three phase

I wouldn't think 0.5% difference would be a problem.

Fluctuations through the day are probably more than that....maybe caused by single phase equipment loading 1 phase more than the others?

In most industrial applications your voltage readings would be considered very acceptable.

All it takes is a slightly unbalanced load to cause the slight difference you see, or even the turns ratio accuracy on the supply transformer.

If you are using 254V (1 phase+Neutral) or 440 Volt( 1Phase between any 2 -R,Y,B ) worry when you get beyond+/_ 10% limit .

If you are using all 3 phases and no Neutral-say into a motor- Either Star Y or Delta, you can use even at +/_ 15% unbalance.Circulating zero sequence currents will correct the unbalances to a large extent.

1.< how to equal voltage??> You Cannot. You need not.-see above

2. not equal mean what is the advantage and disadvantage???

what is the advantage.=No Advantage

disadvantage= Again none if you are within limits -see above

You really have no problem.

If the lines are loaded it is an asymmetry of the different loads, If un-loaded, it is a slight transformer inaccuracy. Nothing to worry about.

Wangito.

Traditionally, in the UK the red phase had more current and a lower voltage than the other two. The reason is that an electrician, faced with connecting single phase loads onto a new distribution board with a 3-phase input and no specification for doing so, started from the top-left and kept going. Red phase was nearly always at the top.

As others have pointed out, from the figures given there is no need to attempt to equalise the voltage across the phases, and it is difficult to conceive the advantage of so doing. The disadvantage would be the installation of equipment with no return on the cost.