Ultrasonic Inspection: Electrical Systems

Introduction

When power leaves the generator at a power plant, it first runs through a transmission substation located at the plant. This substation converts the generator's voltage from a level in the thousands to extremely high voltage levels for long-distance transmission on the power grid. The power then comes off the grid and is stepped down at a power transmission/distribution substation. The distribution substation uses transformers to reduce the voltage from a level of 100's of thousands of volts to less than 10 thousand volts. It also has a "bus" that splits the power off in multiple directions. Often circuit breakers are installed on the substation so that the substation can be disconnected from the main power grid when necessary.

Transformers come in different sizes and have different ratings. You can find small transformers all over the house attached to phones, computers, appliances, etc. The large transformers at a power substation perform the same task but on a much greater scale. Transformers are used to change voltage of alternating current either down or up. Two windings are used called the primary and secondary windings. Current comes into the primary winding at one voltage. The current creates a magnetic field in an iron core that runs along the inside and outside of the winding. The secondary winding wraps around the iron core, where the magnetic field creates current. The voltage in the secondary winding is controlled by the ratio of the number of turns in the two windings. If there are more turns in the primary winding than the secondary, the voltage is stepped down. If there are more turns in the secondary winding, the voltage is stepped up.

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