Difference Between Neutral Voltage (Vn) and Zero Sequence Voltage (V0)

Neutral voltage would be measured in the case of a short from your zero sequence source....

https://cdn.selinc.com/assets/Literature/Publications/White%20Papers/0001_MethodsDetectingGroundFaults_NF_20061010.pdf?v=20150812-085711

http://electrical-engineering-portal.com/earthing-grounding-transformer-voltages-during-a-ground-fault

Answer: the earth loop impedance and the system imbalance.

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Neutral voltage is the net resultant of the vectorial summation of the voltages of all the 3 phases . This is observed during un-balanced loading of the 3 phases or due to the DIFFERENCE in leakage inductance of the line at any sub-station supplying industrial loads .The neutral voltage is expected to be limited up to 5 - 10 % .

The Zero sequence voltage is the voltage with no phase difference with respect to any of the phases . The Zero sequence voltage flowing in all the 3 phases has no no phase difference . The zero sequence voltage can flow in the winding of the 3 phase machine simultaneously with no phase difference . IT can also flow in the metallic enclosure of the machine . It is pre-dominantly observed during the LINE - GROUND fault .