# Overload Control in Smart Transformer-Fed Grid
## 5 Experimental Results  
The overload controller simulated in the previous section has been experimentally verified with the microgrid setup shown in Figure 8. The experimental setup is composed of two three-phase voltage source converters (VSCs) (Danfoss Converter 4 kW) and a variable resistor used to create the overload conditions. The controller of the inverter emulating the ST has been implemented as described in Figure 2. The experimental setup parameters are listed in Table 3, and the controllers of both inverters are implemented in a dSPACE 1106.  
In the DG side converter, the droop controller described in Figure 3 has been implemented.  
Figure 8: Lab experimental setup scheme and picture.  
Figure 7: (**a**) Active power without controller BESS \(A\) (dotted green line), with overload controller (red line), active power without controller BESS \(B\) (dotted black line), with overload controller (blue line); (**b**) ST: Active power without controller (dotted black line), with overload controller (blue line), reactive power without controller (dotted green line), with overload controller (red line).  
The maximum current has been set to 6 \(A_{rms}\), and the frequency and voltage controller threshold have been set to 80% and 90%, respectively. The voltage controller ramp limiter has been set equal to 0.2 V/s. The variable resistor has been operated from 28 \(\Omega\) to 21 \(\Omega\) for about 20 s. In Figure 9 the operations of the overload controller have been plotted. As soon as the ST current exceeds the 4.8 A threshold, the frequency controller is activated, and it decreases the frequency from 50 Hz to 49 Hz. When the current increases above 5.4 A the voltage is decreased, limiting the ST overload to 5.6 A, with a further margin for load increase. The voltage drops from 220 V to 215.5 V during the steady state condition. As soon as the variable resistance increases, the ST current decreases, and the voltage and frequency are slowly restored to the nominal value. A better understanding of the overload controller operations can be obtained looking at Figures 10 and 11. These figures represent the steady-state waveforms before the overload condition and during the controller operation, marked respectively with the orange and green boxes in Figure 9. In Figure 10, the ST controls the voltage waveform \(v_{LV}\) at the nominal amplitude and frequency, being the ST current \(i_{LV}\) lower than the frequency controller threshold. The DG current \(i_{gc}\) amounts to 4 \(A_{peak}\) and the load absorbing about \(i_{load}=11\,A_{peak}\). As soon as the load increases, the overload controller acts on the voltage amplitude and frequency values. The steady-state behavior during the controller operations (Figure 9, green box) is shown in Figure 11. The ST decreases the voltage \(v_{LV}\) amplitude and frequency, and the DG is able to recognize the frequency change and increase the current output up to 5 \(A_{peak}\). The load current consumption is maintained at 14.4 \(A_{peak}\) by the voltage controller, instead to increase to 14.8 \(A_{peak}\). Both frequency and voltage controllers are able to avoid the ST overload.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(V_{LV}^{DC}\) & 650 V & \(V_{DG}^{DC}\) & 650 V \\ \(C_{LV}^{DC}\) & 3.3 mF & \(L_{f}\) & 5.03 mH \\ \(C_{f}\) & 10 μF & \(v_{LV}\) & 220 \(V_{rms}\) \\ \(f_{s}\) & 10 kHz & \(i_{max}\) & 6 A \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental setup parameters.  
Figure 9: Experimental results: ST rms voltage \(v_{LV}\) (magenta line), grid frequency \(f_{LV}\)(yellow line), and ST rms current \(i_{LV}\) (blue line).