Hybrid Foil-Litz Windings for Highly Efficient and Compact Medium-Frequency Transformers

Abstract

Medium-frequency transformers (MFTs) are key components in solid-state transformers (SSTs), where they provide galvanic isolation and a certain voltage transfer ratio between a mediumvoltage (MV) grid and a low-voltage (LV) bus. Typically, MFTs are operated in the kilohertz range, which results in a significantly reduced volume and material usage compared to 50/60 Hz transformers. Foil conductor is an attractive solution for improving the filling factor and the cost-effectiveness of MFTs, however, the insulation clearances increase the ohmic losses due to the current crowding effect; besides, the electric field hotspots at the winding corners require shielding by equipotential rings. In this work, a new hybrid topology is presented, where the first and last turn of the foil winding package is replaced by turns of litz wire connected in series, one above and one below the foil winding, respectively, such that they act as equipotential shielding rings; besides, the current in the litz turns deflects the magnetic field lines away from the foil edges, decreasing the losses due to current crowding. The effect of the litz rings on the losses is modeled and measured on a hybrid and a standard foil winding prototype; a loss decrease higher than 15% is observed. The verified model is used to compare the conductor and footprint saving between windings designed with litz rings, and with standard equipotential shielding rings; in this respect, a minor amount of litz wire allows reducing losses up to 30%, whereas the conductor utilization and winding height can be decreased more than 20%, improving the power density of the MFT.

166 kW Liquid-Insulated Medium-Frequency Transformer With Hybrid Foil-Litz Windings

“…As analyzed in [8], foil windings suffer from additional losses due to the edge effect. Additionally, the sharp corners in the foil winding originate electric field hotspots in the insulation [12]. To circumvent these drawbacks, the new hybrid foil-litz winding topology is chosen [12].…”

“…Additionally, the sharp corners in the foil winding originate electric field hotspots in the insulation [12]. To circumvent these drawbacks, the new hybrid foil-litz winding topology is chosen [12]. In this topology, the first and last turns of the foil winding package are replaced by turns of litz wire connected in series, one above and one below the foil winding, respectively, such that electric field hotspots at the foil winding corners are suppressed.…”

“…However, litz is more expensive and has a lower fill factor than foil conductor. Although the fill factor of foil conductor is higher, in MV applications, the overall window utilization of a foil winding is lower than with litz wire, since the electric field hotspots at the foil edges require higher insulation distance [11], [12]. The disadvantages of foil conductor in MV applications are overcome in the hybrid foil-litz winding, as detailed in [12], [13].…”

“…Although the fill factor of foil conductor is higher, in MV applications, the overall window utilization of a foil winding is lower than with litz wire, since the electric field hotspots at the foil edges require higher insulation distance [11], [12]. The disadvantages of foil conductor in MV applications are overcome in the hybrid foil-litz winding, as detailed in [12], [13]. This topology offers a cost-effective alternative to litz wire, as it is designed with foil as the main conductor.…”

“…Besides, the electric field hotspots near the foil edges are suppressed, allowing for more compact insulation systems. This topology has been preliminarily validated in [12] with a scaled down prototype.…”