If the STATCOM injects both positive- and negative-sequence reactive currents during a fault, then you need to model it with a converter-interfaced resource (CIR) instead of a voltage-controlled current source. In a release of OneLiner v15.5 in late February 2022, we have raised the limits on the slopes and the maximum current to make this possible. However, the CIR object in version 15 of OneLiner does not have the option to sense the voltage at the network side of the unit transformer.