Power Transformers Construction

Power transformers construction involves units to be configured as either a single-phase primary configuration or a three-phase configuration. The size and cost of power transformers construction increases when you move down the listing of primary windings. Single-phase primary configurations include single, dual, quad (2+2), 5-lead, and ladder. A 5-Lead primary requires more copper than a Quad (2+2) primary. A Ladder is the least economical primary configuration. Three-phase transformers are connected in delta or wye configurations. A wye-delta power transformer has its primary winding connected in a wye and its secondary winding connected in a delta. A delta-wye transformer has its primary winding connected in delta and its secondary winding connected in a wye. Three phase configuration choices include delta - delta, delta - wye (Y), wye (Y) – wye (Y), wye (Y) – delta, wye (Y) – single-phase, delta – single phase, and international. Primary frequencies of incoming voltage signal to primaries available for power transformers include 50 Hz, 60 Hz, and 400 Hz. 50 Hz is common for European power. 60 Hz is common in North American power. 400 Hz is most widely used in aerospace applications. The maximum primary voltage rating is another important parameter to consider. Power transformers should be provided with more than one primary winding if it is to be used for several nominal voltages.

Students will learn safe and proper power transformers construction and maintenance/testing procedures on pad-mounted power transformers. The course covers transformers used in commercial and industrial power distribution systems, including oil and dry-type units. Larger power transformers used in utility applications are also covered.

Load and no-load tap changer maintenance, ac and dc testing, routine inspections and oil sampling and testing are covered in detail. Students will learn how to perform routine oil tests, and understand how to perform the major tests that are required. A variety of electrical test equipment from various manufacturer’s will be discussed.

Who Should Attend:
Field and shop technicians, field engineers, supervisors and others responsible for the testing and maintenance of power transformers rated 750kVA to 500MVA and 4.16kV to 500kV.

Upon completion of this course, the participant should be able to:

  1. Explain the basic operation of a transformer.
  2. Discuss turns ratios and calculate terminal voltage and current.
  3. Discuss terminal markings and various single phase and three phase wiring schemes. (WYE vs DELTA).
  4. Explain how to perform a polarity test on a potential transformer.
  5. Discuss the electrical testing performed on transformers such as insulation resistance testing, excitation and power factor testing.
  6. Discuss the various tests performed on insulating oil.


  1. Transformer Principles
    1. Transformers
    2. Right Hand Rule
    3. Magnetic Flux
    4. Magnetic Induction
    5. Left Hand RuleTurns Ratio
    6. Transformer Losses
    7. Transformer Types
  2. Vector Diagrams
    1. Operating Principles
    2. Operation Without Load
    3. Operation with Load
  3. Transformer Classifications
    1. Service Classification
    2. Instrument Transformers
      1. Current
      2. Potential
  4. Transformer Construction (Construction)
    1. Magnetizing Circuits
    2. Losses
      1. Hysteresis Loss
      2. Eddy-Current Loss
    3. Efficiency
  5. Cooling
    1. AirCooledOil-Immersed
      1. Water-Cooled
      2. Forced-Oil Cooling
      3. Self-Cooling with Air Blast
    2. Temperature Limits
    3. Transformer Loading
  6. Tap Changers
    1. No-Load Tap Changer
    2. Tap Changing Under Load
    3. Phase-Angle Control
  7. Transformer Connections
    1. Single Phase Circuits
    2. Two-Phase Circuits
    3. Three-Phase Circuits
  8. Transformer Maintenance
    1. Insulation Testing
    2. High Potential Testing
    3. Turns Ratio Testing
    4. Polarity Testing
    5. Power Factor
    6. Excitation Current
    7. DC Winding Resistance
    8. Polarization Recovery
    9. Insulating Fluid Dielectric
    10. Dissolved Gas Analysis
  9. Transformers and Relaying
    1. Transformer Faults
    2. Differential Relaying
      1. Normal Load
      2. External Faults
      3. Internal Faults


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