Negative-sequence current

In a perfectly balanced system, no negative phase sequence currents would exist. However, it is virtually impossible to acheive this perfectly balanced system in practice and so these negative phase currents need to be considered.  Line voltage imbalances caused by electrical faults or imbalanced loads lead to current imbalances in each conductor. Therefore, the magnetic coupling between windings becomes uneven. A counter rotating field (in respect to the main field) will now exist and the resultant field will cause undesirable eddy currents to flow. The consequenes of this for generators will either be a loss of torque, or depending on the load, will increase the current for the same slip speed and hence raise the temperature of the alternator. 
In a rotating machine, the negative sequence current vector rotates in the same direction as the rotor. It is the magnetic flux produced by the negative sequence current that rotates in the reverse direction of the rotor. Thus, the rotor cuts through the flux at twice the synchronous speed, and the induced current in the rotor is twice the line frequency. Regarding measurement of negative sequence, it is measured by the negative sequence filters within the relays.
The net torque is reduced and if full load is still demanded, then the motor will be forced to operate at a higher slip, thus increasing the rotor losses and heat dissipation.