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Transcript
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?
2005 APPA Conference
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
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
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
2005 APPA Conference
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
2005 APPA Conference
Reduce Catastrophic Failure
2005 APPA Conference
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
2005 APPA Conference
Motivation for Field Testing
 Average age of failure is 17.7 years
2005 APPA Conference
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
2005 APPA Conference
Dielectric Power Factor
Example
2005 APPA Conference
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
2005 APPA Conference
Dielectric Capacitance
Example
2005 APPA Conference
Significance of Measured Capacitance
Capacitance detects movement and deformation of transformer
windings.
Interwinding (CHT) capacitance of an
autotransformer.
Test Date
1965
1968
1974
1982
2005 APPA Conference
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
2005 APPA Conference
Excitation Currents
2005 APPA Conference
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).
2005 APPA Conference
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

Detect short-circuited turn-to-turn insulation

Find open-circuited windings

Find problems with tap changer connections
2005 APPA Conference
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
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
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
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
Leakage Reactance Testing




Similar to Short-Circuit Impedance Test
Obtain benchmark impedance data
Confirm nameplate impedance
Investigate winding deformations
2005 APPA Conference
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
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
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Thank You!!!