A. Dannier

A high performance control technique of power electronic transformers in medium voltage grid-connected PV plants

With reference to medium-voltage grid-connection of large photovoltaic (PV) plants, the paper proposes and analyses a high performance solution based on a power electronic transformer (PET). It Includes dc-links and multilevel converters either in low-voltage or in high-voltage side, and a medium-frequency (MF) transformer. Together with the very reduced sizes of the whole conversion apparatus, an important feature of the proposed topology is the permitted presence of unbalanced voltages on the different dc-links corresponding to the partitions of the PV plant. This is enabled by the multilevel cascaded H-bridge configuration of the low-voltage side converter, in order to take into account the nonuniform distribution either of temperature or (mainly) of solar radiation on the different cells arrays, together with eventual different kind or age of used cells. another advantageous feature is the good level of power quality indexes on grid quantities, ensured by a multilevel NPC front-end: mainly low values of either harmonic distortion, or voltage and current dissymmetry. this occurs even if the commutation frequency of the switching devices of this converter is not high (1÷2 kHz). However, the paper is mainly focused on the control technique of the multilevel converters connected to the primary and secondary windings of the MF transformer, imposing multistep- or square-wave voltages. Some simple control algorithms are presented in order to maximize the power generated by every partition of the plant (also in unbalanced operations) together with the minimization of the joule losses of the central conversion unit in quasi-stationary operating conditions.

An overview of Power Electronic Transformer: Control strategies and topologies

In the last years the interest towards Power Electronic Transformers (PET) is increasing. These new conversion apparatuses perform either voltage transformation or power quality functions, using power electronics both on primary and secondary sides of a transformer operating at medium frequency. In the technical literature are proposed several circuital configurations, with different control strategies. However these are not exhaustive yet, especially if is considered that PET can be utilized in a lot of different fields of application. Simplicity, effectiveness and high-resolution of the control are important aspects which can strongly improve performance of PETs. The paper presents an overview of the main topologies of converters for PET with a Medium Frequency (MF) transformer, and the control strategies are analyzed.

Power Electronic Transformer application to grid connected photovoltaic systems

Grid-connected photovoltaic systems utilize static converters which influence the efficiency of the system. The energy conversion depends on the architecture of the converter, different solutions are available and H-bridge multilevel converters seem to be an optimal solution also for the power quality. In the paper is proposed an architecture that includes a Power Electronic Transformer which is practically an isolated high-frequency link AC/AC converter that substitute a conventional transformer. An MPPT control technique is presented and validated by simulation implemented on a photovoltaic system with H-bridge n-levels converter and Power Electronic Transformer. The simulation results confirm that the control is able to effectively track the Maximum Power Point and to stabilize immediately in the new steady-state condition.

A Sensorless Control of H-bridge Multilevel Converter for Maximum Power Point Tracking in Grid Connected Photovoltaic Systems

There is a high development of grid-connected photovoltaic systems that utilize static converter which influence the efficiency of the system. The energy conversion depends on the architecture of the converter, different solutions are available and H-bridge multilevel converters seem to be an optimal solution also for the power quality. It is important to utilize a proper architecture of converter but also to set up a control optimized to have the energy conversion at maximum efficiency. The persistence of the maximum efficiency condition is related, depending on the atmospheric conditions, to the tracking of the Maximum Power Point (MPP) by modifying the operating conditions of the system (Maximum Power Point Tracking -MPPT). In the paper is proposed a sensorless control set up to deliver the maximum power to the grid in presence of variations of incident irradiation on the photovoltaic arrays. The control technique is presented and validated by simulation implemented on a photovoltaic system with H-bridge 5-levels converter. The simulation results confirm that the control is able to effectively track the MPP and to stabilize immediately in the new steady-state condition.

An Optimized Control Technique of Cascaded H-bridge Multilevel Active Front-ends

The paper is devoted to the investigation of a three-phase multilevel H-bridge PWM-Rectifier operating as a first stage of a back-to-back converter in high-power ac drives. First, a predictive control technique is described with reference to a 5-level converter. Starting from properly defined reference values, the algorithm pointed out allows the direct evaluation of the duty-cycles of all the voltage vectors involved in the VSR modulation. By means of the evaluated switching functions, at the end of every sampling interval the line-currents assume values practically equal to the imposed ones. Some unrestrictive hypotheses are introduced in order to efficaciously simplify the analytical expressions. As a second step, in the paper a specially designed modulation technique is shown. In addition to low values of dc-link voltage oscillations and of line-current distortion in steady-state operations together with high values of the power-factor, this modulation technique allows to keep balanced the different capacitor voltages, by properly using the intrinsic redundancy of the converter topology. Numerical results are presented with reference to some significant operative conditions in order to validate the theoretical considerations.

Maximum Power Point Tracking Algorithm for Grid­tied Photovoltaic Cascaded H­bridge Inverter

Abstract In this article, a maximum power point tracking algorithm properly adapted for a gridtied photovoltaic multi-level inverter is presented. The inverter structure is based on a singlephase cascaded Hbridge configuration, where each power cell is supplied by an individual photovoltaic panel. The dedicated maximum power point tracking algorithm is based on a suitable hysteresis band, which defines the proper boundaries for maximum power point tracking references to ensure both stable inverter operation and maximum photovoltaic power extraction. The inverter control method is a mixed staircase pulse-width modulation based on a sorting algorithm. Some experimental tests have been performed by using a laboratory prototype of a singlephase fivelevel photovoltaic cascaded Hbridge inverter, which is controlled by means of dSPACE realtime hardware platform (ds1006, dSPACE GmbH, Paderborn, Germany), where the control section is implemented on a field-programmable gate array (Xilinx Virtex5, Xilinx, Inc., San Jose, CA). The experimental results confirm that the proposed control is able to efficiently track the maximum power point while assuring good performance in terms of harmonic distortion and power factor in both standard and mismatch conditions.

Numerical and experimental validation of a LiFePO 4 battery model at steady state and transient operations

In the paper some of the battery models proposed in literature are analysed in order to predict the battery performance and, then, make sure that the Battery Management System (BMS) that is a key component to check and control the status of the batteries within their specified safe operating conditions, works in best conditions. In fact to reach an extended battery lifetime and increase the reliability of the system, is necessary to guarantee an accurate evaluation of the state of the whole stack of batteries to track their operations. Therefore, an accurate battery model that captures the dynamics of the battery is required. Lithium batteries are in particular considered because of their high energy density and long lifetime that make them increasingly used in systems such as electric vehicles, space and aircraft power systems and in the energy storage systems for distributed generation systems. In the paper the most interesting models of batteries proposed in literature are analysed and compared with the RC model of battery by means of numerical and experimental testing. The results are discussed in the paper.

Efficiency evaluation of a micro-generation system for energy recovery in water distribution networks

The paper deals with the efficiency evaluation of a micro-generation unit for energy recovery in water distribution networks. The analyzed system includes an electric motor-driven pump working as turbine/generator, connected to the electrical grid through a bi-directional power converter. The micro-generation unit is implemented in an experimental setup which simulates a real water network in a reduced scale. The unit was tested and analyzed in order to monitor some critical electrical quantities as well as to evaluate its generating efficiency. In the measurements campaign, various operating conditions were analyzed. The corresponding results are reported and discussed.

An analysis of modular multilevel converter for full frequency range operations

In this paper an analysis of a power converter based on a modular multilevel topology is discussed. A mathematical model of the MMC is developed in order to evaluate its dynamic behavior and to better understand the incidence of the circulating current phenomenon which could compromise converter performance in variable speed drive applications. In fact, if such problem is not properly addressed reliability problems arise.

An optimized control of PWM-rectifiers with predicted variable duty-cycles

An optimized control of PWM voltage source rectifiers (VSR) is presented. First, in a multi-input look-up table the different converter voltage-vectors are ordered with respect to their aptitude to increase (or decrease) the active power, depending on the angular position of the input voltage vector. A simple procedure allows the quick selection of the most suitable converter-voltage which ensures the correct slope sign of both active and reactive power, in order to satisfy the respective reference values. A predictive procedure evaluates the application time of the selected voltage vector within the actual sampling interval, in order to keep the instantaneous active power inside prefixed hysteresis bands. The proposed algorithm quickly evaluates also the voltage vector to be applied in the remaining part of the sampling interval. Numerical investigations in some heavy operating conditions are carried out at a fixed average value of the switching frequency, especially with reference to current distortion and asymmetry.