## 4 Oscillation Suppressions of FSC-VSPSU  
Based on the oscillation suppression performances presented in Table 1, this section formulates targeted strategies for mitigating oscillations focusing on two key optimization points: (1) the control parameters of FSC-VSPSU; (2) the grid network impedance. Specifically, the oscillation suppression strategy related to control parameters targets the black box controller, wherein the in situ modification of controller parameters is impractical in real-world applications. Consequently, this section introduces an oscillation suppression approach grounded in the optimization of grid network impedance, characterized by its straightforward principle and ease of implementation. From Table 1, reducing \(L_{\text{g}}\) or appropriately increasing \(R_{\text{g}}\) can suppress the oscillation of the grid-connected FSC-VSPSU. However, for the actual operating grid network, reducing its equivalent inductance is difficult to achieve. Therefore, the following will study the oscillation suppression strategy by increasing the equivalent resistance of the grid network, and the corresponding schematic diagram is shown in Figure 16. When the system operates stably, the bypass switch is closed, causing the compensation resistor (\(R_{\text{c0}}\)) to be short-circuited. Furthermore, when the system experiences oscillation, the bypass switch is opened, and the \(R_{\text{c0}}\) is connected to the system in series, which increases the equivalent resistance of the grid network to suppress the occurring oscillation. \begin{table}
\begin{tabular}{c c c c c} \hline Control parameters of the & \(K_{\text{pv}}\) & \(K_{\text{iv}}\) & \(K_{\text{pcl}}\) & \(K_{\text{ic1}}\) \\ \cline{2-5} grid-side converter & \(\times\) & \(\downarrow\) & \(\times\) & \(\downarrow\) \\ \hline Control parameters of the & \(K_{\text{pw}}\) & \(K_{\text{iw}}\) & \(K_{\text{pc2}}\) & \(K_{\text{ic2}}\) \\ \cline{2-5} machine-side converter & \(\times\) & \(\times\) & \(\times\) & \(\times\) \\ \hline Excitation system and turbine & & \(K_{\text{pg}}\) & \(K_{\text{ig}}\) & \(K\) \\ \cline{2-5} governor parameters & & \(\times\) & \(\times\) & \(\times\) \\ \hline Equivalent impedance parameters of & & \(L_{\text{g}}\) & & \(R_{\text{g}}\) \\ \cline{2-5} the grid network & & \(\downarrow\) & & \(\uparrow\) \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of the oscillation analysis results. Figure 16: Schematic diagram of the oscillation suppression strategy by increasing the equivalent resistance of the grid network. It is noteworthy that an increased compensation resistance yields a more pronounced effect on the suppression of oscillations. Nevertheless, this enhancement is accompanied by a reduction in the system's short-circuit ratio (SCR), which poses a threat to its safe operation. Therefore, the compensation resistance has a maximum limitation. The SCR expression is written as  
\[K_{\text{scr}}=\frac{S_{\text{ac}}}{P_{\text{dN}}}=\frac{U_{\text{N}}^{2}}{P_{ \text{dN}}}\cdot\frac{1}{|\mathbf{Z}|}>K_{\text{scr\_min}} \tag{6}\]  
where \(K_{\text{scr}}\) is the SCR value and \(K_{\text{scr\_min}}\) is its minimum limitation; \(S_{\text{ac}}\) is the short-circuit capacity of the grid; \(P_{\text{dN}}\) is the rated DC power of the unit; \(\mathbf{Z}\) is the equivalent impedance of the grid network; and \(U_{\text{N}}\) is the rated voltage of the grid. When \(R_{\text{c0}}\) is connected to the system, the equivalent impedance of the grid network is expressed as  
\[\mathbf{Z}=\text{R}_{\text{g}}+\text{j}X_{\text{g}}+\text{R}_{\text{c0}} \tag{7}\]  
where \(X_{\text{g}}\), written as \(X_{\text{g}}=2\pi f\mathbb{Z}_{\text{g}}\), is the equivalent reactance of the grid network.