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Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Field Testing Program for
Power Transformers
Jeffrey Short, Doble Engineering
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
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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
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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
Cause of Failure for Utility Transformers
2%
1%
0%
1%
Elect Dist
7%
Insulation
29%
13%
Lightning
Maintenance
Elect Connection
Moisture
13%
18%
16%
Overload
Sabotage
Workmanship
Other
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
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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
Insulation
CH + CHL
CH
CHL (UST)
CHL
Localized
CL +CHL
CL
CHL (UST)
CHL
kV
10
10
10
mA
28.5
7.05
21.5
21.45
moisture/contamination
2
39.5
2
17.65
2
21.5
21.85
Watts
1.45
0.38
0.95
1.07
in5.5the
2.85
3.55
2.65
% P.F.
0.39
0.32
0.36
L.V. winding
1.18
1.2
High CL % P.F. & disagreement between HV & LV CHL %P.F.s
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Dielectric Capacitance
Example
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Significance of Measured Capacitance
Capacitance detects movement and deformation of transformer
windings.
Interwinding (CHT) capacitance of an
autotransformer.
Test Date
1965
1968
1974
1982
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20C % PF
0.20
0.29
0.29
0.32
Cap (pF)
2,650
2,756
3,710
5,100
Significance of Measured Capacitance
Capacitance change detects movement and deformation of
transformer windings.
Ae
C =
4pd
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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.
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.
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
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Detect short-circuited turn-to-turn insulation
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Find open-circuited windings
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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)
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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
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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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Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Thank You!!!