96 frequency AC currents and voltages. This allows a single transmission line to be supplied with high-voltage (HV) and high-frequency (HF) excitation, typically in the range of 5-30 kHz, compared to the conventional 50/60 Hz operation (Kasprzak et al., 2022). In the project, the solution was proposed, in which a high-frequency SWET line operating in resonant mode connects the ESS to a three-phase 400 V AC grid. This approach results in significant improvements in the operating parameters of the SWET line. Furthermore, within the POSTEN project, the use of a SWET line also offers an economic advantage over transmission via a conventional three-phase low-voltage overhead line (3 × 400 V). Energy transfer is achieved via a thin, single conductor with a small cross-section, enabled by the relatively high operating voltage of approximately 5 kV. Figure 4 – SWET converter system schematic with key electrical parameters The SWET system consists of three power electronic converters, a single-wire transmission line with grounding, and auxiliary resonant and transformer stages, as shown in Figure 4. Energy is injected into the SWET line by a SiC MOSFET-based resonant inverter INV1 operating at 7-12 kHz and supplying a high-voltage level of approximately 5 kV through a step-up transformer TR1 and LC1 resonant tank. The single-wire overhead line, approximately 800 m long, uses earth return and is supported by lightweight composite poles. On the output side, a step-down transformer TR2, a resonant tank LC2, and a rectifier REC2 produce a regulated DC bus that feeds a three-phase grid-connected inverter INV2, delivering 400 V, 50 Hz output power at approximately 100 kW. Based on preliminary calculations of semiconductor losses, transformer efficiency, and estimated line losses, the overall efficiency of the SWET energy-transfer system is expected to exceed 90%. However, the final value will depend on the optimized LC tank design, the selected switching frequency, and the SWET line grounding conditions. The SWET line offers several advantages that make it a promising candidate for the ESS to grid energy-transfer link. The most important of these include: accurate power-flow control, achieved through the modulation of INV1 and the operating-point control of INV2; use of lowcost, high-strength steel conductors which can replace conventional copper wires while maintaining a small cross-section; very low transmission losses typically below 1% excluding ground-path losses and converter losses; active voltage-drop compensation, ensured by appropriate INV1 control; inherent self-protection against line short circuits, as detuning from resonance naturally reduces the transmitted power and leads to a drop in line voltage.
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