# A Hierarchical Coordinative Control Strategy for Solid State Transformer Based DC Microgrids
## 3 Hierarchical Coordinative Control Strategy Design  
### The Hierarchical Coordinative Control for the SST-Based DC Microgrid System  
With respect to the problems existing in the independent control, a hierarchical coordinative control strategy for the SST and microgrid composite system is proposed. In general, the hierarchical coordinative control mitigates the local microgrid's external power fluctuation and realizes a joint action of the MMC and the CLLC during the dynamic process. The comparison of control logic between independent control and coordinative control is shown in Figure 8.  
The overall coordinative control diagram is shown in Figure 9. This novel control strategy contains two layers: the microgrid layer and the SST layer. In Figure 8a, \(G_{UES}\) and \(G_{IES}\) represent the ESS double-loop PI controller; \(G_{UPV}\) and \(G_{IPV}\) are the control functions for the PV controller; \(K_{PWMES}\) and \(K_{PWMPV}\) are the equivalent gain of the pulse width modulation (PWM) links for ESS and PV, respectively (\(K_{PWMES}\ \approx\ U_{dLES}\), \(K_{PWMPV}\ \approx\ U_{dCPV}\)); and \(d_{ES}\) and \(d_{PV}\) are the equivalent duty cycles. The meaning of the remaining parameters of Figure 9 has been given in Figure 6.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variables** & **Value** & **Variables** & **Value** \\ \hline \(K_{Puo}\) & 0.04 & \(K_{luo}\) & 40 \\ \(K_{Pudc}\) & 4 & \(K_{ldc}\) & 100 \\ \(K_{Pil}\) & 0.5 & \(K_{lid}\) & 50 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the solid state transformer controller.  
Figure 7: Current response Bode diagrams under independent control: (**a**) current response of the high-voltage DC (HVDC) link; (**b**) current response of the low-voltage DC (LVDC) link.  
In the microgrid layer control, the additional control branch feeds the variation of PV outlet current back to the inner current loop of super capacitors inside the ESS, giving full play to the fast regulation capability of super capacitors. Although mechanical and battery energy storage systems are still the key elements in system operation, super capacitors nowadays are drawing an increasing  
Figure 8: Control logic of the independent control and proposed coordinative control.  
Figure 9: The hierarchical coordinative control diagram: (**a**) the microgrid layer control; (**b**) the SST layer control.  
interest [25], due to the advantages of high efficiency and high power density [26]. This paper adopts a hybrid energy storage device combining super capacitors and batteries, as discussed in [27; 28; 29]. In the additional control branch, \(k_{mc}\) is the coordinative control coefficient, which reflects the compensation degree of PV output fluctuation by the ESS. \(G_{mc}\) is the coordinative transfer function, and the fluctuation inspection section represents the instruction-receiving and triggering module.  
In the SST layer control, the control branch 1 feeds the load current of the CLLC forward to the inner current loop as a new control variable to improve the dynamic current response speed of the CLLC stage. The additional control branch 2 not only feeds the output of the CLLC voltage controller back to the inner loop of the MMC, bringing the two parts of the SST together to complete the task of LVDC bus voltage control, but also feeds the CLLC load current back, to eliminate the influence of the CLLC current fluctuation on the HVDC link at the same time. Similar to the microgrid layer control, \(k_{1}\) and \(k_{2}\) are additional control coefficients; \(G_{fb}\) and \(G_{cc}\) are the corresponding transfer functions. The expressions are as follows:  
\[\left\{\begin{array}{c}G_{mc}=\frac{G_{tS}+sl_{ES}}{G_{tS}}\\ G_{fb}=\frac{1}{b}\\ G_{cc}=-\frac{2 bl_{L}(G_{uf}+sl_{0})}{3 nt_{E}g_{Cu}}\end{array}\right. \tag{11}\]