The American Transmission Co. (ATC) Asset Management Department is focused on the life cycle management of transmission assets. The objective is to ensure assets perform the required function in a sustainable manner while managing life cycle cost.
Coordination of design, commissioning, operation, maintenance and replacement strategy is crucial to achieve this objective. Asset Renewal is the “replacement strategy” piece of the asset life cycle. Asset Renewal is driven by public and worker safety, regulatory compliance, reliability and operational performance.
State of Existing Facilities
ATC operates approximately 850 lines with a total of approximately 10,000 circuit miles. A number of facilities have been identified as nearing end of life or having components at end of life during the 10-year assessment period.
Due in large part to our asset management efforts, in 2021, we were one of the top performers in line reliability, ranking first quartile 200kV and above, second quartile for 100 to 199kV and second quartile for up to 99kV circuits in an industry reliability bench-marking study.
ATC operates a fleet of over 200 in-service transformers, of which approximately 40% are more than 35 years old. ATC has not experienced a high failure rate of these aging transformers, but the assessment anticipates that ongoing investment is required to maintain reliability and manage operational and financial risk.
ATC operates a fleet of nearly 11,000 protective relays. Relay systems are critical to the safe and reliable operation of the transmission system. Relay systems will be modernized to meet compliance requirements, improve reliability, minimize inadvertent operation and provide additional information to ATC System Operations to improve restoration time when an outage does occur.
ATC has a fleet of nearly 2,500 circuit breakers in service. Of these, 2,200 use sulfur hexafluoride (SF6) as an arc quenching and insulating medium, 270 are oil circuit breakers and 1 is a vacuum circuit breaker. Asset Management’s focus is to monitor and act as needed to address the reliability and performance concerns of the breaker fleet.
Transmission Line Asset Renewal
Overhead Transmission Line Facilities
ATC monitors the overhead line facilities through a maintenance and inspection program designed to assess the condition of line facilities. Aerial and ground inspections of ATC facilities are performed periodically to identify issues for repair. The goal is to maintain reliable performance as well as public and worker safety. ATC System Operations and Asset Management personnel monitor performance and track the reliability of all ATC lines. Particular attention is paid to frequently outaged lines and improvements that can be targeted at lines nearing the end of their useful lives. The inspection and reliability performance programs are considered the initial drivers for asset renewal upgrades to a facility. The line design is further analyzed by Asset Management for structural and clearance-related issues to complete the list of possible drivers.
Transmission line facilities categorized to have inadequate condition (low health) or poor operational performance history are brought to ATC Planning for consideration. Planning may have specific system reliability or economic needs for the line or geographic area. If Planning and Asset Management both have drivers for a project, a primary need driver is agreed upon and the project progresses as either a network (planning driven) or asset renewal (asset management driven) project. The scope of an asset renewal project can vary from complete rebuild to replacements of problematic components such as poles, structures, cross arms, wires, or insulators. The extent of renewal driven work may be further impacted by importance (risk) of the line to the end customer. Net present value (NPV) analysis is used to evaluate various options and determine the least cost means of obtaining the desired reliability.
ATC has a multi-year assessment program to verify that actual field conditions match design assumptions for line and substation facilities. Aerial surveys and other engineering analysis means were used to evaluate line facility ratings.
Underground Transmission Line Asset Renewal
Our underground transmission system consists of high-pressure fluid filled cable systems (HPFF), High Pressure Gas Filled cable systems (HPGF), XLPE or EPR insulated solid dielectric cable (SD) systems and self-contained fluid filled system (SCFF).
|Cable Type||69-kV Mileage||115, 138, and 161-kV Mileage|
|High Pressure Fluid Filled (HPFF)||24.7||54|
|High Pressure Gas Filled (HPGF)||0||3.4|
|Solid Dielectric (SD)||10.5||20.1|
|Self-Contained Fluid Filled (SCFF), Submarine||0.3||0|
The condition of high-pressure gas filled systems is such that no investment is expected in the 10-year horizon. Several of the solid dielectric cable and HPFF installations are approaching end of life and will require replacement over the 10-year horizon. Pumping plant upgrades are expected in the 10-year horizon to extend the useful life of the HPFF cable system and provide newer technology for control and monitoring functions. Pumping plant additions are also expected at few select sites for redundant pressurizing sources.
Instrument Transformer and Surge Arrester Renewal
Instrument transformers used for monitoring voltages and currents on the ATC system are key to reliable performance. The goal of asset renewal efforts is to manage maintenance costs and avoid end of life failures on the ATC system. Instrument transformers at end of life or with poor operational history are targeted for replacement.
Arresters are installed to prevent outages and protect equipment from lightning and over voltage surges. The goal of asset renewal efforts is to manage maintenance costs and avoid end of life failures on the ATC system. Arresters at end of life or with poor operational history are targeted for replacement in conjunction with other capital work at the station.
Relay and Battery Asset Renewal
Relays and station batteries are the cornerstone of a reliable transmission system. The goal of the ATC relay and battery asset renewal effort is to improve relay performance, provide information for Operations, and reduce maintenance cost. ATC can improve line and equipment capabilities with microprocessor-based relays by eliminating over-reaching mis-operations and increasing capacity load limits. The improved performance of the microprocessor relays allows ATC to address stability issues and increase system reliability and security with the use of carrier and fiber optic communication systems. The new technology has additional benefits of better factory support, improved spare part availability, software upgrades, and technical support to ATC staff.
Microprocessor based relays offer valuable information for ATC Operations. New relay systems provide information that can be used to direct field resources to the problem area and verify which component of the transmission system has failed. Additionally, fault location information is used with the geographic lightning detection network to correlate lightning strikes with line outages. This enables ATC to historically track performance of specific sections of the line to aid in determining transmission line Asset Renewal prioritization.
Relay and battery renewals are being made to satisfy NERC reliability standard requirements and recommendations.
The self-check and remote monitoring capabilities of microprocessor-based relays allow longer maintenance cycles and reduce maintenance costs.
Approximately 11,000 in-service relays with 332 associated station batteries protect the ATC network. The asset renewal program is prioritized by replacing the least reliable relays by type, relays at end-of-life and, and relays with schemes that have a history of inadvertent operations. This includes single relays that require remote back-up and electromechanical relays that do not provide fault location or self-alarm.
ATC plans to renew approximately 200-300 relays per year in the 10-year horizon.
Circuit Breaker and Circuit Switcher Asset Renewal
Circuit breakers are essential to the reliability and safety of the network. ATC has a fleet of nearly 2500 gas, oil, and vacuum circuit breakers and approximately 250 circuit switchers used for fault interrupting, equipment isolation, and capacitor bank switching. The goal of the circuit breaker and circuit switcher asset renewal program is to improve reliability and environmental performance, which includes reducing maintenance outage requirements and the number of unplanned outages while minimizing environmental impacts.
Power Transformer Asset Renewal
The intent of the ATC power transformer asset renewal program is to make a risk assessment of each transformer in the fleet based upon health, operational importance and the probability of failure. This assessment is used to determine a spare or renewal strategy. The strategy options are either an on-site spare, a system wide spare that can be relocated, or a proactive renewal replacement for the subject transformer. ATC has a spare transformer plan and participates in industry spare equipment programs.
- Healthy units with high importance and high probabilities of failure based on age may warrant an on-site spare or a system spare in the general area to minimize transport times.
- Units in poor health with high importance are candidates for renewal.
- Healthy units with low importance will be backed up with a system wide spare.
- Units in poor health and low importance will be backed up with a system wide spare.
The operational importance of a transformer to the transmission network is based upon a series of planning studies that look at the severity of the contingencies following the failure of the specific transformer. Given the high cost, specialty design, and logistical challenges of moving a large power transformer, double-contingency studies provide guidance for the operational impact and prioritization.
Power transformer condition is monitored as part of the asset maintenance program through off-line electrical tests, visual inspection, and tracking of maintenance history. On-line tests include dissolved gas analysis (DGA) oil tests, infrared inspections, vibration analysis and partial discharge condition assessment.
An online monitoring program was approved at ATC in 2020 to provide real time monitoring of select power transformer assets across the fleet. The intent of the monitoring program is to provide real time condition information and allow opportunities for condition-based maintenance in the future.
The expected investment to maintain reliability will require the replacement of approximately two 161/138/115 kV class transformers and one 345/230 kV class transformer every year in the 10-year horizon. Actual replacement of a specific transformer in the fleet will be based on the results of the condition monitoring program, subject matter expert review, and information provided from the Transformer Health Index.
Control buildings contain the protective relaying and control components of the substation including relays, SCADA, AC/DC sources, and batteries. The useful life of a building is estimated at 50 years. Consideration of replacement is given at times of major work in the control building. Major work may include relay replacement or substation expansion. Economies are gained if the control building is replaced when new relaying or a building expansion is required. The new building design is an effective upgrade to physical security at the site.
Remote Terminal Units (RTUs)
Remote Terminal Units (RTUs) provide the interface between the substation equipment and the Energy Management System (EMS) for remote monitoring and control of ATC substation facilities. The RTUs provide a communication link between all intelligent electronic devices in the substation and the EMS. Data links are also important to the Asset Management group by providing real time equipment data like dissolved gas, GIC values, harmonic values, and other equipment data. This data allows the Operators and Asset Management to effectively and efficiently operate the system and monitor performance of important substation assets.
The RTU asset renewal program is prioritized by replacing the older, least reliable RTUs with modern equipment that can meet ATC’s ever-increasing communication and security needs. The program will replace obsolete hardware followed by replacing hardware with slow speed processors. With the upgrades, the station alarms will be modified to meet the current ATC standard.
ATC plans to renew about 20 RTUs per year in the 10-year horizon.
ATC has developed a risk-based physical security program to address the risk of physical attack. The program is designed to provide increased resiliency for intentional attacks and weather events. Substations and facilities have been prioritized and evaluated for importance to the bulk electric system. Implementation has begun and is expected to be complete by 2025.