The order in which the 3-phases attain their peak or maximum value and the three generated electrical voltages or currents rotate is called the phase sequence. Each voltage is separated from the next generated voltage by 120 degrees. The 3-phase has only two possible sequences ABC or ACB, positive and negative sequence respectively.
In normal balanced condition, the 3-phase sequential order is positive i.e. ABC. In unbalanced condition, this sequence changed and turned into negative sequence. Unbalanced loads or asymmetric faults reduce the proportion of positive sequence current and produce negative sequence currents. This negative sequence components results the sequential order to change.
Negative phase may cause damage to rotatory machines i.e. AC machines running on 3-phase.Negative phase sequence components create an opposite rotating magnetic field in the stator and induce voltages in the rotor. These induced voltages have a frequency that is double the system frequency.
In Bangladesh 3-phase motors are widely used in industrial and agricultural purposes. Here very frequent, unbalanced load and asymmetrical faults cause negative phase sequence which may damage rotating electrical machines. If the negative phase sequence occurs, it demands a protecting system which will disconnect the rotating loads from the electrical grid. But before disconnecting the loads, it needs to be ensured the presence of negative phase sequence. Hence, to protect 3-phase rotating load, a phase sequence detection system is very essential for each load.
There are many phase sequence detection system. Micro-controller based 3-phase detection system is very cheap and dynamics in operation. Detection of negative phase sequence using micro-controller is the main focus of this thesis.
Details: B.Sc Thesis