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Partial discharges: keys for condition monitoring and diagnosis of power transformers

Partial discharges: keys for condition monitoring and diagnosis of power transformers

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The combination of thermal and electrical stress, in conjunction with the synergic effects of moisture in the insulation, might dramatically reduce the lifetime of a power transformer, which is roughly estimated to be 40 years if properly maintained, meaning that many transformers do not experience any failures before being dismantled. Mechanical stresses during short-circuits can also play a role leading, in the worst case, to the radial buckling of the winding and damage of the connection cables at the bushing terminations. A number of these factors can, in the course of time, give rise to partial discharge (PD) phenomena which can be a cause of failure or a symptom of degradation. PD measurements can be carried out conventionally, following the standards IEC 60270 and IEC 60076. Alternative methods, such as those presented in the IEC TS 62478 standard based on acoustic or ultra-high frequency sensors, have been proposed and might offer better sensitivity, particularly for monitoring purposes, and the capability to locate the PD sources. For all these methods, identification of the PD source is a key to achieve a correct course of action (run, repair and replace).

Chapter Contents:

  • Abstract
  • 2.1 Introduction
  • 2.2 Dielectric materials used in power transformers
  • 2.3 Effects of ageing in insulation systems of power transformers
  • 2.3.1 Thermal stress
  • 2.3.1.1 Cellulose
  • 2.3.1.2 Oil
  • 2.3.2 Mechanical stress
  • 2.3.3 Electrical stress
  • 2.3.3.1 Tracking on barriers
  • 2.3.3.2 Bubbling
  • 2.3.3.3 Incomplete filling of turrets
  • 2.3.3.4 Metallic objects
  • 2.3.3.5 Improper spacing of tap changer conductors
  • 2.3.3.6 Static electrification
  • 2.3.4 Ambient stress
  • 2.3.4.1 Water leaks
  • 2.3.4.2 Moisture transients
  • 2.3.4.3 Insulating particles
  • 2.3.4.4 Metal particles
  • 2.3.4.5 Oxygen and moisture leaks
  • 2.3.4.6 Sludge
  • 2.4 Condition monitoring techniques in power transformers
  • 2.4.1 Electrical measurements
  • 2.4.1.1 ABC circuit
  • 2.4.2 Apparent charge estimation: quasi-integration and calibration
  • 2.4.2.1 Principle of quasi-integration and calibration
  • 2.4.2.2 Limits
  • 2.4.3 PD detection in transformers
  • 2.4.3.1 Circuits for PD detection
  • 2.4.3.2 Interference and noise
  • 2.4.3.3 Inductive sensors
  • 2.4.4 Unconventional methods of partial discharge measurements in power transformers
  • 2.4.4.1 Acoustic
  • 2.4.4.2 Ultra-high frequency
  • 2.4.5 Methods of partial discharge analysis
  • 2.4.5.1 Phase-resolved partial discharge patterns
  • 2.4.5.2 Time –frequency maps
  • 2.4.5.3 Power-ratio maps
  • 2.4.5.4 Key characteristics identification
  • 2.4.5.5 Machine learning
  • 2.4.5.6 Comments on different approaches
  • 2.5 Conclusions
  • Acknowledgements
  • References

Inspec keywords: buckling; condition monitoring; power transformer insulation; IEC standards; partial discharge measurement; transformer windings; thermal stresses

Other keywords: ultrahigh frequency sensors; thermal stress; electrical stress; PD measurements; radial buckling; mechanical stresses; bushing terminations; connection cables; PD source; partial discharge phenomena; insulation; condition monitoring; winding; IEC 60270 standard; synergic effects; power transformer diagnosis; IEC 60076 standard; short-circuits; IEC TS 62478 standard

Subjects: Transformers and reactors; Insulation and insulating coatings; Dielectric breakdown and discharges

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