Should you be considering performing a condition ranking on your transformer fleet?

Author: Rodney Fickler

02/02/2015

Many utilities these days are considering making the investment in transformer fleet analytics to support better decision making regarding asset management of their transformer fleet.  Many Utilities have introduced condition assessment scoring criteria based upon standardized field tests and inspection data. The process allows a relative ranking of each transformer within the fleet to help prioritize maintenance practices or capital expenditure budgets associated with replacement.

There are many IEEE technical and white papers suggesting the appropriate weighting factors for various field test and inspection data to accurately calculate the relative score. Typical factors include:

  • power factor testing of transformer and bushings dielectrics
  • mineral oil tests, DGA analysis and trends (changes over time)
  • on load tap changer operation and maintenance data (if applicable)
  • winding resistance tests
  • attribute data associated with inspections
  • operational data (load profile, breaker trips, Load tap changer operations, etc.)

The biggest impediment to successfully launch such an initiative to perform scoring of the relative condition of transformer units is data quality. Some of the quality issues are:

  • maintenance history
  • sufficient amount of test data (preference for 5 data points for each metric)
  • time interval between test data is consistent (preference of annual capture)
  • metrics (units of measure) and accuracy of test instruments used for the test data is consistent (avoidance of pass, fail, investigate attribute data)
  • criteria for good, fair and poor condition on inspected components is consistently applied (preference for defined condition scoring)
  • accessibility to transformer loading profile between test data is available
  • completeness of description of repairs performed and results of post installation testing
  • availability of manufacture factory test data or commissioning test data when the unit was placed into service

Many utilities struggle to get sufficient budget to perform tests as well as have the resources to conduct the necessary planning of when tests are executed (both energized and de-energized tests) to achieve both consistency as well as quantity of data required. Data captured from field tests should be periodically vetted by a transformer expert for obvious data entry errors.  Additionally, operational data is not normally within view of maintenance personnel.  Operational data is normally captured by a SCADA system of some type (alarms, temperature, loading, etc.) and extracting the data to complement the testing program may be difficult to achieve. In short to perform a meaningful relative condition assessment of your transformer fleet requires a process of data acquisition to deliver meaningful results and decision making criteria.

 

To avoid some of these pitfalls of lack of data quality, utilities are purchasing transformer monitoring systems with the purchase of new transformers. Although the placement of a transformer monitoring system on a new transformer may seem a bit counter intuitive, the future benefit of these systems addresses many of the data consistency and quality issues mentioned above.  Additionally, the data can be located in both operational and maintenance data bases, and available for future or on-going analysis and ranking of condition.  Although it is often difficult to rationalize the expense of such systems on smaller transformers, utilities should consider purchasing monitoring systems on transformers having a high impact to electrical system reliability.  The remaining transformers should be covered by a manual testing regime that supports combining data (manual testing data and transformer monitoring system data) to allow fleet wide relative condition assessment.

 

Table 1 provides a listing of typical parameters used to perform a relative condition assessment of transformers.

Table 1
 
Periodic data capture
Manual test data measurement
Monitoring system
Technical data
Windings
Turns ration
Available through calculation
Technical data
Windings
Resistance
 
Technical data

Windings

Insulation

Power factor
 
Technical data

Windings

Insulation

Capacitance
 
Technical data
Bushings
Power factor
Available direct measurement
Technical data
Bushings
Capacitance
Available direct measurement
Technical data
Insulating oil
Dielectric strength (ASTM)
 
Technical data
Insulating oil
Neutralization No. (ASTM)
 
Technical data
Insulating oil
Interfacial tension (ASTM)
 
Technical data
Insulating oil
Power factor (ASTM)
 
Technical data
Insulating oil
Water content
Available direct measurement
Technical data
Insulating oil
Total dissolved combustible gas
Available direct measurement
Operational data
Loading
High, low, and average
Available direct measurement
Operational data

LTC

Operations

Count
Available temperature differential, LTC moto load
Operational data

Temperature

Winding

High, low, and average
 
Operational data

Temperature

Alarms

Count
Available, calculated hot spot temperature
Inspection data
Infrared scan (radiators)
Thermal image (hot spots)
 
Inspection data
Tank and accessories
Level of rust, leakage, control, and instrumentation defects
 
Inspection data
Oil preservation system
Level of rust, leakage, control, and instrumentation defects
Calculated oil level, auto leak detection
Inspection data
Cool system
Level of rust, leakage, fin mechanical integrity, pumps, and fans (excessive vibration) and related control defects
Calculated performance measurement
Inspection data
Vintage
Age
Calculated performance measurement
Inspection data
Repair history and experience
Frequency, type, and severity of repairs performed
 

In addition to the parameters outlined in Table 1, analysis of the transformer relative to system power throughput requirements, and comparative analysis to either factory test or as commissioned data, is also recommended to monitor degradation over time as test data is accumulated.

 

In summary, for a utility to benefit from a transformer condition assessment for prioritizing maintenance and capital expenditures, it requires a data acquisition strategy which delivers both data quantity and quality.  Siemens transformer consulting services can help develop a testing regime, scoring criteria and application of transformer monitoring solutions to help plan the data acquisition process and achieve strategic asset management of a transformer fleet. Additionally, Siemens has extensive experience in transformer services and provides a broad range of transformer monitoring solutions to consider.