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Alternator Voltage
What is the problem if run alternator less than required parallel cable in one phase.

One synchronize alternator was found yellow phase shorted with ground. After investigation, Plant people separated the grounded cable where as total 4 cables are parallel in one phase.

Can you explain what will be the problem if run that alternator?

first get the electrical drawings for that installation, and get a qualified electrical specialist involved

I would like to add to the good advice above, that, at its simplest, removing one of four conductors will result in the remaining three conductors having to carry the same current as the other two phases with four conductors. That may require operating the prime mover and generator at less than rated load in order to avoid overloading the three conductors left to carry the current for one phase to prevent overheating the conductors.

The change in resistance/impedance may also impact some of the protective relaying, but I'm not sure of that and can add nothing to the discussion about this possibility.

My biggest concern would be not to possibly overload the three conductors by calculating the difference in current-carrying capability of three- versus four conductors, and if the three conductors cannot safely carry the same amount of amperes as four conductors then the unit output should be limited to protect the remaining three conductors on the one phase.

I would think this would be the absolute minimum consideration when deciding to continue to operate the generator with only three conductors on one phase. A qualified engineer should be consulted for any other possible considerations to be prudent.