Distribution System Training

The objective of this module is to examine the features of distribution transformers. These devices are located in great numbers around the distribution system to provide the points of interface with customers.

After completion of this module, the participant should be able to understand the following concepts, and apply them in day-to-day work practice.

• Different methods of mounting transformers and the appropriate types
• Basic construction of the single-phase pole top mounted distribution transformer
• Transformer cooling by natural radiation of heat
• Transformer protection devices
• The self-protected distribution transformer
• Grounding the transformer tank
• Transformer overload warning light and counter
• Single-phase secondary connections
• The meaning of additive and subtractive polarity
• Distribution transformers used in underground systems
• Three-phase distribution transformers
• Connection of three single-phase transformers to provide a three-phase bank
• Characteristics of different Wye and delta connections
• Effect on the transformer of out-of-balance current due to single-phase loads
• Operating the open delta and open Wye connections
• Phase shift across a delta-Wye (or Wye-Delta) transformer
• Conditions to observe when single phase transformers are connected to form a three-phase bank
• Significance of phase sequence rotation
• Transformer losses
• Distribution transformer sizing to meet customer demand, taking account of diversity factor
• Distribution transformer tolerance for overload
• Distribution transformer tolerance for high magnitude short-circuit current
• Distribution transformer protection against high voltage impulse surges
• Significance of the customer’s load characteristics