Enhancing Microinverter Energy Capture With Submodule Differential Power Processing

Fault-tolerant strategy without redundant switches for PV systems based on differential power processing converters

The characteristic mismatch due to partial shading in photovoltaic (PV) strings comprising multiple panels triggers not only a significant reduction in energy yield but also a malfunction of maximum power point tracking. Differential power processing (DPP) converters have been proposed to prevent negative influences of characteristic mismatch. In large-scale systems, however, conventional DPP converters face various challenges, such as poor extendibility and increased cumulative power conversion loss. This paper proposes a modular DPP architecture with an isolated bus for large-scale systems. Several panels are grouped as a module, each having a DPP converter, and the system can be flexibly scaled up by adding modules without redesigning the DPP converter. Power from unshaded high-voltage modules is directly transferred to shaded low-voltage ones through the isolated bus, reducing the cumulative power conversion loss. A field testing using a prototype for a 1-kW module demonstrated the energy yield enhancement of 5.6% by the proposed DPP converter.

Partial shading on a photovoltaic string consisting of multiple panels triggers a significant reduction in energy yield. Various kinds of differential power processing (DPP) converters have been proposed to prevent the partial shading issues. However, if DPP converters unnecessarily operate under unshaded conditions, no-load losses lower the system efficiency. Meanwhile, in case of short-circuit fault of panels, DPP converters might be destroyed due to over-current. Previous works have proposed DPP resonant converters that realize inherent over-current protection capability in discontinuous conduction mode (DCM). However, the peak currents in DCM tend to be large, resulting in increased conduction losses. This paper proposes a novel control algorithm for the DPP resonant converter to detect partial shading and short-circuit faults. The DPP converter is disabled under unshaded conditions and operates in DCM under short-circuit fault conditions. Experimental results demonstrated the energy yield enhancement under shading conditions and over-current protection capability under short-circuit fault.

Partial shading affects the energy harvested from photovoltaic (PV) modules, leading to a mismatch in PV systems and causing energy losses. For this purpose, differential power processing (DPP) converters are the emerging power electronic-based topologies used to address the mismatch issues. Normally, PV modules are connected in series and DPP converters are used to extract the power from these PV modules by only processing the fraction of power called mismatched power. In this work, a switched-capacitor-inductor (SCL)-based DPP converter is presented, which mitigates the non-ideal conditions in solar PV systems. A proposed SCL-based DPP technique utilizes a simple control strategy to extract the maximum power from the partially shaded PV modules by only processing a fraction of the power. Furthermore, an operational principle and loss analysis for the proposed converter is presented. The proposed topology is examined and compared with the traditional bypass diode technique through simulations and experimental tests. The efficiency of the proposed DPP is validated by the experiment and simulation. The results demonstrate the performance in terms of higher energy yield without bypassing the low-producing PV module by using a simple control. The results indicate that achieved efficiency is higher than 98% under severe mismatch (higher than 50%).

Differential power processing (DPP) architectures employ distributed, low power processing, submodule-integrated converters to mitigate mismatches in photovoltaic (PV) power systems, while introducing no insertion losses. This paper evaluates the effects of the simple voltage-balancing DPP control approach on the submodule-level maximum power point (MPP) efficiency. It is shown that the submodule MPP efficiency of voltage-balancing DPP converters exceeds 98% in the presence of worst-case MPP voltage variations due to irradiance or temperature mismatches. Furthermore, the effects of reduced converter power rating in the isolated-port DPP architecture are investigated by long-term, high-granularity simulations of five representative PV system scenarios. For partially shaded systems, it is shown that the isolated-port DPP architecture offers about two times larger energy yield improvements compared to full power processing (FPP) module-level converters, and that it outperforms module-level FPP approaches even when the power rating of DPP converters is only 20-30% of the PV system peak power. In the cases of aging-related mismatches, more than 90% of the energy yield improvements are obtained with DPP converters rated at only 10% of the PV peak power.

Conventional microinverter or module-integrated converter (MIC)-based photovoltaic (PV) systems are prone to be complex and costly because each MIC requires not only a boost converter to bridge a huge voltage gap between a PV panel and grid but also desirably a differential power processing (DPP) converter to preclude partial shading issues. This paper proposes a novel MIC based on cascaded quasi-Z-source inverters (qZSIs) with DPP capability. A traditional qZSI and voltage multiplier (VM)-based DPP converter are integrated into a single unit with sharing active switches and magnetic components, achieving system- and circuit-level simplifications. In addition, a novel control strategy utilizing two control freedoms of shoot-through duty cycle $d_{st}$ and modulation index M to simultaneously perform maximum power point tracking (MPPT) and DPP function, respectively, is also presented. A 150 W prototype for a standard PV panel consisting of three substrings is built, and experimental tests are performed emulating partial shading conditions. The results demonstrate that the proposed integrated qZSI could perform MPPT with satisfactory preventing partial shading issues while generating ac voltage at the inverter output.

Strings of series connected Photovoltaic (PV) elements suffer from severe power reduction when mismatch between PV elements occurs. Differential power processing (DPP) is used in PV systems to minimize energy conversion losses while optimizing the PV power during mismatch. One way to implement DPP is using the PV-bus DPP architecture where each DPP converter is connected between a PV element and the collective bus. The total processed power in PV-bus system depends on the substring current which is the output current of PV elements. Analytically, there is a unique operating point for this current to minimize the losses and achieve least processed power (LPP) through all DPP converters. This work represents a new methodology to control the substring current for achieving LPP in DPP system using minimum energy conversion stages. Different mismatch levels are analyzed to show that the proposed system can reach LPP resulting in a huge reduction in the processed power.

Partial shading on a photovoltaic (PV) panel is well known to trigger not only significantly reduced power generation but also the occurrence of multiple maximum power points (MPPs). Various kinds of differential power processing (DPP) converters have been proposed and developed to address partial shading issues. Meanwhile, power generation of PV panels substantially decreases as panels deteriorate due to damage, and therefore, PV panels are desirably diagnosed for early detection of degradation and malfunctions. In recent years, autonomous electrical diagnosis techniques based on ac impedance measurement are considered as a promising solution, but conventional electrical diagnosis techniques require expensive instruments. This paper proposes a DPP converter as well as an electrical diagnosis technique using the DPP converter. The proposed DPP converter not only preclude the partial shading issues but also offer the electrical diagnosis capability based on ac impedance measurement. The operation analysis and experimental verification tests using a prototype of the proposed DPP converter were performed. The results demonstrated the improved power yield from partially shaded PV panels and the electrical diagnosis capability of the proposed DPP converter.

Various kinds of differential power processing (DPP) converters have been developed to improve the energy yield of partially-shaded photovoltaic (PV) panels by precluding partial shading issues, such as the reduction in power generation and the occurrence of multiple maximum power points. Meanwhile, PV systems require fault diagnosis tools to detect premature deterioration and malfunctions that lead to a reduction in power generation of the system as a whole and, in the worst case, hazardous situations, such as a fire. This article proposes a novel DPP converter as well as an electrical diagnosis technique using the DPP converter. The proposed DPP converter not only addresses the partial shading issues but also provides the electrical diagnosis capability. In the diagnosis mode, the DPP converter injects ac currents to a panel, and an ac impedance of the panel can be calculated from the voltage response to the injected ac currents. The detailed operation analysis and experimental verification tests using a prototype were performed. The results demonstrated the improved power yield from partially shaded PV panels and the electrical diagnosis capability of the proposed DPP converter.

Analysis of the PV-to-PV architecture with a bidirectional Buck-Boost converter under shading conditions

In photovoltaic (PV) strings based on series-connected PV modules, mismatches among the modules due to partial shading will cause power loss either due to series connection constraint or bypass diodes. This paper proposes a new distributed maximum power point tracking (DMPPT) scheme using multiwinding forward-based converter, which acts as a current balancing differential power processing converter. The converter is configured in the way that each converter port is connected in parallel with individual PV module to enable module-level maximum power extraction. The proposed architecture operates in two modes: local maximum power point tracking (MPPT) mode and current balancing mode. The new maximum power points (MPPs) at module levels are tracked in a local MPPT mode and then dispatched to start current balancing mode of operation. In the current balancing mode, this paper proposes a simple control approach to maximize the output power from partially shaded PV string by maintaining an effective MPPT at module levels and directly transferring the compensation currents toward the shaded modules from modules under normal conditions without any intermediary. The proposed DMPPT scheme is validated by the simulation and experimental results. The results show that using the proposed scheme, the system efficiency is significantly improved compared to other existing topologies.

Differential power processing (DPP) module integrated converters are developed to mitigate the effects of the mismatch losses in series connected photovoltaic (PV) strings. This paper focuses on the configurations used in various DPP converter topologies by categorizing them into two main classifications: 1) Maximum power point tracking configurations and 2) voltage balancing configurations. The implementation of different control approaches on the DPP configurations is discussed and the components sizing used in the different DPP topologies are driven under different mismatching scenarios. Experimental results are presented for the ladder configuration to illustrate the need for developing new DPP configurations.

Tracking efficiency, cost, and reliability are important factors when selecting photovoltaic (PV) architectures and converter topologies. PV systems require power converters to maximize power extraction, for which dc–dc converters are a common choice. Differential power processing (DPP) architectures can achieve higher efficiencies and lower cost by reducing the amount of power passing through these converters, while still providing maximum power point tracking (MPPT) capabilities. Single-phase grid connected PV systems, which are the most popular choice in residential applications, require a large capacitance in the dc bus to minimize the voltage ripple caused by double-line pulsating power, impacting the cost and reliability of the system. This work introduces a new MPPT mode of operation for flyback converters in DPP architectures. The proposed MPPT method shows extremely fast dynamic performance and it is capable of maximizing the power extraction even for extreme variations in the bus voltage. In this way, the method enables a significant reduction in the dc bus capacitance, reducing costs, and facilitating the use of ceramic capacitors, while maintaining excellent tracking efficiency. The analysis incorporates comprehensive models that characterize the large-signal dynamic behavior of ideal and nonideal flyback converters, and it is supported by detailed mathematical procedures. The system performance behavior, and limits are validated through simulation and experimental results.

This paper presents a decentralized control technique, implemented through a differential power processing topology, in hybrid PEM Fuel Cell systems. This strategy allows the control of the power flow in the cells. The purpose is to improve the maximum power point tracking (MPPT) of the different submodules and thus increase the efficiency of the system. In a PEM Fuel Cell device, several cells are connected in series, where the same current passes through all of them. This implies that the degradation of some membrane not only affects the partial performance but also the efficiency of the entire system. To solve the problem of a single fuel cell block, a serial sub-modules application has been considered. Now, this solution needs optimization techniques. Differential Power Processing (DPP) techniques have been implemented in the sub-modules to provide an effective solution to mitigate the mismatch impact on the energy obtained. The DPP architecture consists of a small micro-converter at the submodular level applied to the cell. The main objective is to apply an algorithm oriented to polarize each fuel cell sub-module at its optimum point, which allows balancing between fuel cells, regardless of external parameters. The proposed strategy does not preclude the hybridization of fuel cells with a hybrid energy storage system (HESS) based on batteries or ultracapacitors. This ensures fast power transients in the load while the fuel cell is maintained at this optimum MPPT. The effectiveness of the strategy has been validated by the Matlab/Simulink software simulation. The results obtained, merit figures and conclusions are also presented in this paper.

In this paper, a sub-module differential power processing (DPP) architecture for solar photovoltaic (PV) applications is presented, along with a maximum power point tracking (MPPT) control scheme that requires minimum communication and no local current sensing. The efficiency, size and cost benefit of this architecture are analyzed, and the control challenges associated with this architecture are addressed. A hardware prototype is implemented and tested in a controllable indoor test setup that enables replication of real-world partial shading conditions. We experimentally verified a substantial increase in total system energy capture compared to module-level MPPT solutions, through the use of the proposed DPP architecture and control strategy.