When the currents on the three live wires of a three-phase system are not equal or are not at an exact 120° phase angle, the power-loss is greater than for a perfectly balanced system. The degree of imbalance is expressed by symmetrical components. Three-phase systems are evaluated at generating stations and substations in terms of these three components, of which two are zero in a perfectly balanced system.
http://en.wikipedia.org/wiki/Three-phase_electric_power#Unba...

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Note added at 11 mins (2013-10-31 16:50:04 GMT)
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http://forums.mikeholt.com/showthread.php?t=153127

Any small difference between generator voltages will
result in unbalanced load division (Figure 5) or, in the extreme, circulating current (Figure 6).
http://www.basler.com/downloads/VR_parallel.pdf