| The "double carbon" policy requires the power system to be clean and low-carbon,and gradually increase the proportion of new energy generation.In order to adapt to new energy large-scale grid,flexible DC transmission technology has become increasingly widely applied.In our country,the power grid has become AC/DC hybrid network initially.The flexible DC transmission system contains a large number of power electronic components,has a fast response speed and a unique control strategy,so that when the AC system fails,the characteristics of short circuit current provided by the flexible direct transmission system will change significantly compared with the traditional synchronous generator.The traditional AC based on power frequency electric gas,such as differential protection and distance protection,has reduced its rapidity and reliability,and cannot meet the requirements of"four characteristics" of relay protection.The new type of power system is faced with challenges in relay protection.Therefore,it is urgent to improve the existing protection principle according to the characteristics of flexible HVDC system,or put forward a new protection scheme suitable for flexible direct access AC power grid,to ensure the safety and reliability of power grid operation.First of all,this paper analyzes the basic working principle,mathematical model and control strategy of flexible HVDC system,and then studies the characteristics of DC fault and AC fault of flexible HVDC system and the mutual influence between AC and DC side.At the same time,a two-ended AC/DC transmission system with flexible DC is built in PSCAD,and the theoretical analysis is verified by simulation.The followup research of this paper focuses on AC protection,and analyzes the reasons why distance protection and differential protection are affected by the control strategy of flexible HVDC system and the reliability is reduced,which lays the foundation for the subsequent protection improvement and the proposal of new protection schemes.Secondly,in terms of improvement of protection principle,starting from differential protection,the influence of control strategy of flexible DC transmission system on positive,negative and zero-sequence network in the event of failure is utilized.Three auxiliary criteria are formed by the sudden change of positive sequence current,negative sequence current and zero-sequence current at the installation point of protection on both sides of the line respectively,so as to improve the sensitivity of current differential protection when faults occur in the area.In terms of new protection schemes,the fault current transient which is mainly affected by the busbar’s ground capacitance and line parameters but not by the control strategy of the flexible HVDC system is combined with the idea of different layers of wavelet packet decomposition and differential protection.Two new protection schemes are proposed:First,the fault inside and outside the area is distinguished by the difference of the low frequency and high frequency energy ratio of the fault current under different time Windows and different frequency bands.The second is to use the energy of fault currents on both sides in the characteristic frequency band to form a recognition function,so that it is maximum when the fault is inside the area and minimum when the fault is outside the area,so as to identify faults inside and outside the area.Finally,the feasibility of the improved protection scheme and the new protection scheme is verified in the built simulation system.It has good adaptability in different fault locations,different fault types and different transition resistors,which improves the reliability of the relay protection after the flexible DC transmission system is connected to the AC power grid,and escorts the safe operation of the new power system. |
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