## 4 Simulation and Experiment Results
### Parallel Operation Using Three-Level Communication Simulation  
In a system such as a micro-grid system where large capacity of a power converter is required, there is a limitation in increasing the capacity to a single stack. Thus, to increase the capacity, the parallel operation technique by a number of power converters is required, and the control performance of parallel operation is greatly affected by the communication speed.  
Figure 9 is a simulation circuit diagram for checking the superiority of the control characteristics when applying the proposed three-level communication method to a power converter configured for parallel capacity increase. The circuit configuration consists of one master converter performing voltage control and two converters performing current control. The current command transmission signal generated by the master converter was directly received from the slave DLL controlling the slave converter so that the load sharing operation could be performed. Simulation was conducted under the assumption that the protocol is composed of 19,200 bps, 2-byte data, and 2 byte CRC (Cyclic Redundancy Check) in order to verify the characteristic of the control by the communication speed in parallel operation. If the communication speed is too fast, it is difficult to compare the operation characteristics in the experiment. Therefore, both the simulation and the experiment were performed with the communication speed set to 19,200 bps. The communication cycle by protocol is implemented by using DLL, and simulation is performed by commanding the current command value according to the communication cycle to the slave converters.  
Figure 10 shows the simulation result of the load sharing using the existing communication method and the communication method proposed in this paper. The order of the result waveform is listed above which is in the order of the output voltage, the reactor current of each converter, the current command value by communication, and the magnified waveform of the current command value. Regardless of the communication method, the controller of each converter is set identically, and the simulation is performed by changing the load from 50% to 100%. When load sharing is performed using existing serial communication, the load sharing is normally performed as shown in Figure 10a, but it was confirmed that the current command value of the converter was delayed by approximately 30 ms according to the communication cycle. As the number of power converters increases, the control delay increases due to communication, which causes a large current ripple and voltage ripple, and adversely affects power quality. However, when using the communication method with the proposed communication hardware as discussed in this paper, it is possible to control the load sharing more than  
Figure 9: Parallel operation simulation circuit diagram.  
twice as fast as the conventional method. As shown in Figure 10b, when the communication using the proposed communication hardware is performed, the power quality can be improved as compared to the existing communication method. Additionally, it can be predicted that the difference becomes larger as the number of power converters increases.