| The reverse distribution of power and load makes the demand for long distance power transmission relatively high.The advantages of high-voltage direct current(HVDC),such as large transmission capacity,reliable operation and low active power loss,have made HVDC become an important form of power transmission.Since the commutation process and stable operation of DC system need the support of AC system,the construction of HVDC/AC interconnected system is an inevitable choice for China.However,the interaction between DC and AC system will cause new failure characteristics and lead to the misjudgment of fault nature.Since most faults of AC transmission lines at the DC system power receiving terminal are single phase to ground short circuit,and automatic reclosing is usually disposed.In order to avoid the secondary impact of blind reclosing,research on adaptive single pole autoreclosure technology is significant.Based on which,the interaction mechanism of the HVDC/AC interconnected system,the applicability of the traditional adaptive single pole autoreclosure and a new method for determining the nature of the fault is studied in this thesis.Firstly,this thesis analyzes the mechanism of commutation failure caused by AC faults and concludes the difference between the HVDC/AC interconnected system and AC system.For AC faults which do not lead to commutation failure,the electrical signal characteristics of the HVDC/AC interconnected system are almost the same as those of AC system;the current is mainly dominated by the 50 Hz component.For AC faults which lead to commutation failure,steady-state harmonic will exist in the HVDC/AC interconnected system.Those theories are verified by fast ensemble empirical mode decomposition-Hilbert(FEEMD-Hilbert)spectrum analysis.Secondly,this thesis analyzes the principle of the typical adaptive single pole autoreclosure and verifies the applicability of them in the HVDC/AC interconnected system.According to the essential difference between the HVDC/AC interconnected system and AC system,this thesis proposes a new adaptive single pole autoreclosure principle suitable for the HVDC/AC interconnected system.The reliability of the principle is verified through simulations.Finally,this thesis studies the commutation failure mechanism and harmonic transmission characteristics of multi-infeed direct current(MIDC)system.Based on the above conclusions,the applicability of typical adaptive single pole autoreclosure is analyzed in MIDC system.The thesis points out that regardless of whether the commutation failure occurs,there is the possibility of misjudgment.Therefore,a new adaptive single pole autoreclosure scheme is proposed.Simulations prove the correctness and reliability of this scheme. |
