Carlos Andrés Ramos-Paja

Grid-Connected Photovoltaic Generation Plants: Components and Operation

The main design objective of photovoltaic (PV) systems has been, for a long time, to extract the maximum power from the PV array and inject it into the ac grid. Therefore, the maximum power point tracking (MPPT) of a uniformly irradiated PV array and the maximization of the conversion efficiency have been the main design issues. However, when the PV plant is connected to the grid, special attention has to be paid to the reliability of the system, the power quality, and the implementation of protection and grid synchronization functions. Modern power plants are required to maximize their energy production, requiring suitable control strategies to solve the problems related to the partial shading phenomena and different orientation of the PV modules toward the sun. Moreover, the new policy concerning the injection of reactive power into the grid makes the development of suitable topologies and control algorithms mandatory. A general view of actual solutions for applications of the PV energy systems is presented. This article covers several important issues, including the most reliable models used for simulation, which are useful in the design of control systems, and the MPPT function, particularly in distributed applications. The main topologies used in the PV power processing system and, finally, grid connection aspects are discussed, with emphasis on synchronization, protections, and integration.

A Fast Current-Based MPPT Technique Employing Sliding Mode Control

This paper introduces a novel maximum power point tracking (MPPT) technique aimed at maximizing the power produced by photovoltaic (PV) systems. The largest part of the MPPT approaches presented in the literature are based on the sensing of the PV generator voltage. On the contrary, in this paper, a current-based technique is proposed: the sensing of the current in the capacitor placed in parallel with the PV source is one of the innovative aspects of the proposal. A dual control technique based on an inner current loop plus an outer voltage loop allows to take profit of the fast current tracking capability of the inner current loop while the voltage loop benefits from the logarithmic dependency of the PV voltage on the irradiation level. The features of the proposed algorithm, particularly in terms of tracking of irradiation variations and disturbance rejection, are supported by theoretical analysis, simulations, and experimental results. The technique described in this paper is patent pending.

Minimum Fuel Consumption Strategy for PEM Fuel Cells

This paper proposes a proton exchange membrane fuel cell control strategy to produce the power requested by an electrical load, minimizing the fuel consumption and also providing a regulated DC bus voltage to the load. The power system consists of a hybrid fuel cell/capacitor topology, and the control objective is to follow the minimum fuel consumption points for a given load power profile. This is done by controlling the air pump voltage and regulating the fuel cell current through a DC/DC switching converter. Moreover, the design and control parameters of the output DC bus are discussed, and the calculations are adjusted to a Ballard 1.2-kW Nexa power module. Finally, the control results, fuel consumption, and fuel cell protection against oxygen starvation phenomenon are analyzed and experimentally validated, contrasting its performance with the Nexa power module internal control system.

A PEM Fuel-Cell Model Featuring Oxygen-Excess-Ratio Estimation and Power-Electronics Interaction

In this paper, a polymer-electrolyte-membrane fuel-cell (FC) model that is useful for simulation and control purposes is presented. The model uses both electrical-circuit components and functional blocks to reproduce both static and dynamic FC behaviors. Its main feature is in the reproduction of the oxygen-excess-ratio behavior, but it is also able to interact with any electrical device connected at the FC terminals, e.g., a load or a switching converter. Consequently, the proposed model can be used to develop new control strategies aimed at avoiding the oxygen-starvation effect and/or minimizing the fuel consumption. The model has been customized for a Ballard Nexa 1.2-kW power system, and this has allowed an experimental validation by means of measurements performed on a real FC device.

Minimizing the effects of shadowing in a PV module by means of active voltage sharing

Negative effects in a two-section-module PV system due to shadowing are minimized by using active voltage sharing of section voltages. Instead of conventional bypass diodes, a bidirectional buck-boost-type circuit with current control guarantees equal section voltages yielding a unique maximum power point for the global system. Connecting an external maximum point power tracking (MPPT) converter results in bigger power extraction than the one obtained with the bypass diodes case, the improvement being in some cases up to 40%. Also, voltage values corresponding to the maximum power point (MPP) are less scattered than in the bypass diode solution, this facilitating the MPPT converter operation. To improve the efficiency, the current control strategy permits to identify the conditions to disconnect the bypass. PSIM simulations verify the theoretical predictions of the proposed technique.

Improved Design of Sliding-Mode Controllers Based on the Requirements of MPPT Techniques

In many grid-connected applications, a dc/dc switching converter is usually connected between the PV modules and the inverter. This paper presents an improved procedure to design a sliding controller for the PV system, which drives the PV voltage to follow a reference provided by an external MPPT algorithm and mitigates the perturbations caused by the irradiance changes and oscillations in the bulk voltage. By considering that the switching surface is the linear combination of the input capacitor current and the PV voltage error, the proposed design exhibits advantages in comparison with existing solutions that rely in the linearization of inner current loop dynamics. The proposed integral procedure, by taking also into account the effects in the closed-loop system dynamics of a reference filter, ensures a stable sliding regime in all the desired operation range of the system, while the settling time and overshoot of the PV voltage required by an MPPT algorithm are provided. Differently from a previous similar but less rigorous approach, the switching function and reference filter parameters are obtained by numerically solving a set of nonlinear equations. Simulations and experiments were used to demonstrate the efficiency of the proposed solution in presence of environmental and load perturbations.

Reconfiguration analysis of photovoltaic arrays based on parameters estimation

Abstract A method to determine the photovoltaic (PV) series–parallel array configuration that provides the highest Global Maximum Power Point (GMPP) is proposed in this paper. Such a procedure was designed to only require measurements of voltage and current of each string, which avoids to perform experiments in each module. The ideal single-diode model parameters of each module in the string are obtained from the analysis of the voltage vs. current characteristics of the string. Using the estimated parameters, all feasible PV array configurations are evaluated to determine the array configuration that provides the highest GMPP. Finally, the proposed solution is validated using simulations and experimental data.

PV field distributed maximum power point tracking by means of an active bypass converter

An active bypass structure that was initially proposed as a solution to minimize the reduction in produced power caused by mismatch of photovoltaic modules is compared in terms of efficiency with single and distributed maximum power point solutions based in more conventional dc-dc structures. The analysis and simulations performed under simplified losses assumptions demonstrate that the bypass structure is very competitive with respect to boost, buck or buck-boost based solutions in terms of efficiency.

Mathematical model of total cross-tied photovoltaic arrays in mismatching conditions

This paper presents a mathematical procedure for modeling rectangular (N rows with M modules each) and non-rectangular photovoltaic (PV) arrays in Total Cross-Tied (TCT) configuration operating in uniform and mismatching conditions. The proposed model uses the simple single diode representation for each PV module; then each row of the TCT array is represented as an equivalent non-linear PV circuit with a bypass diode, which allows to represent the TCT array as one string of equivalent PV circuits. The inflection voltages (array voltages that turn off the bypass diodes) of the string are calculated in order to solve only the non-linear equation system related to the active equivalent PV circuits for calculating the array current for a given voltage. Such a strategy reduces the computational burden and improves calculation speed. A TCT array of 4×2 with deep mismatching conditions was implemented in PSIM software to validate the proposed model, obtaining a correlation between model predicted data and the circuital simulation. The accuracy and improved calculation speed of the proposed model allow its use altogether with reconfiguration techniques as well as to reduce the time of energetic evaluations of TCT arrays for PV planning.

Fuel cell MPPT for fuel consumption optimization

In this paper a new approach to the minimization of fuel consumption in Polymer Electrolyte Membrane (PEM) Fuel Cell (FC) systems is presented. It is mainly based on the minimization of the current produced by the FC by means of an adaptive technique, namely the Perturb and Observe (P&O) approach. The controller works in conjunction with the dc/dc converter which processes the power produced by the FC system: the electrical power requested by the load is provided at the minimum current, so that the minimum hydrogen consumption is ensured. Simulation results obtained by means of an experimentally validated Ballard Nexa 1.2kW dynamic model put into evidence the effectiveness of the approach.