# Droop Control Strategy of Utility-Scale Photovoltaic Systems Using Adaptive Dead Band
## 3 Simulation Study and Analysis
### Simulation Results  
In the simulation, the results of the voltage profile for the disturbance are simulated through the PSS(r) E program. To determine the value of the dead band over time, a disturbance in the bus to which the RES is connected is assumed. The time step for each value is set to 0.001 s, so that the values such as rise time and SI are accurately calculated. Figure 10 shows a graph that summarizes the results indicating the voltage profile of the corresponding bus when a large disturbance is simulated on bus 3 in the modified 39 bus system. Fault in bus 3 is more severe than fault in any other PV connected buses, so the adaptive dead band range determined by this simulation is stable against single-phase fault in all locations. The dead band range, which is the difference between V\({}_{\text{dbd,high}}\) and V\({}_{\text{dbd,low}}\) in Figure 1, is expressed in units of p.u. In Figure 10, as an example, a dead band with a length of 0.04 p.u is simulated. Table 3 shows the results of organizing the values of the performance indicators corresponding to three dead bands. The simulation conditions are the same as those presented in Figure 10, and use the load and PV generation data at 12 o'clock. The resultant value of the performance indicator shows that the values of the rise time and SI satisfy all dead band criteria listed in Table 2. Moreover, in case of overshoot, the criterion is satisfied when it is longer than 0.04 p.u. Thus, a simulation is performed on the bus disturbance where all the time and all RESs are connected, and the value of the dead band by hour is determined. The results in Figure 10 and Table 3 correspond to the contingency analysis in proposed flowchart. Contingency analysis is continuously iterated to find dead bands that satisfied the three performance indicators. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Small Disturbance** & **Large Disturbance** \\ \hline \(t_{\text{rise}}\) (s) & \textless{}1–30 & \textless{}0.2 \\ Overshoot (\%) & \textless{}5 & \textless{}3 \\ Stability Indicator (SI) & \textless{}0.0 & \textless{}0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Performance indicator parameter specification. Figure 10: Performance indicators of the simulation result. 1. Small disturbance result  
A small disturbance simulates a phenomenon in which the voltage drops due to a change in the PV output because of climate changes, such as cloud movement. According to the simulation results, the longer the dead band range, the greater the voltage drops. However, even in the worst case, the voltage range is satisfied. Small disturbance is perfomed for four dead band cases, and the voltage profile by the simulation result is shown in Figure 11. Rise time is not a problem as it guarantees a fairly long time for small disturbances. Even in the case of overshoot, the standard is satisfied for all dead band cases, and in the case of a small disturbance, the SI value is zero in all cases because the voltage maintenance range is not exceeded. 2. Large disturbance result  
In case of a large disturbance, the voltage drop phenomenon is simulated owing to a single-phase fault in the bus connected to the PV. The range in which the voltage falls due to a fault is 0.5-0.7 p.u., indicating the difference for each bus. Figure 12 shows the results of a single-phase fault around 13 o'clock, when the PV produces the most active power. The difference is depicted in the enlarged graph for each dead band. The results of the performance indicators for each dead band represented in the figure are summarized in Table 4. The adaptive dead band range determined according to the proposed scheme and flowchart is 0.032 p.u. Comparing the adaptive dead band and other dead band's performance indicators in Table 4, it is confirmed that the overshoot criterion is exceeded when the dead band is shorter than adaptive one and the rise time become longer as the dead band range increases.