Performance Comparison of Phase Shifted PWM and Sorting Method for Modular Multilevel Converters

Modular Multilevel Converters (MMC) are the solution of preference in HVDC applications due to modularity, scalability, low losses and low filtering requirement. Carrier-based (PWM) and carrier-less (nearest level control) modulation can be applied. By using advanced sorting methods focusing on keeping the capacitor voltage ripple under some limit, unnecessary switching events are eliminated leading to reduced switching losses. This paper presents a comparison between the steady-state performances in terms of output voltage THD and equivalent switching frequency of the Phase Shifted Carrier PWM and NLC plus sorting methods

Extended Functionalities of Photovoltaic Systems with Flexible Power Point Tracking:Recent Advances

The power system is experiencing an ever-increasing integration of photovoltaic power plants (PVPPs), which leads demand on the power system operators to force new requirements to sustain with quality and reliability of the grid. Subsequently, a significant quantity of flexible power point tracking (FPPT) algorithms have been proposed in the literature to enhance functionalities PVPPs. The intention of FPPT algorithms is to regulate the PV power to a specific value imposed by the grid codes and operational conditions. This will inevitably interfere the maximum power point tracking (MPPT) operation of PV systems. Nevertheless, the FPPT control makes PVPPs much more grid-friendly. The main contribution of this paper is to comprehensively compare available FPPT algorithms in the literature from different aspects and provide a benchmark for researchers and engineers to select suitable FPPT algorithms for specific applications. A classification and short description of them are provided. The dynamic performances of the investigated algorithms are compared with experimental tests on a scaled-down prototype. Directions for future studies in this area are also presented.MOE (Min. of Education, S’pore)Accepted versio

Wear-out failure analysis of solar optiverter operating with 60- and 72-cell Si crystalline PV modules

A new concept of shade-tolerant PV microinverter named Optiverter is capable of operating with both 60- and 72-cell PV modules. In order to ensure highly reliable operation of the Optiverter, this paper analyzes the wear-out failures of the Optiverter considering both 60- and 72- cells Si crystalline PV modules. The lifetime evaluation of the Optiverter also considers the impact of module degradation rates and different mission profiles into consideration. The evaluation results reveal that operating the Optiverter with a high-power PV module (i.e., 72-cell) results in a lower reliability performance compared with 60-cell PV module. This is mainly due to the higher power and thermal loadings of the power devices in the Optiverter with a 72-cell PV module. When considering the effect of module degradation, using the 72-cell PV module also lead to a higher deviation in the reliability prediction compared to the case without considering the degradation

An overview of photovoltaic microinverters: Topology, efficiency, and reliability

This paper presents an overview of microinverters used in photovoltaic (PV) applications. Conventional PV string inverters cannot effectively track the optimum maximum power point (MPP) of the PV string due to the series configuration (especially, under partial shading conditions). In order to tackle this problem, microinverters make each PV panel operate at its own MPP so that the overall efficiency can be improved. In this paper, a detailed analysis is carried out among commercially-available microinverters in terms of topological structure and operational principle. Moreover, the latest products on the microinverter market and future trends of the microinverters are discussed in terms of efficiency and reliability

Mission profile-based accelerated testing of DC-link capacitors in photovoltaic inverters

The dc-link capacitor is considered as a weak component in Photovoltaic (PV) inverter system and its reliability needs to be evaluated and tested during the product development. Conventional reliability testing methods do not consider the real operating conditions (e.g., mission profile) of the dc-link capacitor during the test. Therefore, the validation of the reliability performance of the dc-link capacitor under its mission profile is still a challenge. To address this issue, a new reliability testing concept for the dc-link capacitor in PV inverters is proposed in this paper. In contrast to the conventional method, the proposed reliability testing method realizes the test profile through the modification of the original mission profile (e.g., solar irradiance and ambient temperature) in order to maintain the test condition as close to the real application as possible. A certain acceleration factor is applied to the solar irradiance amplitude and the ambient temperature level during the mission profile modification in order to increase the thermal stress of the dc-link capacitor during test, and thereby effectively reduce the testing time. The results show that the testing time can be reduced to 2.5 % of the real field operation lifetime, if the solar irradiance amplitude is increased by 20 % and the ambient temperature is elevated to 75 °C

Reliability of DC-link Capacitors in Two-Stage Micro-Inverters under Different PV Module Sizes

The dc-link capacitor is one of the lifetime-limiting components in the photovoltaic (PV) micro-inverters, whose reliability should be evaluated carefully during the design. In micro-inverters, the PV module size (e.g., number of cells) is the parameter that determines the power rating of the PV module. The PV module size employed in the micro-inverter can vary for different manufacturers, and this variation can strongly affect the thermal stress and reliability of the dc-link capacitor in micro-inverters. To address this issue, an experimental-based reliability assessment is carried out in this paper using a two-stage micro-inverter where 60-cell and 72-cell PV modules are considered. Three different daily mission profiles are employed during the experimental test. The thermal stress and reliability of the dc-link capacitor under different operating conditions are evaluated together with the energy yield. The results indicate that employing a 60-cell PV module is more beneficial for the micro-inverter, especially during a clear day, where 19 % more energy can be captured during the entire lifespan of the micro-inverter. Thus, using the 60-cell PV module offers a better tradeoff between the reliability and energy yield of the micro-inverter

Design for Accelerated Testing of DC-Link Capacitors in Photovoltaic Inverters Based on Mission Profiles

The dc-link capacitor is considered as a weak component in photovoltaic (PV) inverter systems and its reliability needs to be evaluated and tested during the product development. Conventional reliability testing methods for capacitors are typically carried out under constant loading conditions, which do not reflect the real operating conditions (e.g., mission profile) of the dc-link capacitor in PV inverters. To address this issue, a new reliability testing concept for the dc-link capacitor in PV inverters is proposed in this article. In contrast to the conventional method, the proposed reliability testing method designs the test profile through the modification of the original mission profile (e.g., solar irradiance and ambient temperature) in order to maintain the test condition as close to the real application as possible. A certain acceleration factor is applied during the mission profile modification based on the lifetime model of the capacitor, in order to increase the thermal stress of the dc-link capacitor during test, and thereby effectively reduce the testing time