A twelve-pulse bridge consists of two six-pulse bridge circuits connected in series or parallel, with their AC connections fed from a supply transformer that produces a 30° phase shift between the two bridges. This cancels many of the characteristic harmonics the six-pulse bridges produce.
The 30 degree phase shift is achieved by using a transformer with two sets of secondary windings, one in star (wye) connection and one in delta connection.
http://upload.wikimedia.org/wikipedia/commons/6/65/12_pulse_bridge.png
INSTRUCTIONS
In the example I have set the inductances of the transformers to be very small (2mH) because large inductances cause DC offset currents in the primary windings at the beginning of the simulation and it takes too long to reach zero. So by doing that I solved that problem.
That may do the trick in the beginning but it cause large AC currents in the three phase source in steady state.
In order to eliminate that problem too, begin the simulation and wait for the circuit to reach steady state, then press pause and set the inductances of all primary windings to 1 kH and continue the simulation. Now the circuit should work as it should.
Now if you plot the line currents of the three phase sources you may notice that they look much more sinusoidal than in the six-pulse rectifier case, so that means that the current harmonics is significantly reduced.
Don't forget to set the inductances back to 2mH if you want to restart the simulation !!!
