Fabricio Bradaschia

Modulation for Three-Phase Transformerless Z-Source Inverter to Reduce Leakage Currents in Photovoltaic Systems

In this paper, a modified Z-source inverter (ZSI) with specific modulation techniques is proposed to reduce leakage currents in three-phase transformerless photovoltaic (PV) systems. The new topology only requires an additional fast-recovery diode when compared with the original structure. On the other hand, the pulsewidth modulation technique is entirely modified in order to reduce the leakage currents through the conduction path. Simulation results for the three-phase transformerless PV system operating in two cases, i.e., connected to a grid and connected to a grounded RL load, are presented. Experimental results of leakage currents in three-phase ZSIs connected to a RL load are obtained to validate the theoretical and simulation models.

Digital Filters for Fast Harmonic Sequence Component Separation of Unbalanced and Distorted Three-Phase Signals

In this paper, two methods for determining the fundamental frequency and harmonic positive- and negative-sequence components of three-phase signals are investigated. Many aspects of the space-vector discrete Fourier transform and generalized delayed signal cancellation (GDSC) such as response time for different possible implementations, frequency adaptation schemes, stability of recursive implementation, and rounding error effects are discussed. A new design procedure for GDSC transformations is presented. New indices for characterizing three-phase unbalanced and distorted signals are proposed. Simulations and experiments are included in order to verify the performances and illustrate the theoretical conclusions.

A Method for Extracting the Fundamental-Frequency Positive-Sequence Voltage Vector Based on Simple Mathematical Transformations

In this paper, a novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage is proposed. The method is based on four simple mathematical transformations; two of them are in the stationary reference frame, which are able to eliminate odd harmonics from the original signals. The other two transformations are implemented in a synchronously rotating reference frame in order to eliminate even harmonics. The output of the last transformation block is the input to a synchronous reference-frame phase-locked loop for detecting the frequency and position of the positive-sequence voltage vector. The proposed algorithm was verified through simulations and experiments by applying distorted and unbalanced signals, containing positive and negative-sequence components. The results are in agreement with those theoretically predicted and indicate that the proposed scheme has a great potential for use in grid-connected converter synchronization algorithms.

A Modulation Technique to Reduce Switching Losses in Matrix Converters

This paper presents a modulation technique based on the generalized pulsewidth-modulation strategy for matrix converters. The proposed technique uses a discontinuous modulation to clamp each output leg of the converter during 120deg of the output voltage period, achieving a reduced number of switchings compared with the traditional modulation techniques. Aside from that, the major attraction of the proposed technique is an additional algorithm that lags the clamping of each output leg of the converter to synchronize it with the peak of the corresponding output current (load current), avoiding high switching losses (switching at high currents). Therefore, this technique reduces the number of switchings as well as guarantees only medium and low current switchings. Simulation and experimental results show the efficiency of the proposed technique.

A Space-Vector Discrete Fourier Transform for Unbalanced and Distorted Three-Phase Signals

In this paper, a space-vector discrete-time Fourier transform is proposed for fast and precise detection of the fundamental-frequency and harmonic positive- and negative-sequence vector components of three-phase input signals. The discrete Fourier transform is applied to the three-phase signals represented by Clarkes αβ vector. It is shown that the complex numbers output from the Fourier transform are the instantaneous values of the positive- and negative-sequence harmonic component vectors of the input three-phase signals. The method allows the computation of any desired positive- or negative-sequence fundamental-frequency or harmonic vector component of the input signal. A recursive algorithm for low-effort online implementation is also presented. The detection performance for variable-frequency and interharmonic input signals is discussed. The proposed and other usual method performances are compared through simulations and experiments.

Transformerless Hybrid Power Filter Based on a Six-Switch Two-Leg Inverter for Improved Harmonic Compensation Performance

One of the most important power quality issues is related to current harmonics generated by nonlinear loads such as diode and thyristor front-end rectifiers. Well-established solutions to overcome this problem, such as active power filters (APFs), have the required high power rating components as a major disadvantage. An alternative, called hybrid power filter (HPF), mixes low power rating active filters with passive filters. Unfortunately, many of these HPF topologies have, as a common disadvantage, a great number of passive components and/or transformers. Based on this fact, new concepts of HPFs, consisting of small-rated inverters and LC filters, have been introduced with wide acceptance. The advantage comes from the fact that these HPFs are connected to the grid without any matching transformer. Recently, some topologies based on dual-converter configurations have been shown to be very attractive, where the APF (or HPF) must deal with highly nonlinear loads with high values of di/dt and supplying the reactive power together with harmonic compensation. On the other hand, the drawback of dual converters is the high number of switch devices. Therefore, this paper proposes a transformerless HPF based on a new six-switch two-leg inverter with an enhanced harmonic compensation capability. Aside from presenting a reduced number of switches when compared with dual topologies, the proposed solution is capable of providing fully compensation even for loads with high harmonic content. Experimental results are presented for an HPF inverter prototype in order to demonstrate that the harmonic compensation performance meets the IEEE 519 standard.

Safe transient operation of microgrids based on master-slave configuration

Master-Slave configuration is a suitable alternative to droop control method used in microgrids. In this configuration, only one inverter is the master, while the others are slaves. The slave inverters are always current controlled whereas the master inverter should have two selectable operation modes: current controlled, when the microgrid is connected to the grid; and voltage controlled, when it is operating in island mode. In grid-connected mode, the master needs a synchronization system to perform the accurate control of its delivered power, and, in island mode, it needs a voltage reference oscillator that serves as a reference to the slave inverters. Based on the master-slave concept, this paper proposes a single system that perform both functions, i.e., it can act as a synchronization system or as a voltage reference oscillator depending on an input selector. Moreover, the system ensures a smoothly transition between the two operation modes, guaranteeing the safety operation of the microgrid. Experimental results are provided to confirm the effectiveness of the proposed system.

Generalized pulse-width-modulation to reduce common-mode voltage in matrix converters

In this paper a simple pulse-width-modulation strategy for matrix converters is studied. It takes advantage of the fact that the space vector modulation switching sequences for a matrix converter are the same as those created by the carrier-based modulation when an adequate zero-sequence component is injected into the carrier system references. A generalized equation for the zero-sequence component, which is a function of a distribution ratio of zero voltage vectors allows the pulse widths to be easily calculated, avoiding the complicated derivation of similar relations for matrix converters. Such generalization is possible by using the two-stage direct power converter topology and an adaptation of the duty cycles for the matrix converter switches. In particular, a modulation technique for reducing the common-mode voltage of the matrix converter is proposed based on this concept and comparison results are exposed, demonstrating its efficacy without a significant degradation of the input currents quality in the matrix converter.

A method for extracting the fundamental frequency positive-sequence voltage vector based on simple mathematical transformations

In this paper a novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage is proposed. The method is based on four simple mathematical transformations, two of them in the stationary reference frame, which are able to eliminate odd harmonics from the original signals. The other two transformations are implemented in a synchronously rotating reference frame in order to eliminate even harmonics. The output of the last transformation block is input to a synchronous reference frame phase locked loop for detecting frequency and position of the positive-sequence voltage vector. The proposed algorithm was verified through simulations and experiments by applying distorted and unbalanced signals, containing positive and negative-sequence components. The results are in agreement with those theoretically predicted and indicate that the proposed scheme has a great potential for using in grid connected converters synchronization algorithms.

Variable-Structure Generalized Delayed Signal Cancellation PLL to Improve Convergence Time

This paper shows a scheme for improving the detection performance of the fundamental-frequency positive-sequence (FFPS) space vector through the generalized delayed signal cancellation phase-locked loop (GDSC-PLL). We implemented a variable-structure scheme in order to reduce the convergence time by changing the traditional datapath whenever even harmonics are not present in the input signal. If this condition is not true, the performances of the variable-structure GDSC-PLL and the traditional GDSC-PLL for detecting the FFPS space vector will be similar. Therefore, the variable-structure GDSC-PLL does not require a previous knowledge about the harmonic components of the three-phase signal. Furthermore, we improved the frequency-adaptive scheme. The performances of the original and proposed schemes were compared through transient test cases based on IEC 61000 and typical electrical events. The new approach led to a reduction of up to 62% in the convergence time.