Research On Analysis And Control Of Ship MVDC System

The ship's integrated power system is the development trend of many ships(commercial ships,warships,cruise ships,etc.)in the future.As the power demand of the ship's electric load increases,the installed capacity of the ship's power grid rises,which drives ship's power grid voltage level rise.In addition,compared with the traditional ship grid using AC system,the use of DC system has many advantages(such as: eliminating reactive power transmission,reducing cable weight and no need to synchronize between generator sets,etc.).In order to meet the future power requirements and efficient operation of the ship's power grid,the researchers began to pay attention to the power distribution scheme in the form of medium voltage DC.At present,the development of marine medium voltage DC system is still in the exploration stage in engineering application.This paper combines the navigation conditions of the ship and conducts research from the following aspects.Firstly,according to the basic components of the ship's medium voltage DC system,mathematical models and simulation models of each part are established,including gas turbine generator set,rectifier unit,propulsion motor and its control unit,ship propeller load unit,energy storage unit,etc.Carry out corresponding simulation experiments under PSCAD to verify its rationality.Secondly,considering the power transmission between the medium/low voltage and the access of the energy storage unit on the medium voltage DC bus,in this paper,the dual active bridge DC/DC converter is selected as the basic unit,and the purpose of voltage division and high power transmission is realized by series and parallel connection of basic units,and then the single/bidirectional power transmission control between medium voltage and low voltage is studied.The designed converters are respectively applied to the distribution transformer function and the energy storage interface function.Under the function of distribution transformer,the voltage on the low voltage side of the distribution transformer is stable during loading.The bidirectional power transmission between the energy storage unit and the medium voltage DC bus is realized under the function of the energy storage interface.Finally,on the whole system simulation platform,the typical working conditions in the ship navigation are simulated and verified,including starting,parking,reverse sailing,frequent changes in the external environment,the propeller being blocked,and the propeller leaving the water.For some working conditions(parking,reverse),due to the reversal characteristics of the propeller,the propulsion system returns energy to the medium voltage DC bus,causing the bus voltage to rise.This paper explores the use of energy storage units and energy braking resistors to control the bus voltage at the same time.The simulation results show that under this method,the DC bus voltage can be stabilized at the rated value for a longer period of time.In addition,the pulse load on the medium-voltage DC bus is studied,and the supercapacitor is supplied to the pulsed load to reduce the impact on the bus voltage,the simulation results show that the impact of the DC bus voltage when controlling the supercapacitor to supply the pulsed load is significantly reduced.