# A Hierarchical Coordinative Control Strategy for Solid State Transformer Based DC Microgrids
## 4 Simulation Results  
The model of the SST-based DC microgrid system, as shown in Figure 3, is built by PSCAD/EMTDC to verify the proposed coordinative control. PSCAD/EMTDC is a professional electromagnetic transient simulation software, which is also suitable for power electronics system simulations. The simulation parameters are listed in Table 2. The rated capacity of the SST is 1.5 MVA, and the distribution grid keeps running under unity power factor. In steady-state operation, the output stage of the SST has a 1 MW three-phase unbalanced load. At this time, the working waveform of the CLLC is shown in Figure 11. It can be seen that the voltage zero-crossing point of the primary-side switch is ahead of the turn-on signal, and the current zero-crossing point of the secondary-side switch is ahead of the turn-off signal, thereby reducing switching losses. Figure 12 shows the efficiency curve of the CLLC resonant converter under different load conditions. It can be seen that, when the power is transmitted bidirectionally, the half-load efficiency can reach above 97%, and the efficiency can reach above 98% at a rated power of 1.5 MW. Figure 11: The steady-state working waveform of the CLLC: (**a**) the zero-voltage-switching waveform of the primary-side switching device; (**b**) the zero-current-switching waveform of the secondary-side switching device. Figure 10: The voltage response Bode diagrams of the HVDC and LVDC links to the CLLC load current: (**a**) the LVDC bus voltage response; (**b**) the HVDC bus voltage response. At 1.5 s, the DC microgrid is connected to the system, the PV output is 0.3 MW, and the DC load is 0.2 MW. Starting from 2.7 s, the output power of the PV supply in the microgrid fluctuates continuously in a wide range, and the variation rule is 0.3 MW-2 MW-0.3 MW-2 MW. In this process, the power flow direction of the SST will change frequently under independent control. Figure 13 shows the response of the LVDC and HVDC bus voltages. It can be seen from the comparison that the hierarchical coordinative control strategy proposed in this paper can quickly stabilize the bus voltage and greatly improve the power quality in the dynamic process. Using traditional control strategy, the maximum transient voltage changes of the LVDC and HVDC links are about 47 V and 580 V, respectively. However, under the proposed coordinative control strategy, the DC bus voltage has almost no fluctuation. \begin{table}
\begin{tabular}{c c} \hline \hline
**Variable Description** & **Value** \\ \hline Distribution network voltage level & 10 kV \\ Distribution grid side filter inductor (\(L_{s}\)) & 10 mH \\ Three-phase arm inductance of MMC (\(L_{d}\)) & 20 mH \\ Number of submodules of MMC and CLLC (\(N\)) & 6 \\ Reference value of HVDC voltage (\(U_{dcref}\)) & 20 kV \\ Primary-side voltage of CLLC submodule (\(U_{dci}\)) & 3.33 kV \\ Secondary-side voltage of CLLC submodule (\(U_{out}\)) & 700 V \\ Turns ratio of HFT (\(n\)) & 4.76:1 \\ HVDC-link series capacitance (\(C_{H}\)) & 3000 uF \\ LVDC-link shunt capacitance (\(C_{L}\)) & 3000 uF \\ Primary-side resonant inductance (\(L_{r1}\)) & 15 uH \\ Primary-side resonant capacitance (\(C_{r1}\)) & 19 uF \\ Secondary-side resonant inductance (\(L_{r2}\)) & 0.216 uH \\ Primary-side resonant capacitance (\(C_{r2}\)) & 1319.42 uF \\ CLLC excitation inductance (\(L_{m}\)) & 2300 uH \\ Coordinative control coefficient \(K_{1}\) & 0.9 \\ Control coefficient \(K_{2}\) & 0.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The simulation parameters of the SST-based DC microgrid system. Figure 12: Efficiency curve of the CLLC under different load conditions. The microgrid layer control utilizes the fast power response of super capacitors to smooth the power fluctuation of the DGs. The current waveform of the SST during power fluctuation is shown in Figure 14. It can be seen that the compensated DG output current does not cause frequent fluctuations in the SST power transmission direction, and the change of SST current is relatively smooth.