## 1 Introduction  
With the gradual depletion of fossil fuel, the increasing environmental pollution, and the large-scale development of distributed energy, the existing grid infrastructure becomes inadequate to meet the future needs of development, which makes the energy internet come into being. American scholar [PERSON] has proposed in the \"Third Industrial Revolution\" that the energy internet is a next-generation smart grid that deeply integrates energy with the Internet. The energy internet is supported by advanced power electronic devices, high-speed digital communication technology, and efficient distributed control technology. It aims to provide a reliable energy support system [1]. In 2011, the University of North Carolina proposed an energy internet architecture based on future renewable electric energy delivery and management (FREEDM) system. Intelligent energy management (IEM) device, as the core of FREEDM, consists of a power electronic transformer (PET) that is based on high-frequency isolation, in order to achieve system interconnection of AC/DC grids as well as flexible control and intelligent management of power flow [2]. Moreover, Europe's universal and flexible power management (UNIFLEX-PM) project utilizes the power converter based on the PET structure, and possesses many advantages. For example, bidirectional flow of active and reactive power can be achieved; power quality and robustness of power grid can meet the international standards of grid connections for distributed energy systems; electrical isolation between different ports can be also achieved; and the operation of modular structure is reliable which also improves the convenience of maintenance [3]. As the core device of energy internet, PETs can provide flexible and standard power electronic interfaces for various distributed energy sources and loads [4; 5]. Meanwhile, real-time acquisition and control of the electrical quantities of each port can be performed through PETs to meet the requirements for grid dispatching and provide data support for the effective and stable operation of energy internet [6]. With the increasing voltage level, the traditional two-level or three-level voltage source converter PET struggles to meet the needs of medium- and high-voltage applications. Hence, different topologies of multilevel converters have been proposed for PET applications. For instance, the cascaded H-bridge multilevel inverter (CHB-MLI) PETs is widely used for medium- and high-voltage applications for its simple control, modularity, and small voltage stress of switching tube [7; 8; 9]. However, the CHB-MLI PET requires plenty of high-frequency transformers and controlled switching devices in three-phase applications, which is uneconomical [10; 11]. Recently, as modular multilevel converters (MMCs) becomes widely applied, the PETs that based on MMC structure has attracted more attention [12; 13; 14] since it applicable to higher voltage levels and able to provide higher power quality and conversion efficiency [15]. The isolated DC/DC converters of existing PETs mainly adopt the ISOP connection, and the converter of every output port connects to the ISOP output side in parallel. In this case, additional DC/DC converters need to be installed to obtain the access to AC/DC grids with different voltage levels, which will complicate the system and reduce economic benefits [16]. On the other hand, the pros and cons of PET control methods can have an immediate impact on the performance of an entire system. Most of the existing research focuses on direct power control or droop control, etc. [17; 18]. However, with an increasing proportion of distributed energy resources in the distribution network, it becomes difficult, or impossible, for traditional control methods to control the grid voltage and frequency, which further reduces the inertia of the grid.