## 1 Introduction  
The need to further advance the development of renewable energy sources points to exploring new ways of implementing power systems for new stationary and mobile applications and integration with energy storage. The conventional structure of DC power systems with renewable sources were constructed such that each source interfaces to the power bus via its own DC-DC converter which controls its mode of operation and adapts its voltage level with the bus voltage. One of the most common examples for such a structure are the stand-alone photovoltaic (PV) power systems which consists of two DC-DC converters: one is unidirectional for PV maximum power point tracking (MPPT) and the other one is bidirectional to control battery charge/discharge operations [1; 2; 3; 4] as seen in Figure 0(a). Recently, multiport converter (MPC) topologies have been shown to provide advantages compared to the convention systems, including lower cost and higher power density. The MPC topology can be fully isolated implementing multi-winding transformers with half-bridge topology [5] or full-bridge topology [6, 7, 8] to allow full isolation between all ports. Other alternatives are partially isolated [9, 10, 11, 12, 13, 14] or non-isolated topologies [15, 16, 17, 18, 19, 20, 21, 22]. The isolated and partly isolated MPCs are better choices for applications that require some sort of galvanic isolation, however the use of transformer may reduce the power density of such converters. On the other hand the non-isolated MPCs can have simpler control design and better efficiency [23] but with limited voltage mismatch level between sources and the power bus. The structure of most of these MPCs are centralized topologies, built based on a central converter that interfaces different power sources and energy storage with the power bus as seen in Figure 1b.The problem with these centralized multiport converters (CMPCs), when a PV-battery system is considered, is that they require strings of a high number of series-connected PV-panels in order to be able to interface to high voltage power buses to minimize converter current rating but this leads to several disadvantages. The most important one is the severe reduction in the throughput power when one or several panels are subject to a partial shading or partial shading-like characteristics due to panel irregularities as investigated in References [24, 25, 26, 27]. Additionally, all the PV-panels within the string must be at the same orientation in order to achieve identical light incident angles and balanced power for all panels within the string. This limits the degree of freedom for mounting the PV-panels which needs to be maximized, especially for the household applications due to the limited availability of space. Similarly, a CMPCs with a higher voltage power bus also requires battery strings with a high number of series-connected battery cells which have several disadvantages related to the need for additional complex charge balancing circuitry and limited utilization of the available capacity of the battery as investigated in References [28, 29, 30, 31, 32]. Additionally, the CMPCs are not fault tolerant as a single failure within the converter will cause the overall system to be unusable. The modular structure of the MPCs shown in Figure 1c can improve system flexibility and fault tolerance as presented in Reference [33], where the structure of a power system based on parallel MPCs has been investigated. However, it kept the centralized structure of PV/battery strings that needs a high number of series-connected PV/battery cells for interfacing to a higher voltage power  
Figure 1: Power system structure with (**a**) multiple single DC-DC converters; (**b**) Conventional centralized multiport DC-DC converter; (**c**) proposed modular multiport DC-DC converter. buses.