# A Hierarchical Coordinative Control Strategy for Solid State Transformer Based DC Microgrids
## 2 Overview of the SST-Based DC Microgrid System
### Problems Existing in the Independent Control Strategy  
In the three-stage independent control, the input-stage MMC adopts voltage and current double-loop control to maintain the HVDC bus voltage and adjust the power factor on the distribution  
Figure 4: The submodule topology of the CLLC resonant converter. grid side; the CLLC adopts single-phase-shift control to regulate the parallel output voltage of the isolation stage. The control diagram is shown in Figure 5. In this figure, \(Q\) is the reactive power of the distribution network; \(G_{\text{{u}dc}}\), \(G_{\text{{i}id}}\), \(G_{q}\), and \(G_{\text{{i}iq}}\) represent the double-loop controller functions of the MMC; and \(G_{\text{{u}o}}\) is the controller function for the CLLC, which adopts single voltage-loop control, so no high-precision current sensor is needed. The part inside the dotted box represents the relation between \(\varphi_{i}\) and \(i_{2i}\), which has been given by the second line of Equation (4). It is obvious that \(G_{\text{{p}s}}\) is a nonlinear function, since the CLLC usually works near the resonant frequency point, which means that \(\theta_{i}\approx 0\). Therefore, a linear approximation for \(G_{\text{{p}s}}\) is carried out in this paper. The approximate expression after linearization is as follows (\(0<\varphi<\pi\)):  
\[\left\{ \begin{array}{l}i_{2i}{=a+b\varphi_{i}}\\ a=\frac{8 nU_{dc}}{\pi^{2}\left|\begin{array}{c}Z_{\text{{e}qi}}\\ \end{array}\right|}\\ b-\frac{16 nU_{dc}}{\pi^{3}\left|\begin{array}{c}Z_{\text{{e}qi}}\\ \end{array}\right|}\end{array} \right. \tag{6}\]  
As mentioned previously, the topology of the grid-connected DC microgrid system is shown in Figure 3. A boost chopper circuit is usually adopted as the interfacing converter for PV to realize the maximum power point tracking (MPPT) control [23; 24]. The topology and control diagram are shown in Figure 6a,b. For the energy storage system, which includes batteries and super capacitors, the interface converter must realize a bidirectional power transmission, so the current reversible chopper circuit is usually used. Common control methods include constant DC voltage control and voltage droop control. The topology and control diagram are shown in Figure 6c,d. Figure 5: The independent control strategy of the SST. In normal operation, the SST acts as the balance node of the system to provide the voltage and frequency support, the DGs output at maximum power, and the energy storage equipment responses to the control-center command. In this case, fast and continuous fluctuations of the distributed power output will be sufficiently compensated in real time by the SST. It can be seen from Figure 5 that the output current of the microgrid is the interference to the CLLC control loop, which means that the change of \(i_{L}\) leads to the fluctuation of \(i_{2i}\) and \(U_{out}\). The current of the CLLC port is the interference to the MMC active power control loop, which means that the change of \(i_{2i}\) will further lead to the fluctuation of \(U_{dc}\). In addition, the difference of power response speed between the MMC and CLLC resonant converter will aggravate the situation. From Figure 5, the transfer function of the CLLC output DC current \(i_{2i}\) to its load current \(i_{Li}\) can be obtained.