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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!