## 3 Control Units of the Hybrid MG
### Control Units of the CVSI  
Figure 6 illustrates the control unit of the CVSI. Voltage-oriented control (VOC) is employed as a control strategy, widely recognized for its effectiveness in managing 3P grid-connected inverters. VOC is valuable in renewable energy systems, where accurate power flow management ensures system stability, efficiency, and compliance with grid standards. The ability of VOC to maintain stable and efficient operation in varying conditions makes  
Figure 5: Control unit the PEMFC.  
it a preferred choice in applications such as solar photovoltaic systems, wind turbines, and other distributed energy resources.  
The core principle of VOC lies in transforming quantities of the 3P alternating current (AC) into a \(d\)-\(q\) reference frame. This transformation is essential as it simplifies the control of AC systems by converting the three-phase currents and voltages into two orthogonal components: \(d\)-axis and \(q\)-axis. In the \(d\)-\(q\)-frame, the active and reactive power components can be decoupled, allowing for the independent control of each component. This decoupling significantly streamlines control operations and enhances the precision with which power flow is managed.  
In the context of VOC, the control objectives are divided into active (\(P_{B2}\)) and reactive (\(Q_{B2}\)) power components. The active power component is responsible for real power transfer to the grid, while the reactive power component is involved in voltage regulation and power factor correction. The control system of the CVSI consists of three primary control loops. The first loop is designed to maintain a constant voltage across the DC link capacitor, while the second and third loops regulate the d-axis and q-axis components of the inverter current, as illustrated in Figure 6. A PI controller manages each of these control loops.  
The control equations governing the VOC strategy, as shown in (18)-(21), define the relationship between the \(d\)-\(q\)-frame variables and the output power components. These equations are critical for implementing VOC in practical applications, ensuring that the inverter operates in harmony with grid requirements while maximizing the efficiency and stability of power conversion processes.  
\[V_{inv}^{d*}=\left(K_{p1}^{CVSI}+\frac{K_{i1}^{CVSI}}{S}\right)\left(i_{B1}^{ d*}-i_{B1}^{d}\right)-\omega\left(L_{f}^{v}+L_{tr}\right)i_{B1}^{d}+V_{B1}^{d} \tag{18}\]  
\[V_{inv}^{d*}=\left(K_{p2}^{CVSI}+\frac{K_{22}^{CVSI}}{S}\right)\left(i_{B1}^{ d*}-i_{B1}^{d}\right)+\omega\left(L_{f}^{v}+L_{tr}\right)i_{B1}^{d}+V_{B1}^{d} \tag{19}\]  
\[i_{B1}^{d*}=\left(K_{pdc}^{CVSI}+\frac{K_{idc}^{CVSI}}{S}\right)(V_{DC}^{*}-V _{DC}) \tag{20}\]  
\[i_{B1}^{d*}=\frac{-2\ Q_{B1}}{3\ V_{B1}^{d}} \tag{21}\]  
The variables \(V_{PPC}^{d}\) and \(V_{PPC}^{q}\) represent the \(d\) and \(q\) components of the grid voltage, respectively. The primary function of the PI controller is to regulate both the DC link voltage and the output currents of the inverter. This regulation is critical for maintaining  
Figure 6: Control unit the CVSI.  
the needed voltage level and true power factor at the PCC with the grid, as depicted in Figure 5. In control system design, it is common to incorporate grid voltage feedforward. This approach involves adding a feedforward term to the voltage reference equation. This strategy enhances the inverter's responsiveness to fluctuations in grid voltage, thereby improving overall system stability and performance.