Sergei Kolesnik

On the Equivalence of Major Variable-Step-Size MPPT Algorithms

Functional equivalence of three nonmodel-based variable-step-size maximum power point tracking (MPPT) algorithms is revealed in this paper by establishing general closed-loop controller-plant structure of corresponding MPPT control loops, allowing us to formulate the problem as a typical stabilization task. The MPPT loop plants may be separated into three main components, two of which are similar and the third one, namely stabilization function estimator, is the main reason for the performance differences of the discussed methods in case the MPPT loop controllers are alike. It is shown that in case the estimator convergence rate is much faster than the MPPT loop bandwidth, similar low-frequency performance may be expected by utilizing any of the three methods.

Design Guidelines for Multiloop Perturbative Maximum Power Point Tracking Algorithms

Due to relatively good performance and simple implementation, fixed-step direct maximum power point tracking (MPPT) techniques such as perturb and observe and incremental conductance are the most popular algorithms aimed to maximize the energy yield of photovoltaic energy conversion systems. In order to optimize the MPPT process performance, two design parameters—perturbation frequency and perturbation step size—need to be set a priori , taking into account the properties of both interfacing power converter and photovoltaic generator. While perturbation frequency is limited by the combined energy conversion system settling time, perturbation step size must be high enough to differentiate system response from that caused by irradiation variation. Recent studies have provided explicit design guidelines for single-loop MPPT structures only, where the algorithm directly sets the interfacing converter duty cycle. It was shown that dynamic resistance of the photovoltaic generator, which is both operation point and environmental conditions dependent, significantly affects the combined energy conversion system settling time. On the other hand, no design guidelines were explicitly given for multiloop MPPT structures, where the algorithm sets the reference signal for photovoltaic generator (PVG) voltage and inner-voltage controller performs the regulation task. This paper introduces perturbation frequency and perturbation step size design guidelines for such systems. It is shown that while perturbation step size design is similar to that of single-loop structures, perturbation frequency design is quite different. It is revealed that once the inner-voltage loop is properly closed, the influence of PVG dynamic resistance on settling time (and thus on perturbation frequency design) is negligible. Experimental results are provided to verify the proposed guidelines validity.

Revisited Perturbation Frequency Design Guideline for Direct Fixed-Step Maximum Power Point Tracking Algorithms

In order to optimize the performance of direct (or perturbative) fixed-step maximum power point tracking algorithms (e.g., perturb and observe and incremental conductance), two design parameters—perturbation frequency and step size—must be selected. The main requirement for perturbation frequency design is ensuring the period between two successive perturbations is longer than settling time of photovoltaic generator power transient. According to existing design guidelines, perturbation frequency should be selected at maximum power point, corresponding to standard test conditions. However, due to finite resolution of digital controllers, maximum power region rather than single maximum power point exists in practice. Therefore, operating point can arbitrarily reside within this region, belonging either to constant-current or constant-voltage I–V curve parts. It is shown that the photovoltaic generator power transient settling process is significantly slower in constant current than maximum power region due to increased value of dynamic resistance. Consequently, perturbation frequency design should be carried out in constant-current region rather than at maximum power point. Short-circuit condition should be selected as worst-case design operation point, where photovoltaic generator dynamic resistance obtains highest value. Then, perturbation frequency design becomes photovoltaic generator independent, influenced only by interfacing converter component values. Experimental results validate presented findings successfully.

Solar Irradiation Independent Expression for Photovoltaic Generator Maximum Power Line

In order to enhance maximum power point tracking (MPPT) speed of photovoltaic generators (PVGs) upon fast irradiation changes, maximum power line (MPL)-based control is often used. MPL is a curve, linking all possible MPP coordinates for a given temperature. In the literature so far, PVG MPL was either assumed linear, which is inaccurate for all irradiation levels, or possessed photocurrent dependence, requiring real-time estimation of the latter. In this letter, an irradiation-independent explicit expression for PVG MPL is derived, valid for all practical irradiation levels, thus allowing real-time implementation without the need of photocurrent estimation.

Sampling frequency design to optimizing MPP-tracking performance for open-loop-operated converters

Fixed-step perturbative maximum-power-point (MPP) tracking algorithms, such as perturb & observe and incremental conductance technique, are the most popular techniques in single and double-stage grid-connected photovoltaic PV systems due to their relatively good performance with a simple implementation. However, in order to optimize the performance of such algorithms, the design parameters - sampling frequency and perturbation step size - need to be designed in respect to interfaced power electronic converter. Recent studies have provided state-of-the-art MPP-tracking design rules for single and two-stage grid-connected PV systems. In perturbation frequency design, the basic guideline is to ensure that the interval between the perturbations is chosen to be long enough so that oscillatory behavior of PV power transient is attenuated to its steady-state value. Unfortunately, the perturbation frequency design in recent studies is treated at the MPP, which does not represent the worst-case from the power settling time point of view. Due to the natural behavior of the perturbative MPP-tracking algorithm, the operation point moves from the MPP into constant-current and constant-voltage regions with significantly different PV power settling time. In this paper, deterministic analysis and experimental results reveal that MPP-tracking design needs to be performed in constant-current region, where the settling time of the PV power transient is the longest. Thus, the design of the perturbation frequency is very deterministic and entirely governed by the design of the converter.

Determining maximum MPP-tracking sampling frequency for input-voltage-controlled PV-interfacing converter

A maximum-power-point tracking (MPPT) algorithm is essential in all controllers of solar power electronic converters due to the nonlinear current-voltage characteristics of a photovoltaic generator. One of the most widely utilized algorithms are perturbative MPPT techniques such as perturb and observe and incremental conductance methods due to their simple implementation with relatively good tracking performance. However, in order to optimize the performance of such algorithms, the design parameters — sampling frequency and perturbation step size — need to be designed in respect to interfaced power electronic converter. Recent studies have provided state-of-art MPP-tracking design rules for single and two-stage grid-connected PV systems. Unfortunately, the analysis of those studies does not provide analytical results for PV power transient response under feedback-controlled converters. This paper provides reduced-order transfer functions for the converters equipped with either I-type or PID-type controllers in order to approximate the maximum sampling or perturbation frequency for MPP-tracking algorithms. The analysis reveals the factors affecting the transient behavior similarly as in open-loop converter providing valuable tools for optimizing MPP-tracking perturbation frequency design.

Comparison of photovoltaic and wind generators as dynamic input sources to power processing interfaces

The paper reveals that while the equivalent circuit, representing the load side reflected low-frequency dynamics of a wind turbine generator, is similar to the electrical equivalent circuit of a photovoltaic generator, their dynamic resistances possess different behavior. While the incremental conductance of a photovoltaic generator does not change sign with terminal voltage variations, zero-crossing dynamic conductance characterizes wind turbine generator. The findings points out the complications arising during interfacing a wind turbine generator by different power processing interfaces.

Real-time state-of-energy estimation of supercapacitor-based energy storage

The paper suggests utilizing a recursive-least-squares identification algorithm for online supercapacitor parameters estimation in order to calculate its state of energy (SOE). The SOE is used as a fuel gauge of the supercapacitor instead of the commonly adopted state of charge (SOC). The proposed method allows to precisely estimate the remaining energy capacity of a supercapacitor utilized e.g. as backup energy device or primary energy storage.