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The potential of each line of a three-phase system is displaced from the other two by 120 electrical degrees. The sequence, or order, in which the potential of each line reaches its peak value is termed the 'phase sequence' of the system.

The 'normal' phase sequence of an alternator is considered to be the 'positive phase sequence', is arbitrarily assigned colours (e.g. red-yellow-blue, etc.), numbers (e.g. 1-2-3), letters (e.g. A-B-C), or a combination of letters or numbers (e.g. L1-L2-L3).

If that alternator should run backwards, or (far more likely) if its terminals were disconnected and then incorrectly reconnected, the the sequence would be A-C-B (etc.), which would be considered to be 'negative phase sequence'.

Before connecting two generators (or two complete transmission/distribution systems) together, their phase sequence must be confirmed as being the same.

Reconnecting a three-phase motor with the wrong phase sequence would result in it running in the reverse direction and, possibly, damaging its load.

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What is total voltage in 3phase supply when one of the phase becomes zero?

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No. It depends on the inductive and capacitive reactance of the load.


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Yes.


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It is very unlikely that you will ever come across an archaic 'two-phase' system, so your question is of academic interest only. A two-phase, three-wire, system is one in which phase voltages are generated 90 electrical degrees apart and whose line voltage is, therefore, 1.414 greater than its phase voltage. So, the answer to your question is no.


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Rephrase your question so that it makes sense.


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How Negative phase sequence relay work