What Causes Under Voltage on Over Head Lines?

one phase of delta side of distribution transformer might be under open circuit fault.

Thanks for your response,do you mean to say that as a result of the open circuit in one of the phases,there was an inrush of current in the healthy phases thereby pushing voltage down?

well i am not sure about that but i guess someone more experienced will give you an explanation.I can assure you there is a solid explanation for anything happens in power systems.

If one phase is open on the delta primary (fuse blows, tree branch breaks wire, etc.) but there isn't a fault that trips the source's circuit breaker, the current will still flow through the delta winding from the two remaining phases, but divided and unbalanced between the windings (Draw the picture - two windings in series, in parallel with the other winding). Thus two of the 3 secondary wye windings will have a proportionally lower voltage due to the phase voltage split across the 2 series primaries. Once the utility restored the 3rd phase, it was back to normal.

This condition ("single-phasing") is particularly bad for motors, and will burn them out quickly. That is why phase imbalance protection is important on larger motors.

will overloaded cause voltage drop?