| Due to the imbalance of economic development and primary energy distribution,the phenomenon of "charge source separation" is becoming more and more prominent in China,which puts forward high requirements for large-capacity and long-distance transmission of electric energy.High voltage direct current transmission technology has been developed rapidly because of its significant advantages in long-distance and large-capacity power transmission.Commutation failure is one of the common faults in LCC-HVDC transmission system,especially continuous commutation failure may cause DC system blocking,which poses a great threat to the safe and stable operation of the system.Commutation failure has an adverse effect on the action characteristics of DC system relay protection.After commutation failure occurs in DC system,the current characteristic quantity which constitutes the action criterion of converter bridge differential protection and valve differential protection will produce irregular distortion,which will lead to incorrect action of DC system converter protection.The DC system will present complex fault characteristics to the AC system on the inverter side,which will adversely affect the correct operation of the differential protection and distance protection of the AC line on the inverter side.Therefore,it is of great significance to carry out in-depth research on the suppression of continuous commutation failure of DC transmission system and the influence of relay protection between AC and DC transmission systems,so as to effectively improve the stability of system operation and the reliability of power supply.The main work of this thesis includes:(1)Research on continuous commutation failure and its suppression method of HVDC transmission system.This thesis analyzes the cause and mechanism of commutation failure,and analyzes the DC current,commutation reactance,commutation voltage,delay trigger angle and zero crossing phase shift of commutation voltage φ The influence of commutation angle and other factors on commutation failure is studied.In this thesis,a control method based on virtual capacitance effect is proposed to suppress the occurrence of continuous commutation failure.This method uses the capacitance characteristics to induce the change of DC voltage,and works with the low voltage current limiting control link to reduce the current command value and increase the commutation margin,so as to achieve the purpose of suppressing continuous commutation failure.At the same time,the control method based on virtual capacitor is further improved.By directly compensating the DC voltage,instead of increasing the commutation margin by reducing the current command value,the factors leading to the decrease of DC voltage after fault are eliminated,which can more effectively suppress the continuous commutation failure and accelerate the recovery of the system after fault.The simulation results based on PSCAD / EMTDC show that the proposed method is effective.(2)For DC transmission system,commutation failure caused by AC system fault at inverter side leads to incorrect operation of converter protection.This thesis analyzes the specific reasons for the incorrect operation of the bridge differential protection and valve differential protection of the converter,and uses the proposed control method based on Virtual Capacitance to improve the operation environment of the protection and the correctness of the protection action.(3)The operation characteristics of AC line differential protection and distance protection on inverter side of HVDC transmission system are analyzed in detail.Aiming at the problem that the differential protection of AC line on inverter side refuses to operate in the fault area,a voltage amplitude criterion is proposed to identify the faults inside and outside the protection area,which can effectively solve the problem of failure refusal to operate in the protection area.The effectiveness of the proposed method is verified by PSCAD/EMTDC simulation. |
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