Abstract
This article presents a novel MPPT method for two stage PV inverters with a single phase connection to the power grid. The method takes advantage of the 100 Hz/120 Hz harmonic present on the DC-bus voltage to guide the MPP search. It consists of detecting the slope of the P-V curve and integrating it to obtain the duty-cycle. The power slope detector (PSD) is able to calculate the P-V slope to command the MPPT even at very low powers, where the amplitude of the oscillations is barely perceptible. Design equations are provided, both of the gain of the PSD and of the gain of the MPPT integrator. It also shows how this PSD-MPPT strategy can be combined with the power control, allowing regulation of powers lower than those of the MPP. The power control loop is analyzed, and its stability is related to a single gain to be designed. The PSD-MPPT is tested in a two stage PV inverter, where the step-up DC-DC converter consists of three parallel boost converters. The results show that the PSD-MPPT method can work without the measurement of the current in the boost converters, which implies a cost reduction. A PV efficiency of about 99.8% is obtained with a usual ripple in the DC-bus of 4% peak-peak. In addition, the PSD-MPPT method is characterized as being extremely fast, both in the MPP search and in the power control, with response times around 50 ms. The PSD-MPPT is a simple algorithm of constant parameters that can be solved in a low cost microcontroller at a sampling frequency of about 2 kHz, requiring only the voltage and current of the PV array.
Highlights
Photovoltaic (PV) inverters are DC-AC converters that allow the extraction of photovoltaic energy from solar panels and its injection to the power grid or any 50/60 Hz AC-bus
The results show that the power slope detector (PSD)-maximum power point tracking (MPPT) method can work without the measurement of the current in the boost converters, which implies a cost reduction
The PSD-MPPT is a simple algorithm of constant parameters that can be solved in a low cost microcontroller at a sampling frequency of about 2 kHz, requiring only the voltage and current of the Keywords: photovoltaic (PV); two stage single phase PV inverter; power slope detector MPPT
Summary
Photovoltaic (PV) inverters are DC-AC converters that allow the extraction of photovoltaic energy from solar panels and its injection to the power grid or any 50/60 Hz AC-bus. When PV voltage is low (less than about 400 V in single phase connection and 800 V in three phase), as occurs in micro-inverters and string-inverters, a DC-DC converter is included inside the PV inverter to raise the voltage on the DC-bus so that the inverter can operate This step-up DC-DC converter is responsible for adjusting the PV voltage to extract maximum power from the array using maximum power point tracking (MPPT) algorithms. In the case of PV inverters with a single phase grid connection, the inherent oscillating feature of the instantaneous power generated in the grid produces a 100/120 Hz oscillation in the DC-bus voltage This oscillation makes it difficult to search for the MPP when the MPPT generates its own disturbance, as both overlap, producing search errors that decrease PV efficiency. It was possible to activate and deactivate converters as a solution for reliability and to improve efficiency, without producing current peaks or modifying operation at the MPP
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