Study On The Characteristics And Countermeasures Of Abnormal Operating Conditions Of Hybrid Multi-terminal UHVDC Transmission System

UHV multi terminal hybrid flexible DC transmission is a new type of DC transmission system integrating the advantages of conventional UHVDC transmission and flexible DC transmission.It not only has the advantages of high voltage level and large capacity,but also can quickly and independently control the active power and reactive power exchanged with the AC system,control the AC voltage at the common connection point,facilitate and flexible power flow reversal,and can self commutation,It has the functions of improving the voltage stability and power angle stability of the AC system,reducing losses,and rapid recovery after an accident.The Kunming Liulong project has applied advanced new technologies such as flexible UHVDC,hybrid DC and multi terminal DC for the first time.There are also technical difficulties such as restraining the tide reversal caused by abnormal working conditions.Therefore,it is necessary to conduct in-depth research on the fault conditions of UHV three terminal hybrid DC transmission system.The first is the theoretical analysis and simulation verification of the measurement anomalies of the relevant measuring points in the system.The measuring points that have great influence mainly include the current of Kun-Liu line and the current of Liulin line.In order to solve the problem of power shortage after abnormal current measurement in Kun-LIU line,the strategy of changing current reference value is put forward:In order to solve the problem of large trigger angle deviation rating after abnormal current measurement in Liulong line,fixed angle control is added as an improvement measure.The proposed strategy can effectively solve the disturbance caused by the measurement anomaly.Secondly,the problem that abnormal measurement of bridge arm current will affect the circulation suppression effect and lead to incomplete suppression of double frequency component is expounded.Combined with simulation,the suppression effect of the double frequency component of circulation is analyzed when the bridge arm current is superimposed with triangular wave or sine wave of different frequencies and amplitudes.The third is the theoretical analysis and simulation verification of the outage measures after abnormal communication in the system.For the overvoltage situation that occurs when the communication of three stations is lost under the common outage strategy,the improvement measure of reducing power first is put forward to suppress the overvoltage phenomenon effectively.Finally,for the situation of the reverse current after the fault outage of Kunbei station under the loss of communication,the limit value of the voltage controller of Longmen station is modified to limit the reverse current,so as to achieve the purpose of stable outage of the system.