| The AC-DC hybrid microgrid is highly concerned with its compatibility with distributed clean energy.Compared with single-supply microgrid,AC-DC hybrid microgrid can operate in both directions,and each subsystem can run independently.Therefore,the distributed power supply,energy storage devices and all kinds of loads can be integrated into the distribution network efficiently through the hybrid grid,which is one of the future development direction of the intelligent microgrid.In view of the control and operation characteristics of AC/DC hybrid microgrids,at present,it is a hot topic to study the topology of microgrids,operation control and protection.However,as one of the key technologies of hybrid microgrid,relay protection technology is still in its infancy.The research on the mechanism of fault propagation in hybrid microgrid has not yet formed a theoretical system,in addition,there is less engineering experience in hybrid microgrid,and the accumulation of fault phenomena and protection is less,therefore,the development of relay protection technology is relatively slow.Based on the analysis of flexible DC transmission faults,the transient process of various types of hybrid microgrid faults is analyzed in detail,and the influence of harmonic disturbance caused by AC unbalanced faults on DC system is discussed.Then,according to the theoretical analysis and simulation results,some suggestions for hybrid microgrid protection are proposed.Finally,based on the existing protection technology and the protection suggestions proposed,a cooperative protection scheme is designed for the hybrid microgrid.In this paper,the transient process of typical DC fault in hybrid microgrid is analyzed in detail.It is pointed out that the rising of the fault line current is mainly caused by the rapid discharge of the DC filter capacitor and it is also revealed that the overcurrent of AC system is caused by DC bus voltage oscillation over zero,causing the converter to enter the full-conduction stage,and thus the AC system is equivalent to a three-phase short circuit,and the AC system is subjected to severe impact caused by fault current.Secondly,combined with typical DG control strategies,this paper takes the second-harmonic disturbance as an example to analyze the influence of harmonic disturbances on DG control system,and the inner loop reference current expression under the state of harmonic disturbance is deduced.The expression indicates that the disturbance causes the inner loop reference current to contain the perturbation component,resulting in a deviation in the control system regulation,and the DG output power is no longer stable.In addition,it is found that under the same fault condition,the harmonic content of the inner-loop reference current under droop control strategy is higher than other control strategies.Therefore,from the point of view of suppressing harmonic,the droop control strategy should be avoided in the hybrid microgrid.Then,the simulation model is used to analyze the response characteristics of the electrical quantities of DC sides under different fault conditions,and the suggestions are provided for the hybrid microgrid protection system according to the simulation results.Finally,combined with the above analysis and protection suggestions,a cooperative protection scheme is designed for AC/DC hybrid microgrid based on the existing protection technology,and the selectivity is guaranteed through selecting the appropriate fault parameters and delay factors. |
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