# A Three-Port Power Electronic Transformer Based on Magnetic Integration
## 1 Introduction  
Power electronic transformer (PET) has drawn increasing attention in distribution system due to the advantages of high flexibility and controllability [1, 2, 3]. Recently, the multi-port PET which can simultaneously interconnect medium-voltage DC (MVDC), medium-voltage AC (MVAC) and low-voltage DC (LVDC) systems has become a hotspot [4, 5]. The MVDC port can be used to integrate large-scale renewable energy into the grid with the features of no reactive power, lower loss, etc. In addition, for the LVDC port, it can directly connect with the low voltage energy storage system (ESS), decentralized renewable energy (PV, wind turbine, etc. ), hydrogen production, EV station and other DC loads. The existing typical three-port PETs are mainly based on cascaded H-bridge (CHB) or MMC, which are used to withstand the medium voltage. The CHB-based PET is investigated in [5, 6] (named as CHB + common HF bus PET), where the CHB connects to an MVAC port while the MVDC and LVDC ports are obtained from a common high-frequency (HF) bus by adopting a large number of paralleled AC/DC cells. The employment of too many power conversion stages limits its application due to high conversion loss and high cost. In comparison, MMC-based multi-port PET can reduce the conversion stage, as discussed in [4, 7] (named as MMC + distributed DABs PET). By adding a dual active bridge (DAB) to each MMC submodule (SM) capacitor, the LVDC port is achieved by parallel connection of the DAB outputs. Note that the number of DABs for forming the LVDC port is still too large (the same as the SM number of MMC), which results in a high component count and cost. Particularly, the HF transformers in the DABs are highly expensive as each of them require an MVDC insulation effort. Moreover, the enclosures, control and communication circuits, as well as the auxiliary circuits including power supply, signalmeasurement, IGBT drivers, cooling device, etc., are also indispensable to each DAB and these costs are proportional with the DAB number. For improvement, in [8,9] (named as MMC + ISOP DABs PET), DABs are directly input-series connected with MVDC port and output-parallel connected to obtain the LVDC port, thus the number of DABs can be reduced to 1/6. However, it still requires at least tens of DABs. Thus, from a cost perspective, these PETs are not suitable for the multi-port applications that moderate amounts of power is tapped from LVDC port, i.e., 100s of kW (to meet the rapidly increasing demand of hydrogen production, DC households, EV charging stations, etc.). In this context, reducing the number of DABs and HF transformers in the multi-port PET is essential. Reference [10] presents a novel simplified multi-port PET topology that an extra HF voltage component is generated by the MMC arms, then the LVDC port is obtained by an AC/DC cell in parallel with each arm via a DC blocking capacitor. In this structure, only six AC/DC cells are used for LVDC port, thus the component count and number of HF transformers are significantly reduced. Furthermore, in [11], the MVDC bus generates HF voltage by injecting the same HF component into all arm modulation signals. Then, the AC/DC cell is further moved to the MVDC bus by a DC-blocking capacitor to obtain an LVDC port, thus only one power cell and HF transformer are needed. However, a high voltage stress capacitor is required to block the MVDC voltage, which leads to concerns such as difficulty in starting-up and insulation design. In [12,13], to reduce magnetic volume, the arm inductors are integrated into the transformer. It uses MMC to convert low-frequency (LF) MVAC into HF MVAC, and then transform to HF LVAC by a transformer, it finally obtains an LVDC port from this LVAC by a three-phase inverter. However, the three-phase inverter is hard-switching, which leads to high switching loss. In addition, the HF LVAC is generated by the three-phase inverter, which adopting SPWM modulation, thus its frequency cannot be high (only 500 Hz in [12,13]), resulting in large transformer volume.