Power Systems

See these threads:

http://cr4.globalspec.com/thread/64674/What-is-Positive-Sequency

http://cr4.globalspec.com/thread/65826/Zero-Sequence-Reactance

- MS

Physically, in a three phase winding a positive sequence set of currents produces a normal rotating field, a negative sequence set produces a field with the opposite rotation, and the zero sequence set produces a field that oscillates but does not rotate between phase conductors. Since these effects can be detected physically with sequence filters, the mathematical tool became the basis for the design of protective relays, which used negative-sequence voltages and currents as a reliable indicator of fault conditions. Such relays may be used to trip circuit breakers or take other steps to protect electrical systems.

Google on Symmetrical Components for more info.

Please read any good text book on Power System Analysis.