New MPPT approach for multi-string PV systems under partial shading

Egyptian researchers have developed a multi-string PV system with a converter control strategy, achieving 99.81% efficiency with a direct duty cycle for maximum power point tracking (MPPT).

MMPPT approach

Image: South Valley University, Scientific Reports, Creative Commons License CC BY 4.0

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Researchers at Egypt’s South Valley University have developed a novel control strategy to deal with partial shading in PV systems. The new approach uses a multi-string PV system with a converter control strategy that utilizes a direct duty cycle to track the maximum power point (MPP) under shade.

“This study proposes a reconfiguration of the PV system to suppress the negative impact of partial shading on the PV-system performance by dividing the PV system into multiple parallel strings,” the scientists explained. “In this work, the proposed system comprises four strings, with each string comprising three paralleled sub-strings.”

The proposed multi-maximum power point tracking (MMPPT) approach involves connecting each string to a DC-DC converter, optimizing power extraction from individual strings of PV solar panels. This differs from the tested single-MPPT (SMPPT) arrangement in a PV system, where a single converter is connected to parallel strings of solar panels.

The MMPPT approach uses a direct duty‑cycle control strategy to find the duty cycles of desired converters. Then, using direct duty-cycle control (DDCC), it adapts the duty-cycle of the DC-DC converter to the calculated results, regulating the produced power.

The shading conditions

Image: South Valley University, Scientific Reports, CC BY 4.0 DEED

Calculating the desired duty cycle involves using the PV system’s existing sensors to estimate radiation, derived from changes in PV cell current and voltage.

“The proposed technique minimizes the overall system cost by reducing the number of the required sensors by utilizing a radiation estimation strategy,” the researchers said. “The DDCC technique boosts the overall system efficiency by the steady-state oscillations elimination, hardware simplification, and ease of implementation. Additionally, the DDCC has a fast-tracking speed for global maximum power point tracking (GMPPT) extraction during partial shading conditions.”

The researchers presented the results of the comparison between the MMPPT and the SMPPT system in “Investigation of single and multiple MPPT structures of solar PV‑system under partial shading conditions considering direct duty‑cycle controller,” which was recently published in Scientific Reports. The systems were simulated in MATLAB/SIMULINK software, under three shade conditions.

All four PV strings were subjected to uniform irradiance of 1,000 W/m2 in the first shading pattern. In the second pattern, however, the first two strings had a constant irradiation profile of 1,000 W/m2, while the other two had a uniform irradiance profile of 1,000 W/m2 for half a second, and then stepped down to 500 W/m2 for another half a second.

In the third pattern, the first two strings were again under a uniform irradiance profile of 1,000 W/m2, while the remaining two started with 1000 W/m2 for 0.3 seconds, changed to 500 W/m2 for another 0.3 seconds and decreased to 250 w/m2 for 0.4 seconds.

“Under these conditions, the average efficiency of the SMPPT system is found to be 98.98%, while the MMPPT system achieves an efficiency of 99.81%. These findings validate the proposed approach,” the researchers concluded. “A real radiation dataset from Benban, a location in southern Egypt, is used in MMPPT configuration. The results demonstrate that the proposed control system enhances overall system effectiveness while reducing installation costs.”

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