## 4 Results and Discussion
### Simulation results  
In order to demonstrate the effectiveness of the proposed control functionalities described in Section 3, a sudden connection of 200 MW and a sudden disconnection of 200 MW in the main power system were considered both for net load and net generation scenarios in the hybrid distribution network. The considered power disturbances originated under- and over-frequency disturbances in the main power system, for both net load and net generation scenarios considered for the hybrid distribution network as represented in Figure 18. The resulting frequency disturbances are intended to evaluate the response of the active power-frequency support capabilities proposed for the ST regarding both under- and over-frequency conditions. It is possible to observe that the frequency disturbances in both net load and net generation scenarios are very similar. Such behavior was expected since the magnitude of the considered load steps (200 MW) is much higher than the rated power of the ST (1 MVA). In order to demonstrate the aggregated response of the active power-frequency support functionalities proposed for the ST and downstream hybrid AC/DC system, the presented results were obtained with the simultaneous operation of all droop controllers incorporated in the ST designed for that purpose. Since the frequency curves shown in Figure 18 are practically identical for both net \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Drop** & **Slope** & **Offset (Net Load/Net Generation)** & **Dead-Band** & **Limits** \\ \hline P(\(f_{MV}\))-Local ESS & \(5\mathrm{e}5\mathrm{W}/\mathrm{Hz}\) & 0 W & 50 \(\pm\) 0.025 Hz & \(\pm\)200 kW \\ \(f_{LV}(f_{MV})\) & \(1\mathrm{Hz}/\mathrm{Hz}\) & 50 Hz & 50 \(\pm\) 0.025 Hz & \(\pm\)0.5 Hz \\ \(V_{LV}(f_{MV})\) & 80 V/Hz & 230 V & 50 \(\pm\) 0.025 Hz & \(\pm\)25 V \\ VDC(\(f_{MV})\) * & 4e3V/Hz & 27 kV & 50 \(\pm\) 0.025 Hz & \(\pm\)5 kV \\ P(\(f_{LV}\))-ESS & \(1\mathrm{e}5\mathrm{W}/\mathrm{Hz}\) & 0 W & 50 \(\pm\) 0.025 Hz & \(\pm\)30 kW \\ P(\(f_{LV}\))-PV ** & 2.5 SP/fn & \(-30\mathrm{ kW}/-200\mathrm{ kW}\) *** & \(\pm\)0\% & \(+\)0/\(-\)30\% \\ P(VDC)-PV ** & 2.5 SP/VDCn & \(-800\mathrm{ kW}/-900\mathrm{ kW}\) *** & \(\pm\)0\% & \(+\)0/\(-\)10\% \\ P(VDC)-EV ** & 2.5 SP/VDCn & 450 kW/250 kW *** & \(\pm\)0\% & \(\pm\)10\% \\ \hline \hline \end{tabular} Note: EV—EV chargers; SP—Setpoint prior to droop actuation; fm—Nominal frequency; VDCn—Nominal DC voltage in DC networks; *set for MV DC network. Affects LV DC network according transformation ratio; **door parameters are function of base active power set-point; **aggregated values.  
\end{table} 
Table 4: Droop Controllers: main characteristics. Figure 18: Frequency variations observed in the MV AC network for the net load (**up**) and net generation scenarios (**down**): under-frequency due to load connection (**left**) and over-frequency due to load disconnection (**right**). load and net generation scenarios, the response of the droop controllers incorporated in the ST was practically identical for both scenarios, as shown in Figure 19.  
The response of the \(f_{LV}(f_{MV})\) and \(V_{LV}(f_{MV})\) droops is shown in Figure 19a,b respectively, where it is possible to observe that the frequency and the RMS value of the voltage in the LV AC network are modulated according to the frequency disturbances observed in the MV AC network. Further, the response of the \(VDC(f_{MV})\) droop is shown in Figure 19c, where it is possible to observe that the voltage in the MV DC network is also modulated according to the frequency disturbances observed in the MV AC network. The voltage regulation performed over the MV DC network affects the voltage in the LV DC network, as shown in Figure 19d, since a fixed transformation ratio is defined for the isolated DC/DC converter. It is worth mentioning that the effect of the droops' dead band is notorious from roughly t = 21 s, since the droop responses stabilize before the frequency returns completely to its nominal value.