Creating
a 4 lines 3 phases net in About operation mode of a Zn - transformer and its design. Typical application schemas of a Zn-transformer is shown in the Fig.1. The Zn-transformer has 6 equal windings which are connected in zig - zag. If the 3 phase load is symetrical ( linear load complex impedances ZA = ZB = ZC ) then the there is no current in the neutlal line ( In = 0 ) and the Zn-transformer runs in the no-load operation mode.
If the 3 phase load is symetrical ( l ZA = ZB = ZC) but non-linear (typical for some rectifier) then the currents IA, IB and Ic can have the following form: iA = I1m sin(ωt)
+I3m sin(3ωt)+I5m sin(5ωt)
+I7m sin(7ωt)
+I9m sin(9ωt)+... and the current in the neutral line is: in = iA + iB + iC = 3 *I3m sin(3ωt) + 3*I9m sin(9ωt) + ... where I3m and I9m are max. values of the 3. and 9. current harmonics. For this operation mode the curents through the Zn transformer are: Ia = ib = ic = in/3 = I3m sin(3ωt) + I9m sin(0ωt) + ... Note that the Zn-transformer works in this operation mode as a very powerful filter of th 4., 9., ... current harmonicd. Assune the rms values of the 3. and 9. harmoc in
all 3 phases of a 3x400V, 50Hz, three phase net are: Run theLlarge Transformer èrogram and set the input as follows in the Fig.2:
Rermsrks:
Normally the worst case for the Zn-transformer is unsymetrical operation mode when only one phase is on load:
iA = I1m sin(ωt)
+ I3m sin(3ωt)+I5m sin(5ωt)
+ I7m sin(7ωt)
+I9m sin(9ωt)+... and the current in the neutral line is: in = iA = I1m sin(ωt) + I3m sin(3ωt) + I9m sin(9ωt) + ... For this operation mode the curents through the Zn transformer are: Ia = ib = ic = in/3 = (I1m/3) sin(ωt) + (I3m/3) sin(3ωt)+(I5m/3) sin(5ωt) + (I7m/3) sin(7ωt) ... Assune the rms values of the current
harmonics in the phase A in a 3x400V, 50Hz net are: Run theLlarge Transformer èrogram and set the input as follows in the Fig.4:
Rermsrks:
Finally note that you can use the Small Transformers Program if there are no current harmonics: iA = I1m sin(ωt) |