Modeling And Control Of Zonal Medium-voltage DC Integrated Shipboard Power System Based On DC-DC Converters

With the advancement of power electronics technology and the growing demand for high-power equipment on ships,the integrated shipboard power system which integrates power generation,distribution,conversion,and energy consumption has gradually become the development direction of shipboard power system(SPS).Based on medium-voltage DC(MVDC)power supply and DC zonal power distribution,zonal MVDC SPS is regarded as the developing trend in the future.Compared with AC system,MVDC architecture has significant advantages in achieving generators decoupling,reducing intermediate transformation,and improving system efficiency and power density.However,there are still many challenges for the MVDC system such as: fault current detection and protection,configuration and optimization of composite energy storage devices,impedance matching of the cascaded power electronic devices,and multi-objective energy regulation for the power distribution system.This thesis takes the new-generation power electronic based zonal MVDC SPS as the research object.Based on the modeling and control of DC-DC converters,the following aspects are studied,including topological structure analysis and steady-state operation analysis,stabilization of constant power loads,and power management in zonal distribution area.The specific research contents are as follows:1)From the perspective of topological analysis,reliability analysis and evaluation method of zonal MVDC SPS is studied.First,based on the topology of SPS,the minimum cut set of each load node is obtained by matrix calculation,its failure rate is quantitatively evaluated in combination with equipment reliability parameters.Meanwhile,a new reliability index for power transmission/distribution lines —probability gain of failure rate(which is,the multiple of load failure probability in case of line fault),is proposed in consideration of the characteristic of SPS.So as to quantify the impact of line faults on the reliability of load nodes,and is expected to provide reference for the design of wiring and protection.Then from the perspective of steady-state analysis,the hybrid AC/DC power flow calculation method which is suitable for MVDC SPS is studied.Considering most of the existing AC/DC power flow calculation methods are based on voltage source converters,which mainly focus on the balance of the AC-DC interfaces.For its DC part,only line impedance model is considered.However,the zonal MVDC SPS contains a large number of DC conversion links.In order to solve the problem of power flow calculation in the DC power distribution part,modeling and analysis of the key equipment such as DC-DC converters must not be ignored.The innovation of this thesis is to model the DC line containing the DC-DC converter in the zonal MVDC SPS,and generate the nodal admittance matrix based on this.Then,according to the different control methods of the DC-DC converter,it is divided into constant duty cycle control and constant voltage control,and the iterative calculation methods of power flow under different control methods are respectively given.Finally,the power flow calculation method based on Newton-Raphson is used to solve the power flow of the zonal MVDC SPS,which verifies the effectiveness and convergence of the proposed method.2)The problem of stability analysis and control of the MVDC bus is studied.There are a large number of constant power loads in the MVDC SPS,such as the propulsion motor whose speed / power is precisely controlled by the frequency converter,and various kinds of electrical loads whose voltage/ power is precisely controlled by the electronic power converters,The negative impedance characteristic of the constant power load in SPS is the main reason for the instability of MVDC bus.At the same time,a large number of DC-DC converters are used to convert DC voltage in MVDC SPS.Taking stable control of MVDC bus in the face of large disturbances as the goal,we can model it as a coupled nonlinear control problem of a source-side multiterminal parallel DC-DC converter with a constant power load.In response to this problem,traditional hardware compensation methods or linear control methods is only suitable for small-signal analysis near its original equilibrium point,while the requirements of large-signal stability under complex working conditions cannot be met.However,most of the researches on nonlinear control methods that meet the large signal requirements of constant power loads are only based on single-input single-output systems.Without considering system reduction,there are few studies on the coupled nonlinear control of multi-terminal parallel DC-DC converters with constant power loads.Aiming at the problem of voltage stabilization and current sharing of the multiple parallel-connected DC-DC converters loaded by constant power loads,a sliding-mode based duty cycle controller is proposed,which is able to ensure large-signal stability of the bus voltage.Meanwhile,droop control strategy with load current feedback or consensus of the source converters is adopted to achieve proportional current sharing between the parallel DC-DC converters.3)The power management method of the zonal distribution system based on distributed control architecture is studied.Intelligent management technology in the face of complex working conditions is needed for the zonal distribution management of SPS,in order to ensure that distributed control can guarantee the basic performance of the system when centralized control fails due to abnormal conditions such as communication obstruction or system failure.Based on distributed control architecture,this thesis models DC-DC converters with different conversion ratios and different types as heterogenous agents,and established a heterogenous multi-agent system model for MVDC SPS in zonal distribution area.Meanwhile,a cooperative control protocol is designed based on flocking algorithm,where each agent declares its requirements according to its load ratings and priorities,the group agents reach to an agreement through information interaction,so as to achieve the group objective,that is,maximizing load demand while satisfying system operating constraints.In addition,an improved navigational feedback is adopted to constrain the upper and lower bounds of each agent,which improves the control effect.Through static topological analysis and steady-state analysis of the system,it can be seen that zonal MVDC SPS has higher reliability and security than radial structure and ring structure.At the same time,the power electronic based converters in MVDC system have brought new stability problems.In consideration of stabilization problem of the source-side parallel-connected DC-DC converters with constant power loads,a slidingmode based duty-cycle controller is proposed,where large-signal stability of bus voltage and proper current/power balancing between the converters are achieved.On the basis of DC bus voltage stability,a multi-agent based load management strategy for the zonal distribution area is designed,so that the distribution system can achieve maximum load power supply under the premise of meeting external operation constraints and internal load requirements.