The Voltage Stability Analysis Of DC Microgrid System Based On Energy Storage Unit

Compared with AC Microgrid,DC Microgrid has many advantages,such as simple connection structures,easier interface with DC loads and energy storage system and low cost and failure rate.Based to the unique advantages,DC Microgrid develops quickly in recent years.Many closed-loop converter loads maintain constant power when the input voltage changes,and can be regarded as constant power loads(CPLs).These loads exhibit negative impedance characteristics and may cause system instability.There is no frequency and phase problem in DC Microgrid system,the stability of the DC bus voltage is a important measure of system stability.The energy storage system can realize the bidirectional power flow and plays an important role for stability in the system.This paper solves the stability problem of bus voltage based on the battery storage system,considering the negative impedance characteristics of constant power loads.The main methods are limiting the charging and discharging power of the battery,the power of constant power loads and optimizing the algorithm of the bidirectional DC-DC converter connected with the battery unit to improve the stability of the system.Because the bus voltage can be maintained by micro source and battery storage system,this paper uses the Brayton-Moser mixed potential function theory to analyze the stability of the system based on the two above working modes.Under the micro source maintaining bus voltage mode,a mixed potential function model is established and the large signal stability criterion is obtained by derivation,that can analyze the effect of battery charging and discharging power,constant power loads power to system stability quantitatively.Similarly under the battery storage system maintaining bus voltage mode,the mixed potential function model is still established,and the large signal stability criterion is deduced.According to the criterion,the influence of the constant power loads power and the control parameters of the battery bidirectional DC-DC converter on the system stability can be quantitatively analyzed.To verify the criterion,use the Simulink simulation software to build a DC Microgrid model,and the large signal stability criterions were verified by simulation.At the same time,an experimental platform was built to verify the large signal stability criterions.This paper derives the large signal stability criterion under the micro source maintaining bus voltage mode and under the battery storage system maintaining bus voltage mode respectively.The criterions quantitatively describe the relationship between parameters such as bus voltage,power of constant power loads,power of battery storage system,bidirectional DC-DC converter control strategy,and the large signal stability of the system,considering the charging and discharging two working modes of the battery storage system.The criterions are very simple and easy to be implemented.These provide design basis for solving the stability problem of the DC Microgrid and has a certain application value.