Faustino Quintanilla PE
& Bradley E. Staley PE
Salt River Project
Salt River Project (SRP) has an Asset Optimization team that manages physical assets to meet its corporate objectives. Key areas include proactively managing the risk of catastrophic failure, reliability, aging infrastructure and lifecycle costs. Power transformer assets were considered first since they are the most critical and capital intensive transmission assets. Salt River Project is identifying and managing risk for its power transformer assets. This risk evaluation process is automated creating a dashboard, looking at several streams of data while avoiding the cumbersome manual snapshot approach for risk analysis that was used in the past. A two-tier risk evaluation approach is being used. The first tier identifies risk for the entire power transformer fleet. The second tier manages risks for individual transformers with a Subject Matter Expert (SME) that develops plans and strategies considering every stage of an asset’s life cycle. The Subject Matter Expert takes into account testing trends from dissolved gas analysis (DGA), fluid quality, power factor and constructability factors for prioritization of transformer replacements. This asset management strategy will be discussed identifying the risk scores for the transformer fleet and reviewing high risk transformers with subject matter experts.
Salt River Project is the oldest multipurpose federal reclamation project in the United States. SRP has been serving central Arizona since 1903. Today, SRP’s water business is one of the largest raw-water suppliers in Arizona. SRP delivers about 800,000 acre-feet of water annually to a 375-square-mile service area and manages a 13,000-square-mile watershed that includes an extensive system of reservoirs, wells, canals and irrigation laterals.
SRP is one of the nation’s largest public power utilities. SRP provides electricity to approximately 1 million retail customers in a 2,900-square-mile service area that spans three Arizona counties, including most of the metropolitan Phoenix area. SRP is a vertically integrated utility, providing generation, transmission and distribution services, as well as metering and billing services.
SRP owns or maintains about 2418 miles of transmission at voltages from 69kV to 500kV. This transmission system requires 275 substations. In addition, SRP owns, operates or maintains 589 transformers, which includes 69-12kV and 115kV distribution substation transformers and 230kV to 500kV transmission and generation step-up transformers (1). The following table summarizes the breakdown of transformers by voltage and use. Participant, spare and mobile units are not included.
- The median age of the entire fleet is 21 years.
- Approximately 65% of the transformers are 69kV high side transformers.
- Approximately 10% of the transformers are over 40 years old.
Since 2014, SRP’s Asset Optimization group has been developing a strategy for managing risk for substation and transmission system assets. It started with power transformer assets and is now developing risk strategies for the proactive replacement for high risk transformers.
SRP’s strategy for managing power transformers over their lifecycle includes optimizing and therefore minimizing life cycle costs while delivering reasonable operating performance and managing risk (2).
Once a power transformer has been placed into commercial operation, the most important asset management strategies include performance standards, maintenance, testing, inspection, performance monitoring and spares. This strategy applies to all power transformers owned, operated or maintained by SRP including:
- 69-500kV primary voltage
- 1 MVA capacity and larger
- Generator step-up power transformers, transmission system transformers and distribution substation transformers
Issues with Power Transformer Asset Management Strategy
In creating the process for transformer asset optimization, a number of notable issues exist. They are:
- Reduction of transformer design margins from advancements in CAD and other software for modern transformers
- Long term impacts of electrical impulse, switching transients and through faults during power system operations.
- Decreasing transformer operating margins while transformer load is increasing
- Aging power transformer population
- Post warranty maintenance support from some original equipment manufacturers (OEMs) has become unavailable
To move forward with SRP’s transformer asset management strategy, an enhanced risk evaluation approach is being implemented while considering the issues described above.
Inspection, maintenance, testing and performance monitoring activities are performed to keep transformers in good working order and to maintain their current condition status.
The focus in this paper is to describe the planning, operation, maintenance, testing, inspection, condition monitoring and spares aspects of transformer asset management. The transformer design, procurement, manufacturing, installation and commissioning strategies will not be discussed.
Asset Management Strategy
SRP is using a two-tier risk evaluation approach for maintenance, testing, inspection and monitoring of transformers. The first tier identifies risk for the entire power transformer fleet. Figure 1 shows how to identify and manage risk over the equipment lifecycle.
In the identification of risk step (first tier evaluation), the probability of failure and consequence of failure of power transformers are quantified which in turn calculates a risk score, from very low to very high levels.
The probability of failure (POF) and consequence of failure (COF) for individual transformers makeup the risk score. The risk scores are calculated for all transformers within the utility fleet. Figure 2 show the identified risk for the transformer fleet.
Probability of Failure:
The probability of failure (POF) is more accurately described as an asset health index. The POF is a condition assessment of the transformer based upon current and available data from visual inspections and maintenance history, testing and monitoring, operational history and in-house subject matter expert knowledge. Table 1 shows the eleven attributes, four major categories and weighted categories utilized to calculate the POF.
The POF scores for transformers are updated daily. The POF score is updated based upon available information from visual inspections, maintenance history, testing of dissolved gas, fluid quality and transformer and bushing power factor. However, the transformer faults and loading are attributes that are updated on an annual basis.
Consequence of Failure:
The consequence of failure (COF) assesses the impact for loss of the transformer to SRP. The impact takes into consideration a number of different factors such as customer, financial, reliability, public perception, environmental and safety impacts. Table 2 shows the ten attributes and six categories utilized to calculate the COF.
The second tier manages risks for individual transformers with higher risk scores utilizing an incremental analysis method with a Subject Matter Expert (SME) that develops plans and strategies considering every stage of an asset’s life cycle. SRP focuses resources where risk is the highest and conversely looks for cost savings where risk is lowest. This process will help determine how much to spend managing risk while balancing a high level of reliability with customer expectations.
Subject matter experts (SMEs) review only the medium, high and very high risk transformers for incremental analysis. The incremental analysis reviews testing trends from DGA, fluid quality, transformer power factor and bushing power factor. Also, testing trends apply forecasting with double exponential smoothing of DGA analysis (3) for individual transformer prioritization. Finally, the constructability is reviewed for the transformers. The constructability reviews the planning, maintenance schedules and operational flexibility along with physical site constraints.
There are separate SMEs for the POF and COF. The POF SME reviews the testing information from elevated risk transformers, and testing data trends. The COF SME reviews the planning studies, site constraint information and project schedules. The POF and COF SMEs review the elevated risk transformers, and determine the priority of transformers for proactive replacement and their budget, job scope and schedule.
The proactive replacement of transformers review of medium, high and very high risk transformers is with subject matter experts. The intent is to reduce the quantity to a smaller pool of transformers for incremental analysis. The transformers selected for incremental analysis have low COF with high POF or low POF with high COF. Figure 3 shows the medium, high and very high risk transformers identified for additional incremental analysis.
Electric Power Research Institute (EPRI) reviewed the DGA data of the SRP transformer fleet for degradation condition status (4) and replacement consideration. EPRI recommends individual transformer prioritized replacement based on normal degradation index, abnormal condition index, institutional knowledge and subject matter expert input. Figure 4 shows the DGA rating of transformers from Delta-X’s Transformer Oil Analyst (TOA) (5) using the Normalized Energy Index (NEI). Conditions 1-4 are as follows:
1 – No abnormality detected from gas sample.
2 – An increase in gassing event from latest DGA sample requiring retesting or extra attention.
3 – A moderate gassing event is in progress as of the latest DGA sample requiring urgent attention.
4 – A severe gassing event is in progress as of the latest DGA sample requiring extremely urgent attention or action.
Condition 4 transformers are scheduled for proactive replacements. Condition 3 transformers will be placed on a transformer watch list and may be considered for replacement in the near term.
Forecasting with Double Exponential Smoothing:
Individual gases from DGA are modeled using forecasting with double exponential smoothing to identify potential concerns. Figure 5 shows the carbon dioxide (CO2) ppm trend history for transformer A in 2015.
Transformer A was identified for incremental risk analysis. The DGA shows high carbon dioxide and ethane gases, while other gases were within acceptable levels. Transformer A was 40 years old with a known problematic load tap changer. The accumulation of load tap changer and other issues raised the transformer POF and risk score.
Very high risk transformers will be replaced. Medium and high risk transformers will have additional inspections, maintenance, testing and monitoring applied as determined by a comprehensive risk evaluation. Cost saving opportunities would be pursued for low and very low risk transformers.
System Planning performs scenarios for loss of elevated risk transformers of various contingencies. The transmission scenarios measure the impact on the transmission and sub transmission grid while maintaining compliance with North American Electric Reliability (NERC) requirements (6). The sub-transmission scenarios may re-configure distribution topography to support the loss of a transformer and measure the impact on neighboring substations and feeders.
The transformer operation risk management strategy includes various measures. Some examples of these measures are:
- Operating above nameplate limits
- Using dynamic ratings for operation
- Minimizing load tap changer operations
Substation Maintenance, Design, and Operations review the substations for site restrictions and constructions requirements. Maintenance plans and capital work schedules are discussed for outage and labor coordination.
The transformer spares strategy is to ensure that a minimum number of spares are available in warehouse inventory. Spares and mobiles can be located near high risk transformers.
Future Plan – Living algorithm
In 2014, SRP developed the POF, COF and Risk algorithms. Since then, SRP has leveraged the risk scores for multiple strategies. However, gaps in the risk analysis still exist. There are a few attributes that are manual inputs and need to be automated. For example, PI historian and synchrophasor data are being considered to identify fault quantity, current magnitude and duration for POF. Also, COF utilization may begin to use PI data for transformer summer (peak) loading.
The application of a proactive replacement strategy addresses the ageing power transformer infrastructure, work prioritization and other risk reduction strategies for the fleet. The SRP approach for identification and management of risk during the transformer’s lifecycle is leveraged to effectively manage power transformer assets. System planning, constructability, capital and O&M budget planning for transformer assets have been significantly improved using this strategy.
The benefits of applying this approach for transformer asset management include the following:
- Developing risk-centric processes which provide a framework for the consistent evaluation of both the probability of failure and consequence of failure.
- Utilizing enhanced transformer monitoring and condition information and related analytical tools.
- Increasing the amount of condition-based maintenance and decreasing time-based maintenance activities.
- Identifying the effective remaining lives of SRP’s transformer asset base to leverage their full capability
- Developing plans for ageing transformer infrastructure
The authors want to thank the following collaborators from SRP for their contribution in the development of the approach described in this paper: Yancy Gill, Scott Anderson, Javier Munoz, Robert Kondziolka, Wes Knuth and Grant Smedley.
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