## 3 Three-Port Electrical Converter Technologies
### Three-Port Converters  
Some circuits with TPC technology instead of two individual DC-DC converters have been proposed to overcome the low efficiency, low power density, and high cost issues of traditional PV systems [59; 60]. DC-DC converters equipped with a PV panel and a BESS were integrated into a TPC, resulting in only one power conversion stage between any two ports. Figure 6 illustrates the power flow scheme of traditional TPCs, which include three ports: a PV input port, an energy storage port, and a load port. The PV input port is a unidirectional port that could be a string or array of PV modules. The energy storage port is a bidirectional port that could be connected to supercapacitors and BESSs, which can supply energy to the load and receive power from the PV source. The load port is unidirectional and could be connected to DC loads, such as LEDs and DC motors, or AC loads or an AC grid, through a DC-AC inverter. Figure 5: Traditional configuration of a PV system with two individual DC-DC converters. The red lines indicate the possible directions of power flow. Generally, the power flows among the three ports of a TPC depend on the relationships among the PV power, the BESS power, and the load power [61]. Figure 7 illustrates the operational modes of a TPC (the arrows indicate the power flows), which can be divided into three modes: single-input dual-output (SIDO), dual-input single-output (DISO), and single-input single-output (SISO) modes. When the generated PV power is higher than the load power, the load receives energy from the PV panel through the TPC and the excess PV power supplies the BESS. Thus, the BESS port performs as an output port, and the system enters SIDO mode (Figure 7a). When the PV power is lower than the load power, the TPC is operated in the DISO mode (Figure 7b), as two input DC sources, the BESS and PV, supply energy to the load. For the SISO mode (Figure 7c-e), only one input source and one output port are turned on. This means that one of these three ports (PV port, load port, and BESS port) in the PV system consumes zero power or does not generate any power. TPCs can be categorized into three types: fully isolated, partly isolated, and non-isolated. * _Fully isolated TPCs_ Traditional full-bridge converters and half-bridge converters are commonly used to construct fully isolated TPCs. Since multi-winding transformers and a group of active power switches in full- and half-bridge converters are used, high voltage gain and galvanic isolation between ports can be achieved. But this type of topology has the disadvantages of higher complexity, higher costs, and a larger size [62; 63; 64; 65] than partly isolated and non-isolated TPCs. Table 2 presents a comparison of fully isolated TPCs. Figure 6: Power flow scheme of traditional TPCs. Figure 7: Power flow scheme of traditional TPCs. Operation modes of a TPC: (**a**) SIDO; (**b**) DISO; (**c**–**e**) SISO. In reference [62], an optimal idling control strategy for multi-port converters (MPCs) was proposed. The advantages of MPCs are their simple system structure and low cost. Moreover, MPCs provide an idling port with zero power if one of the ports is disconnected from the system. To ensure low conduction losses, a control strategy based on phase-shift and PWM controls is proposed with a wide soft-switching operation range. In work [63], an asymmetrical duty cycle control method for a fully isolated TPC with three half-bridge converters is proposed. A wide zero-voltage-switching (ZVS) range for each port under variable ultracapacitor and battery voltages can be achieved with a constant DC bus voltage regulated by asymmetrical duty cycle and phase shift control. Reference [64] presents a system architecture with two modules of a three-port isolated DC-DC converter connected in parallel. A control method with an adapted power sharing algorithm was used to improve the global efficiency of the system.