## 4 Simulation and Experiment
### Large-Signal Stability Simulations of a Multi-Node CSO Undersea DC Power System  
In the last two sections, the large-signal stability of a single-node CSO undersea DC power system and the factors affecting the large-signal stability are studied. This section mainly studies the large-signal stability of a multi-node CSO undersea DC power system and its influencing factors.  
In this paper, the large-signal stability of a multi-node CSO undersea DC power system is simulated by using Saber. The simulation file is shown in Figure 11. The shore station PFE is modeled by an ideal voltage source. The submarine cable is modeled by cascaded lumped parameter equivalent circuits. The undersea station is modeled by CPLs. In this system, there are four CPLs, in which the distance between the CPL4 and the source is the longest, so that CPL4 gets the minimum energy from the source. The total length of the submarine cable is assumed to be 490 km. Every cable segment of 10 km is represented by a \(\pi\)-type lumped parameter equivalent circuit.  
To study the effects of various factors on the large-signal stability of a multi-node CSO undersea DC power system, five simulation sets are needed. The parameter sets for the Saber simulation are shown in Table 6. \(U_{S}\) represents the output voltage of the source, i.e., the shore station PFE. \(R_{0}\), \(L_{0}\) and \(C_{0}\) represent the resistance, inductance and capacitance of a cable segment, respectively. \(P_{L}\) represents the power consumption of the CPL4, i.e., the undersea station with external science payloads. The power consumption of the four CPLs is equal.  
Initially, \(U_{S}\) in the system was 0 V. At 0.1 s, according to experimental needs, the voltage rose from 0 V to 10 kV or 6 kV, and the voltage transient processes were generated.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & \(\mathbf{U_{S}/V}\) & \(\mathbf{P_{L}/W}\) & \(\mathbf{R_{0}/\Omega}\) & \(\mathbf{L_{0}/mH}\) & \(\mathbf{C_{0}/\mu F}\) \\ \hline \#1 & 400 & 82.29 & 10.2 & 4 & 2.03 \\ \#2 & 400 & 82.29 & 10.5 & 400 & 2.03 \\ \#3 & 400 & 82.29 & 10.2 & 4 &Figure 10: Cont.  
The simulation results are shown in Figure 12. It shows the voltage transient process of CPL4, where the green, purple, blue, orange and black lines represent the simulation results for the parameter sets #1, #2, #3, #4 and #5, respectively. As is apparent from Figure 12, the power system of parameter set #1 was stable and the power systems of parameter sets #2, #3, #4 and #5 were unstable, in the case of power-on. This indicates that the large-signal stability of a multi-node undersea DC power system is positively correlated with the output voltage of the shore station PFE and the capacitance of per unit length submarine cable, and negatively correlated with the power consumption of the undersea station and the inductance of per unit length submarine cable. This is consistent with the large-signal stability conclusion of a single-node undersea DC power system.  
Figure 11: The Saber file for the large-signal stability simulation of a multi-node CSO undersea DC power system.  
Figure 10: (**a**) The voltage transient processes in the parameter set #1; (**b**) the current transient processes in the parameter set #1; (**c**) the voltage transient processes in the parameter set #2; (**d**) the current transient processes in the parameter set #2; (**e**) the voltage transient processes in the parameter set #3; (**f**) the current transient processes in the parameter set #3; (**g**) the voltage transient processes in the parameter set #4; (**h**) the current transient processes in the parameter set #4; (**i**) the voltage transient processes in the parameter set #5; (**j**) the current transient processes in the parameter set #5.