Evolution of the Hendrix Line Post Insulator

Introduction

The first pole-top insulators were invented to support telegraph lines in the 1840’s. They were made of glass and functioned both as mechanical support of the wire and to insulate it from the wooden pole and ground plane. The first insulators used for power lines were installed in Manhattan and energized in 1882. As with the earlier telegraph insulators, they were designed to provide mechanical support and to insulate the conductors from supporting structures. The earliest of these were made of glass, although ceramic materials were soon introduced. Glass has excellent electrical properties (higher dielectric strength and more uniform voltage distribution than porcelain), but suffers from potential breakage from sudden physical impacts or vandalism. Porcelain, being a ceramic material, soon became the standard for electric power lines due to its greater strength and ease of manufacturing.

High Density Polyethylene (HDPE) Introduction

HDPE was introduced in the mid-60’s as a material for pin-type insulators. While HDPE has several outstanding properties making it ideal for electrical insulator applications, it was initially introduced for an entirely different reason. The decade of the 1950’s saw the rise of aerial covered conductor, both in Spacer Cable Systems and Tree Wire Systems. The conductor turns angles on insulators, and early systems utilized standard porcelain pin-type insulators. The issue with the porcelain insulators is that they have an inherent dielectric incompatibility with the HDPE covering on the covered conductor. With two dielectric materials in series, there is an impedance discontinuity as the electric field passes from the conductor, through the HDPE covering, and then onto the insulator. Porcelain has a dielectric constant of around 6.9, while the HDPE has a dielectric constant of 2.3. This essentially sets up a voltage divider circuit between the conductor itself and the ground (grounded insulator pin in this case). As dielectric materials are essentially capacitors, the voltage divides in inverse proportion to the impedance. This means that with a porcelain insulator, the majority of the voltage drop is forced across the cable covering, rather than across the insulator. Furthermore, this creates a partial discharge (PD) at the interface between the covered conductor and the porcelain insulator, leading to breakdown of the conductor’s covering. While for a 15 kV system this may not result in failure for decades, at higher voltages the PD can create problems much sooner. At 34.5 kV, failures due to dielectric incompatibility can occur in as little as six months. Considering the dangers of dielectric incompatibility, while a reduction in reliability is problematic, what is entirely unacceptable is the increase in surface current as the PD degrades the insulation, as this will pose a safety hazard for line workers and bystanders alike should they come into contact with the energized conductor.

The solution to the problem is to replace porcelain with HDPE. This creates a smooth voltage drop profile across the conductor covering and insulator itself, eliminating the dielectric incompatibility and attendant destructive PD.

HDPE insulators, in general, are lighter weight than their porcelain counterparts, have excellent UV inhibition, are intrinsically hydrophobic, and are resistant to impact damage, breakage (in shipping and handling), and vandalism. The weight issue is particularly relevant. Porcelain line post insulators, at 34.5 kV, can weigh as much as 18 lbs., compared to an HDPE line post insulator at under 7 lbs. The weight reduction is important since line workers carrying heavy insulators into the workspace are under threat of personal injury from the heavier porcelain units.

Vise Top Line Post Insulators

The next step in line post insulator evolution was to add a clamp on the top of the insulator to secure the conductor. The same benefits exist as for the tie-top line post insulators, such as light weight, ease of installation, increased line worker safety, and the ability to use with both bare wire lines and covered conductor lines. The additional benefit of the clamp-top model accrues from the time and effort saved by not needing to install the tie. The conductor is simply placed on the insulator, the clamp closed, and the installation is complete. Similarly, the clamp-top insulators conform to ANSI C29.5 and C29.6 standards and can be utilized for any application that calls for a pin-type insulator.

Replacing the hand tie or preformed tie with a clamp-top provides major savings in labor. We can break this down to the following. Savings of as little as ten minutes per insulator (putting conductor into clamp-top vs making a hand tie or installing a preformed tie), provides a labor savings of 30 minutes per pole, which is considerable – reducing costs and increasing throughput.

Summary & Conclusions

Hendrix HDPE composite line post insulators are much lighter (up to 70% lighter for 69 kV applications) and are resistant to chipping, cracking or breaking. Linemen prefer composite insulators because they are easier to handle and easier to install, while avoiding concerns for damage during shipping, handling and installation. The electrical characteristics far exceed the specifications for porcelain insulators as well as ANSI Standards requirements. The extremely high and naturally intrinsic hydrophobic properties of HDPE provide reliability far superior to similar voltage class porcelain insulators.

The evolution of polymeric composite insulators culminating in the Tie-Top, Clamp-Top and Trunnion Style line post insulators has motivated electric utilities performing evaluations and seeking approvals for new products. Utilities are inherently slow to change (the standard refrain being “If it ain’t broke, don’t fix it.”), and each utility has different price points for different products. Tie-top insulators have the lowest up-front material cost which seems to suggest they are the cheaper option, but require purchase of other parts (conductor hand tie or preformed tie, and possibly also armor rods) as well as additional labor (making the tie). The bottom line is that tie-tops have a higher installed cost and also introduce room for workmanship errors. Trunnion-style line posts are a mid-price option and offer more movement and flexibility for the conductor with the suspension clamp, but they also require the purchase of an additional part (the trunnion). However, the trunnion style is a little faster and more forgiving during installation. The clamp-top line post insulator has the highest up-front cost, but requires no additional part purchases. It is easy and quick to install and allows more flexibility for the conductor. These factors combine to make the clamp-top line post insulator the option with the lowest installed cost.

Where is the HDPE insulator evolution leading us? Polymeric compounds can be modified to provide a variety of useful characteristics by utilizing additives. Insulator models produced from compounds tolerant of high temperatures are already on the market, allowing continuous operation at 200°C. This comes in handy when legacy lines are called upon to carry heavy loads during transfer periods, which can elevate conductor temperatures above standard operating temperature norms. Similarly, HDPE insulators are produced from compounds designed for use in fire retardant applications for which an insulator ignited by a passing wildfire will self-extinguish after the heat source is removed. As anticipated and observed changes occur in our power grid operating environments, polymers will no doubt take a lead role in advancing line post insulators and their applications.