The Research Of HVDC Commutation Failure Transient Characteristics

Ningdong-Shandong HVDC project will be put into operation, which is a challenge to the security and stability of Shandong power grid. In view of the special operation characteristics of the DC system, DC system fault is a hidden peril for the safety of AC relay protection and AC power gird, it is necessary to study AC system electrical transient characteristics under DC faults. In PSCAD/EMTDC environment, the electromagnetic transient simulation model of Shandong power grid with Ningdong-Shandong HVDC transmission system is established in detail. Based on this model, commutation failure caused by faults of AC line is simulated. Prony algorithm is applied to analyze the electrical transient characteristics of AC lines after commutation failure. The coincidence of fitted waveform and simulated waveform prove the validity of Prony algorithm. Prony analysis results show that DC system injects a large number of transient inter-harmonic components into the AC system after commutation failure, which distort the waveforms of AC line electrical quantities, and the amplitude of some components is great, which may result in the malfunction of AC relay protection. Moreover, the influence of HVDC commutation failure on AC system's electrical transient characteristics is studied when different faults occurs at different positions of AC line near Qingdao substation. According to the comparative analysis of simulation results, the parallel line of the fault lines is most affected, and the distortion of the fault phase current in parallel line differs from fault positions and fault types, and it is more serious when the fault occurs in the middle of the line than at two ends, and it is more serious when single-phase grounding fault occurs in the line than three-phase fault occurs. From the mechanism analysis, the distortion of the current is related to the components of the short-circuit current and the proportion of harmonic current generated by HVDC system in short-circuit current in the line after the fault, and the components of the short-circuit current and their proportions in the whole current are related to two factors, i.e. the distribution of short-circuit current after the fault and the influence of the fault on commutation failure. The former is related to many factors, such as the topology of the whole power grid, the mode of operation, the fault position and so on. The latter is related to the type of the fault. In Shandong power grid and under the specific operation condition, the distortion of the current in the parallel line is the most serious when single-phase grounding fault occurs in the middle of the line.