Positive phase sequence:
A system of vectors is said to have positive sequence if they are all of equal magnitude and are displaced by 120° with same time interval to arrive at fixed axis of reference as that of generated voltage. The positive phase sequence is represented below and the vectors arrive along X-axis in order 1, 2, 3 and conscript P has been used to designate as positive sequence.
Negative phase sequence:
A system of vectors is said to have a negative phase sequence if they are of equal magnitude displaced at an angle of 120° but arrive at the axis of reference at the regular interval same as that of positive phase sequence but in order of 1, 3, 2. That is the order is reversed.
Zero sequence:
A system of vectors in a phase system is said to have zero phase sequence if all the three vectors are not displaced from each other and there will be no phase sequence in such cases. The current or voltages in the 3-phase circuit vary simultaneously in all the 3- phases. Such phase sequence is zero phase sequence.