# A Hierarchical Coordinative Control Strategy for Solid State Transformer Based DC Microgrids
## 1 Introduction  
The microgrid, as an economical and efficient form of electric energy organization, has attracted worldwide attention [1; 2]. A solid state transformer (SST) can serve as a kind of energy router in microgrids [3], which not only coordinates the energy transmission between the utility grid and the microgrid, but also controls the interaction between the AC and DC microgrids. Compared with the traditional transformer, the multi-stage SST provides a direct interface to the DC microgrid, and the energy density can be improved through a high frequency transformer (HFT). The SST is expected as a crucial equipment for utilization of distributed generations (DGs) [4] and construction of the Energy Internet [5; 6]. The multi-stage SST commonly contains three stages: input stage, isolation stage, and output stage, as shown in Figure 1. The input stage mainly adopts a modular multilevel converter (MMC)and a cascaded H-bridge (CHB) [7]. A dual active bridge (DAB) converter is often employed as the isolation stage. At present, the main factors affecting the practical application of SSTs are low-energy transmission efficiency and high cost. The efficiency of an SST can reach 93.72% with a CHB-type input stage [8], which is relatively low for practical application. However, the required number of power electronic switches and high-frequency transformers can be reduced by using an MMC as the input stage, which can significantly reduce the switching losses of power transistors and the cost of the SST [9] as well. Under the voltage level of the distribution network, power loss of the SST isolation stage is usually much higher than that of the input stage. Before extensive commercial use of power devices such as SiC and GaN, CLLC resonant converters can effectively solve for the inherent inefficiency of the DAB converter, whose conversion efficiency exceeds 96% [10]. In [11], a 5 kV CLLC resonant converter prototype was built and its efficiency reached 97%. In this paper, the efficiency of the CLLC resonant converter can reach beyond 98% at the rated power. Due to the randomness and intermittence of DGs, the power flow direction of an SST will change frequently, and the voltage across the port of each stage will suffer fluctuation, flicker, sag, and interruption [12], which brings serious damage to electric motors and, precisely, operated power electronic instruments and other power equipment [13; 14]. Integration of DGs requires a comprehensive control of the SST. As for the multi-stage SST, traditional control at each stage is independent: the input stage is responsible for the voltage stability of the high-voltage DC (HVDC) bus, and the isolation stage is responsible for the voltage stability of the low-voltage DC (LVDC) bus [15; 16; 17; 18]. In [15], an all-SIC device-based multistage SST was established, where the input stage of the SST adopted a front-end converter (FEC) and the isolation stage was with a DAB. The HVDC bus voltage and LVDC bus voltage were set as the control target of FEC and DAB, respectively. The control strategies of FEC and DAB do not interact with each other, which lacks collaborative operation between them. A global system control structure is described in [16], which can handle the different power flow directions and DC voltage unbalance using battery energy storage. The global control strategy is based on independent control strategy, and DC voltage fluctuation can be suppressed with assistance of the battery contribution, but it may increase the investment. In [17], a SiC-based multiple active bridge was chosen as the isolation stage of the SST to reduce the number of power devices, and a first order approximation-based mathematical model was analyzed, which helped to simplify a voltage balance controller design. The balance control of submodule DC voltage was also researched in [18] to solve the problem of unbalanced submodule voltage after using multilevel converter modules.