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C O M E T 2 0 1 7 W E L C O M E T O A

T E C H N I C A L P A P E R P R E S E N T A T I O N

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B Y E M I L I O M O R A L E S

Most Common Transformer

Failures and Their Principles

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A B O U T T H E A U T H O R

Emil io Morales Cruz Emilio Morales is a Technical Application Specialist in Transformer applications at Qualitrol Company LLC. His main focus is to support solutions in comprehensive monitoring for Trans- former applications Emilio attended Nuevo Leon State University in Mexico, receiving his Bachelor of Science degree in Electro Mechanical Engineering in 1980. Emilio has spent his entire career in the power transformer manufacturing industry. His has over 30 years of experience in design which includes transformers up to 500 MVA and 500 kV as well as furnace and rectifier transformers and different type of reactors. He is member of the IEEE/PES Transformer Committee, IEC and CIGRE and actively participating in different task forces. Emilio joined Qualitrol in June 2012 and previously worked with GE-Prolec , Ohio Transformer, Sunbelt Transformer and Efacec Power Transformers.

E m i l i o M o r a l e s C r u z T e c h n i c a l A p p l i c a t i o n S p e c i a l i s t

Q u a l i t r o l

4 C O M E T 2 0 1 7 Mos t common t r ans fo rmer f a i l u res and t he i r p r i nc i p l es

Power transformers play a major role in guarantying the

safe and stable operat ion of power system. Any fai lure of

a power transformer can result in serious consequences

for a power system. Mechanical ly damaged windings,

deter iorat ion of sol id insulat ion, bushing fai lures, faulty

operat ion of the on-load tap changer (OLTC), etc.,

reduces the system rel iabi l i ty and may cause unexpected

power l ine cut-offs and f inancial losses to a company. In

order to minimize the r isk of a fai lure, the data obtained

from various diagnost ic tools needs to be interpreted to

ident i fy the fai lure mode. Knowing the transformer fai lure

modes and their fai lure mechanism principles wi l l help to

ident i fy the transformer problems and propose suitable

act ions for mit igat ion or complete el iminat ion of the

fai lure modes. This paper describes the most common

fai lure modes of power transformers and their fai lure

mechanism principles.

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Mos t common t rans fo rmer fa i l u res and the i r p r inc ip les

P O W E R T R A N S F O R M E R S

System outages due to fai lures in transformers

have a signif icant economic impact on the

operat ion of the power gr id. Therefore, i t is

necessary to be able to do a good condit ion

assessment of transformers. Techniques to

diagnose integri ty through non-invasive tests can

be used to opt imize the maintenance effort and

ensure maximum avai labi l i ty and rel iabi l i ty. With

the increase of average age of transformers

populat ion there is a growing need to understand

their internal state. For this purpose i t is

important recognize the most common principles

leading to fai lure

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Fai lure Stat ist ics The latest transformer reliability study shows the

windings with 45% as the major cause of failing

transformers, followed by tap changers with 26%,

bushings with 17% and lead exits with 7%. All

other major components of transformers are

playing a minor role in failure statistics. Looking

into catastrophic failures, bushings are in 70% of

the cases the cause. There are other components

leading the transformer to fail but with less risk of

a catastrophic failure. bA2.37, CIGRE WG. Transformer Reliability Survey: Interim Report, No. 261, ELECTRA. 2012

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Component: Magnetic Circuit (Core Ground & Magnetic Shields)

Loss of core ground and shield grounds Unintentional core and shield grounds

Hydrogen or multi-gas Core ground current Gas accumulation relay Core hotspot (Fiber) Temperature PD

DGA Gas Accumulation

Rate PD

Thermal Core Ground

Current

Principle leading to failure

Measured signals

Diagnostic Models

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Component: Winding Major & Minor Insulat ion

General overheating

Winding hot spot temperature Top and bottom oil temperatures Ambient temperature Line current Hydrogen or multi-gas Gas accumulation relay

Thermal DGA

Gas Accumulation

Rate

Diagnostic Models

Measured signals

Principle leading to failure

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Component: Winding Major & Minor Insulat ion

Local overheating

Hydrogen or multi-gas

DGA

Diagnostic Models

Measured signals

Principle leading to failure

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Component: Winding Major & Minor Insulat ion

Excessive moisture

Moisture in oil Oil temperature at moisture measurement location Winding hot spot temperature

Moisture in Insulation

Principle leading to failure

Measured signals

Diagnostic Models

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Component: Winding Major & Minor Insulat ion

Partial discharge

Hydrogen or multi-gas PD

Gas accumulation relay

DGA PD Gas

Accumulation Rate

Principle leading to failure

Measured signals

Diagnostic Models

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Component: Main Tank

Oil level drops Moisture

contamination

Oil level in main tank or conservator Nitrogen pressure Conservator tank bladder rupture N2/O2 Ratio

Oil Preservation System

Principle leading to failure

Measured signals

Diagnostic Models

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Component: LTC Diverter switch and/or selector switch and/or reversing switch

Overheating of contacts (coking) Excessive contact wear Loose or worn contacts Transitional impedance burn out Barrier board tracking and cracking

Diverter switch compartment temperature Main tank temperature Line current Tap position indication Dissolved gas-in-oil

Temperature Differential DGA Contact Wear

Principle leading to failure

Measured signals

Diagnostic Models

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Component: LTC Drive Mechanism

Mechanical defect Broken linkage Binding of contacts Worn gears AC supply failure Defective brake Relay malfunction Weak Springs Geneva wheel adjustment Motor failure

LTC motor voltage LTC motor current Relay timing Tap position Indication AC supply Motor run time

Motor Torque

LTC Operational

Principle leading to failure

Measured signals

Diagnostic Models

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Component: Bushing Condenser Core

Moisture ingress Poor oil impregnation Wrinkled paper Delaminating of paper

Leakage current Phase voltage Temperature

PF Capacitance Temperature

Principle leading to failure

Measured signals

Diagnostic Models

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Component: Bushing Condenser Core

Partial breakdowns due to overvoltage stresses

Leakage current Phase voltage

Temperature

PF

Capacitance Temperature

Principle leading to failure

Measured signals

Diagnostic Models

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PHOTO GALLERY WITH YOUR GREAT CONCLUSIONS

C O N C L U S I O N S & F I N D I N G S

The power industry today demands a more

comprehensive monitoring approach in order to

increase the reliability and availability of electrical

assets by detecting the existence of abnormal changes

in the transformer’s internal condition and determine

whether the changes could lead to a failure. Modern

monitoring solutions support monitoring of several

parameters, include analytics models with diagnostics

capabilities, which allow active detection and

identification of incipient faults in transformers.

C O N C L U S I O N

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P A P E R Q & A C O M E T | C O L L A B O R A T E | S H A R E

K N O W L E D G E | P R O G R E S S

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1 - 2 5 4 - 7 8 0 - 5 2 5 0 e m o r a l e s @ q u a l i t r o l c o r p . c o m E m i l i o M o r a l e s

EMILIO MORALES .

QUALITROL

TAS / TECHNICAL APPLICATION SPECIALIST

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