Research On Coupling Effeet In Multi-infeed Direct Current System

Commutation failure occurs frequently in multi-infeed system. Thus, it is necessary to study on commutation failure in LCC-HVDC system and coupling effect in multi-infeed system, which are basic study contents in order to solve this problem.Based on the study on commutation failure from the aspects of failure mechanism and influence factor, a commutation failure preventive method is provided in order to decrease the probability of commutation failure, which comprises upperlimit of trigger angle commutation failure detective method and rapidly beforehand trigger commutation failure preventive method. The former method is based on calculation of upperlimit of trigger angle which is derived from the concept of commutation voltage-time area. The latter method is put forward in order to provide a simple, reliable, rapid preventive command, by means of which commutation failure is prevented though triggering valve in advance quickly. Simulation result in PSCAD/EMTDC shows that commutation failure preventive program can decrease the probability of commutation failure effectively which is caused by single-phase grounded fault.In order to analyze coupling effect in multi-infeed direct current transmission system at the aspect of stable interval, apparent increase in short circuit ratio is adhibited in multi-infeed system, which converts interrelationship between subsystems to equivalent increase in short circuit ratio of AC system. The method of calculation is obtained as well as calculation result, and then trend of operating characteristic is analyzed when each influencing factor changes.Ultimately, apparent increase in short circuit ratio is check out to be effective using in double-infeed system by simulation results.Coupling effect analytical method based on reactive power flow is brought up in order to analyze coupling effect in multi-infeed direct current transmission system at the aspect of voltage stability. The method explains the mechanism of voltage dip in each subsystem when one subsystem suffers from reactive power disturbance through change in reactive power flow. Reactive power support coefficient is brought up as a steady state coefficient to analyze reactive power supportive effect between subsystems quantitatively. The expression of reactive power support coefficient is also derived. Based on coupling effect analytical method brought up above, the expression of voltage stable index in double-infeed DC system is derived. The expression and derivation process reveal the relationship among reactive power support coefficient, multi-infeed interaction factor and voltage stable index. Influence factor of reactive power supportive effect is analyzed combining with expressions. Eventually, simulation in double-infeed DC system under PSCAD environment is carried out to validate theoretical derivation.