Electric Power Principles, 2nd Edition

6Transformers

Transformers are essential parts of most power systems. Their role is to convert electrical energy at one voltage to some other voltage. Transmission lines are typically operated at voltages that are substantially higher than either the generation or utilization voltage, for at least two reasons.

• Because power is voltage multiplied by current, and because transmission line losses are proportional to the square of current, higher voltages generally produce lower transmission line losses.
• Transmission line capacity to carry power is roughly proportional to the square of voltage, so to make intensive use of transmission line right‐of‐way, that is, to enable transmission lines to carry high power levels, high transmission voltages are required. This is true for underground cables as well as for above the ground transmission lines.

For these reasons, a transformer is usually situated right at the output of each generating unit to transform the power from generation voltage, which is usually between 10 and 30 kV, and transmission voltage which is in the high or extra high voltage range, typically between 138 and 765 kV. At substations that connect transmission lines to distribution circuits the power is stepped down in voltage. Distribution circuits generally operate in the range of 6–35 kV. (However, there are still lower voltage distribution primaries and some higher voltage circuits might be classified as distribution.) Then, before electric power is connected ...