## 2 Establishment of Energy Internet with Energy Routing Centers
### Architecture and Functions of Energy Routing Center  
In order to realize flexible energy conversion and power dispatch of multi-types of energy, a multi-energy coupled network is proposed in this paper. The basic structure of the ERC is shown in Figure 2.  
As can be seen in Figure 2, an energy routing center is composed of an energy routing control unit, an energy conversion structure, and several input and output ports. In order to illustrate the structure of the ERC clearly, the ERC is divided into the information layer and the physical layer. The energy routing control unit is located in the information layer; it exchanges information with devices located in the physical layer. The control unit receives information from the exchange structure and transfers control signals to the input/output ports. What is more, as an indispensable part of the ERC, the energy routing control unit undertakes functions of energy management and routing design. Energy management mainly focuses on the energy transmission between different ERCs, that is to say, each energy routing control unit formulates a scheme of its output energy. As for energy routing design, due to the restrictions of real topology of the energy network and upper limit of energy transmission for each connection line, the energy routing path should be carefully designed in order to meet the user demand.  
The energy conversion structures are the basic component of the ERC and the core devices in the physical layer. In order to make the ERC a platform allowing interaction and transformation of various types of energy, different conversion devices are integrated. The proposed ERC adopts gas-fired CHP to realize the conversion from gas to heat and electric, which compensate the demand of thermal and electrical users. Electrical converters including DC/AC converters and AC/AC converters are also adopted to take in electrical power of renewable energy such as photovoltaic, wind turbines, etc.  
Another important part of the physical layer are the input/output ports. The input ports provide access for comprehensive power sources. As can be seen in Figure 2, energy storage devices, the main grid, and renewable energy resources, such as photovoltaic arrays and wind turbines, are connected to the common AC bus through input ports. What is worth mentioning is that, in the consideration of saving fossil fuels, the main grid merely serves as a compensation device for electrical power demand. Additionally, the input ports also absorb power flow from other connected routing centers for the sake of free energy interaction. As for other types of energy resources, natural gas networks are connected  
Figure 2: A basic structure of an energy routing center (ERC).  
to supply heat for thermal loads. The output ports allow plug-and-play of multi-energy users and exports multi-energy to its adjacent routing centers.  
As the bridge connecting inner exchange structures of the ERC and external energy sources and load, the input/output ports should have the function of condition monitoring. During the conversion process, current overshooting and voltage mismatch bring danger to the safe operation of the whole energy routing system and may even cause serious accidents. Thus, before energy sources are connected, current and voltage checking are carried out at the input ports to enhance stability and security. As for output ports, conditions of output current and voltage are assessed as well before being exported from the routing center.