Juha Huusari

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.

Issues on Solar-Generator Interfacing With Current-Fed MPP-Tracking Converters

The large-scale harvesting of solar energy is an important action to decelerate the observed climate changes. Reliably operating solar-energy systems composing of solar arrays and their interfacing converters are of prime importance to maximize solar-energy harvesting. The paper investigates the solar-generator interfacing in terms of current-fed (CF) maximum-power-point (MPP) tracking converter. The investigations show that the CF converter under input-voltage control can usually operate from the short-circuit to open-circuit conditions of the solar generator without stability problems. When the output voltage or current has to be controlled constant, the converter may become unstable at the MPP due to the negative incremental resistance appearing at its input terminals. In practice, this means that the operating range of the CF converter has to be limited to the voltages less than the MPP voltage, when the output-voltage or current control is active. Practical evidence is provided based on a CF superbuck converter derived from the corresponding voltage-fed converter applying duality-transformation methods and supplied by an actual solar panel.

Issues on Solar-Generator Interfacing with Voltage-Fed MPP-Tracking Converters

Abstract The large-scale harvesting of solar energy is an important action to decelerate the observed climate changes and ensuring the availability of energy also in the future. Reliably operating solar-energy systems composing of solar arrays and their interfacing converters are of prime importance to maximize solar-energy harvesting. The main obstacle for the improvements in the reliability is turned out to be the lack of full understanding of the solar-generator interfacing. The paper investigates the interfacing in terms of voltage-fed maximum-power-point (MPP) tracking converters. The investigations confirm that there are two reasons for the observed problems of which the one is the violation of Kirchhoff’s current law governing the operation of a solar generator and the other is the phenomenon known as negative resistance oscillation (NRO) causing instability. In practice, this means that the voltage-fed converter can reliably operate only at the voltages higher than the maximum-power-point (MPP) voltage. Practical evidence is provided based on a coupled-inductor superbuck converter supplied by a real solar panel and loaded with a storage battery.

Implementing current-fed converters by adding an input capacitor at the input of voltage-fed converter for interfacing solar generator

The concern on observed climate change has increased the utilization of renewable energy sources. The harvesting of solar energy is recognized as one of the key issues in reducing green house gas emission. Reliable solar-energy systems composing of solar arrays and their interfacing converters are of prime importance in uninterrupted solar energy production. The interfacing maximum-power-point converters are implemented usually by modifying the conventional voltage-fed converters. Actually, the modifications change the converter into a current-fed converter with corresponding steady-state and dynamic properties. The paper investigates the true properties of these transformed converters based on theory and practical measurements. As an example a direct-duty-ratio-controlled voltage-fed buck converter is shown to be transformed into a current-fed boost-type converter.

Dynamic Properties of Current-Fed Quadratic Full-Bridge Buck Converter for Distributed Photovoltaic MPP-Tracking Systems

Interfacing a single photovoltaic (PV) module into either a single-phase or a three-phase inverter requires an interfacing converter with wide conversion ratio and sufficient dynamic headroom. In terms of avoiding issues with common-mode currents between the PV module and the inverter, a converter with galvanic isolation is preferred. This is especially important with the emerging thin-film modules that have larger cell-to-frame parasitic capacitances compared to traditional modules. This paper presents the dynamic properties and analysis of the current-fed quadratic full-bridge buck converter. The foundation for the proposed topology is based on the concept of true current-fed converters that differ fundamentally from the conventional voltage-fed converters. The relevant analytical steady-state equations and the dynamic model are presented with the operational details of the proposed converter. The analytical claims are verified with extensive measurements from a corresponding 190-W prototype. According to the measurements, the prototype performed well in PV application and reached an efficiency of 92% at the maximum power point of the PV module.

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.

Origin of Cross-Coupling Effects in Distributed DC–DC Converters in Photovoltaic Applications

Long strings of photovoltaic (PV) modules are found to be vulnerable to shading effects, causing significant reduction in the system power output. To overcome this, distributed maximum power point-tracking (DMPPT) schemes have been proposed, in which individual dc-dc converters are connected to each PV module to enable module-wise maximum power extraction. There are two main concepts to implement DMMPT systems: series and parallel configuration, describing the connection of the output terminals of the converters. Both systems are studied intensively, with innovative solutions to encountered operational challenges and novel control methods. However, a comprehensive dynamic model for neither system has been presented so far. This paper fills the gap by presenting small-signal models for both configurations, explaining the observed operational peculiarities. The analytical claims are verified with a practical system comprising two maximum power point-tracking buck-boost converters.

Issues on solar-generator-interfacing with voltage-fed converter

Reliably operating solar-energy systems composing of solar arrays and their interfacing converters are of prime importance to maximize the solar-energy harvesting. The paper investigates the solar-generator-interfacing problems in terms of voltage-fed maximum-power-point tracking converters and shows that there are two main reasons for the observed stability problems of which the one is the violation of Kirchhoffs current law governing the operation of the solar generator and the other the phenomenon known as negative resistance oscillation. In practice, this means that the voltage-fed converter can reliably operate only in the voltages higher than the maximum-powerpoint voltage. Practical evidence is provided based on a coupled-inductor superbuck converter.

On Frequency-Response Measurements of Power-Electronic Systems Applying MIMO Identification Techniques

Switched-mode converters are often the key to operating various electronic systems in everyday life. This means that the reliable operation of the converters is of prime importance and that the functioning must be verified during both the design phase and production. Recent studies have shown that the converters can be fully characterized by a set of frequency responses that can be efficiently used to validate the operation of the converters and analyze the related systems. Several methods have been proposed for quickly and accurately measuring the frequency responses, but surprisingly, the use of multiple-input multiple-output (MIMO) identification techniques has not been considered. Applying MIMO identification techniques, the operating conditions of a converter can be kept constant during the frequency-response measurements, and the overall measurement time can be further shortened. This paper reviews the techniques and proposes an implementation setup. The presented techniques provide efficient means to characterize the switched-mode converters during one measurement cycle. The methods can be used for example in field-programmable gate-array-based controller implementation and in fast online analysis. Experimental measurements are shown from a high-frequency switched-mode converter.

Interfacing constraints of distributed maximum power point tracking converters in photovoltaic applications

Long strings of photovoltaic (PV) modules have been observed to be prone to shading effects causing significant reduction in the system power output. To overcome this, distributed maximum power point-tracking (DMPPT) schemes have been proposed, in which individual dc-dc converters are connected to each PV module to enable maximum power extraction out of the PV modules. The output terminals of these converters are connected either in series or in parallel, followed by a grid-connected inverter. The series concept has been under intensive study with novel topologies and control schemes, yet operational problems compromising its feasibility have been widely reported. The parallel concept does not contain similar operational limitations, but suffers mainly from higher voltage levels. This paper discusses the feasibility of the series and the parallel schemes and introduces fundamental constraints governing the operation of DMPPT converter systems. These constraints have not been truly recognized to be key issues in designing high-performance, high-efficiency photovoltaic systems.