# 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 Islanding Signal  
As mentioned in the above section, the state follower-based approach suppresses the transient step by keeping an equal state value of the two controllers. The state follower can theoretically realize the synchronization of the output states of the controllers. The PI regulator is required to fulfill the outer loop control target and to implement state following. Therefore, the PI regulator settings are difficult to configure, and the control effect is not satisfied. In addition, if the operating state is frequently switched in a short period, the regulator has no time to respond, and the regulation state will change abruptly, which leads to a negative effect on the smooth transition.  
In order to solve these problems, an islanding signal-based smooth transition control has been proposed in this paper. The control scheme is shown in Figure 8.  
The entire control consists of an outer loop and an inner loop. The output signal of the current inner loop is used to generate a control signal of the inverter. \(S\) represents the islanding signal. When micro-grid works in grid-connected mode, \(S=0\) and the power control regulator works. When the micro-grid works in islanding mode, \(S\) becomes 1, and voltage control regulator works. Figure 8 shows the outer loop of the controller. \(Kp\) indicates the proportional part that can respond to the input signal instantly. \(Ki\) indicates the integral part. The static error is eliminated by integrating the input error. The memory of the integral state is the main reason for the transient impact.  
When the micro-grid switches from grid-connected mode to islanding mode, islanding signal \(S\) generates a falling edge from 1 to 0. This falling edge triggers the power control outer loop regulator to renew its state value. The renewed value equals to the detected state value of the voltage control outer loop regulator at that moment. When the micro-grid switches from islanding mode to grid-connected mode, the islanding signal S generates a rising edge from 0 to 1. The rising edge triggers the voltage control outer loop regulator to renew its state value. The renewed value equals the detected state value of the power control outer loop regulator at that moment. The difference is that the rising edge of the islanding signal should work on the power control regulator after a delay. Within the delay, a pre-synchronization control is performed.  
In addition to the sudden change of the regulation state, a voltage and frequency difference between the micro-grid and utility grid can also cause a transient impact in the switching from islanding mode to grid-connected mode. In order to solve this problem, it is necessary to perform a pre-synchronization operation before the switch operation [20]. The pre-synchronization control scheme is shown in Figure 9.  
Figure 8: Scheme of islanding signal-based control.  
Here, _Ugd_ and _Ugq_ refer to the voltage of the utility grid in _d-q_ coordinates. _Umd_ and _Umq_ refer to the voltage of the micro-grid in dq0 coordinates. Through Parker transformation, the three-phase AC voltage is transformed into a DC representation in the dq0 coordinates, which avoids the direct adjustment of voltage and frequency. In addition, the transformation realizes the linear control of the voltage. When an islanding signal is detected, the pre-synchronization control is put into operation. When the voltage and frequency deviation of both sides are in the acceptable range, the PCC switches off and the pre-synchronization control stops working. The micro-grid switches to grid-connected mode successfully.  
In this way, the output states of the two sets of controllers are exactly the same at the transient instant, and the impact on smooth transition can be solved. In addition, the setting of regulator parameter has been greatly simplified, since the outer loop is only employed to achieve the control target, and step three of parameter setting can be skipped.