The Impact Of AC Fault Occurring Point On Commutation Failure In HVDC System

Considering the imbalance in the distribution between energy resources and load demands in China, developing HVDC transmission technology to achieve high-power and long-distance transmission is an inevitable choice for the strategy of "power from west to east". Until now, there are 24 HVDC transmission lines commissioned in Chinese power system and the AC/DC interconnected power grid has come into being. However, faults in AC system may cause commutation failure in HVDC system and consequential commutation failure may even lead to HVDC mono-polar and bi-polar block which will interrupt the power transmission. On one hand, the power shortage may impact voltage and frequency stability of AC system. On the other hand, the consequent extensive power flow transferring from HVDC lines to adjacent transmission lines will happen, which may cause cascading failures and threaten the safety of the whole system. The effect of AC fault occurring point on commutation failure is studied, and other influence factors of commutation failure are also analyzed. The study of this thesis has a certain value for accurate prediction of commutation failure, system parameter settings and system stable operation in both theoretical and technical aspects.In order to achieve quantitative analysis for the time behavior of commutation failure, aiming at the first commutation after three phase and single phase grounding fault, the formula of △ V is derived and verified according to whether the AC fault happens during one commutation or between two adjacent commutations. △V is the minimum voltage reduction required to produce the onset of commutation failure. The theoretical curve of △Vover time is obtained in order to analyze its time behavior.Commutation failure may not happen in the first commutation following the AC fault, while the follow-up ones may be impacted and lead to commutation failures. So follow-up commutations are studied in order to predict commutation failure more accurately. For the follow-up commutations, the effect of control system is considered. Simulation tests are also done, and the test data is processed in order to get a more complete characteristic curve of △Vover time, which is helpful for more accurate prediction of commutation failure.A further derivation is done, which lead more parameters including commutating reactance into the formula of △ V. The influence degree of one same factor on commutation failure will be different when AC fault happens in different stages of commutation. So the relationship between AC fault occurring point and other influence factors is studied.