# Droop Control Strategy of Utility-Scale Photovoltaic Systems Using Adaptive Dead Band
## 3 Simulation Study and Analysis
### System Disturbance  
The simulation assumes two disturbances for the dead band setting of the plant by time and identifies the performance. The disturbance can be divided into small and large disturbances, and the difference between the two lies in whether the system voltage is within the maintenance range depending on the degree of disturbance.  
* Small disturbance: Network voltage is maintained within the range of 0.95-1.05 p.u after disturbance
* Large disturbance: Network voltage is out of the voltage maintenance range after disturbance  
The small disturbance relies on the actual PV data to check for changes in the system voltage due to a sudden decrease in the active power output. Large disturbances occur in various ways depending on the fault type, and this simulation assumes a single-phase fault. In case of a serious three-phase fault, the voltage falls below 0.3 p.u, and situations such as the ride-through mode need to be considered.  
To confirm the performance of the voltage profile in response to the disturbance, several evaluation indicators are applied to confirm the result of each dead band. The values of rise time, overshoot, and stability indicator (SI) are used to check the performance. Rise time (\(t_{\text{rise}}\)) refers to the time taken by the voltage to reach 95% after the plant controller of the RESs reacts (\(t_{\text{rea}}\)) after a fault occurs (\(t_{i}\)). Overshoot calculates the ratio of the operating voltage to the maximum voltage value reached by increasing the voltage beyond the operating voltage during the voltage recovery process. SI calculates the sum of the areas where the voltage profile falls outside the allowable maintaining range and determines whether this value is within a certain value. The value of the voltage deviates the allowable maintenance range and is expressed as \(\Delta V_{dev}\); the SI value is derived through the integral sum of the values and expressed as Equations (14) and (15).  
\[\Delta V_{dev,\tau}=\left\{\begin{array}{ccc}V_{\tau}-1.05V_{op}&(&V_{\tau} \geq 1.05V_{op}&)\\ 0&(&0.95V_{op}\leq V_{\tau}\leq 1.05V_{op}&)\\ 0.95V_{op}-V_{\tau}&(&V_{\tau}\leq 0.95V_{op}&)\\ \end{array}\right. \tag{14}\]  
\[SI_{t_{simal}}=\int_{0}^{t_{simal}}\Delta V_{dev,\tau}\;d\tau \tag{15}\]  
Figure 9 shows the performance indicator for disturbance. In case of rise time, it corresponds to the difference between the reaction time and the time taken by the voltage to reach 0.95 p.u. The overshoot, represented by the green dotted line, represents the value corresponding to the point that is the largest distance from the operating voltage (in percentage). The SI, represented by the blue area, corresponds to the value calculated by the extent beyond the maintenance range. The SI calculates the area of the voltage out of the maintenance range, 0.95-1.05 p.u; thus, it can express how much the voltage profile is out of the limits in numerical form.  
Figure 9: Voltage profile performance indicator.  
Table 2 shows the specifications that must be satisfied by the three parameters measured in the voltage profile for both disturbances. For the information on rise time and overshoot, the value specified in the NERC inverter-based resource performance guideline is used [30]. The planning coordinator or transmission planner defines the regulation of overshoot for large disturbances to have a different regulation value for each system, but here it is defined as the same value as that of a small disturbance.