Apparent increase in Short Circuit Ratio (AISCR) as a measure of the impact of VSC converters on LCC converters in multi-infeed HVDC systems with VSC and LCC infeeds

A hybrid multi-infeed HVDC system, in which Voltage Sourced Converter based HVDC (VSC-HVDC) and linecommutated converter based HVDC (LCC-HVDC) interconnected by transmission tie line, is investigated. The “Apparent Increase in Short Circuit Ratio” or AISCR index is employed to quantify the impact of VSC converters on LCC converters. Then, steady state and transient aspects, such as the maximum available power (MAP), temporary overvoltage (TOV) and "Commutation Failure Immunity Index" (CFII), are considered to further analyze the performance improvement of LCC converters. The results show that VSCHVDC has the ability to improve the maximum available power, regulate the ac voltage, make the LCC-HVDC less susceptible to commutation failure and enhance the system strength. However, the longer the tie line, the less improvement for LCC-HVDC link.