Power Electronic Transformer with Open-End Winding Electric Drive for Wind Energy Conversion Systems

University of Minnesota Ph.D. dissertation. May 2016. Major: Electrical Engineering. Advisor: Ned Mohan. 1 computer file (PDF); xii, 187 pages.Power Electronic Transformers (PETs) provide a reduction in size over line frequency transformers by operating at much higher frequencies than line frequency or grid frequency. Due to their smaller size, they could be useful in renewable energy systems where an interface with the grid is needed. As the name suggests, a power electronic interface is needed to convert line frequency voltages to high frequency voltages before they can be fed to the transformer. A PET topology that has simple control and less number of high voltage devices would be considered desirable due to lower total device cost and easy control implementation. A push-pull based PET topology has been proposed in the past which contains only two high voltage controlled switching devices and the control of those two devices is very simple. This topology could be configured for single stage ac to dc power conversion, to which an open-end winding dc to ac converter could be connected. Alternatively, it could be configured for direct ac to ac power conversion using dual matrix converters. In the first part of this thesis, the aforementioned push-pull based power electronic topology has been studied for power conversion from ac to dc and vice versa. Both single phase ac to dc and three phase ac to dc variants of the topology have been analyzed for power transfer, rms currents and soft switching. They provide attractive features which include single stage ac to dc bidirectional power conversion, unity power factor operation in open loop and control of dc side voltage using simple PI controllers. The other part of this thesis deals with open-end winding drives for suppression of common mode voltages at machine terminals. Switching frequency Common Mode Voltages (CMV) are generated by conventional Pulse Width Modulated (PWM) drives at machine terminals, which cause shaft voltage build up leading to bearing currents. These bearing currents are harmful for the machine and also cause Electromagnetic Interference (EMI). Open-end winding drives consist of one electric drive connected on each end of a three phase electric motor with the stator neutral opened up to give three more terminals. Open-end winding drives can be controlled to suppress switching frequency CMV at machine terminals. In this thesis, open-end winding two level Voltage Source Inverter (VSI) drive and open-end winding two level Matrix Converter (MC) drives have been investigated. Carrier based PWM techniques have been proposed for each of these drives for suppressing CMV. In addition, an improved four step commutation method has been proposed for the open-end winding matrix converter drive to suppress CMV spikes during the commutation process. Finally, a circuit consisting of the reduced switch PET connected with an open-end winding MC drive has been studied for single stage ac to ac power conversion with open loop power factor control

A Bidirectional Soft-Switched DAB-Based Single-Stage Three-Phase AC–DC Converter for V2G Application

In vehicle-to-grid applications, the battery charger of the electric vehicle (EV) needs to have a bidirectional power flow capability. Galvanic isolation is necessary for safety. An ac-dc bidirectional power converter with high-frequency isolation results in high power density, a key requirement for an on-board charger of an EV. Dual-active-bridge (DAB) converters are preferred in medium power and high voltage isolated dc-dc converters due to high power density and better efficiency. This paper presents a DAB-based three-phase ac-dc isolated converter with a novel modulation strategy that results in: 1) single-stage power conversion with no electrolytic capacitor, improving the reliability and power density; 2) open-loop power factor correction; 3) soft-switching of all semiconductor devices; and 4) a simple linear relationship between the control variable and the transferred active power. This paper presents a detailed analysis of the proposed operation, along with simulation results and experimental verification

Fixed-Time Stable Proximal Dynamical System for Solving MVIPs

In this paper, a novel modified proximal dynamical system is proposed to compute the solution of a mixed variational inequality problem (MVIP) within a fixed time, where the time of convergence is finite, and is uniformly bounded for all initial conditions. Under the assumptions of strong monotonicity and Lipschitz continuity, it is shown that a solution of the modified proximal dynamical system exists, is uniquely determined and converges to the unique solution of the associated MVIP within a fixed time. As a special case for solving variational inequality problems, the modified proximal dynamical system reduces to a fixed-time stable projected dynamical system. Furthermore, the fixed-time stability of the modified projected dynamical system continues to hold, even if the assumption of strong monotonicity is relaxed to that of strong pseudomonotonicity. Connections to convex optimization problems are discussed, and commonly studied dynamical systems in the continuous-time optimization literature follow as special limiting cases of the modified proximal dynamical system proposed in this paper. Finally, it is shown that the solution obtained using the forward-Euler discretization of the proposed modified proximal dynamical system converges to an arbitrarily small neighborhood of the solution of the associated MVIP within a fixed number of time steps, independent of the initial conditions. Two numerical examples are presented to substantiate the theoretical convergence guarantees.Comment: 12 pages, 5 figure

A Reduced Switch Count Single-Stage Three-Phase Bidirectional Rectifier With High-Frequency Isolation

Dual active bridge (DAB) based converters offer the benefit of smaller volume due to high-frequency isolation and controllability of active power flow, making them attractive for various applications, such as renewable energy generation, plug-in hybrid vehicles, and distribution systems. This paper presents a novel converter topology along with a modulation strategy for a DAB-based three-phase ac to dc converter. The major benefits are single-stage conversion, no unreliable intermediate dc-link capacitor, reduced number of switches, i.e., only two active switches on the ac side, simple control scheme, open-loop unity power factor operation, bidirectional power flow, and partial soft-switching. This paper presents the analysis of all the operating modes of the converter, resulting in the analytical estimation of power transfer and rms winding current and investigation of soft-switching conditions for the power devices. Simulation and experimental results have been presented to demonstrate the advantages of the proposed technique and accuracy of the analysis

A Dual-Active-Bridge-Based Single-Phase AC to DC Power Electronic Transformer With Advanced Features

Power electronic transformers (PETs) offer the advantage of size and weight reduction compared to line-frequency transformers by operating at much higher frequencies than line frequency. In this paper, a push-pull-based ac/dc PET has been proposed and analyzed. The PET offers bidirectional power flow between single-phase ac and dc, using the dual-active bridge principle. Such a system may find applications in interfacing plug-in hybrid and electric vehicles to the grid. The proposed PET offers advantages of open-loop unity power factor operation, soft switching of secondary-side converter power switches for all operating points, high power density owing to use of a high-frequency transformer, and high utilization factor (UF), compared to previous work. Analysis has been done for power transfer, UF, and soft switching. Simulation and experimental results have been provided to demonstrate the operation of the PET

Congenital rubella syndrome surveillance in India, 2016–21: Analysis of five years surveillance data

Background: In India, facility-based surveillance for congenital rubella syndrome (CRS) was initiated in 2016 to estimate the burden and monitor the progress made in rubella control. We analyzed the surveillance data for 2016–2021 from 14 sentinel sites to describe the epidemiology of CRS. Method: We analyzed the surveillance data to describe the distribution of suspected and laboratory confirmed CRS patients by time, place and person characteristics. We compared clinical signs of laboratory confirmed CRS and discarded case-patients to find independent predictors of CRS using logistic regression analysis and developed a risk prediction model. Results: During 2016–21, surveillance sites enrolled 3940 suspected CRS case-patients (Age 3.5 months, SD: 3.5). About one-fifth (n = 813, 20.6%) were enrolled during newborn examination. Of the suspected CRS patients, 493 (12.5%) had laboratory evidence of rubella infection. The proportion of laboratory confirmed CRS cases declined from 26% in 2017 to 8.7% in 2021. Laboratory confirmed patients had higher odds of having hearing impairment (Odds ratio [OR] = 9.5, 95% confidence interval [CI]: 5.6–16.2), cataract (OR = 7.8, 95% CI: 5.4–11.2), pigmentary retinopathy (OR = 6.7, 95 CI: 3.3–13.6), structural heart defect with hearing impairment (OR = 3.8, 95% CI: 1.2–12.2) and glaucoma (OR = 3.1, 95% CI: 1.2–8.1). Nomogram, along with a web version, was developed. Conclusions: Rubella continues to be a significant public health issue in India. The declining trend of test positivity among suspected CRS case-patients needs to be monitored through continued surveillance in these sentinel sites