Teuvo Suntio

• PhD
• Professor Emeritus at Tampere University

This paper provides a simple and effective method to model the closed-loop output impedance of a buck converter under direct-duty-ratio (DDR) and peak-current-mode (PCM) control as well as under PCM control with output-current-feedforward (OCF), which can be used to study analytically the time-domain response of the output voltage to a change in th...

The heuristic perturb-and-observe-based maximum-power-point tracking (MPPT) algorithm of photovoltaic (PV) generator is still the most popular technique in use, despite the broad spectrum of developed other MPPT algorithms. The correct direction of the next perturbation step requires that the previous perturbation is settled down properly and the a...

Renewable energy-based generation of electrical energy is currently experiencing rapid growth in electrical grids [...]

The papers in this special section focus on power conversion and control in photovoltaic power plants. The power grid is undergoing significant changes with the rapid adoption of renewable energy. Solar power generation, and photovoltaics (PV) in particular, has been one of the fastest growing sectors in the power industry and has now reached price...

Impedance-ratio-based interaction analyses in terms of stability and performance of DC-DC converters is well established. Similar methods are applied to grid-connected three-phase converters as well, but the multivariable nature of the converters and the grid makes these analyses very complex. This paper surveys the state of the interaction analyse...

Owing to the good trade-off between implementation and performance, fixed-step direct maximum power point tracking techniques (e.g., perturb and observe and incremental conductance algorithms) have gained popularity over the years. In order to optimize their performance, perturbation frequency and perturbation step size are usually determined a pri...

Peak current-mode (PCM) control has been a very popular control method in power electronic converters. The small-signal modeling of the dynamics associated with PCM control has turned out to be extremely challenging. Most of the modeling attempts have been dedicated to the converters operating in continuous conduction mode (CCM) and just a few to t...

In this paper, a novel method of maximum power point tracking of renewable energy generators is proposed, utilizing the sum of dynamic and static conductance as maximum power point tracking loop variable. This allows to formulate the maximum power point tracking problem as a typical closed-loop stabilization task of non-linear static plant with zer...

Peak-current-mode (PCM) control was proposed in 1978. The observed peculiar behavior caused by the application of PCM-control in the behavior of a switched-mode converter, which operates in continuous conduction mode (CCM), has led to a multitude of attempts to capture the dynamics associated to it. Only a few similar models have been published for...

The application of a resistor as a load of the pulse-width-modulated DC-DC converters has dominated the dynamic modelling since the development of the modelling methods in early 1970s. In 1990s, the research in the source and load interactions was very active providing valuable information to justify the necessity to develop unterminated dynamic mo...

Solar energy is an important source of renewable energy due to the easy access to the source. One drawback in this system is achieving the maximum efficiency of the system. The photovoltaic (PV) cell is the fundamental unit in the power conversion of the solar system. The extracted maximum output power of PV cell reduces when it exposes to partial...

Filling the need for a reference that explains the behavior of power electronic converters, this book provides information currently unavailable in similar texts on power electronics. Clearly organized into four parts, the first part treats the dynamics and control of conventional converters, while the second part covers the dynamics and control of...

Voltage-type sources have dominated as an input source for power electronics converters for a long type. The existence of duality implies that there are also current-type sources. The growing application of renewable energy sources such as wind and solar energy has evidently shown that the current-type input sources exist in reality such as photovo...

The application of constant power control and inclusion of energy storage in grid-connected photovoltaic (PV) energy systems may increase the use of two-stage system structures composed of DC–DC-converter-interfaced PV generator and grid-connected inverter connected in cascade. A typical PV-generator-interfacing DC–DC converter is a boost-power-sta...

In order to enhance maximum power point tracking (MPPT) speed of photovoltaic generators (PVGs) upon fast irradiation changes, maximum power line (MPL)-based control is often used. MPL is a curve, linking all possible MPP coordinates for a given temperature. In the literature so far, PVG MPL was either assumed linear, which is inaccurate for all ir...

LCL-filter is commonly used to attenuate the switching harmonics of grid-connected converters. LCL-filter creates resonances in the converter dynamics which shall be damped for ensuring robust performance of the converter. Active damping methods can be used to attenuate the resonant behavior effectively. Accordingly, the output impedance is affecte...

Due to relatively good performance and simple implementation, fixed-step direct maximum power point tracking techniques such as perturb & observe and incremental conductance are the most popular algorithms aimed to maximize the energy yield of photovoltaic energy conversion systems. In order to optimize maximum power point tracking process performa...

In order to optimize the performance of direct (or perturbative) fixed-step maximum power point tracking algorithms (e.g. perturb & observe and incremental conductance), two design parameters – perturbation frequency and step size – must be selected. The main requirement for perturbation frequency design is ensuring the period between two successiv...

There are numerous maximum power point tracking (MPPT) algorithms for improving the energy efficiency of solar photovoltaic (PV) systems. The main differences between these algorithms are digital or analog implementation, simplicity of the design, sensor requirements, convergence speed, range of effectiveness, as well as hardware costs. Therefore,...

LCL-filter is commonly used as an output-side filter in a grid-connected converter due to its good attenuation capability at the switching harmonic frequencies. LCL-filter creates several resonances in the converter dynamics which shall be damped for ensuring robust performance of the converter. The resonant behavior introduced by the LCL-filter ca...

The power fed to the grid by a single-phase photovoltaic (PV) inverter fluctuates at twice the grid frequency. This fluctuation causes double-line-frequency ripple component to the DC-link voltage in two-stage PV inverter. If the DC/DC converter is not able to attenuate this ripple, also the PV generator voltage will fluctuate decreasing the max...

Grid-connected three-phase inverters are increas- ingly used to interface renewable energy sources to distribution networks. From the perspective of power quality the output impedance of the inverter should be as large as possible since inverters are usually operated under output-current control mode. Feedforward from grid voltages has been demonst...

Photovoltaic arrays have been shown to possess significant influence on the dynamic performance of coupled source-converter-load system while its dynamic resistance is the most important parameter, contributing to the overall system dynamics. In this paper, a method of deriving the parameters of linearized photovoltaic equivalent circuit from manuf...

The manuscript presents a method of operating renewable energy generators (REG) by a new method of robust maximum power point tracking (MPPT). The method utilizes the sum of REG static and dynamic conductances as the main MPPT loop regulation variable. Static and dynamic conductances are both operating point and environmental variables dependent. N...

Grid-parallel inverters are typically used to connect renewable energy sources to a power grid. The impedance mismatch between the grid and the interfacing circuit can generate harmonic resonances which may lead to reduced power quality. The impedance mismatch can be mitigated by applying an inverter that employs feedforward control. This paper pre...

Three-phase photovoltaic inverters are usually equipped with an LCL-type output filter to reduce cost and size of the converter compared to a simple L-type output filter. However, an LCL-filter has an inherent resonance which has to be damped by a passive or active method to avoid instability. This paper presents an accurate small-signal model of...

This paper proposes a multivariable small-signal model of a three-phase grid-connected renewable energy inverter in dq-domain. The model allows predicting the shape of inverter output admittance with an arbitrary power factor and includes the dynamic effects of DC-voltage control, AC-current control, phase-locked-loop and decoupling terms. There is...

Nonlinear characteristics of photovoltaic generators were recently shown to significantly influence the dynamics of interfacing power stages. Moreover, since the dynamic resistance of photovoltaic generators is both operating point and environmental variables dependent, the combined dynamics exhibits these dependencies as well, burdening control ch...

In this paper, dynamic analysis of a multi-battery dual mode charger, powered by a single solar array and suitable for lead-acid and lithium-ion cell-based batteries is presented. Each battery is interfaced to the solar array by means of a current-controlled buck power stage, operating either in constant power or constant voltage mode. Operation in...

The optimal design of a VSI based photovoltaic (PV) inverter has been studied extensively during the last years. The focus in these studies has been in the selection of the reactive components of the LCL-filter, leaving the inductor design out from the discussion. However, the inductor design plays important role when the design target is to minimi...

Three-phase photovoltaic inverters are usually equipped with an LCL-type output filter to reduce cost and size of the converter compared to a simple L-type output filter. The LCL-filter has an inherent resonance which has to be damped by a passive or active method to avoid instability. This paper presents an accurate full-order small-signal model...

In transformer-less grid-connected renewable energy systems, interfacing of photovoltaic (PV) generators is typically implemented by means of DC-DC boost-power-stage converter, mainly because of its voltage-boosting capability. In order to track the maximum power point of a PV generator, input voltage of the converter is usually feedback controlled...

In this paper, “An Efficient Partial Power Processing DC/DC Converter for Distributed PV Architectures” by Agamy et al. (IEEE Trans. Power Electron., vol. 29, no. 2, Feb. 2014), a dc-dc converter for distributed photovoltaic plant architectures, capable of performing series-type partial power processing under maximum power point control was propose...

This paper investigates the dynamic properties of the photovoltaic-generator-interfacing voltage-controlled buck power stage operating in both the maximum and limited power point tracking modes. The photovoltaic generator (PVG) is known to possess both current- and voltage-source properties with respect to its maximum power point. While voltage-fed...

In this paper, design process and functionality of a portable single-panel dual-battery solar charger prototype are presented, achieving energy density of 571 W h kg−1 during a typical 3-day infantry mission. The device may instantaneously charge up to two Li-ion MR-2791 batteries, supporting plug-and-play operation. The system consists of a lightw...

There are numerous Maximum Power Point Tracking (MPPT) algorithms for improving the energy efficiency of solar photovoltaic (PV) systems. The main differences between these algorithms are digital or analog implementation, simplicity of the design, sensor requirements, convergence speed, range of effectiveness, as well as hardware costs. Therefore,...

Voltage-type sources such as storage battery, ac grid, and constant-output-voltage-regulated converters have dominated as input sources for power electronic interfaces for a long time, leading to the development of different power stages dedicated to voltage-type input sources. Recent penetration of renewable energy sources has initiated the use of...

The power losses of three-level neutral-point-clamped (NPC) converters and their AC-filters with iron and amorphous core inductors designed for a bipolar low voltage direct current (LVDC) distribution network are evaluated. The main objectives of the LVDC distribution network are improved energy efficiency, power quality and reliability and higher...

This paper investigates the benefits which can be obtained if the maximum output current of a photovoltaic (PV) generator is taken into account in the design of a boost-power-stage converter. The investigations clearly show that the benefits are smaller inductor core size and more uniform temperature distribution among the power electronic componen...

The low voltage direct current (LVDC) distribution network is a promising solution to be considered in future smart grids. For facilitating the design of cost effective LVDC distribution networks, the total losses of the network with different configurations are evaluated and compared. The possibility to connect the customer loads asymmetrically, w...

The amount of grid-connected inverters has been growing steadily over the past decade due to increase in renewable power generation. These inverters have been reported to degrade power quality in the grid in areas where the amount renewable power generation is large. However, the reasons behind the power quality issues are not yet extensively repor...

This paper investigates the physical issues affecting the load transient response both from the power-stage-component-selection and control-design point of views. A conventional buck converter under three different control schemes - direct-duty-ratio control, peak-current control and peak-current-control with load-current-feedforward control - is u...

It is well known that the application of peak-current-mode (PCM) control produces highly desirable properties in the conventional or voltage-fed converters but requires to compensate the inductor-current loop for extending the active duty ratio beyond 50%. This paper investigates the dynamical properties and operational constraints of a PCM-control...

This paper introduces a simple method to measure the output impedance of a three-phase grid-connected inverter in dq-domain. The impedance measurements are most often used for model verification purposes but they can be also utilized to study impedance-based interactions between three-phase converters and the utility grid. Therefore, the methods to...

Voltage-type sources such as grid, constant-voltage-regulated power supplies, and storage batteries have dominated as input sources for power electronic converters leading to development of multitude of different power stages dedicated for such input sources. The growing application of renewable energy sources has revealed that there are also curre...

In this paper the small-signal model of a three-phase photovoltaic inverter is upgraded to include the grid-voltage feedforward. The feedforward is shown to increase d and q-components of the inverter output impedance which makes the inverter more insensitive to impedance-based interactions and explains the reported improvement in the output curren...

Grid interfacing of photovoltaic generators using three-phase inverters offers the advantage of constant power flow allowing smaller capacitance values to be used in the dc-link compared to single-phase inverters. Electrolytic capacitors, used in the dc-link, are often considered to decrease reliability. Reliability could be improved by using film...

A converter circuit includes first and second input terminals, first and second capacitors, the second capacitor having a first terminal connected to the second input terminal and a second terminal forming a positive voltage node, first and third semiconductor components connected in series between the first input terminal and the positive voltage...

All the interconnected regulated systems are prone to impedance-based interactions making them sensitive to instability and transient-performance degradation. The applied control method affects significantly the characteristics of the converter in terms of sensitivity to different impedance interactions. This paper provides for the first time the w...

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 ext...

The main design objective of photovoltaic (PV) systems has been, for a long time, to extract the maximum power from the PV array and inject it into the ac grid. Therefore, the maximum power point tracking (MPPT) of a uniformly irradiated PV array and the maximization of the conversion efficiency have been the main design issues. However, when the P...

Photovoltaic generator is often interfaced with the utility grid using a two-stage inverter. The two-stage inverter comprises of a dc-dc boost power-stage converter and a three-phase inverter. The duty ratio of the dc-dc converter can be given directly by a MPPT-algorithm or, alternatively, by a controller which regulates the voltage of the PV gene...

The objective of this paper is to provide performance metrics for small-signal stability assessment of a given system architecture. The stability margins are stated utilizing a concept of maximum peak criteria (MPC) derived from the behavior of an impedance-based sensitivity function. For each minor-loop gain defined at every system interface, a si...

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,...

Exemplary systems and methods provide a transformer-isolated current-fed quadratic full-bridge converter topology. The optimal interfacing of a current source, such as a solar panel, can be implemented by using current-fed converters. The current-fed converter can operate within the whole range of a UI curve from short-circuit to open-circuit condi...

Z-source inverter (ZSI) is a recently proposed buck-boost type inverter topology that has wider input voltage range compared to a commonly used voltage-source inverter (VSI). In photovoltaic (PV) applications, the wide input voltage range increases maximum power point tracking efficiency of the interfacing converter and makes it more suitable for d...

Photovoltaic power has to be converted from DC into AC in grid-connected applications. The conversion is done by using a single or three-phase inverter. Phase angle and frequency of the injected current and the grid voltage have to match to achieve unity power factor. This has been commonly accomplished by using a phase-locked-loop (PLL). The PLL h...

This Special Issue aims to address current challenges in the design and operation of Solar or photovoltaic (PV) energy systems, to report on novel ideas for enabling the wider utilization of PV energy and to explore optimal energy management control strategies for the PV-based Power Electronic System. The readers of this Special Issue will finally...

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 m...

The impedance-based stability-assessment method has turned out to be a very effective tool and its usage is rapidly growing in different applications ranging from the conventional interconnected dc/dc systems to the grid-connected renewable energy systems. The results are sometime given as a certain forbidden region in the complex plane out of whic...

The optimization of power architectures is a complex problem due to the plethora of different ways to connect various system components. This issue has been addressed by developing a methodology to design and optimize power architectures in terms of the most fundamental system features: size, cost and efficiency. The process assumes various simplif...

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 espec...

A common practice in the analysis of voltage source inverters is the assumption of a voltage-type input source even in the case of renewable energy sources (RES). When a converter is supplied by an RES, such as a photovoltaic (PV) generator, the dc-link voltage is usually controlled by the inverter to extract maximum power from the source. This stu...

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 impor...

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 o...

Dc-dc interfacing of photovoltaic (PV) modules into the downstream system is usually done by using a boost-power-stage converter with an added input capacitor. Its dynamic properties are often assumed to be equal to those of a conventional boost converter. The input voltage of the converter is most often feedback controlled to achieve maximum power...

Dynamic modelling of a switched-mode converter describes the internal dynamics of the converter and is computed using ideal source and load systems. Only by taking into account the effect of non-ideal source and load impedances can the converter dynamics be described reliably. This can be easily done for DC-DC converters. So far only the source-eff...

The paper presents a method to analyze robust stability and transient performance of a distributed power system consisting of commercial converter modules interconnected through a common input filter. The method is based on the use of four transfer functions, which are measurable from the converter input and output terminals. It is shown that these...

This paper investigates DC-link voltage control in a singlephase photovoltaic (PV) inverter. Recent research has shown that a right-half-plane (RHP) zero exists in the output current control dynamics of a voltage source inverter (VSI) and that the sign of current control loop changes when the VSI is fed from a current source, such as PV generator....

The utility-grid connection of a photovoltaic (PV) generator can be implemented by using a single or double-stage inverter. The single-stage inverter is connected directly to the PV generator, which is observed to significantly change the control dynamics of the inverter depending on the operating point. In the double-stage scheme, a dc/dc converte...

The number of power electronic converters is increasing in the grid due to favourable political climates regarding renewable energy resources. Grid-parallel inverters have been shown to suffer from reliability problems that may not be solved with pure time domain analysis. Therefore, it is important to analyse the dynamics of these converters also...

In practical photovoltaic installations, difficulties with partial shading and module have been observed to greatly reduce the energy yield. These difficulties are typically encountered in built environment, where the system is frequently shaded e.g. by nearby buildings. A widely recognized solution to overcome these difficulties is to utilize modu...

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 app...

A buck-boost-type converter is required to interface a photovoltaic (PV) generator to a voltage bus, which has lower or higher voltage than the voltage of the PV generator. Due to the semiquadratic nature of the proposed converter, it can be used in transformerless modular PV energy systems needing wide conversion ratio. Dynamic analysis provides t...

The urge to utilize green energy supplies increases the number of grid-connected photovoltaic (PV) energy systems. Transformerless inverter technology is a key for efficient and low cost solutions. In order to achieve the high DC link voltage required by the transformerless single-stage inverter, numerous photovoltaic modules has to be connected in...

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 interes...

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...

This paper introduces a method to analyze and predict stability and transient performance of a distributed system where COTS (Commercial-off-the-shelf) modules share an input filter. The presented procedure is based on the measured data from the input and output terminals of the power modules. The required information for the analysis is obtained b...

Interfacing a single photovoltaic (PV) module into either a single-phase or a three-phase inverter requires an interfacing converter with wide conversion range 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 espec...

A photovoltaic generator is internally a current source having features resembling either a current or voltage source depending on the operating point in respect to its maximum power point (MPP). Therefore, the MPP-tracking converter can be implemented either as a voltage-fed (VF) or current-fed (CF) converter. This paper investigates the dynamic p...

Interfacing a single photovoltaic (PV) module into either a single-phase or a three-phase inverter requires an interfacing converter with wide conversion range 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 espec...

The interfacing of photovoltaic (PV) energy source is known to be problematic due to the highly varying terminal characteristics of the energy harvesting unit, generally a PV generator. Therefore, its power flow needs to be regulated and a certain maximum power point (MPP) traced. The switchedmode dc-dc converters are conventionally used to interfa...

This paper presents the general dynamic descriptions of current-fed converters applied in the solar or photovoltaic applications including an impedance-based method to assess the stability of the corresponding interconnected system. Several earlier unknown general parameters are indentified governing either the low-frequency behavior of the closedl...