# An Islanding Signal-Based Smooth Transition Control in AC/DC Hybrid Micro-Grids
## 3 A Smooth Transition Control Based on Islanding Signal
### Transient Impulse Suppression Based on Regulation State Follower  
In order to solve the problem of abrupt changes of regulation state, a method based on a regulation state follower has been proposed. When the micro-grid is switched from grid-connected mode to islanding mode, DGs are switched from power control to voltage control. The output of the power control regulator should be applied as the input reference of the voltage control regulator. The output of both regulators always remains in the same state [20]. The scheme of a regulation state follower is shown in Figure 6. Here, \(K_{i}\) (\(i\)=1,2,3,4) is the switch.  
Figure 5: PI regulator structure.  
During the grid-connected period, K1 and K4 are switched off, and K2 and K3 are switched on. At this time, a power controller is used to control the DG's output power. The voltage controller follows the output state of the power controller. When it is detected that the micro-grid disconnects from the utility grid, K2 and K3 are switched off, and K1 and K4 are switch on. The control mode is switched, and the micro-grid turns into islanding mode. Because the state follower maintains the output value of the voltage control regulator as equal to the output of the power control regulator, there is no abrupt change in the reference value of the inner current loop at the instant of switching, which reduces the transient impact.  
When a micro-grid is switched from islanding mode to grid-connected mode, DGs need to switch from voltage control to power control. The output of the voltage control regulator is applied as the input reference of the power control regulator. In this way, the power regulator state can always follow the voltage regulator state [20]. The scheme of a state follower is shown in Figure 7.  
The mechanism of the above two state followers is similar. During the islanding period, K1 and K4 are switched off, and K2 and K3 are switched on. At this time, the purpose of the voltage controller is to control the voltage and frequency of the AC sub-network, as well as the voltage of the DC sub-network. The power controller follows the output state of the voltage controller. When it is detected that the PCC is switched off, K2 and K3 are switched off, and K1 and K4 are switched on. The control mode is switched, and the micro-grid works in grid-connected mode. Because the state follower always keeps the output value of the power control regulator equal to the output of the voltage control regulator, there is no abrupt change in the reference value of the inner current loop at the moment of switching.  
In this suppression control, the most difficult thing is to set the PI parameters. Firstly, the proportional gain \(K_{i}\) is set to achieve a critical oscillation state. Then, the integral gain \(K_{p}\) is calculated by formula or trial test. Now, the parameters for each controller can be settled. However, one controller has to follow the state with another, and parameter correction should be made from time to time to meet the state tracking requirement. Since the parameter correction is greatly dependent on experience, it is hard to determine an appropriate correction; even worse, such parameters do not exist in some cases, and thus the entire process takes long time.  
Figure 6: Scheme of regulation state follower from grid-connected mode to islanding mode.  
Figure 7: Scheme of regulation state follower from islanding mode to grid-connected mode.