Research On Protection Schemes Of The AC/DC Hybrid Microgrid

In recent years,people focus on the application of distributed generation such as wind power or photovoltaic due to the worldwide shortages of energy supplies.Distributed generation not only possesses the characteristics of environmental protection and high usage of energy,but also it can provide users with higher power quality and more economic benefits.However,the output of distributed generation is random,intermittent and hard to control,which lead to many difficulties to the large scale application.To overcome the above shortcomings,the microgrid is introduced and developed.The micro-grid have AC/DC distributed sources and AC/DC loads,In order to improve the utilization of energy and reduce the energy loss,more and more attention fouces on the AC/DC hybrid microgrid.Via AC and DC buses,the AC/DC hybrid micro-grid can connect AC/DC distributed soures and loads to improve the utilization of energy in micro-grid.Researches on the microgrid protection have been attracted more attention for a long time.This paper focuses on the protection technology of AC/DC hybrid microgrid.The main work can be summarized as follows:This paper introduces the basic working principle of photovoltaic power generation unit,wind power generation unit and battery energy storage system in AC/DC hybrid micro grid,analysis grid connected model and corresponding control system of the above units in AC sub-microgrid and DC sub-microgrid in special,and modeling the photovoltaic power generation unit,wind power generation unit and battery energy storage system in PSCAD/EMTDC,finally introduced the overall control strategy of the ac/dc hybrid microgrid.The above work lay the foundation for the following fault analysis and verification of the protection scheme.For the AC sub-microgrid in the AC/DC hybrid microgrid,the influence of internal faults on the traditional protection scheme is analyzed,and a brief introduction is made to the grounding methods of the AC sub-microgrid.In this foundation an anti-transition resistance protection scheme based on the active power of 0-frame or d-frame is proposed for the lines of the AC microgrid,and a model is established in PSCAD/EMTDC to verify the effectiveness of this protection scheme in grid-connected mode and islanded mode.At last,the protection schemes based on communication and local are configured in AC sub-microgrid to achieve reliable,fast,sensitive and selective removal of the faults.For the DC sub-microgrid in the AC/DC hybrid microgrid,the line to line fault an line to groud fault are analyzed in first,and an inverse-time protection method based on absolute value of transient fault current derivative is proposed for cable faults in DC sub-microgrid,the rate of transient fault current derivative is introduced to improve the speed of protection action,so as to realize the fast removal of cable faults.To ensure the safe and reliable operation of the bipolar dc bus in the low voltage DC sub-microgrid,a protection scheme of dc bus based on transient harmonic energy is proposed,which is complementary to the feeder protection scheme.Finally,the protection method is verified with PSCAD/EMTDC simulation.The verification shows that the two protection schemes can be fast and sensitive to remove the fault in the grid connected and islanded operation mode,so as to realize the effective protection of the DC sub-microgrid.