Lalichan Andrews

Application of Computational Fluid Dynamics to Reduce the New Product Development Cycle Time of the

This paper describes thermal modeling, using computational fluid dynamics (CFD), to predict the temperature rise of a high-voltage SF6 gas circuit breaker during a heat-run test. In electrical equipment, maximum temperature rise in various components is limited by their material properties and, hence, the heat-run test forms part of type test. A methodology to predict the temperature rise at the design stage is developed using ANSYS-CFX and found to be beneficial to reduce the number of heat-run tests during the product development process. The developed methodology has been implemented on a 72.5-kV SF6 gas circuit breaker and validated with the experimental temperature rise results of the existing design at two current ratings: 1000 A and 2000 A. The methodology was further used to predict thermal performance of the breaker at a higher current rating of 3150 A. The prediction helped to understand temperature rise in various components and hot-spot locations. Various design modifications were then tested virtually to evaluate the temperature rise. Thereafter, the final modified design where the temperature rise of all the components was within the permissible limits and which also satisfied other constraints of manufacturing feasibility and cost was fabricated and physically tested. Good agreement is found between the CFD and experimental results.

{\rm SF}_{6}

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.

Gas Circuit Breaker

Vacuum Interrupters could be connected in series to increase the voltage withstand capacity. This series connected vacuum interrupters would especially be useful for application of vacuum circuit breakers at transmission voltages (145 kV, 245 kV). An important aspect of series connected vacuum interrupters is sharing of voltage across the two gaps. This paper presents results of power frequency and impulse experiments on series connected vacuum interrupters.

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

Concept of series connected vacuum interrupters

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.

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

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.

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

In the vacuum interrupters employing the radial magnetic field (RMF) type of contacts, the constricted arc column is made to move on the surface of the contacts. The motion of the arc ensures that no particular area of the contact surface is overheated thus minimizing the emission of metal vapor and temperature rise of the contact surface and hence enhancing the current interruption performance. The arc motion is the result of the interaction of the arc current and the magnetic field produced by the flow of the current through the contacts. The velocity of the arc motion would thus be governed by the magnitude of arc current and the design of the contact. Higher the velocity of motion of the arc, higher would be the probability of a successful interruption. A wide range of the values of this velocity and the empirical formulae for the velocity have been reported in the literature. This paper reports the results of a research work which aims to arrive at the value of the arc velocity through experimentation and also through a modeling-simulation approach. The research also aims to establish a correlation between the arc velocity, magnitude of current and the contact design. This correlation is also presented in this paper.

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

The vacuum interrupter is extensively employed in the medium voltage switchgear for the interruption of the short-circuit current. During the interruption process an arc is formed between the opening pair of the contacts. A metal shield is used to protect the ceramic from the metal vapours depositing on it during arcing. A voltage gets induced in the shield during the arcing process. The magnitude of this voltage could be an indication of the arc mode and arc motion, which would in turn give indications of the performance of the contacts of vacuum interrupter. This paper investigates the relationship of the shield potential with the arc voltage, arc current, travel of the moving contact, type of contact and the vacuum interrupter geometry. Through this study the paper would explore the utilization of shield potential as a diagnostic parameter during short-circuit evaluation of the vacuum interrupter.

Estimation of the velocity of arc motion in vacuum interrupters with radial magnetic field type of contacts

The Vacuum interrupter is the arc extinction chamber of the vacuum circuit breaker which is widely used in the medium voltage networks for interruption of short -circuit current. In the field of vacuum switching devices, the contact material is the most important parameter determining the switching behavior. During arcing, the contacts are eroded and hence there is a change in the surface microstructure. The comparison of the microstructure before and after short circuit operations reveals information about the effect of the arc on the contacts. The microstructure properties like grain structure of the parent metals, the distribution of the grains and grain boundaries in the contact are important in the current interruption process. These properties are studied through the scanning electron microscope (SEM). Even the effect of the contact manufacturing process on the above mentioned properties can be studied through the SEM analysis. This paper compares the post short circuit current microstructures of the similar contacts subjected to different values of short circuit current. The paper also compares the microstructures of contacts manufactured with different processes and subjected to the same short circuit current regime. The paper concludes by establishing the correlation between the micro structural properties and the arcing performance of the contacts of the vacuum interrupters.

Study of shield potential to understand the arcing behaviour of vacuum interrupters during short-circuit current interruption

A vacuum interrupter utilises a magnetic field for effective arc extinction. Based on the type of field, the vacuum interrupters are classified as radial or axial magnetic type of vacuum interrupters. This paper focuses on the axial magnetic field type of vacuum interrupters. The magnitude and distribution of the axial magnetic field is a function of the design of the contact system. It also depends on the orientations of the movable and fixed contact systems with respect to each other. This paper investigates the dependence of arcing and erosion performance of the contact on the magnitude and distribution of this axially oriented magnetic field. The experimental observations are well supported by electromagnetic simulations.