Investigation of Variable Switching Frequency in Finite Control Set Model Predictive Control on Grid-Connected Inverters

Finite control set model predictive control (FCS-MPC) has been widely studied and applied to the power converters and motor drives. It provides the power electronics system with fast dynamic response, nonlinear system formulation, and flexible objectives and constraints integration. However, its variable switching frequency feature also induces severe concerns on the power loss, the thermal profile, and the filter design. Stemming from these concerns, this article investigates the variable switching frequency characteristics of FCS-MPC on the grid-connected inverters. An intuitive relationship between the switching frequency and the magnitude of the converter output voltage is proposed through the geometry analysis, where the switching frequency is maximized when the converter output voltage is around one-third of the DC bus voltage and decreasing when the output voltage moves away from this value. The impacts of this variable switching frequency property on the power loss and current harmonics are also analyzed. Simulation and experimental results both verify the proposed property. With this intrinsic property, FCS-MPC can autonomously achieve a less-varying temperature profile of power modules and an improved reliability compared with the conventional control strategy

Centralized Thermal Stress Oriented Dispatch Strategy for Paralleled Grid-Connected Inverters Considering Mission Profiles

One of the major failure causes in the power modules comes from the severe thermal stress in power semiconductor devices. Recently, some local control level methods have been developed to balance the power loss, dealing with the harsh mission profile, in order to reduce the thermal stress. However, there is not any specific system level strategy to leverage these local control level methods responding to the multiple inverters situation. Besides, the impacts of these methods on the thermal cycle and lifetime of the power modules in the long-term time scale have not been evaluated and compared yet. Hence, in this article, a centralized thermal stress oriented dispatch (TSOD) strategy is proposed to take full advantage of these local control level methods, including the switching frequency variation and the reactive power injection, to reduce the thermal stresses for multiple inverters. In addition to the PI controller, the finite control set model predictive control (FCS-MPC) is also explored to synergize with the proposed strategy. The results from the real-time model-in-the-loop testing on a four-paralleled-inverters platform, the reliability assessment, and the experiments all validate the effectiveness of the proposed centralized TSOD strategy on the thermal stress reduction

Recent Advances of Wind-Solar Hybrid Renewable Energy Systems for Power Generation: A Review

A hybrid renewable energy source (HRES) consists of two or more renewable energy sources, such as wind turbines and photovoltaic systems, utilized together to provide increased system efficiency and improved stability in energy supply to a certain degree. The objective of this study is to present a comprehensive review of wind-solar HRES from the perspectives of power architectures, mathematical modeling, power electronic converter topologies, and design optimization algorithms. Since the uncertainty of HRES can be reduced further by including an energy storage system, this paper presents several hybrid energy storage system coupling technologies, highlighting their major advantages and disadvantages. Various HRES power converters and control strategies from the state-of-the-art have been discussed. Different types of energy source combinations, modeling, power converter architectures, sizing, and optimization techniques used in the existing HRES are reviewed in this work, which intends to serve as a comprehensive reference for researchers, engineers, and policymakers in this field. This article also discusses the technical challenges associated with HRES as well as the scope of future advances and research on HRES

Battery Train Fire Risk on a Steel Warehouse Structure

Lithium ion battery fire hazard has been well-documented in a variety of applications. Recently, battery train technology has been introduced as a clean energy concept for railway. In the case of heavy locomotives such as trains, the massive collection of battery stacks required to meet energy demands may pose a significant hazard. The objective of this paper is to review the risk evaluation processes for train fires and investigate the propagation of lithium ion battery fire to a neighboring steel warehouse structure at a rail repair shop through a case study. The methodology of the analyses conducted include a Monte Carlo-based dynamic modeling of fire propagation potentials, an expert-based fire impact analysis, and a finite element (FE) nonlinear fire analysis on the structural frame. The case study is presented as a demonstration of a holistic fire risk analysis for the lithium ion battery fire and results indicate that significant battery fire mitigations strategies should be considered

Gate-Compatible Circuit QED in a Three-Dimensional Cavity Architecture

Semiconductor-based superconducting qubits offer a versatile platform for studying hybrid quantum devices in circuit quantum electrodynamics (cQED) architecture. Most of these cQED experiments utilize coplanar waveguides, where the incorporation of DC gate lines is straightforward. Here, we present a technique for probing gate-tunable hybrid devices using a three-dimensional (3D) microwave cavity. A recess is machined inside the cavity wall for the placement of devices and gate lines. We validate this design using a hybrid device based on an InAs-Al nanowire Josephson junction. The coupling between the device and the cavity is facilitated by a long superconducting strip, the antenna. The Josephson junction and the antenna together form a gatemon qubit. We further demonstrate the gate-tunable cavity shift and two-tone qubit spectroscopy. This technique could be used to probe various quantum devices and materials in a 3D cQED architecture that requires DC gate voltages

A High-Efficiency Low-Wearing Hybrid Voltage Regulator for Utility Applications

Step voltage regulator (SVR) has been utilized in power distribution systems for decades as the voltage regulation device. Due to the increasing integration of distributed energy resources, the conventional SVR is severely challenged by the modern power distribution pattern with high renewable energy penetration. The induced arc from the conventional SVR tap change and more frequent tap changes due to voltage instability from the renewable energy impose constraints on the conventional SVRs lifetime. Meanwhile, the conventional SVR device cannot regulate the voltage accurately since the SVR regulates the voltage step-by-step. This article proposed a hybrid voltage regulator with high-efficiency and low contact wearing, which can achieve arcless tap change and stepless voltage regulation by using a fractionally rated back-to-back power converter. The accurate load voltage regulation is guaranteed, while the tap changer mechanism remains in the system, which helps to promote the upgrade to the existing power distribution systems. The power converter capacity in the proposed topology is only 0.31% of the distribution transformer rating to achieve a stepless voltage regulation range of ±10%, significantly reducing the system cost compared with the full power electronics solutions and projects high total system efficiency. The proposed hybrid voltage regulator was simulated and experimentally validated. The experimental results demonstrate arcless tap change operation and stepless voltage regulation. Collaborative operation between the conventional mechanical tap change and the power converter operation is also demonstrated to acquire large voltage regulation with fast-acting voltage control

Modeling and Simulation of a 20kV Ultra-Fast DC Circuit Breaker for Electric Shipboard Applications

A novel hybrid circuit breaker for medium voltage dc (MVDC) electric shipboard power systems is proposed. The breaker combines the benefits of the efficiency of a mechanical breaker and the interruption speed of a solid-state breaker. The proposed breaker utilizes a fast-ramping current source with a fast-actuating vacuum interrupter (VI) to provided ultra-fast response time and high on-state efficiency. During normal operation, nominal load current flows through the vacuum interrupter in the main conduction branch, providing a low-resistance path with negligible losses. During a fault, a current zero crossing is achieved by the use of a controllable resonant current source (RCS). By leveraging the high switching frequency capabilities of Silicon Carbide (SiC) devices, the current source achieves higher frequency of resonance than previously possible with the silicon counterparts. After interruption, the surge arrester in the energy absorption branch clamps overvoltages and dissipates all residual system energy. Simulation results from the PLECS software environment are presented to verify the functionality of this proposed breaker in a 20 kV MVDC system for electric shipboard applications

Factors Influencing the Demand for Long-term Family Doctor Contract in Community Health Service Centers

Background The number of patients with chronic diseases in the community of China is increasing annually. Family doctor contract service is an important way to promote people's health in China, and long-term family doctor contract will help to improve the continuity of health management and the treatment outcomes. Analyzing patient demand for long-term family doctor contract is critical to the advancement of family medicine services in China. Objective To investigate the demand for long-term family doctor contract among community patients in Beijing Chaoyang Distrct and analyze its influencing factors, so as to provide a theoretical basis for achieving high quality family medicine services. Methods Using continuous enrollment, 500 patients who were enrolled in family doctor contract service at Baliqiao community health service center, Taiyanggong community health service center and Jiangtai community health service center in Chaoyang District, Beijing from January 2020 to January 2021 were included as the study subjects and categorized into the agreement group (those willing to sign long-term contracts) and the refusal group (those unwilling to sign long-term contracts). Data on patient demographics, chronic diseases, educational level, income status, medical expenses, health status, distance from home to the community health service center, compliance, living arrangements, and other indicators were collected through online system retrieval of health records and supplemented by questionnaire surveys from January 2020 to January 2021. From January to March 2021, a questionnaire survey was conducted to collect general information, service quality evaluation, and willingness to sign long-term family doctor contracts. The SERVQUAL scale was designed based on literature retrieval, expert consultation, and preliminary open-ended questionnaire survey results to evaluate the quality of family doctor services. Scores were assigned for expected quality, perceived quality, and importance, ranging from 1 to 5. Expectation scores and perception scores were used to calculate the service quality (SQ) score, and the latter was used to correct importance and calculate the corrected SQ (cSQ) score. Multivariate Logistic regression analysis was used to explore the influencing factors of the demand for long-term family doctor contract services. Results A total of 500 questionnaires were distributed, and 423 were collected, including WeChat platform (73, 17.2%), offline questionnaire (190, 44.9%) and telephone interview (160, 37.8%), the validity rate was 84.6%. Among the respondents, 283 belonged to the agreement group (66.9%), and 140 belonged to the refusal group (33.1%). The average score for the SERVQUAL scale was -0.54, with perceived and expected average scores of 3.98 and 4.52, respectively. Statistically significant differences were observed in the cSQ scores and average scores for each dimension between the two groups (P<0.05). The results of multivariate Logistic regression analysis showed that distance from home to the community health service center (OR=1.077, 95%CI=1.013-1.145, P=0.018), compliance (OR=0.291, 95%CI=0.137-0.617, P=0.001), living alone (OR=4.132, 95%CI=1.997-8.550, P<0.001), and cSQ (OR=0.983, 95%CI=0.980-0.986, P<0.001) were independent influencing factors for patients' willingness to sign long-term family doctor contracts. Conclusion The corrected family doctor service quality, distance from home to community health service center, living arrangements, and compliance are independent factors influencing patients' willingness to sign long-term family doctor contracts. The SERVQUAL scale can effectively evaluate family doctor service quality and aids in developing strategies for improving family doctor services

Application of Wide Bandgap Devices in Renewable Energy Systems – Benefits and Challenges

The rapid development of renewable energy systems (RES), especially photovoltaic (PV) energy and wind energy, poses increasing requirements for highpower, low-loss, fast-switching, and reliable semiconductor devices to improve system power capacity, efficiency, power density and reliability. The recent commercialization of wide bandgap (WBG) devices, specifically Silicon Carbide (SiC) and Gallium Nitride (GaN) devices, provides very promising opportunities for meeting such requirements with their attractive features of high voltage blocking capability, ultra-low switching losses, fast switching speed, and high allowable operating temperatures. This paper analyzed the performance benefits and application challenges of using SiC or GaN devices in both PV and wind energy conversion systems. Solutions to these challenges of using WBG devices in various RES were reviewed and proposed, and the benefits of using such emerging devices were confirmed in simulation based on a 250 kW commercial-scale PV inverter and a 250 kW doubly fed induction generator wind turbine system