## 2 Parallel Operation of the Power Converter
### Method by Serial Communication  
Systems that support remotely monitored or controlled services have emerged for the efficient operation of power facilities such as micro-grid, and power protocols are needed to process various information of the system generated from power equipment. Unlike communication protocols typically used in computers, power protocols require reliable communication networks; high data integrity, data consistency, and fast response times are important. As a result, protocol limitations exist according to the specifications of power devices or systems, and various protocols have been developed to satisfy various elements required in the power system.  
A brief summary of the purpose of each protocol used in the power system is as follows. There is the DNP (Distributed Network Protocol) 3.0 protocol, an industrial protocol utilized in existing power systems, and the IEC 61850 protocol, which is gradually extended from the current substation automation purpose to the power utility automation purpose. In addition, the control area network (CAN) and IEEE (Institute of Electrical and Electronics Engineers) 1451 are used for simple sensor data measurement, and the Modbus protocol is used for simple data acquisition and control in distributed power or renewable energy sources. Table 1 shows communication specifications of power converters for renewable energy power converter manufacturers, which are used in various renewable energy systems.  
As can be seen from Table 1, power converters of many manufacturers still adopt the Modbus protocol. Accordingly, the basic driving method for the parallel operation method using the Modbus protocol adopted by many manufacturers is described below.  
If the line impedance of the power converter performing voltage control is assumed to be similar, the same load current can be supplied to all the power converters by performing the output voltage control [25; 26]. However, in real systems, the difference in track impedance and characteristics of the device causes circulating currents between the power converters, and as the load increases, it causes a breakdown by increasing unbalance currents [27]. Therefore, it is advisable to configure each module with a current controller to solve these problems [28; 29]. In short, this means that in a parallel operation control method using serial communication, the master module must transmit current command generated through voltage control to the slave module. As devices increase as shown in Figure 2a, master-slave communication using Modbus communication increases the overall communication cycle, as shown in Figure 2b. If the communication cycle increases, the number of power converters capable of stable parallel operation is limited. Therefore, high-speed communication is essential for large capacity through stable parallel operation of the power converter.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Manufacture** & **Model No.** & **Usage** & **Communication Method** & **Baud Rate (bps)** \\ \hline ABB & PVI String Series & String Inverter & Modbus RTU etc. & 19,200 \\ LS is & SV Series & Solar Inverter & Modbus RTU etc. & 1200–19,200 \\ Huawei & SUN2000 & String Inverter & Modbus RTU etc. & 19,200 \\ Fronius & SYMO Series & String Inverter & Modbus RTU etc. & 2400–19,200 \\ Solar Edge & SE Series & String Inverter & Modbus RTU etc. & 9600–115,200 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Communication method of power converter according to manufacturer.