On-line&Off-line FT

Transformers Field Testing

Overview

  • Conducted by: Robert Brusetti, P.E.

  • Date: February 16 – 20, 2026

  • Location: San Diego, California, USA

  • Organization: Doble Engineering

  • Copyright: © 2026 Doble Engineering. All Rights Reserved.

Purpose of Testing

  • Testing purposes vary depending on the phase of the transformer life cycle:

    • Factory Testing:

    • Verify that the transformer conforms to customer specifications and industry standards.

    • Transportation Phase:

    • Confirm geometric integrity following shipment to ensure safe arrival.

    • Installation/Configuration Phase:

    • Validate correct configuration post-installation.

    • Service Period:

    • Establish benchmark performance over time.

Importance of Benchmark Testing

  • Establishing a benchmark significantly increases the diagnostic value of future tests.

  • IEEE C57.152 states: "Optimum interpretation of the field tests described herein requires access to the original tests to quickly identify deviations or trends."

  • Benchmark Test Definition:

    • A series of on and offline measurements captured to serve as a reference for future tests.

    • Typically performed on:

    • New and re-manufactured equipment.

    • After significant maintenance including replacement of components or relocation.

    • In absence of previous results.

  • Key Question:

    • Does the organization have a benchmark?

Types of Tests

  • Frequency Response Analysis:

    • Conducted in shipping configuration when tests are performed during transit.

    • Data includes various analyses such as Winding Resistance, Short Circuit Impedance, Exciting Current and Loss, Turns Ratio.

    • Recommendations to establish a benchmark in the current service configuration.

Off-Line vs On-Line Testing

  • Off-Line Testing:

    • Definition: Apparatus is de-energized and isolated from the power grid, requiring an ancillary power source.

    • Limitations: Leads and sensors typically not permanently installed on apparatus.

  • On-Line Testing:

    • Definition: Apparatus is energized and connected to the power grid; sensors may be permanently installed.

    • Periodic Monitoring vs Continuous Monitoring:

    • Periodic: specific tasks completed to obtain measurement at set intervals.

    • Continuous: provides up-to-date asset assessment, incurs large volume of data that requires management.

On-Line Measurement Techniques

  • Dissolved Gas Analysis (DGA):

    • Standard: IEEE C57.104

    • Monitors gas levels in transformer oil to indicate degradation.

  • Oil Quality Testing:

    • Standard: IEEE C57.106

    • Assesses condition of oil insulation.

  • Furanic Compound Analysis:

    • Standards: ASTM D5837 and IEC 61198

    • Evaluates cellulose insulation degradation.

  • Partial Discharge Detection:

    • Techniques: Acoustic emission, UHF detection (e.g., using probes), RF surveys.

    • Frequency Ranges:

    • Acoustic: 1kHz to 1MHz.

    • Electrical UHF: 100MHz to 2GHz.

    • Electro Magnetic Interference: 10kHz to 1GHz.

  • Infrared Thermography:

    • Standard: ASTM E1934 - 99a(2010).

  • Radio Frequency Survey:

    • Typical frequency scan: 5MHz to 1GHz.

On-Line Continuous Monitoring

  • Objective: Identify deterioration in bushing insulation systems.

  • Metrics Monitored:

    • Capacitance, Power Factor, Moisture, Partial Discharge.

  • Sensors Used:

    • In-tank sensors, bushing tap adapters, acoustic sensors.

Capacitance and Power Factor Measurement

  • Measurement Methodology:

    • Relies on extracting real and reactive components of leakage current phasors through insulation medium.

    • Absolute Measurement: Uses applied voltage as a reference, typically from a test instrument or voltage transformer.

    • Relative Measurement: Utilizes leakage currents of bushings to obtain a vector.

  • Evaluation Criteria:

    • Changes in system voltage impact bushing leakage current assessments; extensive trending needed to differentiate peripheral influences from actual changes.

Continuous Moisture Monitoring

  • Rationale for Monitoring:

    • Moisture content is dynamic, migrating between oil and cellulosic materials significantly affected by temperature and thermal history.

  • Benefits of Monitoring:

    • Real-time condition assessment of dielectric state, enabling proactive maintenance and assessments.

Effective Monitoring Program Requirements

  • Considerations for Implementation:

    • Selection of appropriate technology and understanding of the measurement process (e.g., signal extraction, filtration).

    • Maintenance needs as well as software and hardware training requirements - Ensure efficient use of selected tools and support structures.

Diagnostic Tests Overview

  • Standards Addressed:

    • Dissipation Factor, Insulation Resistance, Leakage Reactance, Short Circuit Impedance, Frequency Response Analysis, Electrical Partial Discharge.

  • Equipment Setup:

    • Confirmed through IEEE C57.152 for variations in assessments.

Dielectric Loss Measurement

  • Concept:

    • Measured in Watts (W = V^2/R), indicates efficiency of an insulation system.

    • Lower loss indicates a better-quality insulation system; deterioration raises losses.

  • Observations:

    • Dielectric losses depend on material volume and change based on contamination or deterioration.

Power Factor Measurements and Interpretation

  • Typical Thresholds:

    • Power Factor ranges for transformers:

    • Liquid Filled Power Transformers <230kV: ≤0.5% (Good), ≤0.7% (Deteriorated).

    • >230 kV: ≤0.4% (Good), ≤0.6% (Deteriorated).

  • Changes exceeding 5% from previous or benchmark results necessitate investigation.

Transformer Turns Ratio:

  • Importance: Confirms voltage ratio and detects insulation issues between turns. Typically performed at power frequency. Methods include:

    • Low Voltage Method: Verify turns number and test continuity.

    • High Voltage Method: Stress turn-to-turn insulation.

  • Analysis: Variations of +/- 0.5% of nameplate calculations considered acceptable, with factory benchmark offering refined evaluation.

Winding Resistance Assessment

  • Purpose: Identifies loose connections and resistance issues in windings. Indicative results should be compared against original measurements for standards.

Case Studies and Example Results

  • Various tests and their interpretations illustrated through results comparisons, historical data, and direct observations across a variety of transformer configurations.

Summary of Testing Procedure

  • Proper testing helps in identifying transformer condition and maintenance needs, encouraging a systematic and analytical approach towards transformer diagnostics and monitoring. Each test should be tailored to diagnose specific issues and healthy practices employed for routine assessment.

Conclusion

  • Emphasizing the need for regular condition assessments, incorporating diverse diagnostic techniques, and taking proactive measures to enhance transformer longevity and reliability.

Copyright

  • Content derived from Doble Engineering © 2026, All Rights Reserved.