Research On The Stabilizing Techniques For The All Electric Ship MVDC Power System

All electric ship is the future trend for the ships of our country,which has several advantages such as,a higher operating efficiency,a better controllability,and a higher reliability.There are quite a lot of converter fed loads in the all electric ship medium voltage direct current(MVDC)power system,which may exhibit constant power load characteristics.The constant power load will introduce a negative incremental resistance to the system,which may impact the system stability.Stability is the basic requirement for the all electric ship MVDC power system.Therefore,it is necessary to conduct research on the stabilization techniques for the all electric ship MVDC power system.The following of this thesis will be organized as follows:Firstly,the paper briefly review the current research states of the all electric ship and its corresponding stability analysis and stabilizing strategy.The paper summarizes the existing problems in the system stability,stability analysis method,and stabilization method for the shipboard MVDC integrated power system,followed by the main contents and total structure of this thesis clearly clarified.Secondly,the paper conduct research on the system component and operating mechanism for the all electric ship MVDC power system.The following system models are constructed:the gas turbine and its speed controller,the generation system,the propulsion system,the zonal load system,and the power distribution system.The constructed system mathematical model lay the great foundation for studying the all electric ship system stabilization strategy.Thirdly,the paper conduct a research on the all electric ship MVDC power system stability analysis method.The voltage terminal and current terminal classification method is provided based on the system terminal impedance,which solves the problem of system classification contradiction caused by the unfixed power flow in the system.Based on the voltage terminal and current terminal classification methods,the Nyquist impedance stability criterion for the MVDC power system for the all electric ship is provided,which lays the theoretical foundation for the system impedance stability analysis.Then,a system mathematical model based terminal impedance extracting method proposed.With the proposed extracting method,the terminal impedances of the generation system under droop control and propulsion system under direct torque control are studied.Simulations are conducted for validating the effectiveness of the proposed stability analysis method.The terminal impedance,stability analysis result and simulation result of the system under the dc bus capacitance perturbation are provided,which validates the effectiveness of the proposed stability analysis method for the all electric ship MVDC power system.For solving the stability problem in the all electric ship MVDC power system,the paper take a further research on the system stabilization methods.A modified Ⅴ-Ⅰ droop control method is proposed for the main generation system for solving the static deviation problem in the dc bus voltage caused by the traditional droop control method.A dc bus voltage restoration controller is designed with the parameters designated based on the Nyquist impedance stability analysis method.For solving the poor dynamics in the dc bus voltage regulation for the all electric ship MVDC power system,a disturbance estimation based feedforward compensation control method is proposed in this paper.A nonlinear disturbance observer is designed for estimating the load current for the auxiliary generation system.The estimated result is used for designing the disturbance estimation feedforward compensation term,which alleviates the load current disturbance impact on the dc bus voltage,thus solving the problem of poor regulating dynamics in the dc bus voltage.For solving the energy storage system output voltage stability problem caused by the line inductance in the zonal load system,the paper proposes a virtual negative inductor stabilizing strategy for the energy storage system.A virtual negative inductor is built on the output side of the energy storage system through the droop control method,which counteracts the line inductance,thus alleviating the ESS output voltage instability problem caused by the line inductance.Finally,the paper conduct simulating researches for validating the effectiveness of all electric ship power system stabilizing strategy.A simulation based validating scheme for the all electric ship MVDC power system is proposed.Based on the APPSIM real time simulating system,a simulation based validating platform for the all electric ship MVDC power system is constructed.The effectiveness of the proposed stabilizing strategy under multiple operating conditions are studied.The validating simulation results show that the shipboard MVDC integrated power system can operate in a stable sense with expected control effect under the multiple operating conditions,which further validate the effectiveness of the proposed system stabilization strategy proposed in this paper for the all electric ship MVDC power system.