Aim:
This project is to make high efficient digital three phase volt meter , with following specification using micro controller

Accuracy (+ or -) 0.25% F.S. (+ or -) 1 digit
Sampling Time Abt. 0.8 sec. typically
Aux. power source AC 110/220V (+ or -) 10%m 50/60Hz
Power Consumption <AC 3 VA
Operating Temp. range 0-60 deg. C
Storage Temp. range -10 -70 deg. C
Temperature coefficient <100PPM/deg. C
Max. relative humidity 95%
Dielectric strength (IEC688)
AC 2KV/1Min
Input/power/output terminals
AC 3KV/1min
All terminals to case
Impulse (IEC 255-4) Common mode 1.2 x 50 micro-sec
4KV
Display 14.2mm (0.56") H red LED

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i suggest you people need to read
http://circuitstodayac-voltmeters and http://elec-toolboxusefulinfo/xfmr-3ph.htm