This answer comes from Richard P. Schulz and Navin B. Bhatt of the American Electric Power Co., an investor-owned utility based in Columbus, Ohio.

The phase differences among synchronous power generators are directly related to electromagnetic fields, which are used by conventional alternating current systems to create, transmit and distribute electric energy. A simple analogy illustrates this:

Envision two strong bar magnets at rest, magnetically coupled to each other on opposite sides of a thin piece of glass. Were there no friction between the magnets and the glass, the magnets would become aligned because each one contributes to and is linked to the magnetic field of the other. And they would line up so that the path of their common magnetic field would be at a minimum; in other words, their alignment would minimize the distortion and the energy in the field.

If one magnet were moved across the glass and there was no friction, the other magnet would follow and align itself again. If one magnet were held while the other magnet was moved, the force on one magnet would be an exact mirror of the force on the other. The energy needed to move or hold a magnet, which depends on the force required and distance involved, increases the magnetic field energy. So if a magnet were twisted on an axis perpendicular to the glass, the forces involved would be torque and the displacements would be angles.

A generator in an electrical power plant relies on the same principle. Direct current running through coils on the generator shaft --called the field winding--create part of the magnetic field. The other part is created by currents running through coils on the stationary part of the generator, called the armature winding. Both coils are built so that, when the generator is not turning, a current in one produces a magnetic field that crosses the other axially.

The magnetic field from the field winding turns with the generator rotor. When the field winding is rotated at a certain speed, its magnetic field spins past the (stationary) armature windings, thereby inducing voltages on the armature. If the generator is "open circuited"--meaning there are no connections to the armature windings--then the induced voltages appear on the generator terminals.

A three-phase alternating current in the stator also produces a magnetic field that spins around the generator's axis at a speed corresponding to the current's frequency. Under normal circumstances, when the generator is connected to a transmission network, the magnetic field contributed by these currents spins in sync with the one generated by the field winding. When the generator shaft is not rotating, the two magnetic fields will be exactly aligned--a situation similar to two bar magnets on frictionless glass. But as power is applied, the shaft and its magnetic field move ahead of the combined magnetic field. Thus, the shaft and its field winding "pull" on the magnetic fluxes and the induced voltages in the armature.

North American Power Distribution

Image: American Electric Power

This "pulling" action sets the rotor about 40 to 75 degrees ahead of the armature current. This phase advance is much like that occuring when one bar magnet turns another on the other side of the glass. It is in this way that energy transfers from the shaft and its magnetic field to the armature and into the transmission system.

The electrical power from the generator moves through transformers and across power lines to the users. Because large transformers and high-voltage transmission lines are built to cause very low losses, they have low resistance to electric currents flowing through them. Power currents flowing through transmission lines and transformers create magnetic fields around the wires in the lines and in the transformer windings. These fields cause an impedance to the flow of current.

In high-voltage transmission lines (those over approximately 100 kilovolts), this inductive impedance is greater than the effect of resistance by at least a factor of 10 and more likely, 20. The power currents flowing through the inductive impedance of transmission lines and transformers cause a phase delay. That is, the receiving-end voltage lags behind the sending-end voltage.

In transmission lines and transformers, power transfers are fundamentally linked to the phase shift of the voltage from end to end. The switching stations--where generators, transformers, lines, and customers are connected to the power system--include large conducting structures called "buses." Here measurements such as voltage and voltage phase angles are made. For power to flow on the network or grid, each bus must be somewhat out of phase with other buses. This is akin to the fact that for air to flow from one location to another in a weather system, there must be a pressure difference between the two spots.

Four large power networks in North America run with synchronous transmission systems operated at 60 cycles per second. Three of the synchronous networks (illustration) are interconnections of many utilities; the fourth, Quebec, connects to only Hydro Quebec. Other smaller networks exist in Alaska, Hawaii, Mexico, Puerto Rico and elsewhere.