G. Brando

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.

A comparison between some control algorithms of parallel active filtering

This paper deals with parallel active filtering by means of IGBT-Voltage Source Inverters. The control algorithm is based on the evaluation of active and reactive power by detecting line voltages and load currents. Then these powers are split in their average and alternative components using proper low-pass filters. The features linked to some different techniques of current control are investigated with reference to time behaviour and frequency spectrum of the resultant line current. Simulation results are discussed and compared with reference to a specific case-study.

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 algorithm for torque oscillation reduction in DTC-induction motor drives using 3-level NPC inverter

The paper presents a direct torque control DTC for induction motor drive using a 3-level NPC inverter. The control is optimized for torque oscillation reduction. The control is described evidencing the good performance validated by means of simulation. The results are reported and discussed in the paper.

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.

A predictive control technique for PWM-rectifier in vector controlled induction motor drives

In the paper, an optimized control technique of an active front-end is described with reference to induction motor drives with a voltage source inverter combined with an active voltage source rectifier. Both the motor-side and line-side PWM power converters employ IGBTs as switching devices. Based on a simplified analytical solution of the system mathematical model in a discrete digital control, a coupled control plus modulation technique is pointed out. It is able to estimate the values of the interesting electrical quantities easily in some switching instants within the control sampling period. This allows one to evaluate the width of the different modulation intervals directly. A very simple method to recognize the modulation sector quickly in the complex plane is also proposed. A numerical analysis is performed in order to test the reaction capability of the active front-end to different heavy variations of the operating conditions of the drive.

Permanent magnet brushless drives controlled by sensorless predictive algorithm

A speed and high resolution position identification algorithm, suitable for brushless drives, is presented in this paper. The algorithm, which is developed basing upon the Popovs Hyperstability Theory, guarantees the convergence of the estimated signals to their corresponding actual values by means of the measured currents. The algorithm is proposed for brushless DC (BLDC) motors which are characterized by emfs shapes different from the ideal trapezoidal ones. The identification procedure can be performed starting from the knowledge of both low resolution rotor position Hall signals and emf shape. The identification algorithm is properly tested on a BLDC drive controlled by a predictive algorithm, by performing a simulation study in the Matlab-Simulink environment. The corresponding results have highlighted the effectiveness of the proposed sensorless control system at both low and high speed operation.

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.

Power quality problems in unbalanced operations of fault tolerant H-bridge multilevel active front-ends

The paper deals with active voltage source rectifiers (PWM-VSR) having multilevel cascaded H-bridge configuration. With reference to operations with unsymmetrical supply voltages and/or unbalanced three-phase loads, some optimized control strategies are presented and compared. Particular attention is devoted to the most effective control algorithm in terms of distortion of the input-currents and limitation of the line-current unbalancing. The algorithms are represented by a set of algebraic equations able to generate in a very simple way the actual reference values for the dynamic control of VSRs. Some heavy transient operations are analyzed in order to show the validity of the proposed techniques and the capability to limit negative influence on the line power-quality indexes by means of optimized control of cascaded H-bridge active front-ends.