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Future Technology to Improve Safety, Security, Resiliency, and Reliability

Presented By:
Randy Williams
ABB Inc.
TechCon 2017

Abstract

This paper addresses the trend of the US market from oil to dry type condensers and porcelain to silicone insulators on condenser bushings to address the US Utility and Industrial end users concerns with safety, security, resiliency, and reliability.

Introduction

Grid blackouts as well as recent physical attacks on power stations have made everyone aware of the vulnerability of key components in the electric transmission system. The assumption of electricity always being available in our daily lives is a given. Thus, electric demand continues and an effective way to move power into major metropolitan areas is essential, or the system will become unstable. The industry is expending considerable resources into the protection of the system – from both a cyber and physical perspective. Those efforts are essential to improve the resiliency of the system.

What Is a Bushing?

First, for clarification, we must establish what bushings are in definition and description.

The Institute of Electrical and Electronics Engineers (IEEE) defines a stud or bulk type bushing as “a non-capacitive graded bushing in which the major insulation is provided by a ceramic or analogous material placed around the energized conductor.” Many bulk type bushings are considered a low-voltage bushing and are utilized at 15 kV through 34 kV applications on Power Transformers. Refer to Figure 1a for a bulk type bushing profile.

The definition of a condenser type bushing or capacitive graded bushing per the IEEE Standards is “a bushing in which metallic or nonmetallic conducting layers are arranged within the insulating material for the purpose of controlling the distribution of the electric field both axially and radially.” Condenser type bushings range from a low voltage of 15 kV through high voltage at 800 kV. Refer to Figure 1b for a condenser type bushing profile.

A bushing is a device that allows electrical current to pass through a barrier and provide an electrical connection on each side. The center conductor is usually made of copper or aluminum tube or bar. The current rating is based on the size, conductivity, and design of the materials used. The inside conductor is surrounded by some type of electrical insulation. Lower voltage bushings are typically made up of a solid material like porcelain or some type of resin, epoxy, or polymer composite material. In higher voltage bushings, the inside has a different, more complex design insulation system that deals with the unevenness of the voltage gradient. The inside consists of concentric layers of insulation and layers of conductive materials. This allows the voltage to be graded in a uniform method. These layers of insulation and conductive materials form a concentric capacitor between the high-voltage center core and the bushing flange at ground potential.

Bushings are the most critical component on a piece of major power equipment. This is because the voltage gradients are much more compact. In other words, the physical distance between the high-voltage point and the low voltage (i.e. ground) is much smaller compared to the distance from the high-voltage components inside the tank to the tank wall.”

Fig 1a and Fig 1b Types of Bushings

Man-Made Events

Physical impact to the transmission system can result from intentional, malicious actions. The first step is to determine the level of protection desired based on the method of equipment attack. A practical consideration will be required, as a dedicated person with transmission system knowledge and adequate time and resources can likely damage equipment no matter the protection steps.

Transmission and distribution equipment has often been subjected to small arms fire. This ranges from bored hunters to those wanting to damage the equipment. UL Standard 752 has 10 different levels for resisting damage from a range of small arms fire from handguns to .50 caliber rifle full metal jacket ammunition. Level 8 if often chosen as a reasonable objective, as it provides protection from five shots of 7.62mm (.308 caliber) rifle 9.7 gram lead core full metal copper jacket military projectiles with velocity up to 3025 feet per second. Some may desire protection to the highest small caliber ammunition, defined as Level 10. Level 10 protection leaves equipment operational when exposed to a single .50 caliber rifle 45.9 gram lead core full metal copper jacket M2 military ball projectile with velocity up to 3091 feet per second.

Following is a table containing the full range of protection levels in UL 752.

table containing the full range of protection levels in UL 752

Beyond small arms ammunition in the table above, a determined person or persons may choose rockets, mortars, and larger projectiles. Aircraft can directly impact or drop a payload that can damage transmission and distribution equipment. Also, basic explosives or explosives combined with shaped charges can cause significant equipment damage.

Dry bushings – avoiding the catastrophic consequences of the bushing failure

Today, it is believed that approximately 19 percent of transformer failures on the transmission network result from bushing failures. This is in part because most porcelain bushings used today either contain their own oil or share oil with the transformer. Though oil is a critical part of most bushings, responsible for both cooling and dielectric insulation, it is susceptible to catching fire in case of damage or misapplication of the bushing. Another consideration is that most porcelain bushings are held together using several thousand pounds of spring force. They are designed in this manner for a variety of reasons including accommodating cantilever loading and thermal expansion/contraction. A final consideration is that although porcelain is a great dielectric insulator, it is brittle and tends to shatter when impacted.

Recently experienced acts of vandalism and terrorism have triggered examinations of different bushing technologies. The focus is to harden the transformer from such attacks making it safer and more resilient. The resilient alternative to porcelain bushings is the dry bushing in which the primary insulation system is made of polymers. There are various technologies and methods to constructing these bushings, but one key advantage shared between them is the lack of oil. Another key advantage is the forgiving nature of the polymer insulation system to impact.

Damage caused by vandalism, electrical force, or even seismic activity to a porcelain bushing can lead to long and expensive outages. Often when the porcelain of a bushing is compromised, it will first explode due to the spring force and then the conductor and lead will fall into the transformer. The explosion of the porcelain insulator can send shards flying 40 or more yards with enough force to damage other equipment and hurt or kill anyone working near the transformer. If that were not bad enough, when the remaining conducting pieces of the bushing drop into the transformer, sparks from the resulting electrical short can ignite the oil leading to a massive fire. If the fire cannot be quickly extinguished, the entire transformer as well as nearby equipment can be destroyed. Even if a fire does not occur, there is now an opportunity for moisture to enter into the transformer and degrade the insulation. Getting a new transformer to site can take a long time, with lead-times ranging from six to 18 months.

On the other hand, such problems are generally avoided when using a dry insulated bushing. A dry bushing may ultimately need to be replaced in case of a ballistic attack, but since the polymer insulator is more resilient with respect to brittleness, the bushing will not explode nor will the bottom part fall into the transformer as the bushing is designed with a solid structure not relying on the spring force to keep it together. The risk of failure to the transformer associated with explosion of a porcelain bushing is greatly reduced when using dry bushings.

Another advantage of using dry bushings to achieve transformer resiliency is a reduction of the time needed to restore service in case of damage to the bushing. In the case where a bushing is damaged but no damage occurs to the transformer, a shattered porcelain bushing is likely to disperse pieces of the damaged bushing into the transformer. In order to restore the transformer back to service, the debris from the porcelain needs to be removed and a replacement bushing located and installed. Cleaning and reprocessing a transformer often takes several days, and that does not include the time needed to get specialized crews and equipment onsite to do the cleaning and processing. Moreover, if a spare bushing is not readily available, lead-times can sometimes range up to one year, since it is often necessary to procure porcelain from a foreign country.

Equipment owners can store spare oil-filled bushings but care must be taken to ensure proper orientation or the bushing will degrade making it unusable as a replacement. With domestically produced dry bushings the reliance on long lead-time porcelain is eliminated. Storage conditions for dry bushings vary greatly, but bushings that use Resin Impregnated Synthetic (RIS) offer ease of storage as they do not require special orientation, nor will they readily absorb moisture.

In summary, dry bushings offer the best solution for a resilient transformer strategy by increasing safety and hardening the transformer as a whole. Dry bushings are widely available for a variety of applications and contain no oil, are less fragile, and have the ability to be produced quickly should a failure occur. Of course, for the quickest replacement, domestically produced dry bushings must be considered.

Figure 5 – Example of Transformer Fire Caused by Porcelain Bushing Failure

Figure 5 Example of Transformer Fire Caused by Porcelain Bushing Failure

Conclusion

The next generation of condenser bushing technology is here and the concerns of Safety, Resiliency, and Security are much higher with the many security breaches that have occurred within the last few years. When reviewing and researching bushing technologies to make a decision on the technology that is best for the application, consider the goal and/or task at hand to make the right choice and consider bushing apparatus manufacturers to totally understand their products in design, manufacturing, and materials utilized to make the right choice.

References

IEEE Standards C57.19.01.2000

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