Figure 2.16 shows three phase rectifier transformers (12-pulse transformer) with Y and delta connections at the secondary side of the transformer. The waveforms of 12-pulse input voltage and current are shown in Figure 2.17, where the harmonics in the current waveform have been significantly reduced.

The output current through Bridges 1 and 2 can be expressed in equations (2-22) and (2-23), respectively.

The current in Bridge 1:

Figure 2.16 Three phase with 12-pulse rectifier transformer with Y and delta connections at the secondary side

The current (30° forward) in Bridge 2:

Combining the current in Bridge 1 and Bridge 2, the following output current can be obtained as:

Harmonics 5, 7, 17, 19,... generated from Bridges 1 and 2 will cancel each other. Only harmonics 11, 13, 23, 25 remain.

A comparison of harmonics generated in 6-pulse and 12-pulse rectifier transformers is presented in Table 2.2, where the 12-pulse rectifier transformer has a significant harmonics reduction.

Obviously, the harmonics of the input current and flux, the effect and the principle of interphase reactor in rectifying circuit can be analyzed by HBFEM. The detailed HBFEM computation model and simulation results will be introduced in Chapters 4 and 6.

Figure 2.17 Waveforms of input voltage and current for 12-pulse rectifier transformer

Table 2.2 Harmonic components in 6-pulse and 12-pulse rectifier transformers