Investigation of Cascaded Commutation Failures in Multiple LCC-HVdc Inverters Caused by Rectifier Side AC Grid Faults

In earlier work, cascaded commutation failure (CF) in multiple LCC-HVdc inverters of the multi-infeed system has been attributed to inverter side ac grid faults. However, this paper discusses a phenomenon where the cascaded CF is caused by rectifier side ac faults. The risk of this phenomenon is evaluated considering various fault inception instants, types, durations, and severities by electromagnetic transient simulations. The underlying mechanism of the phenomenon is identified by calculating the area under the commutating voltage curve. It is found that the commutating voltage waveform distortion arising from asymmetrical operation of the primary inverter after the fault clearance causes the initial CF. This CF subsequently introduces significant low-order harmonics into common inverter side ac grid, which propagate to the remote inverter ac bus and consequently induce the cascaded CF. It is also shown that if the tie-line impedance between inverter ac buses limits the propagation of low-order harmonics, the cascaded CF can be avoided. The dependency of the phenomenon on ac grid parameters is further quantified using the CF immunity index (CFII). It is inferred that the phenomenon is alleviated with the increased rectifier and inverter side ac grid strength as well as tie-line impedance magnitude at fundamental frequency.