Cuauhtemoc Rodriguez

Long-Lifetime Power Inverter for Photovoltaic AC Modules

This paper presents a power inverter tailored for low-power photovoltaic (PV) systems. The inverter features high reliability, thanks to a circuit topology that obviates aluminum electrolytic capacitors from the circuit. Moreover, all components, including logic and control, have been designed to exhibit high reliability at high temperatures. Three conversion stages form the power topology. First, a full bridge connected to a high-frequency transformer and a full-bridge rectifier amplifies the voltage of the PV panel to approximately 475 V. This stage is controlled by using a phase-shift pulsewidth-modulation controller that permits zero-voltage switching, thereby minimizing losses. Second, a buck converter is connected in series with the rectifier and is controlled by using current mode in order to shape the current injection into a rectified sine wave. Last, a full bridge is operated at line frequency to unfold the current injection. The amplification stage has a proportional compensator that maintains the voltage at the PV terminals constant. The current injection stage has a proportional-derivative compensator that controls the amplitude of the grid current so that the dc-link average voltage is maintained constant. Experimental results show that the peak efficiency of the system is 89%, and the total current harmonic distortion is below 5%. Finally, analyses show a designed lifetime of approximately ten years.

Analytic Solution to the Photovoltaic Maximum Power Point Problem

Photovoltaic (PV) power has been successfully used for over five decades. Whether in dc or ac form, photovoltaic cells provide power for systems in many applications on earth and space. Its principles of operation are therefore well understood, and circuit equivalents have been developed that accurately model the nonlinear relationship between the current and voltage of a photovoltaic cell. With the improved efficiencies of power electronics converters, it is now possible to operate photovoltaic system about its maximum power point (MPP) in order to improve the overall system efficiency. Hitherto, this problem has been tackled using tracking (MPPT) algorithms that iteratively find the point of maximum power and respond to changes in solar irradiance accordingly. A mathematical manipulation that uses the mean value theorem is presented here that provides the analytic solution of a point in a close neighborhood of the MPP. It is thoroughly proved that this point is enclosed in a ball of small radius that also contains the MPP and therefore can practically be considered as the MPP. Since the solution is analytic, no iterative schemes are necessary, and only a periodic measurement is required to adjust to changes in solar irradiance. A circuit is implemented that shows the validity of the theory and the accuracy of the solution.

Dynamic stability of grid-connected photovoltaic systems

A mathematical model of grid-connected photovoltaic energy sources suitable for stability studies is presented. The power electronic conditioning unit is modelled from basic power transfer relations. Using this model, it is demonstrated that there exist two solutions for a given power output, one of which is unstable. By doing eigenvalue and eigenvector analysis, dynamic orbits are presented that help visualize any potential problem that may occur under disturbances. Simulations are carried out showing instances where the voltage at the photovoltaic panel collapses, in particular when operating close to the maximum power point.

Energy control for long lifetime photovoltaic ac module inverter

Photovoltaic power generation is spreading all around the globe with increasing support from the public and governments. Among the obstacles yet to overcome are higher cell efficiency, lower cost, and longer lifetime. The latter is the focus of the research presented here as it provides an alternative photovoltaic ac module inverter with long lifetime. Four commercial inverters analyzed in our laboratory contained electrolytic capacitors whose lifetimes are limited to 12000 hours. In the proposed solution these components are obviated by using an energy control scheme that stores all the energy fluctuation in the dc-link polypropylene capacitor. The energy control strategy has been applied in a prototype power conditioning unit that comprises a phase-shift full-bridge dc-ac-dc converter connected in series with a current source inverter. The grid current magnitude is controlled using a proportional derivative compensator that maintains the average value of the dc-link capacitor at a voltage level of 475V, while allowing oscillations with a peak value of 125V. Simultaneously, a pulse-width-modulation integrated circuit controller maintains a steady dc voltage at the photovoltaic panels terminals, hence enhancing the performance of the maximum power point tracking algorithm. The sinusoidal shape of the current injection is attained using current-mode-control. It is shown that the converter operates at high efficiency and that the energy fluctuation is absorbed at the dc-link capacitor. In addition, it is shown that the perturb and observe maximum power point tracking algorithm performs well over a day of variable irradiance conditions.

Organic Architecture for Small- to Large-Scale Photovoltaic Power Stations

Increased widespread deployment of power generation from photovoltaics is consistent with binding agreements to reduce carbon emissions and increase the penetration of electricity from renewables and political aspirations to increase security of energy supply. However, in order for these generation facilities to compete in increasingly open power markets, they must be low cost and provide high-quality and high-quantity outputs. The organic architecture suggested in this paper proposes a solution that provides these advantages, using modular-power-electronic and energy-storage components, to facilitate scalable plants, from kilowatt to megawatt size. Specifically, the inclusion of power-conversion building blocks (PCBBs), grid-interactive power units (GPUs), and power-system control units allow efficient transfer of power from the point of energy conversion to the point of common coupling. A specific example of a 24-kW plant illustrates that, through optimum switching of PCBBs, the GPU can transfer 95.46% of the daily available energy to the transmission grid.

Series connected photovoltaic power inverter

In this paper we introduce a series connected photovoltaic inverter that delivers both real and reactive power to a consumer load. It operates in series with the grid supply and injects power to the load along with the grid. Since the installed PV capacity is less than the load, the grid real power supply drops by an equivalent amount thereby resulting in energy savings. The inverter can also provide reactive power compensation so that the load voltage is kept constant even when the grid voltage fluctuates by around 6% of its rated voltage. The inverter serves a dual purpose during nighttime, when the circuitry is configured as a reactive power compensator. This way the proposed inverter is used to provide voltage compensation to the load all the time throughout day and night.

Using strong sustainability to optimize electricity generation fuel mixes

This work represents a contribution to the field of sustainable electricity system design by using an optimization tool to specify the final mix composition, subject to the constraints of: emissions that are within the biocapacity of the region; a diverse and robust electricity supply system; and supply that at least meets current demand. The 25-country European Union (EU-25) is used as a case study. All the goals, save diversity, can be met by re-structuring the current fuel mix, thus maintaining current consumption levels. The diversity target is only met when consumption is reduced by 10-15% and the constraint on maximum material throughput is relaxed. Re-structuring the mix and reducing consumption is insufficient to achieve a sustainable EU carbon footprint. However, the solution proposed singlehandedly allows the EU to meet its Kyoto emissions target as well as its 2007 policy of a reduction of 20% in greenhouse gas emissions by 2020.