## 3 Three-Port Electrical Converter Technologies
### Three-Port Converters
In reference [65], an energy storage-integrated DC-DC converter with a combination of high power density and multiple inputs was proposed. Soft switching can be achieved without auxiliary devices and components. With the equipment of a half-bridge to each port, the power flow between the input and output is achieved by the regulation of the phase shift angles of the voltages across the two sides of the transformer. * _Partly isolated TPCs_ As only two ports in TPCs need to be isolated, partly isolated TPCs can be used [66; 67; 68; 69]. Partly isolated TPCs can fulfil the galvanic isolated requirements between two of the three ports. Compared to isolated TPCs, generally fewer components are utilized, resulting in high power density and small volume. Since the voltage difference between the PV port and the energy storage port is usually small, circuit topology with isolation between the PV and energy storage ports and the load port is widely used. Table 3 presents a comparison of partly isolated TPCs. Reference [67] presented a proposal of a full-bridge TPC with PWM plus secondary phase shift. A control strategy with two control degrees of freedom is used to regulate the voltage and the power and to achieve ZVS of both the primary and secondary side switches. In reference [66], a full bridge TPC with the integration of two buck-boost converters into the primary side of the full-bridge topology was proposed. The advantages of this topology are that the power conversion between any two of the three ports is single-stage, and ZVS of all the primary side switches are achieved, resulting in high power density and efficiency. In [68], based on an improved flyback-forward circuit, a partly isolated circuit was proposed with a decoupled port control for stand-alone PV systems. The advantages of this topology are high power capability, system simplicity, and low cost. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Topologies** & **Diodes** & **Inductors** & **Switches** & **Capacitors** & \
\begin{tabular}{c} **Rated** \\ **Power (W)** \\ \end{tabular} &
\begin{tabular}{c} **Efficiencies** \\ **(\%)** \\ \end{tabular} \\ \hline
[62] & 0 & 1 & 12 & 1 & 1000 & 88–96 \\
[63] & 0 & 3 & 6 & 6 & 2500 & 90–92 \\
[64] & 1 & 2 & 12 & 3 & 220 & 78–91 \\
[65] & 0 & 1 & 6 & 7 & 6000 & 86–91 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of fully isolated TPCs. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Topologies** & **Diodes** & **Inductors** & **Switches** & **Capacitors** & \
\begin{tabular}{c} **Rated** \\ **Power (W)** \\ \end{tabular} &
\begin{tabular}{c} **Efficiencies** \\ **(\%)** \\ \end{tabular} \\ \hline
[67] & 2 & 3 & 6 & 3 & 800 & 89–94 \\
[66] & 4 & 3 & 4 & 3 & 400 & 90–93 \\
[68] & 3 & 2 & 4 & 3 & 250 & 87–91.3 \\
[69] & 4 & 3 & 3 & 5 & 500 & 50–96 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of partly isolated TPCs. Reference [69] presented a partly isolated topology with an input boost converter for PV-battery power supply applications. By adding an input boost converter based on magnetic switching, the PV current is continuous, and the input voltage is stepped up. Additionally, zero current switching (ZCS) technology is used to achieve high power conversion efficiency. * _Non-isolated TPCs_ In non-isolated TPCs, galvanic isolation is not critically required [70; 71; 72; 73; 74; 75]. The topology is mostly based on buck, boost, and buck-boost converters, resulting in limitations of the voltage gain. To achieve high voltage gain, some non-isolated TPCs with coupled inductors are proposed. Compared to fully and partly isolated TPCs, non-isolated TPCs use fewer components, resulting in higher efficiency, higher power density, smaller size, lower cost, and simpler control. In reference [18], a TPC for energy storage-integrated PV systems was proposed with only one inductor and two power switches. Due to the employment of partial power regulation, the power conversion efficiency is improved.