Stefano Bifaretti

Advanced Power Electronic Conversion and Control System for Universal and Flexible Power Management

The future electricity network has to be able to manage energy coming from different grids as well as from renewable energy sources (RES) and other distributed generation (DG) systems. Advanced power electronic converters can provide the means to control power flow and ensure proper and secure operation of future networks. This paper presents analysis, design, and experimental validation of a back-to-back three-phase ac-dc-ac multilevel converter employed for universal and flexible power management (UNIFLEX-PM) of future electrical grids and its advanced control technique. The proposed system has been successfully tested for bidirectional power flow operation with different grid operating conditions such as voltage unbalance, frequency variation, harmonic distortion, and faults due to short circuits.

Modulated Model Predictive Control for a Seven-Level Cascaded H-Bridge Back-to-Back Converter

Multilevel converters are known to have many advantages for electricity network applications. In particular, cascaded H-bridge converters are attractive because of their inherent modularity and scalability. Predictive control for power converters is advantageous as a result of its applicability to discrete system and fast response. In this paper, a novel control technique, named modulated model predictive control, is introduced with the aim to increase the performance of model predictive control. The proposed controller addresses a modulation scheme as part of the minimization process. The proposed control technique is described in detail, validated through simulation and experimental testing, and compared with dead-beat and traditional model predictive control. The results show the increased performance of the modulated model predictive control with respect to the classic finite control set model predictive control in terms of current waveform total harmonic distortion (THD). Moreover, the proposed controller allows a multi-objective control, with respect to dead-beat control that does not present this capability.

Modulated Model Predictive Control for a Three-Phase Active Rectifier

Model predictive control (MPC) has a number of desirable attributes which are difficult to achieve with classical converter control techniques. Unfortunately, the nature of power electronics imposes restriction to the method, as a result of the limited number of available converter states. This, combined with the spread spectrum nature of harmonics inherent with the strategy, complicates further design. This paper presents a method for removing this characteristic without compromising the desirable functionality of predictive control. The method, named modulated MPC, is applied to a two-level three-phase converter and compared with a number of similar approaches. Experimental results are used to support theoretical analysis and simulation studies.

Active DC Voltage Balancing PWM Technique for High-Power Cascaded Multilevel Converters

In this paper, a dedicated pulse width modulation (PWM) technique specifically designed for single-phase (or four wire three-phase) multilevel Cascaded H-Bridge Converters is presented. The aim of the proposed technique is to minimize the DC-Link voltage unbalance, independently from the amplitude of the DC-Link voltage reference, and compensate the switching device voltage drops and on-state resistances. Such compensation can be used to achieve an increase in the waveform quality of the converter. This is particularly useful in high-power low supply voltage applications where a low switching frequency is used. The DC-Link voltage balancing capability of the method removes the requirement for additional control loops to actively balance the DC-Link voltage on each H-Bridge, simplifying the control structure. The proposed modulation technique has been validated through the use of simulation and extensive experimental testing to confirm its effectiveness.

A new predictive control method for cascaded multilevel converters with intrinsic modulation scheme

Multilevel Converters are known to have many advantages for electricity network applications. In particular Cascaded H-Bridge Converters are attractive because of their inherent modularity and scalability. Predictive control for power converters is advantageous as a result of its applicability to discrete system and fast response. In this paper a novel control technique, named Modulated Model Predictive Control, is introduced with the aim to increase the performances of the Predictive control. The proposed controller is described in detail and validated through simulation and experimental testing, in comparison with Dead-Beat and Model Predictive Control.

Global Learning Position Controls for Permanent-Magnet Step Motors

Permanent-magnet step motors offer several advantages such as high efficiency, high power density, high torque-to-inertia ratio, and excellent durability and serviceability, as well as the absence of external rotor excitation and windings. The nonuniformity in the developed torque due to the nonsinusoidal flux distribution in the airgap is, however, the major obstacle in achieving global high-precision position tracking. When the position reference profile is a periodic signal of known period, such an obstacle may be however overcome by using recent learning control techniques, which require neither high gains in the inner speed/position control loops nor resetting procedures. An experimental comparison of two different recently designed learning position controls (“adaptive” and “iterative”) is, for the first time, carried out with reference to the same low-speed robotic application. Benefits and drawbacks of the two learning approaches are analyzed in detail.

Multiobjective Modulated Model Predictive Control for a Multilevel Solid-State Transformer

Finite control set model predictive control (FCS-MPC) offers many advantages over more traditional control techniques, such as the ability to avoid cascaded control loops, easy inclusion of constraint, and fast transient response of the control system. This control scheme has been recently applied to several power conversion systems, such as two, three, or more level converters, matrix converters, etc. Unfortunately, because of the lack of the presence of a modulation strategy, this approach produces spread spectrum harmonics which are difficult to filter effectively. This may result in a degraded power quality when compared to more traditional control schemes. Furthermore, high switching frequencies may be needed, considering the limited number of switching states in the converter. This paper presents a novel multiobjective modulated predictive control strategy, which preserves the desired characteristics of FCS-MPC but produces superior waveform quality. The proposed method is validated by experimental tests on a seven-level cascaded H-bridge back-to-back converter and compared to a classic MPC scheme.

Power flow control through a multi-level H-bridge based power converter for Universal and Flexible Power Management in future electrical grids

The paper proposes a novel power conversion system for universal and flexible power management (UNIFLEX-PM) in future electricity network. The structure is based on three AC-DC converters each one connected to a different grid, (representing the main grid and/or various distributed generation systems) on the AC side, and linked together at DC side by suitable DC isolation modules. Each port of the UNIFLEX-PM system employs a conversion structure based on a three-phase 7-level AC-DC cascaded converter. Effective and accurate power flow control is demonstrated through simulation in Matlab and Simulink environment on a simplified model based on a two-port structure and using a stationery reference frame based control solution. Control of different power flow profiles has been successfully tested in numerous network conditions such as voltage unbalance, frequency excursions and harmonic distortion.

Performances of a PLL based digital filter for double-conversion UPS

In uninterruptible power supplies (UPS) using an automatic bypass switch, to reduce the transients due to the switch commutations, it is convenient that the voltage waveforms produced by the inverter are synchronized with the grid ones. In three-phase UPS, the inverter controller employs, as reference signals, the components of the grid voltage phasor, referred to a fix reference frame, suitably multiplied by an amplitude correction factor. Generally, the reference signals are affected by harmonics and amplitude unbalances. Therefore, improper synchronization between the inverter outputs and the grid voltages can arise. To improve the synchronization, many solutions based on a phase locked loop (PLL) system have been proposed in literature. The paper proposes a different PLL structure employing a Steady-State Linear Kalman filter (SSLKF), based on a third-order linear and time-invariant observation model. Such filter is able to provide an accurate tracking of the grid voltage phasor also in critical operating conditions. The paper describes, at first, different PLL structures and then the proposed architecture based on a prediction-correction filter and its implementation on a DSP controller. Finally, a comparison, obtained by simulation, with traditional filters and some significant experimental results on a pre-production prototype are carried out.

Global MPPT method for partially shaded photovoltaic modules

Among Renewable Energy Sources, the solar energy represents an attractive alternative solution to traditional sources because is available everywhere; moreover, photovoltaic (PV) modules are nowadays becoming more and more attractive as the price per watt of the photovoltaic modules is decreasing. The output voltage furnished by a PV module is quite low; therefore, a series connection of different modules has to be employed in order to obtain a sufficient output dc voltage. In non-uniform irradiation conditions of a PV array, traditional Maximum Power Point Tracking (MPPT) algorithms can converge to a relative maximum point, thus output power becomes lower than the absolute maximum. Since the conversion efficiency of commercial photovoltaic modules is still rather low, to increase the power produced, also under partially shaded conditions, is necessary to employ an efficiently, fast and simple MPPT. The paper proposes, for a Switched Capacitor-Boost converter structure, a non-conventional MPPT method able to track the absolute Maximum Power Point (MPP).