Distribution System Training

The objective of this module is to present the main features of operation and construction of distribution substation transformers. Note that the fundamental concepts of the transformer were discussed in module 4 of this series.

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

• The basic construction and physical arrangement of the oil filed transformer
• Sources of heat generated in a loaded transformer; i.e. iron loss and copper loss
• Cooling arrangements: ONAN, ONAF, OFAF
• The significance of transformer cooling on its rated capacity
• The reason for constructing the iron core of laminated sheets
• Arrangements to accommodate the change in volume of transformer oil under load changes
• Function of the conservator tank
• Nitrogen capped transformers
• Transformer insulation
• The significance of BIL (Basic Impulse Insulation Level)
• Transformer terminal connections
• Three-phase configurations
• The Delta/Wye phase shift
• Winding polarity
• The effect of overloading on the transformer life
• The effects of ambient temperature and pre-loading conditions on transformer overload capacity
• The effect of high magnitude fault currents near to the transformer
• Transformer short circuit capability
• Transformer protection; i.e. built-in devices, and external relays and protection schemes
• Transformer excitation current
• The effect of transformer in-rush current