Francisco Paz

Zero Oscillation and Irradiance Slope Tracking for Photovoltaic MPPT

Maximum power point tracking (MPPT) strategies in photovoltaic (PV) systems ensure efficient utilization of PV arrays. Among different strategies, the perturb and observe (P&O) algorithm has gained wide popularity due to its intuitive nature and simple implementation. However, such simplicity in P&O introduces two inherent issues, namely, an artificial perturbation that creates losses in steady-state operation and a limited ability to track transients in changing environmental conditions. This paper develops and discusses in detail an MPPT algorithm with zero oscillation and slope tracking to address those technical challenges. The strategy combines three techniques to improve steady-state behavior and transient operation: 1) idle operation on the maximum power point (MPP); 2) identification of the irradiance change through a natural perturbation; and 3) a simple multilevel adaptive tracking step. Two key elements, which form the foundation of the proposed solution, are investigated: 1) the suppression of the artificial perturb at the MPP; and 2) the indirect identification of irradiance change through a current-monitoring algorithm, which acts as a natural perturbation. The zero-oscillation adaptive step P&O strategy builds on these mechanisms to identify relevant information and to produce efficiency gains. As a result, the combined techniques achieve superior overall performance while maintaining simplicity of implementation. Simulations and experimental results are provided to validate the proposed strategy, and to illustrate its behavior in steady and transient operations.

High-Performance Solar MPPT Using Switching Ripple Identification Based on a Lock-In Amplifier

Photovoltaic (PV) power converters and maximum power point tracking (MPPT) algorithms are required to ensure maximum energy transfer between the PV panel and the load. The requirements for the MPPT algorithms have increased over the years-the algorithms are required to be increasingly accurate, fast, and versatile, while reducing the intrusiveness on the overall performance of the PV panel and converter. The family of hill-climbing algorithms such as incremental conductance (InCond) and perturb and observe (P&O) has gained popularity given their simplicity and accuracy, but it requires the injection of a perturbation that changes the operating point even in steady state and are prone to errors during changing environmental conditions. In recent literature, the use of the switching ripple has been proposed to replace the perturbation in the hill-climbing algorithms given its inherent presence in the system and speed. The constant work toward smaller and faster ripples presents challenges to the signal detection involved in this kind of algorithm. This paper develops and implements a new InCond MPPT technique based on switching ripple detection using a digital lock-in amplifier (LIA) to extract the amplitude of the oscillation ripple even in the presence of noise. The use of this advanced technique allows to push forward the reduction of the ripple in order to virtually eliminate the oscillation in steady state maximizing the efficiency. The accurate detection allows for adaptive-step features for fast tracking of changing environmental conditions while keeping the efficiency at maximum during the steady state. Detailed mathematical analysis of the proposed technique is provided. Overall, the use of the proposed LIA allows to push the reduction of the ripple even more while keeping accuracy and delivering superior performance. Simulations and experimental results are provided for the proposed technique and the InCond technique in order to validate the proposed approach.

Zero-oscillation adaptive-step solar maximum power point tracking for rapid irradiance tracking and steady-state losses minimization

This paper develops the theory for an adaptive Maximum Power Point Tracking (MPPT) strategy to reduce extraction losses and other issues typically introduced by Perturb and Observe (P&O) algorithms. Three techniques to improve steady-state behavior and transient operation are discussed in detail: 1) idle operation on the Maximum Power Point (MPP), 2) irradiance direction change identification and 3) multi-level adaptive tracking step. As a result, these strategies are combined to achieve superior overall performance while maintaining a simplicity of implementation. Two key elements which form the foundation of the techniques are discussed: the suppression of perturb oscillations at the MPP and the indirect identification of irradiance change through a current-monitoring algorithm. The Zero-oscillation, Adaptive-step Perturb and Observe (ZA-P&O) strategy is studied with simulation and validated with experimental results. The mechanism for power extraction gains is evident, making the combined techniques an excellent solution to enhance MPPT performance.

An embedded impedance measurement for DC microgrids based on a Lock-In Amplifier

The advancements in DC microgrid architectures show a trend towards the use of multiple converters from distributed sources (photovoltaic, wind, battery) connected to different loads (passive and active) coexisting in a weak network. Since these topologies lack the large inertia of big AC generators, the stability of the microgrid can be compromised by the presence of tightly regulated active loads that can behave as constant power loads (CPLs). These loads present a characteristic negative incremental resistance, in contrast to the positive resistance of the passive loads. Detecting the nature of the loads as seen by the source converters, as well as the magnitude, can lead to improvements in the stability of the system, as indicated by the Middlebrook criterion. In this work, a novel incremental impedance measurement technique based on a Lock-In Amplifier (LIA) is presented. The LIA uses a perturbation of a known frequency to extract the equivalent incremental impedance of the load circuit very accurately. The characteristics of this method enable embedded implementation in real-time in the power converter, providing an extra tool to improve the stability of the converters in the microgrid. The proposed embedded instrument allows the incremental impedance of the network to be accurately measured, as seen by the source converter, with reduced complexity and sensor requirements. Simulations of the proposed measurement technique are presented in order to illustrate its behavior. Experimental results for different kinds of loads and transients are presented to validate the proposed strategy.

Fast and efficient solar incremental conductance MPPT using lock-in amplifier

Peak energy harvesting in Photovoltaic (PV) systems requires fast and effective Maximum Power Point Tracking (MPPT) detection. Incremental Conductance (InCond) MPPT is one of the most popular detection methods, given its simple implementation and accuracy. In this paper, a new InCond technique is proposed based on small-signal identification and adaptive-step using a Lock-In Amplifier (LIA). The use of small-signal identification virtually eliminates the losses typically encountered in traditional large-signal MPPT perturbations. This feature improves the steady-state efficiency, while the LIA allows for robust and accurate measurement of the equivalent AC resistance to achieve maximum power extraction, even in the presence of noise. The proposed algorithm enables fast tracking during both static and changing environmental conditions, as well as smooth operation in steady-state. The proposed implementation reduces the adaptive-step InCond to a simple Discrete-Time Integral Controller, simplifying its analysis and configuration. Overall, the proposed implementation delivers superior results with similar hardware both during transients and in steady state. Simulations and experimental results are provided to validate the proposed implementation, and to illustrate its behavior in steady and transient operations.

Improving Solar Power PV Plants Using Multivariate Design Optimization

The proliferation of photovoltaic (PV) installations across the globe has accelerated dramatically in the past decade covering home, rural, mobile, industrial, and utility-scale applications. In all these cases, improving payback time and energy production for PV installations is a very complex design tradeoff that involves multiple variables such as irradiance fluctuations, inverter efficiency, operating temperature variation, and PV panel type. In this paper, a detailed multivariate study of PV plant design is presented, resulting in an improved technique to increase the potential benefits of solar plants with lower capital costs. This new approach includes detailed consideration of the probabilistic hourly temperature and solar irradiation profile of the installation site, the efficiencies and operating areas of different grid-tie inverters, and detailed models of different PV modules in the optimal design process. The harvested energy, total costs, and payback time are the objective functions in this approach, while the number of series and parallel panels, the tilt angle, and inverter topology and PV module type are determined from a list of possible candidates. The optimization process is implemented for a sample system, and the results are compared to both a traditional and design software approach. It is seen that by applying the proposed approach with lower capital costs, the harvested energy, financial benefits, and the payback time can be improved by 9.3%, 1%, and 6.95%, respectively. Several case studies are then presented to investigate the sensitivity and robustness of the design with regard to the ambient temperature variation, solar irradiation fluctuation, and available surface area for PV module installation.

Direct MPPT control of PWM converters for extreme transient PV applications

Solar panels require of Maximum Power Point Tracking algorithms in order to ensure the amount of power extracted is maximized. The control of the power stage connected to the photovoltaic panel, and the MPPT algorithm are traditionally implemented as two independent building blocks, which derives in a relatively slow response. A novel approach is introduced in this work by embedding the MPPT algorithm into the power converter control, allowing faster dynamics, smaller reactive components and reduced switching losses. The scheme being introduced combines MPPT concepts with largesignal geometric control to achieve a high-performance, reliable solution. The theoretical analysis is supported by detailed mathematical procedures and validated by simulations and experimental results.

Open-loop maximum power point tracking strategy for Marine Current Turbines based on resource prediction

This paper presents the theory for a model-based Maximum Power Point Tracking (MPPTs) strategy for Marine Current Turbines (MCTs) that computes the optimal operating point using resource prediction and a look-up table. The high predictability of the Marine Currents (MCs) is exploited in order to produce a simple algorithm that calculates offline the optimal operating point for an evolving date and time. This eliminates the use of sensors to measure the current or the use of a perturbation to scan the curve. Two MPPT methods based on this strategy are discussed in detail: 1) fixed rotating speed with variable pitch angle and 2) fixed pitch angle with variable rotating speed. As a result, the two proposed MPPT methods improve the efficiency of the MCT creating a sensor-less, open loop strategy with precision and simplicity. The information needed to predict the MC is obtained through site characterization, a process required for any investment in green energy. The effectiveness of the proposed strategy is demonstrated through simulations.

Embedded fault location in DC microgrid systems based on a Lock-In Amplifier

Advancements in microgrids and distributed generated systems have created an impetus to distribute protection and management functions throughout a grid. Instead of using a centralized scheduling and protection system, each converter in the grid is expected to contribute to these functions. Many techniques based on impedance detection have been proposed as means to locate faults on a microgrid; however, many of them are not suitable or are too intrusive for a DC microgrid. An accurate detection of the grid impedance (both in magnitude and phase) allows for the accurate detection of the fault, and reduces the effort needed to clear it, in the event that it is necessary to do so. In this work, a novel fault location technique based on a Lock-In Amplifier (LIA) is introduced. This technique makes use of advanced digital algorithms to allow for the accurate detection of the fault location with minimal perturbation of the systems operations. By using both the magnitude and phase of the impedance, the algorithm is able to determine a location based on resistance and inductance, thereby mitigating the effects of errors from any single source. The proposed technique provides three key benefits: 1) high noise immunity, 2) low computational cost, and 3) low perturbation size; moreover, it provides these benefits while also maintaining a high level of accuracy. Simulations of the proposed measurement technique are presented in order to illustrate its behavior, along with experimental validation under different faults.

High-Accuracy Impedance Detection to Improve Transient Stability in Microgrids

The advancements in dc microgrids and the increase in distributed generation systems have led to a new trend toward the coexistence of multiple power converters from different sources (renewable, storage, etc.) supplying a variety of loads of different natures in a weak network. The loads can behave as passive loads (resistances) or be implemented by tightly regulated power converters, leading to constant power load (CPL) behavior. The CPLs present a characteristic negative incremental resistance that can alter the response of the system, even causing instability. In this work, a novel embedded technique based on a digital lock-in amplifier is proposed that enables the real-time detection of the dynamic impedance present in a power converter. The proposed technique uses a very efficient algorithm, along with standard sensors available in the converter, to measure the magnitude and phase of the dynamic load, and uses this information to improve the performance of the converter. A sample application of the proposed technique in an adaptive control system is described. Although the total output power of the converter is independent of the nature of the load, the converters dynamic response is not. The interaction of the CPL, passive load, and control loop will determine not only the stability but also the transient response. The proposed instrument allows the incremental load of the converter to be accurately measured while reducing the complexity and sensor requirements, and improving the performance of the controller. Simulations of the proposed technique are presented to illustrate its behavior. Experimental results for different kinds of loads are presented to validate the proposed strategy.