Research On Commutation Failure In Multi-infeed HVDC Systems

Commutation failre is a very frequent fault in HVDC transmission system,which can cause issues such as overstressing of dc equipments,reduction of transmitted power and overvoltage of the receiving ac system.With the rapid development of HVDC technology and the implementation of “West Power to East” and “national network” energy strategies,situations have arisen with two or more HVDC inverters terminated in close proximity of a common ac system in the East China Power Grid and the China Southern Power Grid. A multi-infeed HVDC system configuration can expand transmission capacity and increase the flexibility of system operating modes.However,it gives rise to the more complex commutation failure behaviors because of the inter-converter interactions as well compared to the single-infeed HVDC systems.So it is of important practical significance to study commutation failure behaviors in multi-infeed HVDC systems.On the basis of the baic operating principle of 6-pulse thyristor inverters,the mechanism of commutation failures was analyzed. Then,a dual-infeed HVDC system configuration was established based on “CIGRE HVDC Benchmark model” in PSCAD/EMTDC platform.And based on the “commutation failure immunity index”(CFII),the impact of ac system parameters,dc system parameters and controller parameters on commutation failure in inverters were analyzed in detail and the general rules of how various factors affect commutation failure in multi-infeed HVDC systems were also obtained.A general and efficient approach to quantify commutation failure immunity levels in multi-infeed HVDC systems was presented.Based on the minimum extinction angle criteria,the analytical expressions of “local commutation failure immunity index”(LCFII) and “concurrent commutation failure immunity index”(CCFII) were derived.Meanwhile, in order to give a profound insight into how inter-inverter interactions influence the concurrent commutation failure behaviors,a quantified index “weak coupled multi-infeed interaction factor”(WCMIIF) was proposed.The validity and accuracy of the analytical expressions and proposed index were verified by the simulated results based on a dual-infeed HVDC system.With the ac line fault location regarded as a generalized node,the “voltage correlation factor”(VCF) was defined based on generalized node impedance matrix.Based on the minimum extinction angle criteria,the analytical expressions of “critical voltage correlation factor”(CVCF) was derived,through which the immunity of inverters in HVDC systems to commutation failure could be effectively characterized.Moreover,the correlation between VCF and the variability of ac line fault locations was expressed,further,a rapid identification method of weak areas in power grid based on commutation failure analysis in multi-infeed HVDC systems was put forward.The validity of proposed index and the applicability of aforementioned method were verified by the calculation results based on the multi-infeed HVDC system model of China Southern Grid.