Issues in Design of Maximum-Power-Point-Tracking Control – Power Electronics Perspective

This thesis provides a comprehensive study of the dynamic characteristics and operation of maximum-power-point-tracking (MPPT) dc-dc converters, especially for those parts that concern the MPPT-control design. The study concentrates on the widely-utilized heuristic perturb-based MPPT algorithms and their design constraints when equipped with photovoltaic-interfacing converter. The main objective is to provide an explicit formulation of the input-power dynamics of the photovoltaic-generator-interfacing dcdc converter for addressing the MPP-tracking control. The dynamics introduce design constraints for the aforementioned MPPT-control algorithms and provide tools for deterministic MPPT design.A photovoltaic (PV) generator has nonlinear current-voltage characteristics with a particular maximum power point (MPP), which depends on the environmental factors such as temperature and irradiation. Thus, to ensure the maximization of the power extracted from the PV source, the interfacing power converter must be capable of controlling its parameters, i.e., changing its input voltage and current levels based on the MPP of the PV generator. That is done by implementing an MPPT controller, which generates the reference control signal for the interfacing converter. Despite the way of implementation, the fundamental operation is to nd the electrical operating point, i.e., the voltage and the current, at which the PV generator either generates the maximum power or follows a given power reference at every time instant. However, the dynamic characteristics of a photovoltaic generator are determined by the environmental conditions as well as the dynamics of the interfacing converter, which creates limitations for the MPPT-control design. It has been noticed recently that the characteristic curve of a PV generator can be separated into three dierent operation regions each having their distinct characteristics. Thus, to ensure reliability and eciency of a maximum-power tracking, all of these regions should be analyzed separately and choose the condition corresponding to the slowest settling dynamics of the PV system. Up to now, that is not completely recognized, and deterministic analytical models are missing to provide design guidelines for the MPPT-control design.This thesis presents a detailed dynamic model for PV-generator power dynamics in case of open-loop and closed-loop-operated switched-mode dc-dc converter. Two common design examples of closed-loop-operated converters were provided, where the closed-loop dynamics of the converter was slow and fast by adjusting the control bandwidth and phase margin of the feedback loop. With the developed models, a proper evaluation of the MPPT control imposed by the converter dynamics was presented. Thus, previously developed design guidelines were revised, or new guidelines were established

Design Issues in Implementing Maximum-Power-Point Tracking Algorithms for PV Applications

A photovoltaic generator (PVG) has a nonlinear current-voltage characteristic with a special maximum power point (MPP), which depends on the environmental factors such as temperature and irradiation. In order to obtain maximum amount of energy from PVG, the power electronic converter connected to the PVG need to utilize some sort of technique for maximum power point tracking (MPPT). The aim of the thesis was to study different MPPT techniques to find the design rules, which offer the balance between the complexity and speed of the MPPT algorithm. Despite a significant amount of developed MPPT algorithms, perturbative MPPT algorithms and their corresponding improved versions were analyzed more thoroughly in this thesis due to the fact that they have been shown to provide good balance between complexity and MPPT performance. These algorithms were tested in steady-state and dynamic conditions. The conventional perturbative MPPT algorithms have a drawback of trade-off between steady-state oscillations and fast dynamics. Therefore, the design variables the perturbation step size and the sampling frequency need to be optimized carefully to ensure proper operation yielding the highest possible efficiency. Sampling frequency of the perturbative algorithm should be selected as fast as possible to obtain the fastest dynamics in varying atmospheric conditions. However, the sampling frequency should not be selected faster than the PVG power settling time to guarantee that oscillatory behavior do not affect the decision process of perturbation sign. In contrast, the perturbation step-size has a significant effect on steady-state MPPT efficiency and on performance in dynamic atmospheric condition. To ensure proper operation in all atmospheric conditions, the power change in PVG caused by perturbation needs to be larger than the power change caused by any other external source such as irradiance variation, output voltage fluctuation and uncertainty factors in the measurement circuit. Based on the simulations, high MPPT efficiency can be achieved even with conventional perturbative algorithms if these are properly optimized. Moreover, the experimental measurements have shown that the uncertainty factors such as noise and quantization errors in the measurement circuit play a significant role in the operation of perturbative algorithm. Therefore, the minimization of uncertainty must be focused on the noise sources that would influence most the decision process of the MPPT

Design Issues in Implementing Maximum-Power-Point Tracking Algorithms for PV Applications

A photovoltaic generator (PVG) has a nonlinear current-voltage characteristic with a special maximum power point (MPP), which depends on the environmental factors such as temperature and irradiation. In order to obtain maximum amount of energy from PVG, the power electronic converter connected to the PVG need to utilize some sort of technique for maximum power point tracking (MPPT). The aim of the thesis was to study different MPPT techniques to find the design rules, which offer the balance between the complexity and speed of the MPPT algorithm. Despite a significant amount of developed MPPT algorithms, perturbative MPPT algorithms and their corresponding improved versions were analyzed more thoroughly in this thesis due to the fact that they have been shown to provide good balance between complexity and MPPT performance. These algorithms were tested in steady-state and dynamic conditions. The conventional perturbative MPPT algorithms have a drawback of trade-off between steady-state oscillations and fast dynamics. Therefore, the design variables the perturbation step size and the sampling frequency need to be optimized carefully to ensure proper operation yielding the highest possible efficiency. Sampling frequency of the perturbative algorithm should be selected as fast as possible to obtain the fastest dynamics in varying atmospheric conditions. However, the sampling frequency should not be selected faster than the PVG power settling time to guarantee that oscillatory behavior do not affect the decision process of perturbation sign. In contrast, the perturbation step-size has a significant effect on steady-state MPPT efficiency and on performance in dynamic atmospheric condition. To ensure proper operation in all atmospheric conditions, the power change in PVG caused by perturbation needs to be larger than the power change caused by any other external source such as irradiance variation, output voltage fluctuation and uncertainty factors in the measurement circuit. Based on the simulations, high MPPT efficiency can be achieved even with conventional perturbative algorithms if these are properly optimized. Moreover, the experimental measurements have shown that the uncertainty factors such as noise and quantization errors in the measurement circuit play a significant role in the operation of perturbative algorithm. Therefore, the minimization of uncertainty must be focused on the noise sources that would influence most the decision process of the MPPT

Issues in Design of Maximum-Power-Point-Tracking Control – Power Electronics Perspective

This thesis provides a comprehensive study of the dynamic characteristics and operation of maximum-power-point-tracking (MPPT) dc-dc converters, especially for those parts that concern the MPPT-control design. The study concentrates on the widely-utilized heuristic perturb-based MPPT algorithms and their design constraints when equipped with photovoltaic-interfacing converter. The main objective is to provide an explicit formulation of the input-power dynamics of the photovoltaic-generator-interfacing dcdc converter for addressing the MPP-tracking control. The dynamics introduce design constraints for the aforementioned MPPT-control algorithms and provide tools for deterministic MPPT design.A photovoltaic (PV) generator has nonlinear current-voltage characteristics with a particular maximum power point (MPP), which depends on the environmental factors such as temperature and irradiation. Thus, to ensure the maximization of the power extracted from the PV source, the interfacing power converter must be capable of controlling its parameters, i.e., changing its input voltage and current levels based on the MPP of the PV generator. That is done by implementing an MPPT controller, which generates the reference control signal for the interfacing converter. Despite the way of implementation, the fundamental operation is to nd the electrical operating point, i.e., the voltage and the current, at which the PV generator either generates the maximum power or follows a given power reference at every time instant. However, the dynamic characteristics of a photovoltaic generator are determined by the environmental conditions as well as the dynamics of the interfacing converter, which creates limitations for the MPPT-control design. It has been noticed recently that the characteristic curve of a PV generator can be separated into three dierent operation regions each having their distinct characteristics. Thus, to ensure reliability and eciency of a maximum-power tracking, all of these regions should be analyzed separately and choose the condition corresponding to the slowest settling dynamics of the PV system. Up to now, that is not completely recognized, and deterministic analytical models are missing to provide design guidelines for the MPPT-control design.This thesis presents a detailed dynamic model for PV-generator power dynamics in case of open-loop and closed-loop-operated switched-mode dc-dc converter. Two common design examples of closed-loop-operated converters were provided, where the closed-loop dynamics of the converter was slow and fast by adjusting the control bandwidth and phase margin of the feedback loop. With the developed models, a proper evaluation of the MPPT control imposed by the converter dynamics was presented. Thus, previously developed design guidelines were revised, or new guidelines were established

Modeling of load-transient response of direct-duty-ratio-controlled buck converter

This paper provides a simple analytical model for the closed-loop output impedance of a direct-duty-ratio-controlled buck converter, which can be used to predict the behavior of the output voltage during a load-current transient. The modeling method utilizes standard control theory to obtain the model for the sensitivity function based on the crossover frequency and phase margin of the output-voltage feedback loop as well as on a clever estimate for the corresponding open-loop output impedance. The modeling method is validated by means of simulations and experimental tests.acceptedVersionPeer reviewe

Factors affecting validity of PVG-power settling time estimation in designing MPP-tracking perturbation frequency

An open-loop and closed-loop operating boost-power-stage converter with relatively low damping factor exhibit resonant behavior in transient conditions. Such an undamped transient characteristic introduces overshoot to the control-to-output-variable transfer function, which is also visible in the inductor current transient behavior. Therefore, due to the either too large duty ratio or voltage-reference step change, the inductor current can move from continuous conduction mode to discontinuous conduction mode. That transforms the second-order system into an equivalent first-order dynamic system extending the PV-power settling time significantly and reducing power tracking performance of the system. This paper introduces design guidelines to determine maximum perturbation step size for duty ratio and input-voltage reference under open-loop and closed-loop operation, respectively. Two different closed-loop design examples are considered in this paper, based on the application of pure integral controller with phase margin (PM) close to 90 degrees and proportional-integral-derivative controller with PM close to 40 degrees, respectively. The closed-loop system dynamics is known to be characterized by the dominating poles and zeros, which locate closest to the origin. This means that the closed-loop system can be usually characterized by the well-known second-order transfer function. Therefore, the minimum and maximum overshoot of the inductor current can be well approximated as demonstrated by deterministic analysis and experimental results.acceptedVersionPeer reviewe

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.acceptedVersionPeer reviewe

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.acceptedVersionPeer reviewe

Design Guidelines for Multi-Loop Perturbative Maximum Power Point Tracking Algorithms

Due to relatively good performance and simple implementation, fixed-step direct maximum power point tracking techniques such as perturb & observe and incremental conductance are the most popular algorithms aimed to maximize the energy yield of photovoltaic energy conversion systems. In order to optimize maximum power point tracking 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 maximum power point tracking 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 multi-loop maximum power point tracking structures, where the algorithm sets the reference signal for photovoltaic generator 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 photovoltaic generator dynamic resistance on settling time (and thus on perturbation frequency design) is negligible. Experimental results are provided to verify the proposed guidelines validity.acceptedVersionPeer reviewe

Review of PV Generator as an Input Source for Power Electronic Converters

Voltage-type sources have dominated as an input source for power electronics converters for a long type. The existence of duality implies that there are also current-type sources. The growing application of renewable energy sources such as wind and solar energy has evidently shown that the current-type input sources exist in reality such as photovoltaic (PV) generator or the feedback technique used in controlling the power electronics converters in the renewable energy systems changes the power electronic converters to behaving as such. The recent research on renewable energy systems has indicated that the current-type input sources are very challenging input sources affecting the dynamics of the interfacing converters profoundly. This paper provides a comprehensive survey of the effects of the PV generator on the dynamic behavior of the corresponding interfacing power electronic converters