Negative sequence currents arise whenever there is unbalance in the system. A negative sequence component produces a resultant reaction field which rotates counter to the DC field system and a flux which cuts the rotor at twice the rotational velocity. This therefore induces double frequency currents in the field system and the rotor body. This effect is not present in the balanced system and accounts for the difference between positive and negative phase sequence reactances quoted by manufacturers.
Regards
Marmite
