## 1 Introduction  
Photovoltaic (PV) systems are well known for their potential in green energy with the advantages of pollution-free power generation and low-cost operation and maintenance [1, 2]. The main problem with PV systems is the uncertainty of the generated power, which depends on the weather [3, 4]. To avoid energy waste and fulfil output requirements, energy storage-integrated PV power systems are used, normally with two individual DC-DC power converters [5, 6, 7]. One of the converters delivers the PV power to the load and/or battery with the implementation of maximum power point tracking (MPPT) algorithms. The other converter functions as a load power regulator. All the power generated by PV modules and/or battery is processed through these DC-DC converters, leading to limited efficiency of the whole PV system, low power density, and high costs [8, 9]. Up to now, a variety of three-port converters (TPCs)--with a renewable energy input port, an energy storage input/output port, and a load port--have been proposed to improve power conversion efficiency and the power density of the overall circuit [10, 11]. They can be categorized into three types: fully isolated, partly isolated, and non-isolated TPCs [12, 13, 14]. With the advantages of galvanic isolation between the ports and high voltage gain, fully isolated TPCs are commonly equipped with multi-winding transformers and groups of active power switches [15]. However, using transformers and multiple switches increases both the system's volume and cost. In applications where not all ports need isolation, partly isolated TPCs are an option, allowing for higher power density and reduced volume [16]. In cases where galvanic isolation is unnecessary, non-isolated TPCsare preferred, offering benefits such as greater efficiency, higher power density, smaller size, lower costs, and simpler control compared to the other types of TPCs [17]. In addition, some TPCs with partial power regulation (PPR) have been proposed to further improve the power conversion efficiency of the whole PV power system [18; 19]. Moreover, PV panels are usually connected in series to cope with high voltage and power requirements [20]. For these internal connections of PV panels, partial shading situations are frequently encountered, causing some issues such as the diminution of the output voltage and power, some hot spots in the panels, and reduction of the system lifetime [21]. To overcome these problems, the distributed PV system with distributed maximum power point tracking (DMPPT) was introduced [22; 23]. In this system, each solar panel is connected to an individual DC-DC converter to supply the energy to the load. With such a configuration, every PV panel can be operated at the maximum power point (MPP) by regulating its associated converter, avoiding energy waste. However, since the energy generated by the PV panel highly depends on the weather conditions, the PV power varies with the time. Mismatches between the generated PV power and the required load power may happen. When the power generated by the PV panels is higher than the load power, to maintain the generated and delivered power at the same level, either a PV power curtailment algorithm [24] is needed to reduce the generated PV power to the load power level, or a BESS is required to be connected to the PV systems to absorb the surplus PV power. Compared to the employment of a PV power curtailment algorithm, the PV power in BESS-integrated PV systems is optimally used, avoiding energy waste [25]. This work explores three-port and partial power conversion technologies for PV generators. The main contributions include a detailed discussion and comparison of various TPCs, focusing on power conversion efficiency and component characteristics. It also analyzes TPCs with partial power regulation through prototype examples, highlighting their advantages and disadvantages. Additionally, the benefits and drawbacks of distributed PV systems with and without a BESS are examined in detail.