Xiao Yuan Chen

Cryogenic Power Conversion for SMES Application in a Liquid Hydrogen Powered Fuel Cell Electric Vehicle

Cryogenic power conversion for superconducting magnetic energy storage (SMES) application in a liquid hydrogen (LH2) powered fuel cell electric vehicle (FCEV) is investigated. Principle and operation strategy of the SMES-based onboard energy system are presented for various operational models. A typical FCEV system equipped with a 720-kJ SMES device is conceptually designed and theoretically modeled with a bridge-type cryogenic chopper, which consists of four metal-oxide- semiconductor field-effect transistors (MOSFETs) cooled by low-temperature gas hydrogen (GH2). The bridge-type cryogenic chopper has higher energy storage and utilization efficiencies than the conventional one because the MOSFETs have much less thermal loss compared with normal operations of the MOSFETs and diodes. Both the start-up time and the regenerative braking time of the FCEV are significantly reduced with the introduction of the SMES. The design and tests of an experimental energy exchange prototype are also presented to verify the feasibility of the proposed high-efficiency SMES system incorporated with the FCEV.

A Superconducting Magnetic Energy Exchange Model Based on Circuit-Field-Superconductor-Coupled Method

A superconducting magnetic energy exchange (SMEE) model based on a circuit-field-superconductor-coupled method is presented. The objective power system problems to be solved, e.g., power and load fluctuations, are transformed into equivalent energy exchange demanding values for the superconducting coil used in a superconducting magnetic energy storage (SMES) device. With the model, the buffering effects for the external power and load fluctuations and the superconducting properties of the superconducting coil itself can be obtained equivalently for optimizing and evaluating the designed SMES device. Four coil-current-dependent formulas are derived to estimate the hysteresis loss, coupling current loss, eddy current loss, and flux flow loss of the superconducting coil under different energy exchange operation conditions. A case study has been carried out using a Bi-2223 coil. The obtained results verify the effectiveness of the SMEE model and its feasibility for practical applications.

Energy Efficiency Analysis and Energy Management of a Superconducting LVDC Network

A novel superconducting low-voltage direct-current (LVDC) power transmission and distribution network consisting of multiple superconducting dc cables and a superconducting magnetic energy storage (SMES) based hybrid energy storage system (HESS) is presented. The superconducting dc cables are favored for both high-efficiency power transmission under normal conditions and self-triggering fault-current-limiting operation under fault conditions. The SMES-based HESS is integrated, with the merits of fast response speed and high-power density from the SMES, and high-energy density and high-economic efficiency from conventional battery energy storage (BES), and thus, is much more effective a sole SMES or BES device for smoothing the line voltage fluctuations caused by some sudden load variations or short-circuit faults. The simulation results obtained from energy efficiency analyses and energy management simulations show that the proposed superconducting LVDC network has significant advantages in high-efficiency, high-quality power transmission and distribution compared to conventional ac networks.

A Superconducting Air-Core DC Reactor for Voltage Smoothing and Fault Current Limiting Applications

A superconducting air-core DC reactor (SACDCR) operated for both DC voltage smoothing under normal condition and fault current limitation under fault condition is presented. The novel SACDCR is formed by one superconducting air-core solenoidal inductor unit and one series-connected superconducting noninductive solenoidal resistor unit. The air gaps between the terminal layers of the superconducting air-core solenoid are optimized to achieve a higher critical current based on anisotropic traits and magnetic field distributions. The superconducting noninductive solenoid, which is inserted into the inner center of the superconducting air-core solenoid, experiences an increased background field under the fault condition and thus achieves better fault current limiting features compared with the same type of superconducting fault current limiter (SFCL) without a background field.

An Efficient Boost Chopper Integrated With Cryogenic MOSFETs and HTS Inductor

A novel efficient boost chopper integrated with two cryogenic metal-oxide semiconductor field-effect transistors (MOSFETs) and one high-temperature superconducting (HTS) inductor is presented. State-of-the-art CoolMOS MOSFETs are introduced and immersed in liquid nitrogen to save about 85% of conduction losses. Considering the anisotropy of gadolinium barium copper oxide (GdBCO) tapes, ferromagnetic disks with raised edges are optimized to enhance the critical current and reduce the hysteresis loss of the HTS smoothing inductor. As compared to the conventional boost chopper composed of ambient-temperature power electronics and a copper inductor, the total operational loss from the cryogenic MOSFETs and the HTS inductor in the proposed cryogenic boost chopper is reduced by 87% in the case of a 40-kW chopper.

Effects of inner air gaps on the leakage field optimization of HTS transformer windings

High temperature superconducting (HTS) transformer is favored to reduce the energy loss by replacing normal conductor windings with superconducting windings. In order to reduce perpendicular field component around the HTS windings, the air gaps insider the windings are optimized and discussed to suit practical HTS transformer development.

Performance Improvement of a Bi-2223 Solenoid Coil With Optimal Ferromagnetic Disks

Solenoid-type superconducting magnetic energy storage (SMES) coils wound by Bi-2223 tapes have strong anisotropic magnetic field dependence due to the fundamental anisotropic property inherited from Bi-2223 materials. The maximum perpendicular field components appearing around the turns located at the two coil ends degrade the critical current and energy storage capacity of the whole coil, and even cause significant AC loss and possible hot-spots during practical SMES operations. To enhance the critical current and reduce the ac loss of the turns located at the two coil ends, this paper investigates a novel ferromagnetic raised-edge disk structure for adjusting the magnetic field orientation to be as close as possible to a parallel angle. Finite element simulations and performance comparisons with a test 0.2-H Bi-2223 solenoid coil without any ferromagnetic disk, with a conventional flat disk, and with the proposed raised-edge disk are presented. The results obtained show that the proposed raised-edge disk has much better effects towards critical current improvement and AC loss reduction compared to a conventional flat disk, especially for the terminal coil turns near the inner wall of the whole coil.

Influence of Flux Diverter on Magnetic Field Distribution for HTS Transformer Windings

High-temperature superconducting (HTS) transformers employ highly efficient HTS windings to replace the conventional windings. However, the magnetic field in an HTS transformer affects the transformers HTS winding performances. The relationship between HTS tape characteristics and the magnetic field has been studied. The HTS windings critical current can be increased with the perpendicular magnetic field decreasing. A concept of unbalanced degree is introduced in this paper. The unbalanced degree of critical current distributions at the ends of windings decreases with the perpendicular magnetic field and the unbalanced degree of perpendicular magnetic field distributions decreasing. For the effects of flux diverter, the unbalanced degrees of perpendicular magnetic field and critical current distributions are analyzed and optimized.

The research on derivational topologies and control of switched-capacitor converter

This paper presents a combined topology based on switched-capacitor converter. This topology can adjust output voltage. It eliminates inrush current and has high power density. Through parallel connection, the topology can output high power.

Design and digital control of a cryogenic boost-buck chopper

Cryogenic operation of power electronics provides many promising benefits for power conversions compared to their ambient-temperature counterparts. This paper presents a cryogenic boost-buck chopper integrated with cryogenic MOSFETs and high temperature superconducting (HTS) inductor. The results obtained show that the proposed boost-buck chopper has much higher operational efficiency than the conventional ambient-temperature boost-buck chopper.