Study On Stability DC Power System With Photovoltaic

As the world is experiencing the third energy revolution,photovoltaic,wind power and other renewable resource are developing rapidly.China is also promoting the development of new power system based on renewable resource.Due to the advantages of excellent power quality,large power supply capacity,high efficiency,high reliability,easy access to renewable energy sources,and flexible voltage control,DC power system has been widely used in data center,electric vehicle,more electric aircraft and other industrial fields,especially the multi-voltage DC power system has become the basis of building the global renewable power system.However,the multi-voltage DC power system has complex topology and contains a large number of power electronic converters at the source,network and load side,which has brought the following problems: firstly,large-scale power system may cause the problem of modelling complexity;secondly,the impedance mismatch between converters leads to the voltage instability of DC bus voltage;thirdly,When the system is disturbed by large signal such as load fluctuation,the key parameters affecting the bus voltage self-balancing ability are not clear.Therefore,this paper will study the reduced order model and stability analysis of multi-voltage DC power system.The main parts of this dissertation are as follows:1.In order to solve the problem of modelling complexity and difficulty in calculation,the closed-loop state space matrix of the system is established by analyzing the working mode of the hybrid photovoltaic energy storage system.Based on the participation factor method,it is concluded that the independent variables of the photovoltaic module are irrelevant to the stability of DC bus voltage,and then the reduced order small signal mode of the system is proposed.Finally,the simulation model is used to verify the correctness of the proposed reduced order model.2.The existing impedance ratio stability criteria are only applicable to single voltage level DC system.This paper,a general impedance ratio stability criterion based on subsystem partition method is proposed for multi-voltage DC system.Firstly,system’s equivalent loop gain is obtained by analyzing all the input and output transfer functions of the system,which can be considered as the necessary and sufficient conditions of the system’s stability.Secondly,the multi-voltage DC system is equivalent to a two-stage cascade system by defining subsystems,and then the impedance ratio criterion of the whole system is obtained.Based on the equivalent loop gain of the system,the correctness of the proposed impedance ratio criterion is verified.Finally,the simulation model and experimental platform of medium and low voltage DC system are built to further verify the effectiveness of the proposed impedance ratio criterion.3.At present,the large signal stability evaluation method based on mixed potential function has not been applied to multi-voltage DC system.Therefore,this paper constricted the mixed potential function of three-stage cascaded DC system and obtained the unified simplified model.Then,the large signal stability criteria under different control modes are calculated according to the stability theorem of mixed potential function.Through further analysis,the key parameters that affect the stability of large signal are obtained,and the large signal stability region of the system is drawn.Finally,the validity of the proposed stability criterion is verified by the simulation model.