Field Testing Program for Power Transformers Jeffrey Short, Doble Engineering

1. Field Testing Program for Power TransformersJeffrey Short, Doble Engineering

2. What are the benefits of starting a testing program for transformers? What is the motivation for having the program? What tests can be performed to assess the condition of transformers? Introduction 2005 APPA Conference

3. Benefits of Testing Program • Enhance System Reliability • Minimize Damage to Apparatus • Enhances Safety to Personnel • Minimize Loss of Revenue • Extension of Apparatus Life • Degradation of Insulation, if detected before failure, can generally be restored to its original condition • Defer replacement costs 2005 APPA Conference

4. Benefits of Testing Program • Better Utilization of Resources • Acceptance of New Apparatus • Verify that new apparatus meets purchased specification and agrees with factory test reports • Assures proper field Assembly 2005 APPA Conference

5. Field Testing provides the maintenance engineer a group of tools to assess the condition of the transformer Benefit for each test must be clear and results able to be interpreted Data may relate to different areas Dielectric Thermal Mechanical Engineer should choose the right tool for the right job Benefits of Testing Program 2005 APPA Conference

6. Acceptance Testing Establish a Baseline Routinely to Establish a Condition Trend Determine Dryness of Insulation Assess Condition after Electrical Disturbance Isolate a Problem Area Assess Condition after a Relocation Asset Management - Ranking and Prioritizing Reduce Catastrophic Failures Motivation for Field Testing 2005 APPA Conference

7. Reduce Catastrophic Failure 2005 APPA Conference

8. Transformer Failures • Hartford Steam Boiler Insurance Co. Statistics 2005 APPA Conference

9. Motivation for Field Testing • Average age of failure is 17.7 years 2005 APPA Conference

10. Dielectric tests - Power Factor and Capacitance Perform on Overall Windings Bushings Liquid Insulation Surge Arresters -Field test on insulation portion of arrester Assess the condition of the insulation and physical properties of the transformer Condition Assessment Tests 2005 APPA Conference

11. Dielectric Power Factor Example 2005 APPA Conference

12. Problem Revealed in LV Winding FPE, 3-, 2-winding , -Y transformer 13.8/4.3 kV, 7 MVA Localized moisture/contamination in the L.V. winding High CL % P.F. & disagreement between HV & LV CHL %P.F.s 2005 APPA Conference

13. Dielectric CapacitanceExample 2005 APPA Conference

14. Significance of Measured Capacitance Capacitance detects movement and deformation of transformer windings. 2005 APPA Conference

15. Ae 4pd Significance of Measured Capacitance Capacitance change detects movement and deformation of transformer windings. C= 2005 APPA Conference

16. Excitation Currents 2005 APPA Conference

17. Excitation Current and Loss Factory Tests at Rated Voltage. Field Tests at the Lesser of Rated Voltage or Highest Capability of the Test Set. Simple measurement of single-phase current on one side of the transformer, usually the HV side, with the other side left floating (with the exception of a grounded neutral). Field Testing Magnetic Circuit and Winding Tests 2005 APPA Conference

18. Types of Problems found with Excitation Current Windings (includes high, low, tertiary, preventative auto and series windings) • Turn-to-turn winding insulation failure causing a short or high resistance connection. • Winding to ground short for a grounded winding. • Open windings (main, tap, reactor). • High resistance conductor connections. • Phase to phase electrical tracking. • These conditions result in a change in the effective reluctance of the magnetic circuit, which affects the current required to force a given flux through the core.

19. Types of Problems found with Excitation Current • Tap Changer • Misalignment, mechanical problems, coking and wear of LTC and DETC contacts. • Core • Abnormal circulating currents in the core, clamping components or through multiple core grounds. • Core laminations insulation damage. • Core joint dislocations.

20. Winding Turns Ratio Ratio of the number of turns in a higher voltage winding to that in a lower voltage winding Factory and Field Low Voltage Method High Voltage Capacitance Reference Method Purposes Confirm nameplate ratios Detect short-circuited turn-to-turn insulation Find open-circuited windings Find problems with tap changer connections Field Testing 2005 APPA Conference

21. Mechanical Assessment Purpose • Assess Mechanical Condition (physical distortions) • Detect Core and Winding Movement • Due to large electromagnetic forces from fault currents • Winding Shrinkage causing release of clamping pressure • Transformer Relocations or Shipping 2005 APPA Conference

22. Mechanical Assessment Current Techniques • Frequency Response Analysis (FRA) • Leakage Reactance • Capacitance • Excitation Current • These independent diagnostic methods have their place in ascertaining transformer condition 2005 APPA Conference

23. Frequency Response Analysis • Simply Stated: To measure the frequency response of passive elements (RLC) for various types of power apparatus. The result is a transfer function which produces a fingerprint related to the mechanical geometry for a given apparatus (i.e. transformers, reactors, generators, and motors). • Sweep Frequency Response Analysis (SFRA) • Apply a 10 volt peak to peak sinusoidal signal from a network analyzer (M5100) to a winding and measure the signal input/output from other end of same winding, or from another winding as function of frequency • Low-Voltage Impulse Method (LVI) • Apply impulse to a winding and measure input & response signals using high speed digitizer. Perform FFT calculation to convert time domain into frequency domain 2005 APPA Conference

24. Frequency Response Analysis • Experience to date shows that certain frequency bands indicate different problem conditions • 2kHz scan sensitive to core deformation, open circuits, shorted turns & residual magnetism • 20kHz scan is sensitive mainly to bulk winding movement relative to each other • 200kHz and 2MHz scans are sensitive to deformation within the windings • 10MHz scan is sensitive to movement of winding leads 2005 APPA Conference

25. Leakage Reactance Testing • Similar to Short-Circuit Impedance Test • Obtain benchmark impedance data • Confirm nameplate impedance • Investigate winding deformations 2005 APPA Conference

26. Transformer Test Summary • Each test is sensitive to a specific set of problems. • Motivation for testing should always be determined before arbitrarily performing a variety of tests • Routine testing should provide owner with a high level of comfort with transformer condition • Investigative testing needs to be more focused and thorough • Test results should always be scrutinized and taken seriously 2005 APPA Conference

27. Transformer Test Summary • Planning discussions for contingencies are important in making good decisions • There are cases where some tests will fail to identify a problem. 2005 APPA Conference

28. Thank You!!!