## 2 Methodology  
The architecture of the proposed system is illustrated in Figure 1. It consists of a low-voltage DC-link interconnecting two independent three-phase inverters: one interfacing with the grid side and the other supplying the customer side. This structure enables the decoupling of grid-side disturbances from the sensitive customer loads while supporting multi-frequency power transfer and efficient energy management. The growing use of distributed energy resources (DERs), including PV systems and BESSs, has prompted the creation of hybrid AC/DC microgrid architectures that provide better flexibility and efficiency. LVDC links have established themselves as essential components for linking various power sources while enabling independent management of power generation and load consumption. The idea of multi-frequency power transfer has received notable interest because researchers intentionally add the 5 th and 7 th harmonic frequencies to the fundamental component to create multiple parallel energy paths in one network structure.  
Traditional systems approach harmonics as unwanted disturbances that need to be eliminated. Research shows that superimposing frequency components enables better transmission capacity usage while creating possibilities for energy packet transfer. This research develops an innovative system design that uses an LVDC link to divide grid functionality from customer functionality and enables multi-frequency operation.  
A three-phase inverter on the grid side maintains DC-link voltage stability and performs power exchange with the grid through fundamental frequency and 5 th and 7 th harmonic operations. The implementation of a Dynamic Voltage Restorer (DVR) at the point of common coupling (PCC) allows active harmonic voltage injection for proper control of multi-frequency signals. A different three-phase inverter located on the consumer side generates a pure fundamental-frequency (50 Hz) voltage supply to loads while blocking upstream voltage disturbances but transferring total power from the grid side. The system design provides sensitive customer loads with high power quality through fundamental-frequency supply while maintaining flexible and efficient grid interaction.  
Figure 1: System configuration diagram.  
The multi-frequency grid voltage consists of 50 Hz and 350 Hz positive sequence, and 250 Hz negative sequence, which maintains a balanced three-phase configuration. The grid voltage after harmonic injection can be formulated by these relations:  
\[v_{a}(t)=V_{1}sin(\omega t)+V_{5}sin(5\omega t)+V_{7}sin(7\omega t)\]  
\[v_{b}(t)=V_{1}sin(\omega t-120^{\circ})+V_{5}sin(5\omega t+120^{\circ})++V_{7} sin(7\omega t-120^{\circ}) \tag{1}\]  
\[v_{c}(t)=V_{1}sin(\omega t+120^{\circ})+V_{5}sin(5\omega t-120^{\circ})++V_{7} sin(7\omega t+120^{\circ})\]  
where \(V_{1}\), \(V_{5}\), and \(V_{7}\) are the fundamental, fifth, and seventh harmonic voltage amplitudes, respectively, and \(\omega\) is the fundamental angular frequency.  
The proposed system allows PV and BESS components to be integrated into the DC-link through bidirectional energy flow, which improves microgrid resilience. The system maintains strong decoupling between grid dynamics and end-user environment by using multi-frequency control for grid-side operations combined with fundamental-frequency regulation for customer-side operations. Through its LVDC link function, the system creates both an energy storage capability and an interface that splits areas with harmonic presence from areas without harmonics.  
The research describes modeling approaches alongside control design methods and analysis for this architecture while discussing difficulties and resolution strategies related to multi-frequency power transfer and DC-link voltage stability and DVR-based harmonic injection and customer-side voltage regulation.