Xianrui Huang

Performance of an HTS Generator Field Coil Under System Fault Conditions

High-temperature superconducting (HTS) coils are generally stable against transient thermal disturbances. Protection against spontaneous quenches is not a main design issue for an HTS coil. However, HTS coils used in many electric devices such as motors, generators, transformers, and current limiters will operate under over-current fault conditions, which may result in a coil quench and thermal runaway. Those electric devices should be able to ride through some grid fault conditions and remain functional. This requires a certain over-current capability of the HTS coils. This paper discusses the over-current requirements from grid faults and the thermal transient responses of a BSCCO coil. It presents the analysis results of the coil subjected to over-current pulses at different operating conditions. The study also investigates the thermal runaway current and its relationship with the over-current pulse

Experimental Layer-Wound Mock-Up Coil for HTS MRI Magnet Using BSCCO Tape

Typical coils with BSCCO tape are wound in a pancake or double-pancake style to minimize the strain in the tape by reducing or eliminating the edge-wise bending. Stainless steel reinforced tapes are frequently used in the winding process to increase the strength and reduce the strain due to winding tension and handling. However, an MRI magnet requires high current density in the winding pack. This high current density in the winding pack gives a higher field in the imaging volume and also allows for a reduction in the overall magnet size. Layer winding was preferred for a better tolerance control and for a reduction in the number of joints, which are known sources of resistance and therefore locations of instability in the coil. A mock-up coil was wound using a high-current-density type of BSCCO tape without the typical stainless steel reinforcement. The coil was layer-wound which involved a few inline lap joints embedded in the winding pack. The test of the coil reveals a few issues that need to be addressed. Investigations and analysis lead to a better understanding of the issues. This paper discusses the lessons learned and solutions for using non-reinforced tape in a layer-wound coil, while controlling insulation dimensions within the build.

Iterative EM Design of an MRI Magnet Using HTS Materials

Conventional superconducting MRI magnet electromagnetic (EM) design involves the critical parameters of the magnet field and dimensional requirements, plus the low temperature superconductor (LTS) wire properties. When using HTS material for an MRI magnet, in addition to the conventional characteristics of a superconductor, the wire and the joint resistance need also to be considered. This is mainly due to the relatively low n-values of the BSCCO tape when compared to that of a conventional LTS wire, and it is also due to the more resistive joint nature for HTS joints. This paper discusses an iterative EM design procedure that calculates the resistance of the HTS wire/tape for the entire magnet, which is a strong function of the magnetic fields at different locations in the coils. The resistance is fed back to the regular MRI EM design for optimization, in order to meet resistance target and the ordinary MRI design goals. This iterative design process results in an optimized HTS MRI design with overall low resistance for magnet drifting and power supply requirements.

The Thermal Performance of a 1.5 MVA HTS Generator

A 1.5‐MVA high temperature superconducting ( HTS ) generator of novel design has been designed, built and successfully tested by the General Electric Company. The 1.5‐ MVA generator has served as the engineering prototype for a much larger 100‐MVA beta unit now under design.The HTS coil in the 1.5 ‐ MVA demonstrator is designed to operate in the range of 20–40 K and is cooled with a closed‐cycle helium refrigeration system employing GM type cryocoolers. This paper will discuss the calculation of the thermal loads to the rotor from all anticipated sources. These sources include conduction losses through the coil suspension system, radiative heat loads to the cold‐system components, residual gas conduction losses, helium‐transfer coupling losses and lead losses. These predicted losses were compared to those measured during actual electrical testing of the rotor at 3600 RPM in order to validate the predictive calculations employed for the 100 MVA machine.

An Active Quench Protection System for MRI Magnets

An active quench protection system is developed to safely protect a 1.5 T superconducting MRI magnet. The protection system includes a digital quench detection controller, a set of quench heaters and voltage taps, a battery backed DC power supply and solid-state switches. The quench controller takes the voltage signals from the voltage taps over superconducting coils and monitor a quench onset. To distinguish a quench signal from operational signals, the quench controller uses a pre-determined moving average voltage over 50 ms as the threshold signal for quench detection. The threshold signal is more than 10 times larger than any of the normal operation signals to avoid false quenching. Once detected moving average voltage exceeds the threshold, the quench controller will close solid-state relays and power quench heaters to accelerate the quench propagation over the entire magnet. The system has been tested in a MRI magnet test module.

AC losses in a high temperature superconducting generator

As part of the DOE-SPI funded project a commercial HTS utility-size generator is being developed based on GEs iron core superconducting generator technology. The iron core concept has significant advantages over air core designs. The rotor consists of a cold superconducting field coil and coil supports and a warm iron core, which takes the torque and transmits to the shafts. Heat load due to AC losses in the cold mass of the rotor are a key design constraint. Analyses have been performed both at the component and system levels. AC loss tests have been conducted on an HTS generator. This paper presents and discusses the analytical and test results.

Transient Capability of Superconducting Devices on Electric Power Systems

Superconducting devices operating within a power system are expected to go through transient overload conditions during which the superconducting coil has to carry currents above the rated values. Designing the coil to remain superconducting through any possible fault scenario can be cost prohibitive, necessitating operation beyond the critical current for short periods. In order to set operating limits and design adequate protection systems for superconducting devices connected to a power system, the region of safe operation of these devices has to be described with general capability curves. Existing standards that define limits for these over-current situations are based on copper winding experience that do not apply to devices with superconducting components because of the highly nonlinear interaction between magnetic fields, operating temperature, and current density in the superconductor, and the rapidly varying material properties at cryogenic temperatures. In this paper, the behavior of superconducting coils during over-currents is discussed and a simplified capability curve is described to help standardize device capabilities. These curves are necessary to aid power system designers in appropriately designing the system and associated protection systems.

Quench Test of HTS Coils for Generator Application at GE

When a synchronous generator connected to the power grid experiences a fault, it is required to stay on line, ride through the fault, and be able to carry full rated field current when the fault is cleared. The peak current during these events could be 2 times higher than the normal operating current. This may cause an HTS rotor coil to go into normal state and generate Joule heating. If the fault event is short enough and the heat dumped can be carried away by the cooling system, the coil may recover to the superconducting state at the end of the fault. Otherwise, the coil may thermally run away, or dasiaquenchpsila. To investigate the quench behavior of the HTS rotor coil of the 100 MVA generator at GE Global Research Center, a 1.5 MVA prototype coil was developed and tested to quench under different conditions. The experiment design, set up, tests and test results are presented in this paper.

The Cryogenics of an MRI Demonstrator Based on HTS Technology With Minimum Coolant Inventory Technology

We introduce an advanced and optimized cryogenic cooling concept featuring minimum coolant inventory requirements for small high temperature superconducting (HTS) magnets based on results obtained with an experimental model. Experience gained from these experiments led to a new design that will be experimentally verified by the end of this year. Winding of the HTS magnet has already begun and will be completed shortly. New components, current status and cryogenic scope of this new engineering model are described.

Transient Perturbation to Permanent Magnetic Field by Gradient Pulses in MRI Magnets

Open MRI magnets are generally designed with ferromagnetic poles to contain and shape the magnetic flux and to reduce conductor cost. Permanent magnet MR magnets have blocks of PM and bulk ferromagnetic materials on or close to the pole face. These electrically conducting regions are sources of eddy currents that affect the image quality because of their relatively long time constants and close proximity to the imaging volume. The impact on image quality can be minimized by appropriate segmentation and/or lamination of these components. Detailed eddy current diffusion models are necessary to quantify the field distortion and time constants of the resulting field to perform design studies. The three dimensional frequency or time domain models required to accurately predict effects of eddy currents due to gradient fields are not computationally economical. This paper describes modeling of a PM imaging system using simplified 2D models with appropriate assumptions to evaluate the impact of these eddy currents. Experimental validation of some of the results with a prototype magnet is provided