Moshe Sitbon

Disturbance Observer-Based Voltage Regulation of Current-Mode-Boost-Converter-Interfaced Photovoltaic Generator

In this paper, a robust control method based on a disturbance observer is proposed to regulate the terminal voltage of a photovoltaic generator, interfaced by a current mode-controlled boost dc-dc converter. The combined generator-converter-load system possesses a nonlinear behavior, highly dependent on operation point and environmental variables, thus burdening the control task. It is shown that employing a typical linear controller, designed according to a single nominal operating point, results in a closed-loop performance, varying from a highly overdamped near open-circuit condition to greatly underdamped around short-circuit conditions. On the other hand, when the proposed robust controller is utilized, the closed-loop performance remains nearly nominal throughout the whole operation range. In addition, it is shown that, by sacrificing the performance in the vicinity of the open-circuit point, it is possible to implement the controller using a single op-amp with a reduced part count. Simulation and experimental results are presented to verify the proposed method.

Dynamics of Photovoltaic-Generator-Interfacing Voltage-Controlled Buck Power Stage

This paper investigates the dynamic properties of the photovoltaic-generator-interfacing voltage-controlled buck power stage operating in both the maximum and limited power point tracking modes. The photovoltaic generator (PVG) is known to possess both current- and voltage-source properties with respect to its maximum power point. While voltage-fed operation is conventional, current-fed action is nontrivial and is thoroughly analyzed in this paper. The photovoltaic-generator-interfacing converter is formed by adding a capacitor at conventional voltage-fed converter input terminals, turning it into a current-fed power stage. During the maximum power point tracking phase, converter input voltage is regulated, possessing nontrivial dynamics. The situation is burdened further when output-voltage control should be alternatively realized to limit the voltage of the converter terminating the energy storage element. It is shown that both the photovoltaic generator and the terminating energy storage greatly affect the combined system dynamics. Parallel as well as cascaded control arrangements are proposed to support dual-mode system operation. Extended experimental results are shown to enforce presented theory and reveal nontrivial dynamics-related issues.

Rapid Prototyping of a Low-Cost Solar Array Simulator Using an Off-the-Shelf DC Power Supply

A method of rapid prototyping of a solar array simulator, based on low cost, off-the-shelf components is proposed in the paper. A commercial constant output voltage switching power supply is utilized as a power stage. It is shown that it is possible to gain control over output voltage of such a device by injecting variable analog voltage into the voltage feedback loop of the supply. As a result, by sensing the power supply output current and varying the injected voltage it is possible to change the output voltage according to a predefined relation and hence any static I-V curve may be emulated by the device. For simulating a solar array output characteristics, the desired I-V curve may be either digitized from a manufacturer provided datasheet, obtained experimentally or estimated from three basic current-voltage pairs (open circuit, short circuit, and maximum power points) using a dedicated algorithm. In order to demonstrate the proposed method, a prototype was designed and built based on available low-cost commercial components. Dynamic characteristics of the prototype were experimentally evaluated and three static I-V curves of a commercial solar panel were simulated. The resulting I-V output characteristics were shown to closely resemble datasheet I-V curves.

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.

Transient response enhancement of PFC front end

An enhancement of a conventional PI controller used on grid connected power factor corrector (PFC) systems is suggested in this paper. The transient length is reduced by applying two different methods. The first one consists of an auxiliary unit which increases the error signal, while the second one is a hybrid controller which completely replaces the basic PI controller when DC-link voltage drop is detected. The applied control method is based on hybrid controller proposed in the literature for dc-dc power converters. In order to apply this method to the PCF case the controller was properly analyzed for variable input voltage and simulation results demonstrate the theoretical findings.

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.