Jyri Kivimaki

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.

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.

Design of boost-power-stage converter for PV generator interfacing

This paper investigates the benefits which can be obtained if the maximum output current of a photovoltaic (PV) generator is taken into account in the design of a boost-power-stage converter. The investigations clearly show that the benefits are smaller inductor core size and more uniform temperature distribution among the power electronic components compared to the conventional design method. It will also be shown how to make the design with small input capacitor and how to take the power decoupling into account in the design process.

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.

Physical insight into the factors affecting the load-transient response of a buck converter

This paper investigates the physical issues affecting the load transient response both from the power-stage-component-selection and control-design point of views. A conventional buck converter under three different control schemes - direct-duty-ratio control, peak-current control and peak-current-control with load-current-feedforward control - is used as an example. The outcomes of the investigation are validated by simulations and experimental tests.