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8/20/2019 NPTI Presentation General
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Transformer Testing
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Life of a transformer…
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Life of a transformer – With proper maintenance…
Service/repair;• Oil regeneration• Drying• New/refurbished bushings• New/refurbished LTC
• Etc...
Extra years in service!Diagnostic testing
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Transformer stress factors
Electrical Stress• Develops within the insulation material separating conductors at different
potentials. It ages the dielectric materiel (even at rated voltages)
Thermal Stress• Develops across all assembly components. It ages the dielectric material
(even at rated temperatures). Stress to insulation worsened by hightemperature and repeated changes in temperature.
Mechanical Stress
• Deforms/displaces assembly components. High shock events during shippingdisplaces components. High current events deform the windings.
Chemical Stress• Oil additives have deteriorated insulation components.
Environmental Stress (contaminants)• Salt and other corrosive substances in the environment have deterioratedtanks and peripheral equipment.
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Applicable Standards
IEEE C57.12.90
• Standard test code IEEE 62
• Field test guide
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IEEE 62
byMegger
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Transformer testing tool-box from Megger!
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MTO
Winding resistance measurements• 1 or 2-ch measurement
• Single or dual current injection (HV and
LV simultaneously)
Tap-changer testing• Winding resistance/tap
• Timing/Maker-before-break verification
Automatic demagnetization of
transformer core• Important to perform before all kinds on
excitation current tests (Delta, TTR,FRAX)
• Recommended in IEEE as standard
procedure before taking the transformerin service
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TTR
Transformer turn-ratio measurement
Polarity and phase deviation test Excitation current measurement
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MIT/S1
DC insulation testing
• Insulation resistance
• Polarization index
• Step voltage
• Dielectric discharge
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Delta
10 kV insulation testing @ 50/60 Hz• Tan delta/power factor and capacitance
• Winding – winding
• Winding – tank
• Bushings
• Oil
Excitation current measurement
HV TTR
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IDAX
Insulation diagnostics(DFR/FDS) from mHz to kHz
• Tan delta/power factor andcapacitance
• Winding – winding
• Winding – tank
• Bushings
• Oil
Automatic moisture assessmentof cellulose insulation (MODS)
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MLR
Short-circuit impedance/leakagereactance measurement @ 50/60Hz
Capacitor bank testing
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FRAX
Sweep frequency response analysisfrom Hz to MHz
Excitation impedance measurement
Short-circuit impedance testing
FRSL (Frequency response of stray
losses) measurements
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Low resistance measurements of;
• HV leads and connections
• Ground connections
• weep frequency response analysisfrom Hz to MHz
Winding resistance on small
transformers (DLRO10), < 10kVA
Microohmmeters
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Oil testing
Dielectric strength (OTS)
Moisture content (KF)
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Transformer testing tool-box from Megger!