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Implement three single-phase, two-winding transformers where all terminals are accessible

The Three-Phase Transformer 12 Terminals block implements three single-phase, two-winding linear transformers where all the twelve winding connectors are accessible. The block can be used in place of the Three-Phase Transformer (Two Windings) block to implement a three-phase transformer when primary and secondary are not necessarily connected in Y or Delta.

**[Three-phase rated power Frequency]**The total nominal power of the three phases, in volt-amperes (VA), and the nominal frequency, in hertz (Hz).

**Winding 1: [phase voltage R X]**The nominal voltage of the three primary windings (labeled 1) in volts RMS (Vrms), the winding resistances, in pu, and the winding leakage reactances, in pu.

**Winding 2: [phase voltage R X]**The nominal voltage of the three secondary windings (labeled 2) in volts RMS (Vrms), the winding resistances, in pu, and the winding leakage reactances, in pu.

**Magnetizing branch: [Rm Xm]**The resistance and reactance simulating the core active and reactive losses, both in pu. For example, to specify 0.2% of active and reactive core losses, at nominal voltage, use Rm = 500 pu and Lm = 500 pu. Lm can be set to

`inf`(no reactive core losses), but Rm must have a finite value.**Note**Refer to the Linear Transformer for explanations on the per unit system.

See `power_3phPWM``power_3phPWM` for
an example of use of the Three-Phase Transformer 12 Terminals block
in a three-phase double-bridge voltage-sourced converter.

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