Harmonic Analysis of the Transformer in Renewable Energy Systems and Microgrids

In renewable energy systems and microgrids, the transformer windings always use a Y connection at the LV side, and Д connection at the distribution network side or high-voltage side, with the voltage around 10-12 kV, as shown in Figure 6.25. Therefore, the harmonic analysis model of the transformer can be considered as either a voltage source-driven or current source-driven HBFEM model from the LV side.

The waveforms of voltage and current in the LV side contain harmonics due to the transformer being connected to the DC/AC inverters or power electronic drives, are illustrated in Figure 6.26.

Transformer with Y/Д connected to the grid

Figure 6.25 Transformer with Y/Д connected to the grid

Waveforms of voltage and current in the LV side due to power electronic devices

Figure 6.26 Waveforms of voltage and current in the LV side due to power electronic devices

Based on Maxwell equations and a quasi-static magnetic field expressed in Equation (6-2), the magnetic vector potential A on the arbitrary node i in the HBFEM descretized system and the source current density Js can be respectively expressed as:

where harmonic k includes all kinds of harmonics generated by power electronic devices, but only a finite number is required in the real system.

 
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