TRANSFORMER FAILURES AND CASE STUDIES

By

Veerabrahmam Bathini

Power Research & Development Consultants Pvt. Ltd., #5, 11th Cross, 2nd Stage, West of Chord Road,

Bangalore – 560086, Karnataka, India

OUTLINE

• Classification of Transformer Failures

• Major Failures in Transformers

• Design Aspects of Transformers

• Protection of Transformers against Problems

• Case Studies

• Conclusions

Classification of Transformer Failures

1. Failure Attributed by Users

Prolonged over loading

Single phase loading

Un-balanced loading

Faulty terminations

Power theft by hooking

Faulty earth connection to tank body as well as LV terminal

Failures due to external short-circuit

Less maintenance

Improper installation

Classification of Transformer Failures

2. Failure Causes at Manufacturer’s end

Faulty design

Poor quality of material

Bad workmanship

Improper transportation

Sharp edges of conductor

Incomplete drying

Bad insulation covering on conductor

Improper joints or connection

Fig. 1. Winding Insulating Process

Fig. 2. Improper Joint

Classification of Transformer Failures

3. Failure During Working Condition

Deterioration of oil

Faults in magnetic circuit

Inadequate pre shrinkage of the winding

Inter turn faults

System Transients and Overvoltages,

Fig. 3. Failure due to Moisture

content in Oil Fig. 4. Internal Faults

Fig. 5. Core Insulation Failure Fig. 6. Failure due to Paper Block Shrinkage

Transients in Power Systems

10-4

10-3

10-2

10-1

100

101

102

103

104

105

106

Frequency f in Hz

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Freq

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Very Fast Transients, SF6 Transients

Fast Transients, Lightning

Slow Front Transients, Switching

Temporary Overvoltages, ferroresonance

Steady State Overvoltages

Subsynchronous resonance

Transient Stability: Machine–rotor Dynamics

Interarea Oscillations

Mid term & long term Stability: Automatic Generation Control

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Classification of Overvoltages

• Steady state

• Temporary

• Slow front

• Fast front

• Very fast front

Major Failures in Power Transformers

• Winding Failure

• Bushing Flashover

• Tap Changer Failure

• Core Failure

• Tank Failures

• Protection System Failure

• Cooling System Failure

Design Aspects of Power Transformers • Accurate electromagnetic field calculations

—Minimization eddy and stray losses —High rate of loss capitalization

• Withstanding against system Overvoltages —Reliable high voltage insulation design

• Stray loss evaluation and Control • Short Circuit Stresses and Strength

—Analysis of Dynamic Behavior

• Elimination of hotspots in —Core , Winding and structural parts

• Robust Structural Design —Seismic withstand, transport induced stresses

• Design of Cooling Systems • Lower Noise Levels

Protection of Transformers against problems

• Overfluxing Conditions

• Part winding resonance

• System generated Overvoltages (FFO,VFTO, SFO)

• Ferroresonance

• Power System harmonics

• Geomagnetic Disturbances

• Static electrification phenomenon

Case Studies • Bushing Failure due to combustible gases formation in Oil

Case Studies • Transformer failure due to Part winding resonance

Case Studies • Transformer Insulation Failure Wind Energy Applications

Conclusions

• Proper Design

• Proper Workmanship

• Proper Maintenance

• Properly rated Surge Protective Devices

• Protective Relays

Discussions?