## 4 Hardware and Implementation Results
The House Load-Inverter (HLI) voltages remain clean and purely sinusoidal at 50 Hz, demonstrating continued effective isolation through the low-voltage DC-link. The grid current shows greater distortion, while the HLI load currents stay unaffected. The harmonic spectrum reveals prominent peaks at both 250 Hz and 350 Hz for the grid-side measurements, while the HLI side remains free of harmonic content. The total active power on the grid side is about 738 W, but the fundamental active power drops significantly to around 566 W due to the added harmonic distortion, whereas the HLI side maintains stable power values with negligible difference between total and fundamental powers. The DC-link voltage is consistently regulated around 376 V, ensuring reliable and isolated operation under severe harmonic grid conditions. Figure 16: System response to a 5 th harmonic voltage. The proposed system introduces a new architecture that enables controlled multi-frequency power transfer through a low-voltage DC back-to-back converter interface while conventional LVDC or hybrid AC/DC systems operate at the fundamental frequency only. The system actively controls harmonic components, including the 5 th and 7 th harmonics together with the fundamental frequency, to enable parallel power channels without exceeding peak voltage limits. The system improves line utilization while maintaining high power quality on the load side through DC-link isolation and grid-forming inverter operation. The dual-mode capability, which combines grid-connected and battery-supported operations, has been experimentally validated under dynamic load conditions to demonstrate both technical feasibility and practical robustness. This paper presents the first experimental proof of a multi-frequency three-phase LVDC system that uses decoupled harmonic power routing and delivers clean-load voltage. The simulation results, together with experimental data, show that the proposed system enables controlled multi-frequency power transfer and maintains strict load-side power quality. The LVDC link architecture proved successful in isolating harmonic-rich grid voltages from sensitive customer loads because it produced a clean 50 Hz output throughout both steady-state and dynamic operating conditions. The DVR-based harmonic injection strategy with multi-resonant controllers achieved exact synthesis of 5 th and 7 th harmonic voltages, which allowed improved line utilization without reaching peak voltage boundaries. The system maintained strong operation during mode changes and load variations, which confirmed the effectiveness of the control system design and the practicality of using DC-link isolation for multi-frequency energy transfer. The proposed system uses 5 th (negative-sequence) and 7 th (positive-sequence) harmonics together with the fundamental component to create parallel power channels without raising peak voltage levels. The selected harmonics function as orthogonal components to both the fundamental frequency and each other in sequence terms. The independent control of each frequency component becomes possible through dedicated resonant controllers because they prevent mutual interference during power transfer. The control structure includes dedicated resonant controllers that maintain system stability for each harmonic. The harmonic voltages injected into the system maintain the peak amplitudes to ensure compatibility with grid-side equipment insulation levels. However, the harmonic content  
Figure 17: System response to the 5 th and 7 th harmonics voltages. will be greater than the levels set by grid codes and standards (e.g., EN50160). Therefore, in these multi-frequency systems, we propose an LVDC back-to-back converter interface that decouples the load from the grid. Thus, the load will be isolated from the harmonics, and a standard-compliant voltage will be ensured for the loads. The DC-link operates as a frequency-selective barrier, which enables the load-side inverter to produce a clean 50 Hz output that protects end-user loads from harmonic effects.