Lari Nousiainen

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.

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.

Effect of minimizing input capacitance in VSI-based renewable energy source converters

Most of single or three-phase converters used in interfacing renewable energy sources (RES) to the power grid originate from VSI-based topologies. A common practice is to connect a capacitor between the RES and the VSI switch matrix to filter the pulsating current drawn by the converter. When a converter is supplied by a RES, one is usually interested in controlling the converter input voltage for maximal power transfer. The VSI incorporates a RHP-zero in its control-to-output transfer function when the operating point is moved from the constant voltage to constant current region of the source. The frequency of the zero can be given based on the input capacitance, voltage and current. The RHP-zero will turn into a RHP-pole in the input voltage control loop, which leads to constraints between the capacitor sizing and control system design. This paper gives scientific insight into the capacitor sizing and control design, of which the latter is often ignored in the analyses but is an essential part of interfacing RES into the grid.

Dynamics of current-fed converters and stability-assessment of solar-generator interfacing

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. It has been observed that the maximum-power-point (MPP) tracking converters have high contribution on the reliability problems in the photovoltaic energy systems because of misunderstanding of their real nature. The paper shows that most of the applied converters are current-fed converters but usually assumed to be conventional voltage-fed converters. The general dynamic properties of a current-fed converter and the method to assess the stability of an interconnected system composing of a solar generator and a MPP-tracking converter are introduced. Theoretical findings are supported with experimental evidence.

Change of paradigm in power electronic converters used in renewable energy applications

Recent investigations on the nature of power electronic converters in renewable energy applications have revealed that most of the converters are not what they are claimed to be. The voltage-type sources have dominated as power sources especially in the past. Therefore, most of the power electronic converters are developed and designed for such applications even if some of them are known as current-sourced converters. The rapidly growing number of renewable-energy-based distributed systems has forced to pay closer attention to the structure of those systems and especially on the nature of power electronic converters within the systems. This paper summarizes the recent findings and states that most of the power electronic converters within the renewable energy systems are real current-fed converters, which have totally different static and dynamic properties compared to the conventional voltage-fed converters.

Appearance of a RHP-zero in VSI-based photovoltaic converter control dynamics

A typical grid-connected voltage source inverter (VSI) in its simplest form consists of an input voltage source, switch matrix, output inductor and grid, which acts as a voltage type load. This arrangement yields a converter with first-order control dynamics, a buck-type converter behavior and discontinuous input current. A photovoltaic (PV) array can provide a specific maximum current depending on the irradiation conditions. This implies that if the peak value of the pulsating input current of the converter tries to exceed the PV array current, the array voltage will collapse to a value dictated by the output voltage of the converter while the output current is determined by the array current. Furthermore, when the discontinuous input current of the converter is zero, the PV array operates in an open circuit. As a consequence, it may be obvious that an energy storage element, i.e. an input capacitor, is needed in parallel with the PV array in order to filter the pulsating input current drawn by the VSI. A vast majority of VSI analysis in PV applications assumes a constant voltage input source and current control dynamics depending only on the filtering elements at the output, e.g. first order dynamics with a mere inductor. However, this assumption is not well justified as will be proven in this paper.

Photovoltaic generator as an input source for power electronic converters

A photovoltaic (PV) generator is internally a power limited non-linear current source having both constant current and 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 one commercial solar array simulator as an example in terms of the defined properties. Investigations are based on extensive practical measurements from dc-dc as well as three- and single-phase dc-ac converters.

Designing MLBS Excitation for the Frequency-Response Measurement of AC-Connected Power Electronics Systems

Abstract Renewable energy, such as solar and wind, is usually connected to a power grid through grid-parallel inverters. The impedance mismatch between the grid and the interfacing circuit often generates harmonic resonances which leads to reduced power quality. Recent studies have shown that the problem can be approached through impedance models that may be obtained by broadband excitation and cross-correlation technique. However, the inverters are affected by the sinusoidal (AC) grid voltage, which necessitates modifications to the state-of-art techniques designed for DC systems. This paper considers the modifications and proposes methods for obtaining impedance models for AC-connected systems.