Nicola Femia

Optimization of perturb and observe maximum power point tracking method

Maximum power point tracking (MPPT) techniques are used in photovoltaic (PV) systems to maximize the PV array output power by tracking continuously the maximum power point (MPP) which depends on panels temperature and on irradiance conditions. The issue of MPPT has been addressed in different ways in the literature but, especially for low-cost implementations, the perturb and observe (P&O) maximum power point tracking algorithm is the most commonly used method due to its ease of implementation. A drawback of P&O is that, at steady state, the operating point oscillates around the MPP giving rise to the waste of some amount of available energy; moreover, it is well known that the P&O algorithm can be confused during those time intervals characterized by rapidly changing atmospheric conditions. In this paper it is shown that, in order to limit the negative effects associated to the above drawbacks, the P&O MPPT parameters must be customized to the dynamic behavior of the specific converter adopted. A theoretical analysis allowing the optimal choice of such parameters is also carried out. Results of experimental measurements are in agreement with the predictions of theoretical analysis.

Distributed Maximum Power Point Tracking of Photovoltaic Arrays: Novel Approach and System Analysis

One of the major drawbacks of photovoltaic (PV) systems is represented by the effect of module mismatching and of partial shading of the PV field. Distributed maximum power point tracking (DMPPT) is a very promising technique that allows the increase of efficiency and reliability of such systems. Modeling and designing a PV system with DMPPT is remarkably more complex than implementing a standard MPPT technique. In this paper, a DMPPT system for PV arrays is proposed and analyzed. A dc and small-signal ac model is derived to analyze steady-state behavior, as well as dynamics and stability, of the whole system. Finally, simulation results are reported and discussed.

A Technique for Improving P&O MPPT Performances of Double-Stage Grid-Connected Photovoltaic Systems

In double-stage grid-connected photovoltaic (PV) inverters, the dynamic interactions among the DC/DC and DC/AC stages and the maximum power point tracking (MPPT) controller may reduce the system performances. In this paper, the detrimental effects, particularly in terms of system efficiency and MPPT performances, of the oscillations of the PV array voltage, taking place at the second harmonic of the grid frequency are evidenced. The use of a proper compensation network acting on the error signal between a reference signal provided by the MPPT controller and a signal that is proportional to the PV array voltage is proposed. The guidelines for the proper joint design of the compensation network (which is able to cancel out the PV voltage oscillations) and of the main MPPT parameters are provided in this paper. Simulation results and experimental measurements confirm the effectiveness of the proposed approach.

Optimized one-cycle control in photovoltaic grid connected applications

The use of one-cycle control (OCC) for maximum power point tracking (MPPT) and power factor correction (PFC) in grid connected photovoltaic (PV) applications is discussed. Circuit and operating parameters of the one cycle-based controller of a cost-effective single-stage inverter are optimized in order to obtain the best performances of the system under different irradiance levels. Firstly, design constraints are formulated which allow to get a very efficient OCC operation in terms of power extracted from the PV array, stability, and PFC. Afterwards, such constraints are used to perform the parametric optimization of the one cycle controller by means of suitable heuristic approaches. Various selection criteria of the best parameters set under different conditions are discussed and applied. Finally, a customized perturb and observe (P&O) control is applied to the optimized one cycle controlled single-stage inverter in order to perform a real MPPT in presence of varying irradiance conditions. Subjects described here are covered by the Italian Patent Application SA2005A000014-13.07.2005 and PCT Application PCT/IT2005/000747-20.12.2005

Power Electronics and Control Techniques for Maximum Energy Harvesting in Photovoltaic Systems

“... very innovative ... [a] rigorous analytical treatment starting from the modeling of the PV field and the power converter stages as well as the dynamics of the overall system, including MPPT control. This in-depth analytical description allows the design of power converters and DMPPT algorithms improving the overall efficiency of the whole PV system operating under mismatching conditions.” —Francesc Guinjoan, Polytechnic University of Catalonia

True worst-case circuit tolerance analysis using genetic algorithms and affine arithmetic

In this paper, a new approach to the calculation of the true worst case in circuit tolerance analysis (TWC-CTA) with parameters characterized by large uncertainties is presented. It is based on the joint use of genetic algorithms (GAs) and affine arithmetic (AA). The GAs are used to minimize the underestimation error which affects stochastic methods while the AA is applied to minimize the overestimation error which affects interval arithmetic methods in TWC-CTA problems. The joint GA-AA approach presented in this paper ensures a reliable TWC evaluation.

Perturb and observe MPPT technique robustness improved

In presence of rapidly changing atmospheric conditions, the perturb and observe (P&O) maximum power point tracking (MPPT) algorithm can be confused due to the fact that it is not able to distinguish the variations of the photovoltaic array output power caused by the duty cycle modulation from those ones caused by the irradiance variation. In this paper, it is shown that the negative effects associated to such a drawback can be greatly reduced if the magnitude of the duty-cycle perturbations is customized to the dynamic behavior of the specific dc-dc converter adopted to realize the P&O MPPT. As an example, a boost battery charger has been studied.

Design of dc/dc Converters for DMPPT PV Applications Based on the Concept of Energetic Efficiency

Distributed maximum power point tracking (DMPPT) is one of the most promising solutions to overcome the drawbacks associated with mismatching phenomena in photovoltaic (PV) applications. DMPPT is based on the adoption of a dc/dc converter dedicated to each PV module. The design of the power stage of such a converter is a challenging task because of the very high efficiency requirements and of the continuous changes of the operating point during the day, depending on the sun irradiation conditions. In this paper the guidelines for the design of dc-dc converters for DMPPT applications are presented and discussed.

Efficiency and reliability comparison of DC-DC converters for single phase grid connected photovoltaic inverters

The three basic nonisolated DC-DC converter topologies are here compared in a single phase grid tied photovoltaic application, from an efficiency and reliability perspective. A general design procedure which attempts to maximize the switching frequency has been established and followed for all the topologies. A MATLAB® model representing the sources of losses has been developed to evaluate the efficiency, while MIL-HDBK-217F has served as guide to reliability estimation. Graphical and numerical representations of the main results are given, together with a simple figure of merit that synthetically summarize them.

Selection of optimal closed-loop controllers for DC-DC voltage regulators based on nominal and tolerance design

This paper discusses a tolerance design approach for the feedback compensation networks of DC/DC switching regulators, identifying the most reliable solutions among different feasible alternatives that fulfil closed-loop design constraints. A voltage-mode-regulated DC/DC buck converter is considered as a case study. Given the performance and stability constraints, as tolerance ranges for crossover frequency and phase margin, feasible design solutions are sought by means of Monte Carlo and interval arithmetic computations. The search space is a set of available commercial values of RC parameters and related tolerances. Best design is identified by a weighted fitness function, exploring the set of solutions provided by different design approaches. The results presented in the paper highlight that the tolerance design approach allows one to find compensation networks that fit the given performance/robustness priorities better than those ones found by means of the classical nominal design approach.