## 1 Introduction  
Since ancient times, mankind has never given up on the exploration of the ocean, and the tools for observing the ocean have been constantly innovating, such as the scientific cabled seafloor observatories (CSOs). It is an emerging means of ocean observation that collects continuous real-time data over long periods by laying a regional observing system on the ocean floor. Since 2006, more and more scientific CSOs have been established around the world for oceanography research, e.g., MARS (Monetary Accelerated Research System) [1], OOI (Ocean Observatory Initiative) [2], ALOHA cabled observatory [3], VENUS (Victoria Experimental Network Under the Sea) [4], NEPTUNE (North-East Pacific Time-series Underwater Networked Experiment) [5], DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) [6], S-net (Seafloorobservation network for earthquakes and tsunamis along the Japan Trench) [7], ESONET (European Seas Observatory Network) [8], Xiaoqushan seafloor observatory [9] and MACHO (Marine Cable Hosted Observatory) [10]. Moreover, the East China Sea (ECS) and the South China Sea (SCS) experimental observatories were built in 2015 and 2016, respectively. In 2017, the Chinese national scientific seafloor observatory (CNSSO) was launched to realize the long-term observation of the complex processes operating within the oceans. In typical scientific CSOs, the shore station converts the alternating current (AC) power from the terrestrial electrical grid into the direct current (DC) power, rated up to \(-10\) kV and delivers it to the undersea stations through backbone cables, branching units (BUs) and spur cables, as shown in Figure 1[11]. A DC/DC converter is installed in each undersea station, converting \(-10\) kV DC power from the spur cable into 375 V DC power for scientific instrument interface modules (SIIMs). The SIIMs convert 375 V DC power into DC power under 48 V required by science instruments (SIs). In this way, the long-distance electrical power transmission and conversion for undersea distributed science payloads are completed. An undersea DC power system of scientific CSO has a large-scale DC distributed power system (DPS) that contains many cascaded power electronic converters, connected by hundreds to thousands of kilometers of submarine cables [12]. Each point of load (POL) converter, installed in undersea stations, acts as a constant power load (CPL), and the distributed parameter characteristics of long-distance submarine cables greatly affect the stability of the undersea DC power system [12; 13; 14; 15]. In the event of instability, the undersea DC power system will oscillate, or even collapse, especially under large disturbance conditions. Because of the harsh subsea environment, CSOs have low accessibility and are  
Figure 1: A typical system diagram of scientific cabled seafloor observatories (CSOs). very expensive to construct and repair, time-consuming and labor-intensive. Therefore, the stability of the CSO undersea DC power system must be analyzed during design and operation. At present, there is less stability analysis of the undersea DC power system of CSOs worldwide. In 2006, Dr. [PERSON] analyzed the stability of the NEPTUNE observatory power system, but it was lacking in simulation and experimental verification [12]. At present, the stability has been studied in small-scale DC DPSs, e.g., space stations, aircraft, ships and electric vehicles [15; 16; 17; 18; 19]. Compared to these DC DPSs, the undersea DC power system of scientific CSOs are more complicated, mainly because of the long-distance submarine cables. This paper focuses on the large-signal stability of the CSO undersea DC power system, which is a high-order nonlinear system. Large-signal stability means that the power system can go back to its original equilibrium state or transition to a new equilibrium state, after encountering a large disturbance during normal operating conditions [20]. For second or lower-order nonlinear systems, large-signal stability analysis can use the phase plane method [19].