Josep Balcells

Interfacing Renewable Energy Sources to the Utility Grid Using a Three-Level Inverter

This paper presents a novel approach for the connection of renewable energy sources to the utility grid. Due to the increasing power capability of the available generation systems, a three-level three-phase neutral-point-clamped voltage-source inverter is selected as the heart of the interfacing system. A multivariable control law is used for the regulator because of the intrinsic multivariable structure of the system. A current source (playing the role of a generic renewable energy source) is connected to the grid using a three-level inverter in order to verify the good performance of the proposed approach. Large- and small-signal d-q state-space averaged models of the system are obtained and used to calculate the multivariable controller based on the linear quadratic regulator technique. This controller simultaneously regulates the dc-link voltage (to operate at the maximum power point of the renewable energy source), the mains power factor (the power is delivered to the grid at unity power factor), and the dc-link neutral-point voltage balance. With the model and regulator presented, a specific switching strategy to control the dc-link neutral-point voltage is not required. The proposed controller can be used for any application, since its nature makes possible the control of any system variable. The good performance of the presented interfacing solution in both steady-state and transient operation is verified through simulation and experimentation using a 1-kW neutral-point-clamped voltage-source-inverter prototype, where a PC-embedded digital signal processor board is used for the controller implementation

Improved Waveform Quality in the Direct Torque Control of Matrix-Converter-Fed PMSM Drives

Despite the ability of matrix converters (MCs) to generate a higher number of voltage vectors compared with standard voltage-source inverters, most of the applications reported in the literature utilize only those having larger amplitudes. This paper investigates the use of MC input voltages with different amplitudes in order to reduce the inherent torque ripple that appears when direct torque control (DTC) is used to drive ac machines, particularly permanent-magnet synchronous motors, as its stator inductance is typically half of that of an induction machine of similar ratings. Utilizing a wider range of input voltage vectors for the MC, but not using the rotating vectors, a new lookup table that distinguishes between small and large torque errors is developed, leading to an enhanced MC-fed DTC. The enhancement enables a reduction in the electromagnetic torque ripple and output-current total harmonic distortion. Furthermore, the proposed control strategy improves the MC voltage transfer ratio up to 86.6% compared with 50% achieved by the conventional DTC using MCs at the expense, however, of slightly decreasing the input power factor control capability. The proposed enhanced MC DTC was tested experimentally, and results comparing its performance with MC DTC using an adapted lookup table are shown.

EMI reduction in switched power converters using frequency Modulation techniques

Frequency-modulation techniques have been used to reduce electromagnetic interference (EMI) produced by the clock of digital systems working in the range of hundreds of megahertz. The working principle consists of modulating the original constant clock frequency in order to spread the energy of each single harmonic into a certain frequency band, thus reducing the peak amplitude of EMI at harmonic frequencies. Nowadays, the switching frequency of power converters has increased up to values that make interesting the application of such techniques to reduce EMI emissions due to switching of power circuits. This paper presents the theoretical principles of frequency modulation using deterministic profiles for the modulating function. It shows the effectiveness of such methods in terms of EMI reduction for different modulation profiles and other parameters. The method is compared with other methods using random modulation. Tests carried out on a buck converter are presented for experimental validation of the method. A short discussion on optimal modulation profiles and parameters is also included.

Evaluation of switching frequency modulation in EMI emissions reduction applied to power converters

Although most of the power converters are currently designed to operate by using a constant switching frequency and a variable duty-cycle, some attempts were made in order to verify the effect of modulating the switching frequency [F. Lin et al., 1992] and how this modulation affected the power converters EMI emissions. As known, spread spectrum clock generation (SSCG) modulates the originally constant switching frequency by following a certain modulation profile in order to spread the single harmonic energy into an amount of side-band harmonics having the same energy but much smaller amplitudes. This reduction technique has been used and implemented for high frequencies (as those related to clock frequencies in communications and microprocessors systems). This paper is dedicated to SSCG applied to the reduction of EMI emissions in power converters, focusing on the effectiveness of frequency modulation in EMI reduction as a function of the different switching frequency ranges and modulation profiles. Theoretical results were obtained just modulating a sine pure wave following several modulation profiles [A. Santolaria et al., 2002], this one representing each one of the harmonics composing the real square PWM-signal controlling the power converter. A practical arrangement was implemented to generate and measure EMI emissions.

Reduction of Output Common Mode Voltage Using a Novel SVM Implementation in Matrix Converters for Improved Motor Lifetime

This paper presents the study of an alternative space vector modulation (SVM) implementation for matrix converters (MCs), which reduces the output common mode (CM) voltage. The strategy is based on replacing the MC zero vectors with rotating ones. In doing this, the CM voltage (CMV) can be reduced, which, in turn, reduces the CM leakage current. By reducing the CM current, which flows inside the motor through the bearings and windings, the induction motor (IM) deterioration can be slowed down. This paper describes the SVM pattern and analyzes the CMV and the leakage current paths. Simulation and experimental results based on an MC-IM drive are provided to corroborate the presented approach.

Wavelet-Based Performance Evaluation of Power Converters Operating With Modulated Switching Frequency

It has been demonstrated that modulating the switching frequency of a power converter is a valuable way for reducing the electromagnetic interference (EMI) due to the switching process. Since we are considering a signal whose frequency content varies with time, wavelets are well suited to analyze the performance of such techniques. In this paper, we evaluate the performance of spread spectrum frequency modulation (SSFM) applied to the EMI reduction of a real power converter that uses periodic pattern switching frequency modulation. The performance of the converter under investigation includes the analysis of the switching voltage spectrum (as the main source of EMI) and the output voltage ripple. This evaluation is performed with two coefficients, i.e., maximum energy ratio (MER) and energy dispersion ratio (EDR), which are figures of merit defined in this paper using time-dependent energy density distribution in frequency, obtained from the scalograms of the analyzed signals. Such figures of merit allow comparison in the time-frequency domain of different modulation techniques and the choice of the best solution for each case in terms of reduction of the peak of noise spectrum.

Speed Anti-Windup PI strategies review for Field Oriented Control of Permanent Magnet Synchronous Machines

The work presented in this paper addresses the unwanted windup phenomenon reviewing and comparing different PI anti-windup strategies employed in speed control of electric drives. The tuning process of PI controllers is usually carried out considering the system as linear and therefore disregarding its physical limits such as maximum current and voltage. To safeguard the systems integrity, the PI output is normally limited eventually causing the windup phenomenon characterized by long periods of overshoots which may even result in instability.

EMI reduction in switched power converters by means of spread spectrum modulation techniques

Spread spectrum clock generation techniques (SSCG) were originally developed to reduce EMI in communications and microprocessor systems working in the range of hundreds of MHz. The working principle consists of modulating the original constant clock frequency, in order to spread each single harmonic energy into a certain frequency band, thus reducing amplitudes at each individual frequency. Nowadays, the switching frequency of power converters has been increasing up to values that make feasible the application of such techniques to reduce EMI emissions in power circuits. This paper deals with SSCG applied to reduction of EMI emissions in switched power converters, following certain modulation profiles. It is focussed on studying the effectiveness of EMI reduction as a function of frequency ranges, modulation profiles and other modulation parameters. A buck converter, whose switching frequency can be swept from 100 kHz to 1 MHz, has been used for experimental validation. Conducted disturbances produced by the converter, using different modulation parameters, were measured and compared with those produced by the same converter when driven with the classical constant frequency, variable duty, switching pattern. Significant reductions in conducted EMI may be observed. Optimal modulation profiles and parameters are identified.

Common Mode EMI Model for a Direct Matrix Converter

This paper presents an electromagnetic interference (EMI) model to evaluate the conducted common mode (CM) disturbances produced by matrix converters (MCs). The model is based on obtaining a high frequency (HF) equivalent circuit with HF sources representing the switching devices. The circuit resolution in the frequency domain allows the calculation of any EMI parameter with very low computational burden and avoids convergence problems, which are common in time domain methods. The original contribution lies in the source model implementation and placement. The simulation and experimental results for CM leakage currents validate the EMI model.

Wavelet-Based Approach to Evaluation of Signal Integrity

In this paper, we present a new approach to evaluation of signal integrity that is based on signal energy density as a function of time and frequency, represented by its wavelet scalogram. Using signal integrity ratio and cumulative energy ratio, we illustrate signal integrity analysis with simulated examples, followed by the demonstration of their usefulness through analysis of experimental data of a real audio amplifier. These figures of merit represent the extent to which the integrity of a signal is diminished by the electromagnetic interference effects and/or nonlinear processes.