Seppo Valkealahti | Tampere University (original) (raw)

Papers by Seppo Valkealahti

TAMPERE UNIVERSITY OF TECHNOLOGY JULIUS SCHNABEL: Compensation of PV Generator Power Fluctuations... more TAMPERE UNIVERSITY OF TECHNOLOGY JULIUS SCHNABEL: Compensation of PV Generator Power Fluctuations Using Energy Storage Systems Master’s Thesis, 55 pages, 2 appendix pages August 2015 Master’s Degree Program in Electrical Engineering Major: Renewable Electrical Energy Technologies Examiner: Professor Seppo Valkealahti

2019 International Conference on Clean Electrical Power (ICCEP), 2019

IEEE Journal of Photovoltaics, 2020

The single diode equivalent circuit allows describing photovoltaic panels behavior quite accurate... more The single diode equivalent circuit allows describing photovoltaic panels behavior quite accurately. The values of the five parameters appearing in this model are usually identified in Standard Test Conditions, at which irradiance and temperature values are given. Instead, when this identification has to be performed on field, suitable environmental sensors are required. In this paper, a method is presented for identifying the single diode model parameters as well as the irradiance and temperature conditions at which the panel is actually working. The method has the measured panel current vs voltage curve as the only input and it is suitable for being implemented on an embedded system for field applications. The method is validated through a large database of experimental results.

2020 47th IEEE Photovoltaic Specialists Conference (PVSC), 2020

PV panel current-voltage curves provided by measurement devices usually suffer from noise and oth... more PV panel current-voltage curves provided by measurement devices usually suffer from noise and other inaccuracies such as uneven distribution of the measurement points. This might complicate the curve fitting process by deteriorating the fit quality and making the diagnosis based on the curve misleading. This paper provides a stepwise preprocessing method for PV panel measured current-voltage curves. The method improves the quality of the measured current-voltage characteristic data, which has been verified by examining its effects on the fitting process.

Current-voltage curve measurements are a potential tool for efficient monitoring and diagnosis of... more Current-voltage curve measurements are a potential tool for efficient monitoring and diagnosis of photovoltaic (PV) panels and systems. To determine indicators of aging, degradation and other such phenomena of PV panels, an attractive option is to fit an electrical model of the PV panels to measured data in order to detect changes. However, raw measurement data usually contains measurement noise and other factors that can distort the analysis. The fitting process is challenging for instance, by the fact that the measurement points are unevenly distributed on the current-voltage curve, being mainly clustered on the open-circuit region. This unintentional weight can distort the fit. To address these issues, the present paper introduces a procedure for fitting an electrical model of the PV panels to the measured current-voltage curves so as to alleviate the above problems. The proposed approach is a prerequisite for a reliable diagnosis of the PV system status.

Under uniform operating conditions, a photovoltaic (PV) array has only one maximum power point (M... more Under uniform operating conditions, a photovoltaic (PV) array has only one maximum power point (MPP) and successful MPP tracking is simple to implement. However, under non-uniform operating conditions, such as partial shading, multiple MPPs can exist on the characteristic of the PV array and the global MPP can vary over a wide voltage range. When multiple MPPs exist, conventional MPP tracking algorithms can be trapped to operate at a local MPP instead of the global one, and consequently the energy yield of the array can be considerably reduced. It is essential to know the operational MPP voltage range of the installed PV array in order to adjust the voltage range of the inverter properly. In this article, the behaviour of the global MPP of partially shaded PV strings is studied experimentally based on measured current–voltage curves of two strings consisting of 17 and 6 series-connected PV modules. The results of this study show that the voltage of the global MPP varies over a wide voltage range and changes in the voltage and power of the global MPP can be very fast. The largest measured changes in the GMPP voltage and power during one second were 60% and 32%, respectively.

Solar Energy, 2017

The mismatch losses of photovoltaic (PV) systems are mainly caused by partial shading and the lar... more The mismatch losses of photovoltaic (PV) systems are mainly caused by partial shading and the largest mismatch losses are caused by sharp shadows. However, in large scale PV plants majority of shading events is caused by moving clouds which lead to gentle irradiance transitions causing typically only minor irradiance differences between adjacent PV modules. Irradiance transitions caused by the edges of cloud shadows have an average length of almost 150 m meaning that even the largest PV power plants are widely affected by them. In addition of mismatch losses, these irradiance transitions can lead to failures in maximum power point tracking and cause significant fluctuations in the output power of PV systems. In this paper, the effects of PV array shape, electrical configuration and orientation on mismatch losses caused by moving clouds were studied based on apparent velocity and other measured characteristics of roughly 27,000 irradiance transitions. The study was conducted using a mathematical model and parametrisation method of irradiance transitions and an experimentally verified simulation model of a PV module based on the well-known one-diode model of a PV cell. The studied electrical PV array configurations were series-parallel, total-cross-tied and multi-string. The results of this study confirmed a prior conclusion, namely, that the mismatch losses decrease with decreasing PV string length. It was also found that the array orientation has a considerable effect on the mismatch losses of the studied array layouts. The mismatch losses were the smallest when the dominant direction of movement of the shadow edges was perpendicular to the PV strings. The differences in the mismatch losses between the studied electrical array configurations were very small. The results indicated that the mismatch losses caused by moving clouds have only a minor effect on the overall efficiency of PV arrays.

IET Conference Publications, 2009

The scope of this paper is on magnetic field exposure of workers in indoor medium to low voltage ... more The scope of this paper is on magnetic field exposure of workers in indoor medium to low voltage (MV/LV) distribution substations. In this paper we present magnetic field hazard evaluation based on measurements, specifications to execute them and specifications for risks assessment procedure. Action values were exceeded in several working tasks but limit values were not exceeded.

Journal of Applied Physics, 1989

This is an electronic reprint of the original article. This reprint may differ from the original ... more This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail.

Under non-uniform operating conditions, photovoltaic (PV) generators can have multiple maximum po... more Under non-uniform operating conditions, photovoltaic (PV) generators can have multiple maximum power points (MPP) which may cause problems for MPP tracking. Since highly varying global MPP (GMPP) voltage causes large fluctuations in the inverter reference voltage, it would be beneficial to keep the operating point of the inverter all the time close to the nominal MPP voltage. In this way, operation of the PV system would be more predictable and straightforward. This paper presents an experimental study based on measured current–voltage curves of two PV strings of a scenario in which the MPP closest to the nominal MPP voltage is used all the time as the operating point instead of the GMPP. In total, 432000 I–U curves measured over 120 hours were analysed. The effects of inverter sizing on the selection of the operating point of the PV strings were also studied. The experimental results presented in this paper demonstrate that the wide operating voltage range when the GMPP is followed...

Applied Sciences, 2017

There is no natural inertia in a photovoltaic (PV) generator and changes in irradiation can be se... more There is no natural inertia in a photovoltaic (PV) generator and changes in irradiation can be seen immediately at the output power. Moving cloud shadows are the dominant reason for fast PV power fluctuations taking place typically within a minute between 20 to 100% of the clear sky value roughly 100 times a day, on average. Therefore, operating a utility scale grid connected PV power plant is challenging. Currently, in many regions, renewable energy sources such as solar and wind receive feed-in tariffs that ensure a certain price for the energy. On the other hand, electricity markets operate on a supply-demand principle and a typical imbalance settlement period is one hour. This paper presents the energy, power and corresponding requirements for an energy storage system in a solar PV power plant to feed the power to the grid meeting the electricity spot markets practices. An ideal PV energy production forecast is assumed to be available to define reference powers of the system for...

Under non-uniform operating conditions, like partial shading, several maximum power points (MPPs)... more Under non-uniform operating conditions, like partial shading, several maximum power points (MPPs) may exist on the electrical characteristic of a photovoltaic (PV) array and the global MPP may vary over a wide voltage range. When multiple MPPs exist, conventional MPP tracking algorithms can be trapped to operate at a local MPP instead of the global one, and consequently the energy yield of the PV array can be considerably reduced. The partial shading of well-designed large-scale PV power plants is mainly due to shadows of moving clouds. In this article, the operation of various PV arrays at the largest MPP voltage instead of the global MPP voltage has been studied during measured irradiance transitions caused by clouds. The voltage range of the MPP with the largest voltage was found to be clearly narrower than that of the global MPP and the energy losses resulting from the operation at the largest voltage MPP instead of the global one during cloud partial shading events were found t...

Surface Science, 1986

... Vacancy-type defect distributions near argon sputtered Al(100) surface studied by variable-en... more ... Vacancy-type defect distributions near argon sputtered Al(100) surface studied by variable-energy positrons and molecular dynamics simulations. J. Mäkinen a , A. Vehanen a , P. Hautojärvi a , H. Huomo a , J. Lahtinen a , RM Nieminen b and S. Valkealahti b. ...

Indoor MV/LV substation may cause higher magnetic field around it than the usual background field... more Indoor MV/LV substation may cause higher magnetic field around it than the usual background field. LV connection is often the most important source of magnetic field in a substation. By knowing the structure and route of the connection it is possible to estimate the magnetic field level in rooms around the substation. The aim of the study was to categorize indoor MV/LV distribution substations by LV connection structure in Finland.

Clouds are the dominant source of spatial and temporal variability of solar radiation that is the... more Clouds are the dominant source of spatial and temporal variability of solar radiation that is the main cause of fluctuating photovoltaic power feed into the grid. Hourly solar radiation measurements are no longer sufficient to predict the output power of PV generators and to properly analyze the effects of non-homogeneous irradiance conditions on their performance. There is need of precise information of the instantaneous irradiance conditions affecting to the generators and faster sampling frequencies are required. In this paper, we present a method to generate a spatial irradiance map from an array of solar radiation measurements during shading events caused by moving clouds by analyzing their speed and direction of movement. Furthermore, the irradiance map is validated with measurements from TUT solar PV power station research plant and compared to the map obtained by applying natural neighbor interpolation of the instantaneous irradiance measurements directly.

Irradiance transitions caused by overpassing cloud shadows can cause significant fluctuations in ... more Irradiance transitions caused by overpassing cloud shadows can cause significant fluctuations in the output power of photovoltaic (PV) systems. With fast growth of PV power production, there is a growing potential of PV output power variability having a negative effect on the power quality and reliability in the grid. This issue is of special importance locally and in small grids with high PV penetration. Furthermore, irradiance transitions caused by edges of overpassing cloud shadows cause mismatch losses and can lead to situations where the grid inverter is not able to follow the global maximum power point causing extra losses. In this paper, the effects of different irradiance transition characteristics on PV output power variation and the relation between irradiance and output power fluctuations were studied. The study was conducted by using a mathematical model of irradiance transitions and an experimentally verified MATLAB Simulink model of a PV module based on the well-known ...

Applied Energy

• MPP characteristics of partially shaded PV strings were studied experimentally. • The global MP... more • MPP characteristics of partially shaded PV strings were studied experimentally. • The global MPP of photovoltaic strings was found to vary over a wide voltage range. • It was found that variations in the global MPP voltage and power can be fast. • Operation at the MPP closest to the nominal MPP voltage was found to have advantages.

Renewable Energy

Abstract Currently, photovoltaic (PV) installations target a maximization of annual energy yield.... more Abstract Currently, photovoltaic (PV) installations target a maximization of annual energy yield. However, as the grid penetration of PV is increasing, PV electricity generation will need to match better with local load profiles. Especially the seasonal variabilities remain challenging. While wind and PV tend to have complementary seasonal variability, wind turbine installation faces limitations especially in densely populated areas. In this paper, we discuss how this challenge may be addressed with climate- and consumption-specific PV module technology. In particular, we demonstrate how the temperature coefficient of a PV system can impact the energy yield throughout the year. In colder climates, higher temperature coefficients allow for a better energy balance, favoring production in colder seasons without a significant reduction of yearly energy yield. Simulations for locations at high latitude, and colder climates, indicate that higher temperature coefficients and improved low-light behavior not only enable a higher energy yield in cold seasons, but also negligible losses in the overall yearly energy yield compared to lower temperature coefficients and slightly better low-light behavior. Simulations show that these results can be obtained using commercial PV modules. More broadly, they indicate how PV module technology may be optimized depending on the location and climate.

CIRED Workshop 2016, 2016