Breakthrough in Pole Type Distribution Transformer Bushings

The electric power industry is facing challenges related to increasing demand and climate change, both dictating a significant migration to renewable energy applications. In some applications, this market shift has prompted new approaches to component designs to meet the new challenges of this technology. Achieving cost-effectiveness while considering all variables in the supply chain is crucial. Central to this chain are distribution transformers and their components, which play an essential role.

These demands drive the development of new equipment and components aimed at enhancing durability, ease of operation, and reducing carbon footprint. Therefore, there is a pursuit to develop new components for transformers and electrical equipment in accordance with international technical standards, to innovate new technologies and raw materials, and to enhance quality standards to the highest level.

Keywords: Epoxy high voltage distribution transformer bushing – Pole type distribution transformer bushing – Leaks in distribution bushings - Bushings Applied to Liquid-Immersed Distribution Transforme

Introduction

The porcelain high-voltage bushing has been widely used in pole-type distribution transformers under IEC and IEEE standards. They offer a reasonable cost-benefit ratio; however, they can also raise concerns regarding the fragility of the porcelain and mechanical performance due to shipping, handling, and installation. In some cases, some users have reported a high radio interference voltage. In addition, performance requirements dictated by the market continue to advance, in some cases surpassing the capabilities of porcelain bulk bushing designs to deliver performance at these enhanced levels. In response to these market developments, the evolution of epoxy formulations for electrical applications has been aggressive in recent years, prompting an increased use of epoxy electrical components in the renewable energy industry where superior dielectric, mechanical and thermal performance is paramount.

Capitalizing on the evolution in epoxy material performance, this knowledge was applied to distribution components culminating in an epoxy HV bushing design for use in distribution transformers. This document describes the mechanical and dielectric performance of epoxy bushings while also highlighting their impact on the environmental footprint.

Bushing leaks

A study provided by NEETRAC, National Electric Energy Testing, Research & Applications Center, shows that approximately 86% of 29 utilities surveyed, have a level of concern regarding the quality of the distribution transformers. One notable concern identified in the study is the occurrence of leaks [1].The epoxy distribution bushing is designed to minimize potential leakage points; the conductor is integrally cast into the epoxy body, limiting possible leak paths to the bushing flange/tank interface and the conductor to epoxy body interface. All potential leak points related to gasketed terminal assemblies are removed. As there is no assembly, leak risk related to improper assembly, or damaging manipulation in the field is mitigated. The epoxy material is more durable than porcelain and is capable of withstanding more rough handling through shipping and installation.

The one-piece design with mounting nut offers ease of installation, thus reducing installation labor and maintenance expenses incurred throughout the life cycle of the transformer.

Dielectric performance

The porcelain distribution bushing is generally manufactured with a copper wire conductor inside of a porcelain insulator, and the primary insulation is air and, in some cases, in combination with oil.  In the case of epoxy bushings, the high-voltage conductor is in direct contact with the epoxy body. No air gaps or supplemental insulation exists within the insulation cross-section. 

The result of an electrical field simulation is shown in Figure 1; it reflects how the electrical field is affected by the medium between the conductor and the porcelain/epoxy body. There is a visible concentration of electrical field in the air gap between the conductor and the porcelain body. In the case of an epoxy bushing, the electrical field is entirely distributed across the epoxy insulation cross-section.

The concentration of equipotential lines across the air gap in the porcelain bushing can result in increased partial discharge activity and potential reduction in insulation performance due to overvoltage and voltage fluctuations. In contrast, the epoxy bushing exhibits improved performance during electrical transients such as harmonics or voltage peaks, attributed to its more homogeneous electrical field distribution.

Mechanical performance and footprint

The compact design of the pole-type distribution bushing provides opportunity for optimization of tank designs and consequently, oil volumes, where the active part is not a limiting factor, notably in single-phase distribution transformers of low capacity.

In addition to the improved mechanical performance of epoxy bushings in application, it should be noted that the robust nature of the epoxy insulation is highly resilient. This material property offers vast improvements vs. porcelain where equipment hardening against vandalism is a concern.

Epoxy performance 

The cycloaliphatic epoxy formulation is ideally suited to these applications. The material provides excellent hydrophobic and surface tracking characteristics and carries a UL-94 V-0 flammability rating allowing for excellent performance in aggressive atmospheric conditions. 

Typical contaminants in application are airborne particulate, fog, and salt in coastal areas. These contaminants can lead to surface tracking due to the action of electric discharges on or close to the insulation surface. Inclined plane tracking testing per ASTM D2303 [8] yields a very robust surface tracking rating of greater than 900 minutes at 3.5kV, making the epoxy material an ideal solution in these environments. 

The epoxy material carries a UL 94 V-0 flammability rating per ASTM D635 [9]; this standard describes the method to test the rate of burning and/ or the time of burning epoxy materials. This method is technically equivalent to IEC 60695-11-10. This standard is intended to measure and describe the response of materials, products, or assemblies to heat and flame.

Environmental Impact 

As summarized in Section I of this document, a one-piece epoxy bushing design significantly reduces the number of potential oil leak paths, thus limiting the likelihood of oil leakage into the environment in the field. 

Additionally, the epoxy bushing design offers a significant  weight reduction as compared to typical porcelain bushings of the same voltage class (0.8kg for epoxy vs. 2.5kg for porcelain). This weight reduction provides a smaller carbon footprint and reduced cost in transit. Based on the average freight truck in the U.S., it emits 161.8 grams of CO2 per ton-mile. Using the same numbers on the weight evaluation, 1.7 kg less weight and 1,042,992 units of liquid-immersed, medium-voltage, single-phase transformers. And an average distance traveled of 2000 km. The total savings of carbon footprint is 287 metric tons.

An estimation made by Environmental Defense found that pollutant gases resulting from transportation, in correlation with the weight, amount to 161.8 grams of CO2 per ton-mile.

In general, the epoxy distribution high-voltage bushing has demonstrated how grid resilience can be improved using the latest technology developed for renewable energy applied in the distribution grid network.

References

[1] IEEE PES Transformer Committee Fall 2018. Distribution Transformer Quality Assurance — Technical Presentation — by Thomas Champion, Yamille E. del Valle, F. Dean Williams.

[2] IEEE Std 693 - Recommended Practice for Seismic Design of Substations. 2016.

[3] IEEE Std C57.12.80 - IEEE Standard Terminology for Power and Distribution Transformers. 2010

[4] DOE - Energy Efficiency and Commercial and Industrial Equipment – Distribution Transformer December 2022.

[5] Environment Defense Founds Organization.

[6] IEEE Std C57.12.00 Standard for General Requirements for Liquid Immersed Distribution, Power, and Regulating Transformers. 2021

[7] IEEE Std C57.19.02 Standard for Design and Performance Requirements for Bushings Applied to Liquid-Immersed Distribution Transformers. 2023.

[8] ASTM D2303 Standard Test Methods for Liquid-Contaminant, Inclined-Plane Tracking and Erosion of Insulating Materials. 2004.

[9] ASTM D635 Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position. 2022.

Mark Pattison  
Product Manager 
The H-J Family of Companies  
Saint Louis, Missouri, USA 

Jeff Door 
Vice-president R&D 
The H-J Family of Companies  
Saint Louis, Missouri, USA  

Amer Krvavac 
Sales Project Manager 
The H-J Family of Companies 
Saint Louis, Missouri, USA 

Jose Gamboa
Design Engineering Manager
The H-J Family of Companies
Saint Louis, Missouri, USA

Barry Beaster
Engineering Consultant
The H-J Family of Companies
Saint Louis, Missouri, USA

Ali A. Ghafourian
Regional Sales Manager
The H-J Family of Companies
Saint Louis, Missouri, USA

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This article was originally published in the June 2024 issue of the Transformers Technology magazine, which you can access here.

To download the PDF version of this article, click here.