# Solid-State Transformer-Based DC Power Distribution Network for Shipboard Applications
## 3 Overall System Control
### Generator Side Voltage Source Rectifier  
DC shipboard power systems commonly employ high voltage levels specially at the generation side; therefore, multilevel converters are necessarily called for this application. Numerous multilevel converters have been presented for various traction and electric ships applications, such as neutral point clamped (NPC) converters [34; 35; 36] and modular multilevel converters (MMCs) for high voltage levels [37], cascaded H-bridge (CHB) converters [38], and flying capacitor converters [39]. In this paper, a three-level NPC converter at the generator side is utilized owing to its advantages, such as low semiconductor stresses, improved output voltage and current quality, dv/dt reduction, low losses, and high efficiency [9; 40]. The three-level NPC has shown promise in practical marine propulsion drives developed by ABB [41] and Siemens [42] for synchronous machines and IMs. Moreover, the developed three-level NPC converter is based on the high-voltage and fast switching wide-bandgap SiC devices [28]. Thus, the same performance with the two-level VSC is ensured, albeit with reduced passive components. Consequently, the converter design becomes less complex, and its cost is lowered.  
The proper operation of an NPC requires a stable DC-link voltage at a constant voltage level. This is accomplished with DC split-capacitors and a feedback voltage control loop. The selection of the DC-link capacitors highly depends on the voltage and power ratings and can be achieved based on analytical expression [35].  
The controller block diagram of the GS-VSR is shown in Figure 4, where the voltage-oriented control (VOC) along with sinusoidal pulse width modulation (SPWM) technique are utilized in this study. The reference value for the HVDC bus voltage, \(V_{HVDC}^{*}\), must be chosen high enough to keep the VSR body diodes blocked. This reference value is compared with the measured HVDC bus voltage, \(V_{HVDC}\), and the voltage error is used to derive the reference direct current component, \(I_{d}^{*}\), of the line currents using a PI controller. Whereas, the reference quadrature current component, \(I_{q}^{*}\), is set to zero to ensure a unity power factor operation. This represents the simplest control technique for this topology. The GS-VSR voltage \(dq\)-components, \(V_{c_{d}}\) and \(V_{c_{q}}\), are given by Equations (1) and (2), respectively:  
\[V_{c_{d}}=V_{g_{d}}+Ri_{d}+L\frac{di_{d}}{dt}+\omega Li_{q} \tag{1}\]  
\[V_{c_{q}}=V_{g_{d}}+Ri_{d}+L\frac{di_{q}}{dt}-\omega Li_{d} \tag{2}\]  
where \(V_{g_{d}}\) and \(V_{g_{q}}\) are the \(dq\)-components of the generator voltage. The line resistance and inductance are \(R\) and \(L\), respectively. Moreover, \(i_{d}\) and \(i_{q}\) are the \(dq\)-components of the line currents. To realize unity power factor operation, the reference quadrature current component, \(I_{q}^{*}\), is set to zero. The PI controller, used to control the HVDC bus voltage (\(V_{HVDC}\)), provides the reference value of the d-component, \(I_{d}^{*}\), of the line current. To independently control \(i_{d}\) and \(i_{q}\), the controller is designed so that the GS-VSR reference voltage components, \(V_{c_{d}}^{*}\) and \(V_{c_{q}}^{*}\), are given by Equations (3) and (4), respectively [43]:  
\[V_{c_{d}}^{*}=V_{g_{d}}+\omega Li_{q}+K_{p}(I_{d}^{*}-i_{d})+K_{i}\int(I_{d}^{* }-i_{d}) \tag{3}\]  
\[V_{c_{q}}^{*}=V_{g_{q}}-\omega Li_{d}+K_{p}\Big{(}I_{q}^{*}-i_{q}\Big{)}+K_{i} \int\Big{(}I_{q}^{*}-i_{q}\Big{)} \tag{4}\]  
where \(K_{p}\)and \(K_{i}\), are the proportional and integral PI controller coefficients. It can be noted that the carrier-based PWM technique is used to control the LS-VSI.