Tianhua Zhu

Analysis and Optimization of Flexible MCPT Strategy in Submodule PV Application

Distributed maximum power point tracking (DMPPT) can effectively increase the power generation ability of photovoltaic (PV) systems in mismatch conditions. To further recover mismatch power loss inside PV panels, submodule realization is developed as an improved penetration of DMPPT concept. This paper proposes an optimized submodule level DMPPT system with a flexible time-sharing maximum current point tracking algorithm which detects the output current of series-connected dc-dc converters. Comparing with current PV optimizer and other submodule DMPPT solutions, the proposal can always guarantee that each PV submodule outputs individual true maximum power regardless of irradiance cases while eliminating the need for several MPPT units and current sensors in the system. Enormous potential of single control IC can be fully developed with fewer components (voltage sensors, current sensors, and relevant AD/DA units) for further integration in PV application.

Analysis of Existence-Judging Criteria for Optimal Power Regions in DMPPT PV Systems

This paper quantitatively evaluates different distributed maximum power point tracking (DMPPT) photovoltaic (PV) systems in mismatch cases. To help designers obtain the full benefits of the DMPPT concept, a general analysis of the optimal power region (OPR) in different DMPPT system configurations is undertaken. Taken into account in the evaluation of OPRs are functional constraints derived from DMPPT converters, mismatch conditions, and system architectures and parameters of related PV units. To simplify the calculation process and enable rapid discovery of OPRs, judging criteria are proposed for 4 DMPPT structures. If the judging criteria are met, DMPPT PV systems can extract ideal maximum power regardless of mismatch issues. Finally, experimental results are provided to show the effectiveness of the proposed static models, formulas, and judging criteria.

Analysis and comparison of FPP and DPP structure based DMPPT PV system

Distributed Maximum Power Point tracking (DMPPT) concept can effectively make each PV unit working on its individual Maximum Power Point (MPP) during mismatch. Therefore, to get the full benefit of DMPPT concept, this paper provides a comparative analysis between two major branches of DMPPT PV systems: FPP structure and DPP structure respectively. Advantages and disadvantages of both structures are analyzed and compared from different aspects.

An Improved Submodule Differential Power Processing-Based PV System With Flexible Multi-MPPT Control

With inhomogeneous irradiance on photovoltaic (PV) units significantly deteriorating the effectiveness of solar energy harvest, distributed maximum power point tracking (DMPPT) concept is considered as an effective solution against resultant mismatch. A comparative analysis of full power processing and differential power processing structures, as two main branches of DMPPT solution, is provided first. Advantages and disadvantages of widely used structures are analyzed and compared from different aspects. To harvest more power and realize better utilization of PV units, this paper presents a submodule level differential power processing PV system with a flexible double stage time-sharing maximum power point tracking (MPPT) control strategy. The proposal can achieve the accurate maximum power points (MPPs) of all PV submodules regardless of mismatch cases with only one adaptive MPPT controller, facilitating system integration and largely reducing system cost and conversion loss. The efficacy of the scheme is analyzed using Simulink and validated by experimental results.

Optimization of time-sharing output Maximum Current Tracking (MCT) strategy in subpanel level Distributed Maximum Power Point Tracking (DMPPT) application

Distributed Maximum Power Point Tracking (DMPPT) has been revealed to be a technique increasing the efficiency and reliability of Photovoltaic (PV) systems in mismatch conditions. To further recover mismatch power loss inside PV panels, subpanel realization of DMPPT solution is developed as an improved penetration of DMPPT concept. This paper proposes a double loop control based subpanel level DMPPT system with a self-adjusted time-sharing Maximum Current Tracking (MCT) algorithm which detects the output current of series connected DC-DC converters. The proposal can always guarantee that all PV units output its individual maximum power regardless of irradiance cases with a greatly reduction of components such as control IC and sensors. Comparing with current commercial panel level solutions, common subpanel level mismatch power loss inside a PV panel can be totally recovered with single current sensor and control IC. The proposal of this paper can be utilized for further integration of PV optimizer.

An improved simulated annealing maximum power point tracking technique for PV array under partial shading conditions

This paper presents an improved Simulated Annealing (SA) based Maximum Power Point Tracking (MPPT) technique designed for photovoltaic (PV) systems under partial shading conditions (PSC). Compared with existing SA MPPT algorithms, the proposed method performs faster convergence speed, higher tracking accuracy and continuously tracking of the global Maximum Power Point (GMPP) under climate changes. The new SA MPPT principle is fully explained, discussed and verified by simulation.

High power DC/AC/DC converter based on different modulation strategies

The recent researches on high voltage and power DC/DC converter mainly focus on topology. The newest topology based on modular multilevel converter(MMC) is composed of a transformer coupling two three-phase MMC, so that it can be easily extended according to the required voltage rating. However, the present topology almost adopt the same modulation strategy at two sides of converter. Because of the different applications of the high power DC/DC converter, the voltages on two sides will be different leading to the number of submodules (SM) different. For the different structure mentioned above, this paper presents the method which takes different modulation strategies at two sides of converter. It respectively selects the optimal modulation strategy according to the SM number at two sides, which could reduce switching losses and harmonic content. Finally, a simulation model of 5kV/500V DC/AC/DC converter is established in MATLAB/Simulink, which verifies the feasibility of the method.

A novel topology of high voltage and high power bidirectional ZCS DC-DC converter based on serial capacitors

With the rapid development of renewable energy systems and Microgrid, more and more DC grids of different voltage level need to be connected by the high voltage and high power DC-DC converter. The traditional topology of high voltage and high power DC-DC converter was Phase Shifted Full Bridge cascaded Modules (PSFBM). Due to the inefficiency and difficulty on manufacturing high frequency transformer of PSFBM, a non-isolated topology, Multilevel Modular Capacitor-Clamped DC-DC Converter (MMCCC), was proposed as an alternative. However, the MMCCC also has some inherent disadvantages, especially, a large number of switching devices and no voltage regulation which limit its application in the connection of DC grids requiring high efficiency and accurate control. In this paper, a novel non-isolated high voltage and high power ZCS DC-DC converter based on serial capacitors is proposed. By energy transferring circuit to transfer the energy between adjacent serial capacitors, it can realize the bidirectional DC-DC conversion with less switching devices. Furthermore, by a control system with independent voltage control of each serial capacitor, It can improve the flexibility and accuracy of output voltage and also can realize a higher input or output voltage without limitation in theory. In this paper, the operation principle and design consideration of the novel topology were presented in detail, and a simulation with ten serial capacitors was also built to validated the effectiveness of the novel topology. Finally, a 1kW test equipment with 250V input voltage and 1000V output voltage was developed in laboratory to verify the simulation result.

An improved control method applied to the DC/DC converter based on MMC

Recent research on high voltage and power DC/DC converter mainly focus on the face-to-face topology based on modular multilevel converters (MMC). The topology consists of two MMC which are coupled by a transformer. It is easy to extend the module number according to the voltage level. However, the present modulation method in this topology cannot regulate the voltage transformation ratio on two dc sides. The ratio of the dc voltage mainly depends on the transformer ratio. This paper proposes a modulation method which can adjust the dc voltage ratio directly. Finally, a simulation model is established to verify the feasibility of the method.

An integrated single inductor-single sensor based photovoltaic optimizer with an optimal current point tracking strategy

In this paper, a single inductor-single sensor based photovoltaic optimizer with an optimal current point tracking strategy is proposed. With only one inductor, one sensor and one MPPT unit, this optimizer can not only make all submodules output their individual maximum powers, but also utilize the least components compared with current solutions, greatly reducing the size and cost of photovoltaic optimizer and increasing the integration level. The effectiveness of the proposed solution is verified by both simulation and experiments.