## 4 Commutation Failure (CF) in a Multi-Infeed LCC-HVDC System
In such cases, the voltage due to a fault is less depressed, but the commutation margin decreases because of the left shift. The insufficient voltage time area leads to commutation failure. This kind of commutation failure happens due to the advancement of a valve commutating voltage, not due to a reduction of voltage magnitude. The reason behind this anomalous commutation failure is harmonic content. The commutation voltage waveform for less severe faults is found to be more distorted than for severe faults. Therefore, concurrent commutation failure for less severe faults occurs due to a distortion in the voltage waveform, and for severe faults, it occurs due to the sudden depression of the voltage magnitude. If a post-commutation failure converter blocking scheme is used, anomalous commutation failure can be avoided. A few categories are reported, depending on the MIIF. If the MIIF is less than 0.06, then a remote converter does not have influence on local faults. If the MIIF is greater than 0.06 and less than 0.15, the local fault may cause remote commutation failure, but it does not have influence on a local converter block or unblock state. If the MIIF is greater than 0.15 and less than 0.6, the CFII is almost inversely proportional to the fault level. If the MIIF is greater than 0.6, every fault at a local converter will cause commutation failure at a remote converter. This corresponds to a constant fault severity because the voltage coupling between converters is too strong. In ref. [38], it is discussed that a high local effective short circuit ratio reduces the likelihood of local commutation failure, and a higher MIIF increases the risk of concurrent commutation failure. Minimum-extinction-angle-based critical values are derived to quickly judge local and concurrent commutation failure. In ref. [39], the evolutional line commutation converter with a thyristor-based full-bridge module is discussed to detect and reduce commutation failure phenomena in single and multi-infeed HVDC systems. In the topology, a thyristor-based full-bridge module is connected in series with each valve of six pulse bridges. A predictive control strategy is proposed in [40] to reduce the chances of commutation failure. The strategy decreases the firing angle on the detection of a fault incident. A phasor measurement unit (PMU) is used to measure the Thevenin impedance. As the Thevenin impedance seen from an inverter bus decreases when the fault occurs, the fault severity and firing angle are estimated based on this Thevenin impedance. An approach for detecting commutation failure based on fault distance from a converter is discussed in [41]; it uses a minimum-extinction-angle-based approach to judge the susceptibility of commutation failure. A fuzzy-controller-based approach to advance the firing angle in the detection of commutation failure is proposed in [42]. A technique for controlling the direct current during a fault is provided in [43]. A capacitor modulation technique for minimizing the commutation failure in a line-commutated converter is discussed in [44]. A power-component-based fault detection method for eradicating the chances of commutation failure is discussed in [45].