Freddy Flores-Bahamonde

Modular-based PFC for low power three-phase wind generator

Hybrid Distributed systems include many sources, storage elements, and various local loads, connected to a common distribution bus. In this work, different matching methods to adapt a 3-phase wind alternator to a variable DC-bus are reviewed, and finally a modular solution based on single-phase Boost-based PFCs is proposed. Nevertheless, Boost modules must be adapted to operate with non-isolated sources, like 3–4 wire, 3-phase generators. Magnetic coupling is introduced here to enhance the isolation among phases, reducing also converter losses and size. Sliding mode approach has been applied to the whole converter as a single unit, to verify that independent regulation of all phase modules was possible, even sharing a common output capacitor. To verify the theoretical analysis, a 1.5 kW prototype has been built confirming independent phase control and good sinusoidal input waveforms.

Grid-connected boost inverter for small-wind urban integration: Analysis and design

Small grid-connected PV-systems are quite usual in domestic roofs, but urban integration is still an open issue for wind systems. Similarly to grid PV systems, direct grid injection in a wind system requires two stages, a PFC boost rectifier and a grid-tie inverter. Nevertheless, although a buck inverter is a common choice in many grid PV systems, this topology imposing a minimum DC-Link voltage complicates the boost stage design, because the voltage variation range of a wind alternator, greater than the variation margin of a PV-array, would be absorbed only by the first converter. Consequently, we propose to share the voltage gain between both processor stages, using step-up voltage inverter instead of a buck one. In this work, boost and buck-boost based inverters are analyzed, and finally, a sliding mode controlled boost inverter with bipolar operation is proposed. To demonstrate the feasibility of this proposal, some relevant experimental results from a 1kW inverter prototype are given.

Predictive control of a single-stage boost DC-AC photovoltaic microinverter

Commercial photovoltaic microinverter topologies are normally composed of two stages: a step-up dc-dc stage and a step-down dc-ac stage. Nevertheless, to achieve high-ratio conversion, a high frequency transformer is used in the dc-dc stage, resulting in a bulky configuration with a high cost of implementation. To minimize these issues, a single stage boost inverter is proposed, composed by two bidirectional boost dc-dc converter. The control of this topology is complex due to its high non-linearity behaviour. Therefore, this paper presents a control methodology based on Finite Control Set Model Predictive Control (FCS-MP) algorithm with predictions of the system variables through the inverter model and an optimization process. Each boost dc-dc converter of the inverter is regulated to achieve an output signal composed of the sinusoidal wave with a dc bias. The main features of the boost inverter and predictive control are analysed. Simulations are shown in order to validate the proposed control and the converter for grid-connected PV applications.

Double voltage step-up photovoltaic microinverter

In conventional photovoltaic microinverters configuration, a single PV module is connected to the grid through two converter stages: a step-up dc-dc stage and a step-down dc-ac stage. In the first stage, a high frequency transformer is generally used to achieve the high step-up voltage ratio conversion to a voltage above the grid peak value, reducing the converter efficiency while increasing its size. More recently single-stage step-up dc-ac configurations have been proposed to overcome these problems. However, they increase control complexity, and efficiency remains an issue due to the high step-up ratio required and being a single stage. Therefore, a two-stage configuration consisting of two consecutive step-up converters is proposed in this paper. With this scheme, it is possible to distribute the elevation effort to improve the global efficiency of the PV microinverter. The proposed topology merges a traditional boost dc-dc converter for the first stage, like in the conventional configuration, but operating with a below-grid-peak-voltage dc-link voltage. A step-up inverter is used for the second stage; it is composed of two boost converters connected in differential mode (dual boost inverter). Simulation results are provided to validate the proposed configuration.

FS-model predictive control of microgrid interface converters for reactive power and harmonic compensation

Microgrids can be controlled to optimize the generation and consumption of energy in a bounded geographical area which, in the global current scenario, facilitates the integration of distributed energy resources to the main grid. Additionally, some ancillary services, such as support of grid instabilities or power quality improvements, can be also obtained modifying the microgrid control scheme. In this paper, the compensation of reactive power and harmonic components in the main grid current using the microgrid interface converter is proposed. Using predictive current control for fast dynamical response and an energy-based model to avoid linear approximation the microgrid converter works as an active filter while the microgrid DC bus is regulated to the desired voltage. Simulations results for a 80 kW microgrid connected to the main grid is presented to support theoretical analysis.

Digital current control of the versatile buck-boost converter for photovoltaic applications

The versatile buck-boost converter has aroused interest because of its advantages such as non-inverting voltage step-up and step-down characteristic, high efficiency, wide bandwidth and regulation of input and output currents, and so forth. Hitherto, the proposed converter has been controlled by means of an analog average current control (ACC) but a digital current implementation has not been reported yet. Digital control has gained more attention due its flexibility and easy implementation. The digital current control presented in this work is based on taking more than one sample per switching period of the instantaneous current error waveform. Moreover, the proposed control allows a dead zone avoidance and mitigation that is a common problem of the non-inverting buck-boost converters. Finally, the digital current control has been tested by means of simulations to demonstrate that it can be nested with an input voltage control to perform a maximum power point tracking (MPPT) in photovoltaic applications.

Flatness-based control of a boost inverter for PV microinverter application

The boost inverter is an attractive solution for photovoltaic (PV) microinverters since it is a single stage architecture. Indeed, in comparison, most of the microinverters topologies consist of two-stage structures, a DC-DC converter for voltage elevation and an inverter for grid connection, resulting in a lower efficiency. The boost inverter consists of two DC-DC boost converters connected in differential mode to the grid. However, due to its high non-linearity behavior the control of this topology is a complex task. To solve this problematic a flatness-based control scheme of the boost inverter is proposed. Simulation results are provided to validate the good behavior of the proposed control scheme for a grid connected PV microinverter.

Ultracapacitor storage enabled global MPPT for photovoltaic central inverters

In most large-scale grid-tied photovoltaic (PV) plants, central inverter configurations are used, mainly due to higher converter efficiency and lower cost per kW. However, compared to other configurations, its Maximum Power Point Tracking (MPPT) efficiency is the lowest since it is the less distributed configuration. Under non-uniform conditions, as mismatch caused by aging and/or partial shading, several local maxima may arise in the PV curve, hence requiring additional actions to maximize the output power of the PV plant. Moreover, tighter grid codes have appeared, requiring for PV systems to limit power fluctuations. This paper presents an alternative to perform Global MPPT (GMPPT) while complying with stiffer grid code limitations. The proposed alternative adds an Energy Storage System (ESS) at inverter level, consisting of an ultracapacitor (UC) bank connected to the DC-link of a PV central inverter through interleaved DC-DC power converters. The proposed configuration is preliminary validated through simulations and tested under extreme conditions. The performance of the system is analyzed and compared to other existing solutions.

Fast maximum power point tracking algorithm based on switching signals modification

Solar PV energy systems have become one of the most researched and developed renewable energy alternatives due to the long lifetime and reduced maintenance of its components. However, considering the low efficiency in the conversion from light to electrical energy, the optimization of the power electronics converters which deliver the energy from the panels to the electrical grid has fundamental importance in terms of overall efficiency. One of the key elements in a PV system, designed to increase the efficiency, is the maximum power point tracking (MPPT) algorithm which finds the operating point where the maximum power is extracted from the panels. Several MPPT algorithms have been proposed in literature, ranging from the simple but powerful perturb and observe to very sophisticated algorithms involving artificial neural networks. All these methods have been designed with a very slow dynamics hence they cannot follow accurately abrupt modifications of the power curve produced when the irradiation changes. In this paper, a simple MPPT algorithm based on the modification of the switching signals and the analysis of the resulting voltage and power deviation is proposed. This algorithm allows to follow accurately the changes of maximum power point produced by fast variations in the irradiation. Simulation results to validate the proposed algorithm and comparisons with previously published algorithm are shown.