# An Islanding Signal-Based Smooth Transition Control in AC/DC Hybrid Micro-Grids
## 4 Simulation and Discussion  
### Comparision of Different Transition Controls  
As mentioned in Section 2, the micro-grid consists of a PV panel, a fuel cell, batteries, and AC and DC loads. The simulation model is built with PSCAD/EMTDC. The maximum load is about 300 kW. The capacity of this micro-grid is balanced in both the AC and DC sub-networks. The AC sub-network is directly connected to the utility grid through the PCC, while the DC sub-network is connected to the AC sub-network through an ILC. In this plant of a relatively large area, the distances between DGs and loads are hundreds of meters. Thus, impedance of transmission lines cannot be neglected. The line parameters are presented as AC line resistance \(r=0.5\)\(\Omega\)/km, inductive reactance \(x=0.35\)\(\Omega\)/km, and DC line resistance \(r=0.325\)\(\Omega\)/km. In order to sufficiently use the renewable energy, PV panels are erected on the roof of the plant. In general, taking into account the impact factors like natural shelter and area utilization, 1 kW of occupied roof area corresponds to 15 to 20 m\({}^{2}\). The plant area is about 1000 m\({}^{2}\), and the PV capacity can reach 50 kW. A fuel cell with a capacity of 50 kW serves as a controllable power supply, which is used to keep continuous supply of important loads under special conditions. The capacity of the batteries in the DC and AC sub-networks are 75 kW and 150 kW, respectively, to ensure continuous power supply to all loads for a certain period during islanding operations, and to mitigate the impact of distributed power and load fluctuations in the micro-grid. The AC bus has a rated voltage of 380 V and a frequency of 50 Hz. The AC sub-network can be directly connected to the utility grid. A unified standard for rating the DC bus voltage has not been approved yet. In the literature, the DC bus voltage is selected from 600 V to 3.5 kV [23; 24; 25; 26]. In this paper, the rated voltage of the DC bus is selected as 800 V.  
In order to validate the effect of the proposed approach, comparisons of direct transition control, traditional method state follower-based, and islanding signal-based smooth transition control are performed. The simulation process is as follows: when \(t=0\)-1.5 s, the micro-grid operates in grid-connected mode. When \(t=1.5\) s, the micro-grid turns to islanding mode, the PCC switch is disconnected, and  
Figure 9: Scheme of pre-synchronization control. the control mode is changed. When \(t=3\) s, the micro-grid turns back to grid-connected mode. After pre-synchronization, the PCC is switched off and the control mode goes back to grid connection. During this process, the PV output power, DC load, and AC load are 50 kW, 112.5 kW, and 75 kW respectively. The DC bus voltage, AC bus voltage, and frequency imply the stability of the hybrid micro-grid, which is shown in Figure 10, Figure 11, and Figure 12, respectively, by using different transition control strategies. The results of the transition controls are listed in Tables 1 and 2 for both the grid  
In order to investigate the performance of the controller during transition, the PI regulation state for each transition approach is observed. Figures 13-15 show the regulator output state of the DGs and ILC in different transition controls. During \(t=0\)-\(1.5\) s, the micro-grid operates in grid-connected mode. Since the direct transition control fails to realize the regulation state following, there is a large state step at the transition instant. Therefore, the DC voltage has a large mutation, and the switching time is long. In traditional smooth transition control, a state follower is used to keep the output states of the two controllers consistent. If the grid-connected controller is working, the islanding controller will follow the output state of the grid-connected controller, as shown in Figure 14. The \(q\)-axis current of DC battery and ILC can provide