Step-lap Joint Research Articles

Reducing losses in distribution transformers

This paper examines three methods of reducing distribution transformer losses. The first method analyzes the effects of using aluminum electromagnetic shields in a distribution transformer. The goal of placing electromagnetic shields in the distribution-transformer tank walls is to reduce the stray losses. A 500 kVA shell-type transformer was used in the experiments. The overall results presented indicate that stray losses can be considerably reduced when electromagnetic shielding is applied in the transformer tank. In the experiment, the tank walls were lined with aluminum foil. The possibility of reducing the dielectric losses was shown through experiments in the second method. The third method of this work analyzes the behavior of wound-cores losses in distribution transformers, as a function of joint configuration design parameters. The joint configuration used in this paper is called step-lap joint.

Reducing Losses in Distribution Transformers

This paper examines three methods of reducing distribution transformer losses. The first method analyzes the effects of using aluminum electromagnetic shields in a distribution transformer. The goal of placing electromagnetic shields in the distribution-transformer tank walls is to reduce the stray losses. A 500 kVA shell-type transformer was used in the experiments. The overall results presented indicate that stray losses can be considerably reduced when electromagnetic shielding is applied in the transformer tank. In the experiment, the tank walls were lined with aluminum foil. The possibility of reducing the dielectric losses was shown through experiments in the second method. The third method analyzes the behavior of wound-cores losses in distribution transformers as a function of joint configuration design parameters. The joint configuration used in this paper is called step-lap joint.

A study of characteristic analysis of the three-phase transformer with step-lap wound-core

In this paper, the analytic algorithm using equivalent magnetization curves is proposed for analysis of the three-phase transformer with wound-core in consideration of a step-lap joint or noncut structure. Influences of differences in step-lap joints and noncut structures on equivalent magnetization curves are examined using the algorithm. Influences on the exciting currents and core losses of the three-phase transformer are also examined.

Magnetic flux distributions in transformer core joints

This paper presents results of a rigorous analytical study performed in order to understand the role which joints play in determining the performance of cores of power and distribution transformers. The paper presents the details of the magnetic flux distribution and redistribution in the core joint region along the core stack height direction as a function of the operating flux density, core joint material, joint configuration, and joint design parameters. The results are presented for both conventional and step-lap joints. The results illustrate the benefits of the step-lap joint in reducing core losses, noise and excitation current. The results also explain the impact of overlap length, the number of laminations per step, and the contribution of the joint region to the whole core performance. The calculated flux density wave shapes in the joint region were found to be in good agreement with measured wave shapes. The results presented in this paper have been used in developing an accurate calculation of core losses for both power and distribution transformers. They have also been used to arrive at the optimum core joint configuration for a transformer core.

Moisture absorption effect on adhesive filled bonded joints

AbstractThe effect of filler type and content on the performance and durability of adhesive bonded joints upon exposure to damp heat (water immersion) has been investigated using concurrently step lap shear and thermo mechanical analysis (TMA) experiments. A TMA based approach is developed in order to predict the strength of the bonded constructions. Two types of fillers were used in order to achieve the objectives: glass beads (GB) and alumina ceramic powder (ACP). It is shown that ACP filled adhesives have higher strength retention than GB filled ones. It is also demonstrated that valuable information can be extracted from TMA measurements. Generally, the increase of the coefficient of thermal expansion (CTE) of the bonded joints is found to be in agreement with the decrease of the step lap joint strengths after exposure. The increase of the CTE of the bonded joints resulting from exposure to hot water seems to increase the stresses, which in turn affect the overall strength. Similarly, bulk adhesive results show that GB filled systems are more susceptible to water than ACP filled ones, which confirms the decrease in strength of GB filled bonded constructions.

Improvement of transformer core magnetic properties using the step-lap design

Magnetic properties of the step-lap joints have been investigated experimentally on two three-phase three-leg transformer cores. Using the step-lap joint design, a reduction of the total core loss of 2 to 4.4% and of the exciting power of 31 to 37% has been obtained.

An adhesive bond strength prediction algorithm based on ultrasonic signal analysis

Abstract The problem of predicting adhesive bond defects for both surface preparation and undercure defects has been studied using an ultrasonic, experimental test bed system. This experimental test bed incorporates the ultrasonic and computer equipment necessary to acquire and process data from various types of adhesively bonded test specimens. The computer hardware and software have been developed to allow the design of reliable pattern recognition algorithms for the evaluation of surface preparation and bond cure. The specific problem studied is the inspection of the adhesive bond in an aluminium/aluminium step-lap joint whose strength could be affected by improper surface preparation or undercure. A set of 164 bond specimens was used to design an algorithm that is 91% reliable for separating the specimens into a good class, those bonds with no defects, or a week class, bonds with poor surface preparation on an undercured adhesive layer. A Fisher Linear Discriminant function was selected by the test bed as the best pattern recognition routine for this classification problem.

An Ultrasonic Evaluation and Quality Control Tool for Adhesive Bonds

The problem of predicting adhesive bond performance for both surface preparation and undercure defects has been studied using an ultrasonic, experimental test bed system. This experimental test bed incorporates the ultrasonic and computer equipment necessary to acquire and process data from various types of adhesively bonded test specimens. The computer hardware and software has been developed to allow the design of reliable pattern recognition algorithms for the evaluation of surface preparation and bond cure. The specific problem studied is the inspection of the adhesive bond in an aluminum to aluminum step-lap joint whose strength could be affected by improper surface preparation or undercure. A set of 154 bond specimens was used to design an algorithm that is 91% reliable for separating the specimens into a good class, those bonds with no defects, or a weak class, bonds with poor surface preparation or undercured adhesive layer. A Fisher Linear Discriminant function was selected by the test bed as the...

Excitation current and power loss characteristics for mitered joint power transformer cores

The 45° mitered-overlap joint is commonly used in the construction of large power transformer cores. There are several variables which affect the performance of such cores with respect to core loss and exciting current. The major variables investigated in this paper are 1) overlap distance, 2) stagger layer thickness, and 3) overlap method, namely, simple staggering versus step-lap. Also investigated to a lesser degree were the effect of frequency and the effect of series air gaps at the corners. One set of results in this paper is not in agreement with a previous paper and reasons for this are speculated. The other sets of results are new, and indicate that staggering beyond two laminations per stagger is not advisable, that the difference between one and two laminations per stagger is marginal, and that the use of a step-lap joint reduces exciting current requirements but not core loss.