Study On Fault Characteristics And Protection Principle Of Hybrid MTDC System

The distribution of primary energy in China is uneven,and with the depletion of traditional energy,the demand for new energy is increasing day by day in the world.The hybrid multi-terminal DC(MTDC)system which combines the conventional DC transmission technology and the flexible DC transmission technology has significant advantages for solving the problems of power transmission,reception and consumption of new energy in China.Based on the topology and operation control of hybrid MTDC system,the fault characteristics and the new protection principle are studied.In this paper,the basic constitution and operation control principle of conventional DC converter station and flexible DC converter station in hybrid MTDC system are introduced.Aiming at the possible short-circuit of valve,inverter export line fault and DC line fault on the DC side of hybrid MTDC,the fault characteristics of different converter stations are studied,which provides the theoretical basis for the design of the following protection principle.Due to the special commutation circuit composed of half controlled devices,the existing overcurrent protection of the line commutated converter(LCC)has the problem of misoperation crossing protection zone.On the basis of analyzing and summarizing the fault characteristics of different protection zones in conventional DC converter station,a new protection scheme to detect fault overcurrent at the same time to judge the number of overcurrent phase is proposed,so as to ensure the selectivity of overcurrent protection.The fast overcurrent fault characteristics of the DC side fault of the voltage source converter(VSC)put forward high requirements for the protection speed.In this paper,by using the information of the first stage fault of two level VSC,a criterion is proposed to judge the direction of fault quickly by using correlation coefficient,and the directional pilot protection scheme of DC line is designed further.The correlation coefficient is derived from the Cauchy inequality,which can effectively characterize the correlation and symbol characteristics between signals.In practical engineering,the line parameters are uniformly distributed along the line and have frequency dependent characteristics.In this paper,two kinds of line models with distributed characteristics are considered: Bergeron model and frequency dependent model.In the Bergeron model,the line parameters are treated as constant coefficients,and the algorithm is simplified under the condition of ensuring a certain calculation accuracy.Frequency dependent model considers the frequency characteristics of line parameters,which make the calculation to be complex.And an effective method to deal with the frequency characteristics of the parameters--Martin algorithm is introduced.In this paper,a new differential protection method and a fault pole selection scheme are proposed based on the Bergeron model,which has considered the frequency characteristics of the line parameters.The differential protection can significantly reduce the protection action delay under the premise of ensuring the accuracy of the calculation.