Marcelo C. Cavalcanti

Modulation Techniques to Eliminate Leakage Currents in Transformerless Three-Phase Photovoltaic Systems

In some photovoltaic (PV) applications, it is possible to remove the transformer of a system in order to reduce losses, cost, and size. In transformerless systems, the PV module parasitic capacitance can introduce leakage currents in which the amplitude depends on the converter topology, on the pulsewidth modulation, and on the resonant circuit comprised by the system components. Based on the common-mode voltage model, modulation techniques are proposed to eliminate the leakage current in transformerless PV systems without requiring any modification on the converter and any additional hardware. The main drawback is that the proposed modulation technique for two-level inverters can only be used with 650-V dc link in the case of a 110-V (rms) grid phase voltage. Comparisons among the modulation techniques are discussed, and it is proven that the proposed modulation for two- and three-level inverters presents the best results. To validate the models used in the simulations, an experimental three-phase inverter is used.

Eliminating Leakage Currents in Neutral Point Clamped Inverters for Photovoltaic Systems

The main contribution of this paper is the proposal of new modulation techniques for three-phase transformerless neutral point clamped inverters to eliminate leakage currents in photovoltaic systems without requiring any modification on the multilevel inverter or any additional hardware. The modulation techniques are capable of reducing the leakage currents in photovoltaic systems by applying three medium vectors or using only two medium vectors and one specific zero vector to compose the reference vector. In addition, to increase the system utilization, the three-phase neutral point clamped inverter can be designed to also provide functions of active filter using the p-q theory. The proposed system provides maximum power point tracking and compensation of current harmonics and reactive power. To validate the simulation models, an experimental three-phase inverter is used to evaluate leakage currents and the dc link voltage control.

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.

A Generalized Delayed Signal Cancellation Method for Detecting Fundamental-Frequency Positive-Sequence Three-Phase Signals

A novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage vector based on a generalization of the delayed signal cancellation method is proposed in this paper. The technique is implemented by sampling and storing the instantaneous αβ voltage vector. A mathematical transformation is then proposed through which the current and delayed voltage vectors are combined. It is shown that the proposed transformation has unity gain for the fundamental-frequency positive-sequence voltage vector, while its gain is equal to zero for some chosen components. Cascaded transformations can then be used for eliminating the fundamental-frequency negative-sequence vector, as well as chosen positive- and negative-sequence harmonic vector components and, thus, for accurately obtaining the fundamental-frequency positive-sequence voltage vector. 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 vector. A proposal for making the scheme frequency adaptive is also presented. The good performance of the proposed method is verified with simulations and experiments by using distorted and unbalanced signals, containing fundamental-frequency as well as positive- and negative-sequence harmonic components. The proposed method frequency adaptation capability is also verified.

A feasible loss model for IGBT in soft-switching inverters

This paper presents a feasible loss model to estimate IGBT losses in a soft switching operation. The loss model is developed based on the experimental determination of the power losses. This paper investigates the power losses in IGBTs as a function of the circuit and its operation parameters, in order to help the device selection for a given application. In addition, it compares the losses produced by hard-switched and quasisquare-wave inverters for different currents. The study takes into account conduction losses, switching losses (turn-on and turn-off losses) and losses in the passive components.

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.

Single-Carrier Modulation for Neutral-Point-Clamped Inverters in Three-Phase Transformerless Photovoltaic Systems

Modulation strategy is one of the most important issues for three-level neutral-point-clamped inverters in three-phase transformerless photovoltaic systems. A challenge for modulation is how to keep the common-mode voltages constant to reduce the leakage currents. A single-carrier modulation strategy is proposed. It has a very simple structure, and the common-mode voltages can be kept constant with no need of complex space-vector modulation or multicarrier pulsewidth modulation. Experimental results verify the theoretical analysis and the effectiveness of the presented method.

Evaluation of maximum power point tracking methods for grid connected photovoltaic systems

This paper presents a study of two maximum power point tracking methods for grid connected photovoltaic systems. The best operation conditions of the perturbation and observation and the incremental conductance are investigated in order to identify the performances of photovoltaic systems. Improvements of these methods can be obtained with the best adjustment of the sampling rate and the perturbation size. Practical considerations about the incremental conductance are discussed and some modifications to overcome its problems are done. A procedure to determine the parameters is explained and it helps to define which method is better to the grid connected photovoltaic system with only one conversion stage. The methods influence the quality of the currents injected in the grid and this influence becomes the main parameter for choosing the incremental conductance. The improvement with the choice of the best parameters is proved by means of simulation and experimental results.

Safe current injection strategies for a STATCOM under asymmetrical grid faults

This paper explores different strategies to set the reference current of a STATCOM under unbalanced grid voltage conditions and determines the maximum deliverable reactive power in each case to guarantee the injected current is permanently within the STATCOM secure operation limits. The paper presents a comprehensive derivation of the proposed STATCOM control strategies to set the reactive current reference under unbalanced grid faults, together with an extensive evaluation using simulation and experimental results from a low-scale laboratory setup in order to verify and validate the dynamic performance achieved by the proposed reactive current limiting algorithms.

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.