New DPP converter to improve yield in PV systems under permanent mismatch

A group of researchers in Brazil has developed a new method based on differential power processing (DPP) to improve electricity yield in PV systems that operate under temporary and permanent mismatch due to shading, or PV modules with distinct specifications, or other factors.

“The concept of DPP has emerged as a prominent solution for mitigating the mismatch problem in photovoltaic (PV) strings comprising several elements,” the research corresponding author, Fernando Lessa Tofoli, told pv magazine. “Its main advantage lies in the ability to process only the power difference between series-connected PV modules, often constituting a minor portion of the total generated power. It is noteworthy that DPP converters are connected in parallel with the modules, this being a scalable solution.”

He also explained that, currently, two main types of power electronic converters are used to obtain DPP: the bidirectional buck-boost (BBB) converter and the switched capacitor (SC) converter. “The arrangement of these converters within the PV series results in three primary architectures: PV-to-PV, PV-to-Bus, and hybrid configurations,” he stated, noting that the proposed DPP converter consists of a hybrid topology that combines the PV-to-PV and PV-to-Bus architectures, thus incorporating elements from both BBB and SC converters.

In a PV-to-bus DPP configuration, a boost converter is used as the central converter, and the string current is regulated to follow the optimal value to achieve the minimum power losses of DPP converters in a real-time mode. In the PV-to-PV DPP architecture, by contrast, the converters are connected in parallel with the PV string to improve power yield.

“Our DPP converter tackles key challenges that impact the PV-to-PV and PV-to-Bus architectures,” said Lessa Toffoli. “Specifically, it addresses issues like the diverted current accumulation in the PV-to-PV architecture and high voltage stresses associated with the PV-to-Bus architecture.”

The proposed approach is said to result in a lower number of components, as only one active switch per module is required, and lower voltage stresses on the semiconductors. “The hard-switching operation of semiconductors in SC converters may lead to high switching losses and poor efficiency,” Lessa Toffoli further explained. “In turn, the introduced solution referred to ReSC converter provides operation under soft-switching conditions while mitigating such undesirable losses.”

Furthermore, the ReSC converter is able to operate under an open-loop condition and will not require additional sensors or complex microcontrollers while relying on simple drivers instead.

“We did not perform an in-depth cost analysis as well, since it falls outside the scope of the work. Anyway, we can positively say that cost will not increase significantly because the converter relies on few semiconductors and passive elements, all of which have low-power ratings,” said Lessa Toffoli. “A return on investment analysis could show that payback time is short compared with the lifespan of the modules while considering the significant increase in energy harvesting.”

Looking forward, the research group said that a patent request is currently under review.

The novel approach was presented in the study “Differential power processing architecture to increase energy harvesting of photovoltaic systems under permanent mismatch,” published in Solar Energy. The research group is composed of scientists from the Federal University of Itajubá and the Federal University of São João del-Rei.