# Multi-Domain Modeling and Analysis of Marine Steam Power System Based on Digital Twin
## 3 Digital Modeling of Marine Steam Power System Based on Modelica  
For each piece of thermal equipment involved in the marine steam power system, including the boilers, main steam turbines, condensers, deaerators, air ejectors, valves, pipes, pumps, etc., according to its mathematical operating characteristics combined with the principle of modular modeling, the corresponding component model and system model library are developed using the MWorks system simulation platform based on the Modelica language, and the digital model of the marine steam power system is integrated. The model development principle is shown in Figure 2.  
Figure 2: The model development principle. After the corresponding component models have been developed on the MWorks simulation platform, in order to ensure the accuracy of the established digital model and the high stability of the digital simulation model of the complete steam power system, and to ensure that the model operating state parameters can accurately reflect the actual marine operating data, it is necessary to conduct a parameter sensitivity analysis, model modification, a confidence evaluation, etc. on the model. The test data and historical operation data of real marine systems are used to verify and check the model. The closer the operation data of the digital model are to the actual marine operation data, the higher the accuracy of the model, which can ensure the accuracy of the subsequent dynamic characteristics analysis, realize the online monitoring and status evaluation of the physical model by the digital model, so as to accurately provide an operation strategy for the actual marine thermal equipment, and realize the virtual-real mapping of the digital model to the physical equipment. The debugging process for the model characteristic parameters is shown in Figure 3.  
At the same time, in debugging the parameters of the system model, we should focus on whether the data interaction of each component model and subsystem model is normal, whether the model connection mechanism and interface are correct, whether the parameter setting for the model characteristic is reasonable, and whether a dynamic characteristic analysis of the system by the digital model can be realized. Through the debugging of the characteristic parameters of the system model, a complete digital model of the marine steam power system is built. The digital model of the marine steam power system is shown in Figure 4.  
Figure 4: Digital model of marine steam power system. Figure 3: The model characteristic parameter debugging process. Through debugging the design parameters and state parameters in the characteristic parameters of the digital model of the marine steam power system, a more accurate digital model has been obtained. In order to verify the accuracy of the digital model of the marine steam power system, this paper uses a type of steam turbine unit to carry out load up test verification. By controlling the opening of the steam valve, it simulated the processes of acceleration and analyzed their dynamic characteristics. An operating state parameter curve for the marine steam turbine from start-up to a 100% working condition is obtained. All state parameters are dimensionless according to their design data. The dimensionless formula is shown in Formula (18). \[\widetilde{y}_{i}=\frac{y_{s}}{y_{d}} \tag{18}\]  
where:  
\(\widetilde{y}_{i}\)--The dimensionless parameter;  
\(y_{s}\)--The simulation or experimental parameter;  
\(y_{d}\)--The design parameter. The steam turbine output power, main steam flow, main steam pressure, main steam temperature, and cooling water outlet and inlet temperatures are shown in Figures 5-10. Figure 8: Main steam temperature. Figure 7: Main steam pressure. Figure 9: Cooling water inlet temperature. It can be seen from the load increase test data for the marine steam turbine that the steam turbine reached a 100% working condition in five steps after starting up.