| Currently,while China's power griddevelops to the goals like the high voltage,long distance and high power,the lack of land resourcesrestricts the power transmission corridors,which results in the gradual increase of the lines on the same tower.As a novel form,the AC/DC lines on the same tower will be applied undoubtedly,but there have been seldom researches on AC/DC lines operation on the same tower,particularly on the AC/DC inter-line faults on the same tower.With the large differences between AC/DC electrical characteristics,the characteristics of the AC/DC inter-linefaults are much more complicated,which brings great challenges to DC and ACprotections and affects the safe and stable operation of the power networksystem.However,thesituation that the AC/DC lines operate on the same tower has not been considered in present AC/DC protection systems.Therefore,this study is aimed at the AC/DC protection systems,and the model for simulating the AC/DC operation on the same tower is established using the software PSCAD/EMTDC to investigate such problems as the DC traveling wave protection under AC/DC inter-line faults,AC/DC inter-line protection,AC current differential protection,operation characteristics of zero-sequence current protection,and sequences coordination between the AC/DC protections.When the AC/DC inter-line faults take place,the electrical transient characteristics of the DC system are similar to those under the grounding fault.It is validated by the simulation in this paper that the traveling wave protection is capable of operating timely even under AC/DC inter-line faults.But different from the zero potentialearth “inter-line”,the instantaneous values of the AC voltagesvary with the time,and thus the traveling wave protection is influenced by the AC/DC inter-line moment.Meanwhile,the characteristic quantities detected by two stations of DC at varied fault locations are different,indicating the effect of fault locations on them.For AC/DC inter-line faults,the operation rate of the traveling wave protection is relatively low.After AC/DC inter-line faults happened,the AC current flows into DC system,and the AC voltage and current componentswill be detected at two ends of the DC lines,which as the characteristics of such faults can be used in the AC/DC inter-line protection.Through the simulation of the inter-line faults under varied DC powers,AC voltages,locations of the faults and their types,features of AC/DC inter-line protection are summarized;and the irrationality of the criterion,values and delay settings in the present AC/DC inter-line protection are pointed out,and measures for their optimization are proposed.Considering theACprotection operation,its protection output is modified so as to help restart the lines of DC system and recover their normal operation after the inter-line faults.Based on the simulation,an overall review of both the AC system current differential protection and zero-sequence current protection under varied AC/DC inter-line faults is made,and it is pointed out that the present current protection is able to operate timely after the inter-line operation.Meanwhile,considering the characteristics of AC/DC power grid,the concept that the priority should be given to the guarantee of the normal operation of the DC system regardless of the AC system is proposed,and based on the identification of AC/DC inter-line faults according to their characteristics,the commissioning and ceasing logic of reclosing system of AC circuit breaker is optimized in the efforts of reducing the outage time of the AC line under with the guarantee of the DC stable operation.Research results of this paper provide reference for ideas and methods to analyze the AC/DC lines operation on the same tower and the DC protection faults for both designers and field operators,and also make suggestions for the design,operation and sequences coordination of the relay protection of AC/DC systems,which are of great significance to the safety and stability of the regional power grid. |
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