Field Testing Program for Power Transformers Jeffrey Short, Doble Engineering

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Field Testing Program for Power
TransformersJeffrey Short, Doble Engineering
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Introduction

• 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?

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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

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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

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Benefits of Testing Program

• 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

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Motivation for Field Testing

• 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

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Reduce Catastrophic Failure
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Transformer Failures

• Hartford Steam Boiler Insurance Co. Statistics

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Motivation for Field Testing

• Average age of failure is 17.7 years

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Condition Assessment Tests

• 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

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Dielectric Power Factor Example
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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
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Dielectric CapacitanceExample
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Significance of Measured Capacitance
Capacitance detects movement and deformation of
transformer windings.
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Significance of Measured Capacitance
Capacitance change detects movement and
deformation of transformer windings.
C
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Excitation Currents
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Field Testing
Magnetic Circuit and Winding Tests

• 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).

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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.

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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.

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Field Testing

• 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

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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

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Mechanical Assessment

• Current Techniques
• Frequency Response Analysis (FRA)
• Leakage Reactance
• Capacitance
• Excitation Current
• These independent diagnostic methods have their
  place in ascertaining transformer condition

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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

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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

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Leakage Reactance Testing

• Similar to Short-Circuit Impedance Test
• Obtain benchmark impedance data
• Confirm nameplate impedance
• Investigate winding deformations

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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

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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.

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Thank You!!!