# Solid-State Transformer-Based DC Power Distribution Network for Shipboard Applications
## 3 Overall System Control
### Dual Active Bridge Converter  
The dual active bridge converter is a DC/DC converter, which steps down the HVDC bus voltage to the LVDC bus voltage. It consists of three main parts: two voltage source active H-bridges interfaced by a high-frequency transformer (HFT) [44]. It is considered as a bidirectional isolated full-bridge DC/DC converter. It can achieve good efficiency using the minimum number of passive components. Hence, the DAB has a simple structure that does not require a resonant network. The design of the passive elements of this DC/DC converter is focused on the DC-link capacitors and the HFT. The HFT is the key building block of the DAB, which is the substitution of the traditional 50/60 Hz power transformer offering a multitude of functionalities over the conventional transformer, including significant size and volume reduction, bidirectional power flow control, and fault current limitation. The detailed design procedures of the HFT are presented in Ref. [45], where the key points in the design are the core magnetic material, the geometry of the transformer, and the structure of the core and the windings. Several modulation techniques have been used to control the power flow in the DAB converter. Amongst them, the phase shift modulation (PSM) technique is used along with PWM control, while the soft switching technique is employed to reduce the switching  
Figure 4: GS-VSR typical controller block diagram. losses. The PSM techniques are classified into single phase shift (SPS), dual phase shift (DPS), extended phase shift (EPS), and triple phase shift (TPS) [46]. The SPS technique is used in this study due to its fast response, high dynamics, and easier soft-switching control. In the SPS technique, the PWM signal of each H-bridge is controlled with a 50% duty cycle to generate square-wave voltages at each side of the HFT. These primary and secondary voltages are phase shifted with a time delay calculated as a ratio, \(D\), of the half cycle of the square-wave voltage. The voltage induced on the DAB inductor due to the phase shift, \(D\), affects the current flow in the DAB converter, which transfers power, \(P_{DAB}\), from the HVDC side to the LVDC side using Equation (5). \[P_{DAB}=\frac{N_{D}\ V_{HVDC}\ V_{LVDC}\ D(1-D)}{2f_{D}\ L_{D}} \tag{5}\]  
where \(N_{D}\), is the HFT turns ratio, \(f_{D}\), is the HFT frequency, and \(L_{D}\) is the DAB leakage inductance. The input voltage, \(V_{HVDC}\), of the DAB converter is regulated by the GS-VSC, which implies that the DAB converter controls the output voltage, \(V_{LVDC}\), using a PI controller, as shown in Figure 5. The circulating current owing to the mismatch between the voltage amplitudes of the two transformer sides as well as the inability to reach the zero-voltage switching (ZVS) in the whole range of the power are the main drawbacks of the SPS modulation technique [47; 48]. Most recent propulsion systems for large vessels are now based on electric motors with multiple three-phase winding. The multi-phase powertrain system ensures high reliability as well as reduces the current rating of the employed semi-conductors by splitting the motor power among a higher number of phases. This technology has recently emerged as a leading technology in hybrid as well as full electric ships [49]. This way, the required multiphase power converter can directly be connected to the HVDC bus of the proposed system, as shown in Figure 3.  
In this paper, an asymmetrical six-phase motor is built and integrated into the system to investigate the effect of the motor dynamics on the performance of the proposed SST-based distribution system. The machine construction can simply be made by rewinding a standard three-phase motor, while two three-level three-phase inverters are used to supply this motor [50]. Accordingly, the well-known V/f control, commonly used in such large power applications, is designed based on the system full-power rating.