AC And DC Interline Fault Analysis At Crossing Place

With the large-scale application of DC transmission system,the crossing situation of AC and DC will be inevitably appear in the condition of the limited transmission corridor.At this time,the interline fault between AC and DC transmission line may be happened.As the AC and DC systems are connected through the interline,the fault characteristic will be unique and complex,which have great impact on the traditional AC and DC protection system and even lead to wrong action.Because of the lack of methods to analyze this fault,this paper intends to explore the characteristic of the fundamental frequency component by using the traditional symmetrical component method,which lays theoretical foundation for comprehensive crossing fault analysis.The difficulty of this paper is how to calculate the equivalent impedance of the converter in fundamental frequency.If exploring the characteristic of the fundamental frequency component,the primary task is to calculate the equivalent impedance of fundamental frequency which is looking from the DC line fault place to the converter side.Since the converter is a non-linear element,the calculation of the equivalent impedance of the converter in fundamental frequency must be analyzed periodically.The method adopted in this paper is to analyze the commutation period and normal conduction period separately,and then combine them to carry out equivalent calculation of fundamental frequency impedance in one cycle.The method this paper is using a fundamental frequency voltage source to measure the equivalent impedance at the DC port of the converter.In this paper,analyzing the influence of cross-line fault to converter angle and commutation angle which is to know about the operation characteristic of the converter under the interline fault.Then this paper employs two steps to calculate the equivalent impedance of the converter in fundamental frequency.In the first step,it is considered that the AC-side condition of the converter is ideal,and the DC current and the current of the DC transmission line in fundamental frequency are calculated,According to the circuit diagram to write electrical equations,and then according to the Fourier decomposition method,using the Taylor formula to take the first three and the equation of drawing a linear approximation of approximate processing according to the operating characteristic of the converter under the interline fault condition.The equivalent impedance of the converter in fundamental frequency is calculated at this time.In the second step,the influence of commutation bus voltage asymmetry and commutation DC bias on the AC side of the converter are considered,and the equivalent impedance of the converter in fundamental frequency is certainly modified.Finally,the error between the calculated and simulated values(about 10%)was analyzed and the cause of the error was analyzed the equivalent impedance of the converter in the fundamental frequency and the application of the symmetrical component method for analyzing the interline fault are simulated,which is for verifying the rationality.