3.4. Phase unbalance
In addition to the aforementioned power quality assessment
criteria that are already defined in the IEC standard, a newcriterion
named ‘phase unbalance’ is proposed in this paper because unbalanced
electric currents and voltages can degenerate the performance
and accelerate damage as well. In practice, this often
occurs due to uneven worn of carbon brushes, rotor/stator winding
defects, or other undesired reasons. The phase unbalance status U
can be estimated from I0(t), i.e.,