# Overload Control in Smart Transformer-Fed Grid
## 4 Simulation Results  
The simulations were carried out implementing the grid described in the previous section in PSCAD. The maximum current allowable in the ST is 174 A, corresponding to the current flowing in the ST working at nominal power (120 kVA) under nominal voltage (230 \(V_{rms}\)). The sum of the load and BESS profiles are shown in Figure 5. Without losing generality on the methodology, a unique load profile for the loads has been considered. The voltage controller ramp limiter has been set in this simulation to 0.5 V/s.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Resource** & **Nominal Power (kVA)** \\ \hline BESS A & 35 \\ BESS B & 25 \\ PV A & 4 \\ PV B & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: BESS (Battery Energy Storage Systems) and PV (photovoltaics) data.  
Figure 4: Grid simulated in PSCAD/EMTDCTM.  
The frequency controller is activated when the current goes above the 80% threshold and the voltage controller follows at 90% of the nominal current. In Figure 6a, the current profile in absence of overload control is plotted (black line). As can be noted, without control, the current exceeds the current maximum limit for more than 20 s in the considered time window. This may lead to damages to power semiconductor and reduction of the ST expected lifetime.  
Applying the ST overload controller, the current is kept below the maximum limit. As plotted in Figure 6, as soon as the current exceeds the frequency threshold, the ST decreases the output frequency until reaching the minimum value of 0.98 p.u. (49 Hz). The current slope is decreased with respect to the no-control case, but due to the limited amount of controllable resources, the current still exceeds the voltage threshold. Here, the voltage amplitude is decreased in order to reduce the load's power consumption. It can be noted in Figure 6 how the current is controlled below the current maximum limit and the voltage is decreased till 0.955 p.u., while the frequency is kept constant to 49 Hz. When the overload condition is solved and the load power request is decreased, the ST restores the voltage to the nominal voltage and controls the current profile with the frequency controller. Until the overload is not completely solved, the ST maintains the frequency controller in the activated state.  
The batteries' power profiles are shown in Figure 7a. The batteries increase the power output when the frequency is modified due to the droop controller action (solid lines) with respect to the base  
Figure 5: BESS \(A\) active power (blue line), BESS \(B\) active power (red line), sum of load active power (dotted black line), sum of load reactive power (dotted green line).  
Figure 6: Overload controller simulations: (**a**) ST current without controller (black line), ST current with the proposed overload controller (blue line), frequency controller threshold (dashed green line), voltage controller threshold (dotted magenta line), current maximum limit (red line); (**b**) grid frequency (dashed green line), ST bus voltage (dotted magenta line).  
case (dotted lines). The effectiveness of the combined action of frequency and voltage controllers can be observed in the ST power profiles in Figure 7b. Without control, the active power alone exceeds the ST size (dotted black line). Applying the ST overload control, both active and reactive power are reduced (continuous lines).