrates how exposure to non-sinusoidal magnetic fields and magnetic flux density exposure values can be compared directly with limit values for internal current densities in a human body. Further, we present how the magnetic field and magnetic field exposure behaves in the vicinity of magnetic field sources within the indoor substation and in the neighborhood. Measured magnetic fields around the substation components have been used to develop a measurement concept by which long-term measurements in the substations were performed. Long-term measurements revealed interesting and partly unexpected dependencies between the measured quantities, which have been further analyzed. The principle of this paper is to substitute a demanding exposure measurement with measurements of the basic quantities like the 50 Hz fundamental magnetic field component, which can be estimated based on the load currents for certain classes of substation lay-out.

Feasibility of density functional methods to predict dielectric properties of polymers.

Feasibility of density functional theory (DFT) to predict dielectric properties such as polarizability of saturated polymers is investigated. Small saturated molecules, methane and propane, which is a monomer of polypropylene chain, are used in testing the methods. Results for polarizabilities based on several density functionals together with different basis sets are compared and contrasted with each other, with results by Hartree-Fock and second-order Moller-Plesset perturbation theory, as well as experimental data. The generalized gradient approximation PW91 method together with the 6-311++G(**) basis set is found to be the most suitable method, in terms of sufficient accuracy and computational efficiency, to calculate polarizabilities for large oligomers of polypropylene. The dielectric constant is then determined using the calculated polarizabilities and the Clausius-Mossotti equation. The molecular DFT methods at the PW916-311++G(**) level together with the Clausius-Mossotti equation give dielectric constants for saturated polymers such as polypropylene in good accordance with the experimental values.

Energy Storage Requirements for PV Power Ramp Rate Control in Northern Europe

Photovoltaic (PV) generators suffer from fluctuating output power due to the highly fluctuating primary energy source. With significant PV penetration, these fluctuations can lead to power system instability and power quality problems. The use of energy storage systems as fluctuation compensators has been proposed as means to mitigate these problems. In this paper, the behavior of PV power fluctuations in Northern European climatic conditions and requirements for sizing the energy storage systems to compensate them have been investigated and compared to similar studies done in Southern European climate. These investigations have been performed through simulations that utilize measurements from the Tampere University of Technology solar PV power station research plant in Finland. An enhanced energy storage charging control strategy has been developed and tested. Energy storage capacity, power, and cycling requirements have been derived for different PV generator sizes and power ramp rate requirements. The developed control strategy leads to lesser performance requirements for the energy storage systems compared to the methods presented earlier. Further, some differences on the operation of PV generators in Northern and Southern European climates have been detected.