Study On Protection Technology Of AC-DC Hybrid Distribution Grid

With the extensive development and application of new energy,distributed generation(DG)has been widespread application.And with the development and change of modern power demand,more DC and variable frequency loads are connected to the power grid,which brings new requirements to the distribution method of power system.The AC-DC hybrid power distribution method retains the advantages of the AC power distribution technology,and has the same excellent performance as the DC power distribution grid in terms of transmission capacity,control flexibility,and power supply reliability.The faults of AC-DC hybrid distribution grids often involve AC and DC side,and cause serious impacts.The lack of reliable relay protection technology restricts the development of AC-DC hybrid distribution grids.Therefore,this paper studies the protection technology of AC-DC hybrid distribution grids based on flexible technology.Based on the analysis of the AC-DC hybrid distribution grids topology,wiring modes,and equipment principles,the fault transient characteristics of the AC side and DC side are studied respectively.The pilot protection methods for AC and DC side are proposed and verified in PSCAD/EMTDC simulations.The details are as follows:1.An AC-DC hybrid power distribution grid model is designed.Based on the introduction of the system topology and wiring modes,the structural parameters and operating principles of the main converter equipment are analyzed.The protection zones and fault isolation methods of the AC-DC hybrid power distribution grids are given.2.A DC side protection scheme for AC-DC hybrid distribution grids based on transient impedance is proposed.According to the structural characteristics of the DC distribution grid,the transient impedance models of the DC solid-state transformer and MMC converter are established,and the transient impedance characteristics of the DC distribution grid are studied.The windowed Fourier transform is used to extract the transient impedance at the characteristic frequency.A fault direction identification criterion is proposed based on the transient impedance characteristic of the fault.The PSCAD model is used to verify the transient impedance models and the proposed protection method.3.An AC side protection scheme for AC-DC hybrid distribution grids based on transient impedance is proposed.According to the structural characteristics of the AC distribution grid,the transient impedance models of doubly-fed induction generator(DFIG)and MMC converter are established,and the transient impedance characteristics of the AC distribution grid are studied.A fault phase selection method based on transient voltage is proposed.And a fault direction identification criterion based on transient impedance is proposed.The PSCAD model was used to verify the transient impedance models of the equipment and the proposed protection scheme.