Extending the Life of Medium-Voltage Vacuum Circuit Breakers
A well-planned program requires long-term vision
By Finley Ledbetter
Vacuum circuit breakers are the dominant switchgear technology used in medium-voltage (1 kV–38 kV) power system applications due to their smaller size, increased service life, and ease of maintenance when compared to other alternatives. Throughout their service life, these circuit breakers must remain in optimum working condition, able to interrupt a fault quickly and reliably even following long periods of inactivity during which the circuit breaker’s mechanical and electrical components remain idle.
With an ever-increasing percentage of medium-voltage vacuum circuit breakers in service nearing or already exceeding their design life, asset owners are facing difficult decisions about how to extend the usable life of their equipment beyond planned obsolescence while at the same time maintaining the safety, reliability, and bottom line of their electrical systems. Simply using a breaker’s age or number of operations to quantify mechanism health and schedule maintenance is ill advised; the effects of outside variables must also be taken into consideration.
A properly structured life-extension program for medium-voltage vacuum circuit breakers (MVVCBs) should seek to continually maintain and upgrade the equipment up to and beyond its planned design life to guarantee that it is operating at the highest levels of dependability while also analyzing the economic impact of every maintenance decision and ensuring that safety is not being compromised. Life extension and modernization of MVVCBs and the electrical equipment they protect is a critical need for the aging power system infrastructure in the United States. As these circuit breakers age, they become more prone to failure, thus increasing the likelihood of an incident involving plant personnel and potentially costly damage to other equipment within the facility. When it comes to life extension of MVVCBs, a well-planned program can provide measurable economic benefits stemming from the added longevity and reliability of the equipment while also providing less quantifiable but very real benefits resulting from improved safety.
Recommended intervals between maintenance can differ based on many variables, including original equipment manufacturer (OEM) guidelines, environmental and operating conditions, age and condition of the MVVCB, number of operations on key components, and the criticality of the circuit. Additionally, the prescribed interval will also be affected by the maintenance philosophy adopted by the equipment operator, facility management, and/or maintenance provider. Industry standards dictate how each of these criteria should be considered when dealing with service-aged MVVCBs to determine the ideal service intervals to maximize safety, reliability, and cost effectiveness.
In addition to recommended maintenance, MVVCBs must also be regularly tested to ensure proper operation during normal operating conditions, and accelerated testing must be performed on equipment subjected to extreme conditions such as high duty cycle applications or interrupting faults. Testing technology has vastly improved since the widespread adoption of vacuum circuit breakers (VCBs) in the 1970s, and many of the tests that were previously too cumbersome or time consuming to perform in the field have made their way out of the service shop and into the hands of field technicians. These tests were either not available or not accounted for when the OEMs were designing these VCBs to a specified service life. Now having the ability to perform these tests in the field quickly and with great accuracy has helped to increase the service life of MVVCBs.
Generally, VCBs will require more maintenance as they age, and new replacement parts from the OEM are not always available in a timely manner, at the right price, or at all. Downtime is very costly and access to high-quality replacement components is essential to getting the equipment repaired quickly and back online. This need has spawned an entire market dedicated to providing replacement or aftermarket parts and assemblies that are often superior to the original components being replaced. Improvements can include use of modern materials, simpler designs with fewer components, or reduced maintenance requirements. For operators looking to maximize the life of their MVVCBs and bring them up to modern standards, this often means making continual upgrades throughout the lifetime of the equipment to ensure it is always up to date.
When properly maintained, MVVCBs can provide longer service lives than originally predicted by the OEM. By adhering to industry standardized maintenance intervals, staying educated about new test techniques and procedures, and incrementally replacing worn out or outdated parts, users can keep their equipment in good working condition to ensure that it will operate safely and reliably.
Not A One-Size-Fits-All Solution
The most basic maintenance strategy is a reactive, or run to failure (RTF), philosophy. Under this system, electrical equipment is deliberately allowed to operate until failure, at which point reactive maintenance is performed and the equipment is repaired or replaced. When discussing MVVCBs, this approach is not prudent due to personnel safety concerns and the potential impact failure would have on the greater electrical system. Yet, too often this policy is practiced in applications where its use endangers personnel and equipment. Only in rare circumstances — when no consequences of failure exist in terms of safety, mission, environment, or security — can maintenance be suspended and the equipment allowed to run until failure before it is repaired or replaced.
An interval-based maintenance philosophy, more often referred to as preventive maintenance (PM), involves scheduled maintenance at preset intervals to ensure safety, reduce the likelihood of operational failures, and obtain as much useful life as possible out of the equipment before failure. This philosophy is based on the assumption that there is a fundamental cause-and-effect relationship between scheduled maintenance and operating reliability, and that the reliability of any equipment is directly related to operating age since all mechanical parts wear out eventually. Therefore, it should follow that the more frequently equipment is maintained, the better protected it is against the likelihood of failure.
However, counterintuitively, studies have shown that many types of failures cannot be prevented no matter how comprehensive the maintenance activities performed are. Additionally, for many items the probability of failure does not increase with age and therefore a maintenance program that is based strictly on the age of the equipment will have little, if any, effect on the failure rate.1, 2
Advances in monitoring and electrical equipment failure analysis have made it possible to identify the precursors of failure, quantify equipment condition, and schedule appropriate repairs with a higher degree of confidence than was previously possible when compared to performing strictly interval-based maintenance. The availability of this data has emphasized the use of a philosophy known as condition-based maintenance (CBM), which in turn has caused a reduction in reliance on strictly interval-based philosophies.
CBM is centered around monitoring equipment wear at measurable points of reference on vital components in real time and predicting the probability of failure, while also taking into account certain outside influences which have an effect on the safety and reliability of the equipment. Once the probability of failure reaches an unacceptable level, repair or replacement is necessary.
Reliability centered maintenance (RCM) is a logical, structured framework for determining the optimum mix of reactive, interval-based, and condition-based maintenance practices needed to sustain the reliability of systems and equipment while ensuring their safe and economical operation. The concept of RCM was originally developed and proven in the aviation industry in the 1960s and has since been gradually adopted by the industrial sector. In the electrical equipment maintenance industry specifically, RCM continues to spread as the most prominent maintenance philosophy due to its endorsement in industry standards and its ability to utilize a combination of existing strategies that take into account all factors influencing maintenance intervals. As shown in Fig. 1, these principal maintenance strategies, rather than being applied independently, are integrated to take advantage of their respective strengths, thus maximizing facility and equipment reliability while minimizing costs.3
Most MVVCB service manuals contain some type of information concerning usage or time-based intervals for PM; however, such “one-size-fits-all” recommendations may not be applicable for certain applications due to environmental and operating conditions, the criticality of the circuit, and/or the condition of the equipment. Properly scheduled maintenance using an RCM methodology will reduce operating costs, increase production, and decrease unplanned outages. Implementation of RCM can be costly up front as a baseline must be developed, requiring breakers to be removed from service, tested, cleaned, inspected, lubricated, and reassembled.
Based on the original criteria OEM marketing and sales departments gave product development and engineering teams, MVVCBs were only designed to have a 20-year service life. However, this does not mean that these MVVCBs will only last for 20 years. In fact, many are now approaching 40 years of reliable service.
But, what will it take to continue this trend and extend the useful life of the MVVCB fleet in the United States even further beyond their original life expectancy, say another 20, 30, or even 50 years? Yes, 50 more years of service is reasonable with proper consideration and planning, and this is really what is meant when discussing life extension: long-term vision.
The infrastructure in the United States is heavily dependent on MVVCBs in the intermediate distribution network for most applications. The MVVCBs in these applications will not simply be replaced; they will be coaxed to reach 100 years of service life by whatever means necessary. This may seem crazy to think about now, only nearing the 50-year mark, but in the future others will be looking at how to reach the 100-year mark for this same equipment. The only way they will get to this point is if we continue to properly implement and improve existing life-extension programs that put a priority on safety, reliability, and value.
One of the most important parts of a properly structured life extension plan for MVVCBs is adhering to an industry-recognized maintenance philosophy. RCM is the preferred maintenance strategy endorsed by the industry standards due to its ability to utilize a combination of existing strategies and take advantage of their respective strengths to better understand all factors influencing maintenance intervals.
Regarding specific maintenance intervals for MVVCBs, all sources point to a maintenance interval not to exceed five years, due largely in part to the limits of lubrication life. The maintenance testing should consist of accurately determining the interrupter’s remaining lifetime and also consist of verifying the breaker trip time.
Continuous switchgear monitoring of insulation integrity and bus temperature is also beneficial to not only ensure breaker integrity but also ensure overall switchgear health. Asset owners can also benefit from non-OEM replacement parts and new technologies to extend service life and ensure reliable operation. UP
Editor’s Note: This article has been adapted from a more in-depth white paper titled “Medium-Voltage Vacuum Circuit Breaker Life Extension: An Approach Utilizing Industry Standardized Maintenance Intervals, New Testing Techniques, and Modern Replacement Parts.” Read the full white paper at www.utilityproducts.com/whitepapers.
The Author: Finley Ledbetter is the chief scientist for Group CBS Inc. with over 40 years of power systems engineering experience. He is a member of the IEEE and was a past president of PEARL.
1. F. S. N. a. H. Heap, Reliability-Centered Maintenance, San Francisco: Dolby Access Press, 1978.
2. J. Cadick, M. Capelli-Schellpfeffer and D. Neitzel, Electrical Safety Handbook, New York: McGraw-Hill, 2006.