Sandeep Kulkarni

Development of 440-V 800-A Resistive-Type Modular Superconducting Fault Current Limiter With YBCO Tapes

CG Global R&D is developing a resistive-type superconducting fault current limiter (SFCL) under the Ministry of Power, National Perspective Plan for R&D in Indian Power Sector. A resistive-type 440-V 800-A single-phase SFCL is developed using YBCO tape in stage I of the project, and an 11-kV 1250-A three-phase SFCL will be developed in stage II. The 440-V 800-A SFCL is assembled with seven parallel modules. Each module has series-connected YBCO tapes to accommodate the design length. The SFCL is tested at 77 K for continuous current of few hours, fault currents at low voltage, and fault currents at 440 V up to 100 ms (5 cycles). The evaluated data will be used to develop an 11-kV 1250-A three-phase SFCL in stage II.

Effect of eddy currents on the current interruption performance of axial magnetic field type of vacuum interrupters

In the axial magnetic field (AMF) type of vacuum interrupters (VI), the design of their contact system is such that the magnetic field is oriented parallel to the arc. AMF keeps the arc diffused, and the value of the current-up to which the arc stays diffused depends on the contact diameter and the magnitude of the axial field. A certain minimum AMF is required to keep the arc diffused. AMF also induces eddy currents in the contact discs. This paper analyses the effect of eddy currents induced in the discs on AMF between the contacts, especially near the current zero region. This analysis incorporates the opening motion of the contacts along with the variation of the coil currentwith time. Initially, a 2-D axis symmetric model of the AMF contact is simulated to determine the radial variation ofthe magnetic field along a plane mid-way between the two separating contacts. A 3-D model of the AMF contact is then analyzed to include the effect of radial slots in the contact plate. The change in the magnetic field profile with the number of slots is studied.

A Resistive-Type Modular Superconducting Fault Current Limiter Fabricated With Stainless-Steel-Stabilized YBCO Tapes

A resistive-type superconductor fault current limiter (SFCL) is experimentally studied with stainless-steel-stabilized 12-mm-wide YBCO tapes. The SFCL is assembled with six noninductive modules, each rated for 120 V, 100 A. The module has several YBCO tapes connected in series that are arranged such that the flow of current in one tape is in the opposite direction from its neighboring tapes. The modules can be connected for different SFCL configurations with series and parallel combinations. In this paper, the SFCL is assembled in two configurations and is tested for continuous, overload, and short-circuit currents. The first configuration is assembled to evaluate current carrying performance in parallel connected modules at 600 A. The second configuration is assembled for short-circuit performance of two series-three parallel connected modules at 240 V, 300 A. The evaluated data will be useful to develop higher rating SFCL with modular construction.

Functionally graded copper chromium based vacuum interrupter contact tip and its interruption ability

Copper-Chromium (CuCr) based contact material is widely used for vacuum interrupter (VI) contacts and has found worldwide acceptance in medium-voltage VIs, especially for high-current interruption. Contact material with chromium content between 25 and 50 wt. % is almost exclusively used. Conventionally, copper and chromium are uniformly distributed throughout the body of the contact. In a few cases, contacts with grading of the material along the thickness have been used. This paper presents a new concept of functionally graded electrical contact which consists of CuCr50 in the bulk region and the remaining petal region consists of CuCr25. The aim of this grading is to improve the interruption ability with optimum erosion characteristics. The paper further reports the results of the investigations of switching behavior of VI with these functionally graded contact tips. The results indicate that the higher thermal and electrical conductivity as well as anti-welding properties at the required locations of the developed functionally graded contact material improved the interruption ability of the VI. The results of the switching tests are seconded by the results of the microstructural analysis (SEM/EDS) of the arced and un-arced contact tips.

Conceptual Design of a 440 V/800 A Resistive-Type Superconducting Fault Current Limiter Based on High

A conceptual design of a high-Tc-based resistive-type superconducting fault current limiter (SFCL) using second-generation coated conductors (CCs) has been carried out in this paper for possible power applications in India. In the design aspect, noninductive SFCL coils made with stabilizer-free CCs have been used for analytical calculations. In electrothermal analysis, a short-circuit characterization has been simulated to get the maximum resistance and temperature achieved during fault limitation in a 440 V/800 A single-phase SFCL. Furthermore, the recovery under no load is studied, and recovery time is calculated.

T_{c}

The vacuum interrupter is extensively employed in the medium voltage switchgear for the interruption of the short-circuit current. The voltage across the arc during current interruption is termed as the arc voltage. The nature and magnitude of this arc voltage is indicative of the performance of the contacts and the vacuum interrupter as a whole. Also, the arc voltage depends on the parameters like the magnitude of short-circuit current, the arcing time, the point of opening of the contacts, the geometry and area of the contacts and the type of magnetic field. This paper investigates the dependency of the arc voltage on some of these parameters. The paper also discusses the usefulness of the arc voltage in diagnosing the performance of the contacts.

Coated Conductors

The Copper-Chromium (CuCr) contact material is the most widely used material for the contacts of the vacuum interrupters (VI). Performance of the VI is greatly influenced by the properties of contact materials. During switching operations, such as short-circuit current interruption, a vacuum arc is generated which in turn produces melted and resolidified layers at the contact surface. Melting followed by welding of CuCr contact results the failure of Vacuum interrupter. This paper correlated the switching performance and the metallurgy of the contacts, specifically in cases where the contacts have welded. The paper reports important factors like microstructure, Cr distribution, conductivity, hardness etc. which significantly affect the welding performance of the contacts. The conductivity and hardness of the contacts were measured by using an eddy current based conductivity meter and Vickers hardness tester respectively. The failure of contacts because of welding has been discussed with respect to conductivity and distribution of Cr as seen in its microstructure. The contacts were also characterized by X-ray diffraction and scanning electron microscopy to correlate the data.

An investigation into the dependence of the arc voltage on the parameters of the short-circuit current and the design of the vacuum interrupter

In the Radial Magnetic Field (RMF) type of Vacuum Interrupters (VI), the contacts are so designed that a magnetic field distribution, oriented in the azimuthal direction to the arc, is produced. The interaction of the arc current with this field produces a circumferential force on the arc. This force rotates the arc, thus minimizing the heating of the contact surfaces. The speed of the rotation depends on the value of the magnetic field. This paper deals with the computation of the magnetic field distribution between spiral RMF VI contacts. An analytical formula is derived for the magnetic field distribution by approximating the limbs of the contact as finite current sheets, using Amperes law. Taking advantage of the fact that the limbs are perpendicular to each other, they contribute to mutually orthogonal components of the magnetic field in a plane parallel to the contacts, and thus the net magnetic field can be easily calculated by taking their norm. The results obtained using the analytical formula are validated by 3D FEM analysis of the spiral contacts using commercial software.

Failure analysis of CuCr contacts of Vacuum interrupter

A two-dimensional axi-symmetric numerical model is developed in order to understand the characteristics of diffused vacuum arc. Vacuum arcs mainly consist of metal vapors which are generated by local evaporation of the contact material due to higher current densities. Strong coupling and high gradients between many parameters like, current densities, temperatures, electrical conductivity, magnetic field and high velocities of plasmas pose many challenges in modeling vacuum arcs. Present work consists of solution of two-fluid equations for electrons and ions. In addition to fluid equations, Maxwells equations are solved to obtain the Lorentz force and Joule heat which are implemented as source to momentum and energy equations respectively. Simulation results show the variation of the ion and electron number densities, their temperatures, velocities and magnetic flux.

On computation of the radial magnetic field in vacuum interrupters

CG Global R&D is currently working on the design and validation of resistive-type superconducting fault current limiters using second-generation-YBa2Cu3O7 coated conductors (YBCO CC). Noninductive modules are made using several series-connected YBCO tapes to accommodate the required design length. Copper clamp-type mechanical bridge joints are employed between these YBCO tapes. These joints are made by sandwiching two parallel tapes between copper plates. This type of configuration gives additional ruggedness to the joint compared to soldered superconducting bridge joints with the superconducting bridging element. The dc and ac resistive characteristics of these joints are studied as a function of the operating current. The ac losses in the YBCO tape are also measured as a function of applied current with line frequency of ~50 Hz.