Sujit Purushothaman

Leakage Inductance Design of Toroidal Transformers by Sector Winding

Toroidal transformers are commonly used in power electronics applications when the volume or weight of a component is at a premium. There are many applications that require toroidal transformers with a specific leakage inductance value. A transformer with a large (or tuned) leakage inductance can be used to eliminate a (series) filter inductor. In this paper, a procedure to control the leakage inductance of toroidal transformers by leaving unwound sectors in the winding is presented. Also, a simple formula is obtained in this paper that can be used to design transformers with a specific leakage inductance value. The leakage inductance formula is expressed as a function of the number of turns, the geometrical dimensions of the toroidal transformer, such as core internal diameter, external diameter, and height, and the angle of the unwound sector. The formula proposed in this paper has been obtained and validated from laboratory experiments and hundreds of three-dimensional finite element simulations. The techniques described in this paper will find applications in the design of transformers that in addition of providing voltage boosting need to double as filters.

Eliminating Subsynchronous Oscillations With an Induction Machine Damping Unit (IMDU)

The IEEE First and Second Benchmark Models for subsynchronous resonance (SSR) are used to analyze the damping properties of an induction machine damping unit (IMDU) coupled to the shaft of a turbo-generator set. This paper investigates the rating and location of the induction machine that, without the aid of any controllers, effectively damps subsynchronous resonance for all line series compensation levels. Eigenvalue analyses are performed on linearized models of the shaft system including the induction machine to find the optimum location. The best location of the IMDU, providing maximum damping, is next to the HP turbine at the end of the shaft. Time domain simulations are conducted to find the adequate rating of the induction machine. It is observed that a small size, high power (about 10% of the generator rating), low energy machine effectively damps SSR. The IMDU reduces peak torques in shaft sections during transients. In the paper, it is demonstrated that the addition of an IMDU at the end of the shaft would have prevented the SSR events of the Mohave Desert shafts.

Heat-Transfer Model for Toroidal Transformers

Toroidal transformers provide increased design flexibility, efficiency, and compact design when compared to traditional shell- or core-type transformers. In this paper, the steady-state thermal analysis for toroidal transformers is conducted using a lumped parameter model which can be applied to small power and distribution-grade toroidal transformers as well. Two cases are considered: 1) when the transformer is kept in open air and 2) when it is installed in sealed enclosures. The detailed model includes the effects of the number of turns of windings, number of layers, insulation properties, and geometric properties of the transformer. The model is capable of finding the hotspots that are of paramount importance for the designer. The model parameters are calculated from the design (geometrical) information; therefore, it is suitable to be included in the design loop of transformer design software. The results are compared with finite-element simulations and lab tests on prototypes of various power ratings fitted with thermocouples to record internal temperatures. The model can also be used with varied external media and encapsulation, such as air, oil, and epoxy.

Closed-Form Analysis of Squirrel-Cage Induction Motors With Anisotropic Modeling of Stator and Rotor

Summary form only given. The paper presents a closed form solution of Maxwells equations in squirrel cage induction motors. The solution is obtained for a cylindrical multi-layer geometry. The squirrel cage is represented by an equivalent anisotropic homogeneous medium. The effect of stator slots and teeth is included by a second anisotropic homogeneous medium. The induction motor is modeled as six concentric cylindrical layers representing the different construction components of the motor. The governing partial differential equations are solved for the excitation source, conducting and non-conducting regions. The obtained formulas allow for efficient calculations of machine performance which may help the motor designer to select the proper parameters of the machine that fit the design requirements. Accuracy of the method is verified against finite element simulations, a commercial motor design program and experimental results.

Thermal Analysis of Cables in Unfilled Troughs: Investigation of the IEC Standard and a Methodical Approach for Cable Rating

A robust algorithm, based on relaxation, is proposed for the implementation of the IEC Standard method for rating cables installed in unfilled troughs. Through hundreds of finite-element simulations, the validity range of the standardized equations is established. Studies are performed by varying the following parameters over a wide range: trough size, ambient air temperature, trough aspect ratio, position of cables, cable operating temperature, and intensity of solar radiation. A physically consistent analog thermal-electric equivalent circuit is proposed for the thermal rating of cables installed in unfilled troughs. In contrast with the standards, the equivalent circuit offers a methodological approach that considers all heat-transfer phenomena involved in cables in troughs, for example, the conduction of heat through the cable layers, the heat convection and radiation inside the trough, the conduction in the trough itself and soil, the convection to the surface air, and the solar radiation. Extensive finite-element verification in steady state and transients demonstrates the accuracy of the proposed equivalent circuit.

Identifying root causes of failures in small electric motors

Accurate identification of root causes helps not only to prevent reoccurring failures but also the efficacy of the insurance claims settling process. Hence FM Global Research has undertaken a study to identify root causes of failures in small electric motors. The study involves a detailed literature review and the development of the Lightning-induced overvoltages (LIOV) matrix. This matrix was developed to evaluate the damage potential of individual indirect strikes by accounting for strike current magnitude and strike distance. This paper provides guidance to aid in identifying the root causes of motor failure under the categories: 1. Electrical insulation aging, 2. LIOVs, or 3. Artificial surges.

GADS-based loss exposure evaluation of transformers

Transformers have been one of the major contributors of loss exposure to power generation units. This study aims to better understand the loss exposure associated with this critical equipment. The study utilized the GADS (Generating Availability Database System) database to analyze transformer-related outage data. Outage data of the last 10 years from the GADS database were extracted as a function of design and operation-related variables such as transformer type, high side voltage, MVA rating, unit loading, and age of the unit. The data were analyzed in different groups considering the types of parameters or ranges of values for each variable. The analysis included the generation of F-N curves based on frequency and risk, and the identification of trends and causal relationships for the observed trends wherever possible. Analyses were also performed for the combinations of important variable groups, for example, transformers of a specific age group and a specific type of loading. All the analyses were performed for the three types of PowerGen units, viz., fossil units, gas turbine units, and hydro units. A summarized list of observations related to the effect of different variables on outages is presented.

Closed form analysis of squirrel cage induction motors with anisotropic modeling of stator and rotor

This paper presents a closed-form solution of Maxwells equations in squirrel-cage induction motors. The solution is obtained for a cylindrical multilayer geometry. The squirrel cage is represented by an equivalent anisotropic homogeneous medium. The effect of stator slots and teeth is included by a second anisotropic homogeneous medium. The induction motor is modeled as six concentric cylindrical layers representing the different construction components of the motor. The governing partial differential equations are solved for the excitation source, conducting, and nonconducting regions. The obtained formulas allow for efficient calculations of machine performance which may help the motor designer to select the proper parameters of the machine that fit the design requirements. Accuracy of the method is verified against finite element simulations, a commercial motor design program, and experimental results.

Eliminating sub-synchronous oscillations with an induction machine damping unit (IMDU)

The IEEE First and Second Benchmark models for Sub-Synchronous Resonance (SSR) are used to analyze the damping properties of an induction machine damping unit (IMDU) coupled to the shaft of a turbo-generator set. This paper investigates the rating and location of the induction machine that, without the aid of any controllers, effectively damps subsynchronous resonance for all line series compensation levels. Eigenvalue analyses are performed on linearized models of the shaft system including the induction machine to find the optimum location. The best location of the IMDU, providing maximum damping, is next to the HP turbine at the end of the shaft. Time domain simulations are conducted to find the adequate rating of the induction machine. It is observed that a small size, high power (about 10% of the generator rating), low energy machine effectively damps SSR. The IMDU reduces peak torques in shaft sections during transients. In the paper, it is demonstrated that the addition of an IMDU at the end of the shaft would have prevented the SSR events of the Mohave Desert shafts.