Lightning Protection: Improving Reliability Through the Use of Surge Arresters

By Jonathan Woodworth, Cooper Power Systems

Atmospheric discharges-better known as lightning-are still among the most common causes of outages on power systems in the world’s high lightning regions. As customers demand fewer interruptions and improved reliability, the issue of lightning protection appears more frequently on the collective radar screen of reliability engineers. Several studies in recent years indicate that outages related to lightning can account for up to 40 percent of the total outages experienced. This translates into millions of dollars in lost revenues and many unsatisfied customers.

One option often considered when trying to mitigate the effects of lightning is surge arrester installation. Many power engineers may believe that surge arrester technology was developed in the late 1800s when power systems first started appearing. They may be surprised to learn that it was telegraph engineers who first identified and implemented solutions to protect overhead lines from lightning. They were patenting and applying surge arresters in the early 1850s.

With the industry now over 150 years old, it would seem that all the issues are solved. Yet every year, due to continuous improvements in the understanding of the problems, new reliability solutions present themselves.

Lightning Protection Drivers

The primary lightning protection decision drivers for utilities are economic in nature. They are: desired outage rates, revenues at risk, cost of the protection selected, cost of the protected equipment and reliability indices. There can be additional drivers in specific cases such as the type of equipment to be protected or local environmental issues.

The technical drivers in decisions regarding lightning protection are: types of equipment being protected, system configurations, environmental conditions, desired outage rates, and historical preferences.

Lightning protection methods for power systems come down to basically four types: no protection, arrester protection, overhead ground wire (OHGW) protection and lightning mast protection. For many years, the installation of an OHGW was the most common choice of protection for 69- to 500-kV transmission systems in areas where lightning was prevalent.

For 3- to 35-kV distribution systems, the most common form of protection has been through the use of surge arresters. A less common form of lightning protection is the use of underground systems (URD). Generally, systems are put underground for aesthetic reasons, but the effect on lightning outages is significant.

For the purpose of this article, we are focusing on arrester protection. Presented here are some of the more common problems and solutions pertaining to the use of surge arresters.

Problems and Solutions

Problem: Improving distribution line outage rates. A common misconception of overhead line protection is that for long runs where improved protection is desired, applying arresters a few times per mile is adequate. Unfortunately, it is not that straight-forward. IEEE standard 1410 “Guide for Improving the Lightning Performance of Electric Power Overhead Distribution Lines” indicates that if the arresters are not mounted on every pole, the system has a high probability of outage if hit directly by lightning.

Solution: Arrester application to the lines. If a zero lightning outage rate is desired on unshielded systems, install an OHGW and a 10-ohm ground at every pole, or install arresters on each phase of each pole with a ground impedance anywhere from 10 to 100 ohms. If a perfect reliability rate is too costly, reliability rates can be determined from formulae found in the aforementioned IEEE standard.

Problem: Arrester-related outages. Wildlife is the single most common cause of arrester-related outages today. This reliability issue is due to the reduced size of recent vintage polymer-housed arresters and a reduction in other arrester failure causes. The animals, unfortunately, are able to touch the hot side of the arrester while perching on the ground side. This action leads to an immediate flashover of the arrester. It generally just causes the line to blink, but on occasion it can lead to a complete arrester failure and outage.

Solution: Use wildlife protectors. To reduce this type of outage, animal-proofing arresters and transformer terminals are the most effective methods of mitigation. For arresters, it is recommended that a wildlife protector be applied to both the top terminal and the hanger nut. It is beneficial to have the terminals both physically and electrically sealed.

Problem: Lead length. When using arresters for equipment protection from lightning strikes, the arrester’s location has a significant effect on how well it protects from lightning. If the arrester is mounted on the cross arm, or on the pole away from the equipment, the inductance in the lead length has the capacity to render the arrester worth much less than desired. It may still be protecting but not at its full capability.

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From Figure 1, the definition of lead length can be ascertained. The direction of the surge can also determine what is considered lead length and what is not.

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In Figure 2, the actual voltage across the terminals of the protected equipment is shown for a 10-kV surge arrester with a 31-kA strike, a rise time of 1.4 uS and a 2.5-foot lead length. The arrester clamps the voltage at 30-plus kV. However, with the inductance of the line lead factored in, the protected equipment is subjected to nearly 70 kV.

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If the lead length is reduced to 0.25 feet, the voltage across the terminals of the protected equipment approaches that of the arrester as shown in Figure 3. This means that the lead length does not reduce the quality of protection when it is kept short.

Solution: Mount the arrester on the terminal. To offer optimum equipment protection, the arrester can be mounted directly on the terminal of the equipment. Even .25 feet of lead length may be too much at times. With the advent of smaller, lighter arresters, this option can even be more cost-effective.

A second approach is to utilize extra space inside the equipment and mount the arrester as an integral part of the device. This mounting arrangement can decrease the lead length to zero. For transformers, underoil arresters are becoming increasingly popular since they also have the added advantage of no installation costs. For more information on this subject, a lead length simulation tool may be downloaded at www.cooperpower.com/Products/Components/Surge/Optimizer.asp.

Problem: Long-duration outages in substations. If an arrester experiences an “end of life event” (EOLE) in a substation from either a lightning strike or sustained overvoltage beyond its capability, it generally becomes a direct short between the buss and ground. It has been a common practice not to use ground or hot lead disconnectors on arresters mounted in substations. Ground and hot lead disconnectors are devices often used on distribution arresters in the event of an EOLE. This device is capable of disconnecting the arrester from the circuit either on the ground or hot side of the arrester with the assistance of a clearing device.

Solution: Use hot lead disconnectors in substations. Detroit Edison has been applying hot lead disconnectors in substations for years with great success. By applying an appropriately sized disconnector to the hot side of substation arresters, the arrester can be removed from the buss immediately after its EOLE. Lead management must become part of the installation process of the arrester. However, with the use of disconnectors on the hot lead, outage times can be reduced in substations. A breaker operation is still required to interrupt the fault current resulting in an EOLE, but after it is interrupted the arresters are isolated from the circuit.

Problem: Transmission line capacity. As right-of-ways become increasingly difficult to obtain and the demand for more power increases, utilities are finding ways to increase transmission capacity without building new lines. Transmission line arresters not only protect the line from lightning, but also allow the user to increase the transmission line capacity. The application of arresters for transmission line protection is the fastest growing lightning protection scheme. The application of arresters in some instances is the only solution since the alternate solutions, such as improving grounds or adding OHGW, are far less cost-effective. It is necessary to complete a system study to optimize the location of the arresters, but the cost of the study is minimal when compared to the benefit of reduced outages.

Solution: Increase the transmission voltage. The installation of an arrester in parallel with each suspension insulator on a transmission line eliminates the need for the insulator to sustain a lightning surge on its own. The arrester in effect gives the insulator infinite resistance to flashover or infinite Basic Impulse Levels (BIL) withstand. This allows for the application of higher power frequency voltage without the increased risk of wet flashover. The net result is that the transmission lines can be used for a higher voltage transmission without changing out the insulators or towers. The only investment is in transformers at either end of the line.

Conclusion

Even though lightning protection technology of power systems has been around for more than 100 years, new ways to mitigate the effects of lightning are being introduced every year. As long as we have a need for transmission of electricity, we will continue to deal with the effects of lightning. Solutions to lightning outages have a cost and require a conscious effort to determine the economic and technical benefits depending on the desired results. ௣à¯£

Jonathan Woodworth is engineering manager for surge protective devices at Cooper Power Systems.

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