Control Strategies for Improving Reliability and Efficiency in Modular Power Converters

The significance of modular power converters has escalated drastically in various applications such as electrical energy distribution, industrial motor drives and More Electric Aircraft (MEA) owing to the benefits such as scalability, design flexibility, higher degree of fault tolerance and better maintenance. One of the main advantages of modular systems is the ability to replace the faulty converter cells during maintenance instead of the entire system. However, such maintenance cycles can result in a system of converter cells with different aging. A system with cells having different aging arises the threats of multiple maintenance, lower reliability and availability, and high maintenance costs. For controlling the thermal-stress based aging of modular power converters, power routing strategy was proposed. The thesis focuses on the different implementation strategies of power routing for modular converters. Power semiconductors are one of the most reliability critical components in power converters, and thermal-stress has been identified as the main cause of their failure. This thesis work concentrates on the power semiconductor reliability improvement algorithms. For improving system lifetime, virtual resistor based power routing algorithms for single stage and multi-stage modular architectures have been investigated through simulations and validated with experiment. A unified framework for routing the power in complex modular converter architectures is defined based on graph theory. Popular converter architectures for Smart Transformer (ST) and MEA applications are modeled as graphs to serve as the basis for developing power flow optimization. The effectiveness of graph theory for optimizing the power flow in modular systems is demonstrated with the help of proposed algorithms

Modular Smart Transformer Topology for the Interconnection of Multiple Isolated AC and DC Grids

The Smart Transformer enables the interconnection of multiple AC and DC grids with potentially different voltage levels. In most configurations, each grid needs to be isolated for preventing the propagation of faults. Actual literature mostly considers a single isolated interconnection, which is not opti-mized for the interconnection of multiple AC and DC grids. By utilizing the modularity of the Cascaded H-Bridge (CHB) converter and the corresponding connected Dual Active Bridge (DAB) to each H-bridge, this work proposes a system design for feeding multiple AC and DC grids. Requirements for the DC grid isolation and integration are discussed and the proposed topology is demonstrated to enable reduced losses compared to the commonly adopted configuration. The commonly adopted and the proposed configuration are compared and experimental results validate reduced losses for a large range of operation compared to the conventional solution

Lifetime Control of Modular Smart Transformers Considering the Maintenance Schedule

Smart Transformers (ST) are power electronic transformers capable of providing grid services and DC connectivity compared to conventional low-frequency transformers. However, failure of the components and consequent maintenance is one of the major challenges compared to it's counterpart, limiting its application. By controlling the power flowing through the cells in a modular ST architecture, the remaining lifetime of these cells can be controlled. The idea is to schedule the maintenance by actively controlling the lifetime of the individual cells using an optimization algorithm. The proposed algorithm has the ability to schedule the maintenances according to predefined maintenance intervals. A potential ST architecture consisting of Cascaded H-bridge (CHB) and Dual-Active Bridge (DAB) converters are analyzed with simulations and experiments to validate the proposed power routing algorithm and its impact on the lifetime and maintenance scheduling

Lifetime-based power routing of smart transformer with CHB and DAB converters

Smart Transformers (ST) are a potential solution for installing intelligent nodes in the electrical distribution grid, which can provide DC connectivity and are capable of providing grid services the traditional low-frequency transformers was not able to provide. However, the reliability of STs is one of the main challenges, limiting its application. To improve the reliability of STs, it is proposed to control the power in a modular ST architecture in order to control the remaining lifetime of its building blocks. Therefore, unequal power sharing through the cells based on the remaining useful lifetime, known as power routing, has been proposed for different converter topologies. However, existing literature has not investigated the impact of power routing on the ST on a system level. This paper focuses on the design and implementation of lifetime-based power routing control based on virtual resistances tailored for an ST consisting of Cascaded H-bridge (CHB) and Dual-Active Bridge (DAB) converters

Comparison of voltage control methods of CHB converters for power routing in smart transformer

Abstract—For Smart Transformers (ST), reliability is one of the major problems when compared to the traditional low frequency transformers. Modular ST with advanced control algorithms could increase the reliability compared to the nonmodular solutions. This could be achieved by distributing differently the power among the cells depending on their aging. This unequal power transfer is a challenge for Cascaded HBridge converters (CHB). ST based on a CHB rectifier with different capacitor voltage balancing schemes are analyzed in the paper. The goal is to evaluate the best scheme for unequal power transfer while maintaining the dc-link voltages constant. The paper focuses on validating the operating power unbalance limits of each cell of the modular system for unbalanced power transfer while maintaining the voltage balance in the medium voltage dc-link stage. The power unbalance limits are established with analytical calculations and verified through simulations and experiment

Active thermal control of a DC/DC GaN-based converter

Advanced packaging technologies in Wide band gap devices like GaN avoid wire bonds thereby making the solder joints more susceptible to thermo-mechanical fatigue. To limit the thermal cycling induced failures, an active thermal control scheme using a two step gate driver for a buck converter is presented in this paper. In contrast to the active thermal control techniques employing variation of switching frequency, this method does not alter the converter operation point. A simple temperature control algorithm which actively varies the device losses is proposed. The effectiveness of the control scheme has been validated through experimental results

Aflatoxins: A Postharvest Associated Challenge and Mitigation Opportunities

In agriculture, Aflatoxins are of major concern as they affect the nutrient quality of crops like Groundnut, Maize, and Coffee which are global economic commodities. Aflatoxin-contaminated products cause substantial financial losses and significant health problems in living beings. Aspergillus produces aflatoxins during environmental stress conditions. The International Agency for Research in Cancer (IARC) conducted studies on aflatoxins and found that Aflatoxin B1 (AFB1), Aflatoxin B2 (AFB2), Aflatoxin G1 (AFG1) and Aflatoxin G2 (AFG2) can cause cancer in both humans and animals and are classified into the Group 1 category of chemical hazards for potentiation mutagens. In India, the Food Corporation of India (FCI) monitors Aflatoxin levels in food and feeds. Aflatoxin contamination reduces the quality of groundnuts, maize, and coffee, affecting their exports. Consumption of aflatoxins contaminated feed induces liver cancer, immune suppressions, shunted growth, and in higher amounts, causes death. The current review provides information based on previous studies and newly adapted guidelines and methods showing the impact of aflatoxins on crops such as groundnut, coffee, and maize. The use of artificial intelligence to detect aflatoxin and mitigation opportunities using technologies such as Aflasafe, Aflaguard, hermetic bags, and Purdue Improved Crop Storage (PICS)

Power routing: a new paradigm for maintenance scheduling

Currently, the necessity of efficient and reliable power systems is also increasing because of the strict requirements that standards and regulations impose, but still costs have to remain low. The monitoring and control of the components' lifetime can lead to reduce maintenance costs. However, overcoming the related challenges is not a straightforward task, as it involves knowledge of power device physics, smart management of electrical quantities, and optimal maintenance planning and scheduling. It represents a multidisciplinary issue being faced in the last decade