Stability Analysis Of The Ship's DC Microgrid

Along with the transition of global energy structure,the shipping industry has become increasingly demanding in terms of energy saving,emission reduction and operational energy efficiency.The integrated shipboard power system has gradually evolved from AC distribution to DC distribution.The shipboard power systems based on DC distribution can also be called DC shipboard microgrids.As the typical power electronic system,the DC shipboard microgrids face greater challenges in its stability.This thesis analyzes the possible stability problems of two typical DC ships with energy storage access including pure electric ships and hybrid electric ships.Which focuses on the small-signal stability analysis among the power sources and between the power source and the load in the DC shipboard microgrid.Specifically,it includes parallel DC-DC power converters of battery packs and cascade systems consisting of DC-DC power converters and electric propulsion loads in pure electric ships,and compound system consisting of DC-AC power converters and synchronous generators in hybrid electric ships,etc.The theoretical basis for designing the DC shipboard microgrid system parameters and controller parameters is derived.This research can effectively promote the development of DC distribution ships and contribute to the dream of Chinese maritime power.The main work of this thesis is as follows:1)By establishing small-signal models of several common DC-DC converter topologies in DC shipboard microgrids,the open-loop characteristics of various DCDC converters are compared and analyzed.2)The small-signal stability of the DC shipboard microgrids for pure electric ships is analyzed.Including cascade system consists of DC-DC power converters and electric propulsion loads,and parallel DC-DC power converters of battery packs.Firstly,the stability of the energy storage bidirectional DC-DC converter and electric propulsion load cascade system was studied by impedance method,and the influence of system parameters and controller parameters on the stability and the power of the electric propulsion loads were obtained.Secondly,the eigenvalue method is used to study the small signal stability of the bidirectional DC-DC converter parallel system using I-V droop control.The main factors affecting the stability of the system are obtained.The design basis of the inner loop bandwidth and the stability range of the droop gains are given.3)The small-signal stability of compound system consisting of the bidirectional DC-AC converter and the diesel generator sets for hybrid DC shipboard microgrid is analyzed.First of all,when the diesel generator sets are used as the main power source,the input impedance of the droop controlled diesel generator sets(which can be regarded as a weak grid)and the output impedance of the PQ controlled bidirectional DC-AC converter are derived using the impedance method.Based on the relationship between input impedance and output impedance,the factors affecting the small signal stability of the cascaded system are analyzed,and the law of the influence of system parameters and controller parameters on system stability is derived.Secondly,the impedance method is used to analyze the small-signal stability of hybrid system consisting of a bidirectional DC-AC converter with a virtual impedance droop control and a PQ controlled diesel generator set when the energy storage battery is used as the main power source.The simulation verification is carried out with the parameters of a hybrid electric real ship.