Research On Control Strategy Of Medium And Low Voltage DC Distribution Network Based On MMC

In recent years,with the increase of the penetration rate of new energy power generation and the large-scale use of DC load,improving the consumption rate of new energy power generation and meeting the growing demand for electricity is one of the challenges facing the current power grid.Compared with the traditional AC distribution network,the DC distribution network has higher controllability,which can reduce the AC/DC conversion links in the system,and improve the transmission efficiency,capacity,range and power quality of the system.A series of power quality problems existing in the traditional AC distribution network have also been solved in the DC power network,such as voltage fluctuation,imbalance,harmonics,etc.At present,the research on DC mainly focuses on high-voltage DC transmission system and low-voltage microgrid,while the research on medium-voltage DC distribution network is less.This thesis focuses on the key control technology of medium-voltage DC distribution network.First of all,analyze the voltage level,grounding mode and topology of the medium and low voltage DC distribution network,determine the DC distribution network structure studied in this thesis,and analyze the system level,converter station level and converter valve level control of the DC distribution network.Secondly,in view of the problems of voltage equalizing control and circulating current suppression in the valve-level control of DC distribution network,improved control strategies are proposed respectively.The traditional voltage equalizing control has the problem of large switching frequency.Incremental switching is used to replace the traditional voltage equalizing control to reduce the switching frequency,and the upper and lower limit control is introduced to reduce the voltage deviation during incremental switching,so as to achieve the lowest possible switching frequency within the allowable range of the voltage of the sub-module.The traditional bubble sorting algorithm is replaced by cardinal sorting algorithm to reduce the complexity of sorting algorithm and reduce its dependence on hardware.The traditional circulation suppression control can only suppress the second harmonic component in the circulation.Aiming at the high-frequency even harmonics in the MMC circulation,the quasi-PR control with SOGI link is proposed to replace the traditional circulation suppressor to reduce the circulation.Finally,in view of the problems of the traditional droop control with fixed droop coefficient in the DC distribution system,such as unreasonable power distribution and large voltage deviation,an improved MMC outer loop control strategy considering power margin and voltage deviation factors and an inner loop control based on internal model control are proposed.Establish a medium and low voltage DC distribution system,and compare and analyze the typical working conditions such as load fluctuation,AC side short circuit fault,power flow reversal,converter station out of operation,and verify that the proposed improved control strategy has the advantages of reasonable power distribution and small voltage deviation.