## 4 Commutation Failure (CF) in a Multi-Infeed LCC-HVDC System  
Commutation failure (CF) in single and multi-infeed HVDCs is an adverse event, which occurs due to the failure of current transfer from one valve to the next in a pattern. The main reason for CF is the reduction in the commutating voltage of a converter due to any significant fault at the converter's commutating bus or near the converter. It might occur due to rapid increase in DC current, voltage waveform shift, harmonics, and maloperation of a control system. CF can cause a temporary suspension of power transfer to AC networks. Practically, the normal operation of a HVDC system includes switching of several thyristors in a predefined manner. If any of the thyristors keep on conducting in their blockage period, then the two phases of a converter transformer create a short circuit. This transformer's short circuit results in the temporary suspension of power transfer to an AC network. In an LCC-HVDC system, the converter station needs a large amount of reactive power during commutation failure. If sufficient reactive power support is not available, the converter moves to a blockage stage. In case of a multi-infeed configuration, if sufficient reactive power supply to a converter station is not available, then the decrease in bus voltage influences the nearby converter's bus voltage, which can ultimately cause commutation failure at a remote converter. In order to mitigate commutation failure, different compensators, such as static synchronous compensator (STATCOM), synchronous condenser (SC), and static VAR compensator (SVC), are linked to the converter's bus [31]. In real-world HVDC projects, a STATCOM and SC are usually connected at an inverter's end in order to improve the voltage profile and reduce the chances of commutation failure under single and three-phase AC faults. The commutation failure is also influenced by the AC system's strength. The higher is the AC system's strength, lesser will be the chances of commutation failure in a converter station and vice versa. In a single infeed LCC-HVDC system, a higher short circuit ratio (SCR) of the AC system results in lower commutation failure. In the case of a multi-infeed LCC-HVDC system, a higher multi-infeed effective short circuit ratio (MIESCR) indicates a lower risk of commutation failure in a converter station. It can be noted that if the voltage interaction between converters is high and no reactive power compensator is installed at any of the converter buses, then there is a greater chance that commutation failure in one converter can cause CF in the neighborhood converter station. In an LCC-HVDC system, harmonics are produced at the DC and AC sides. The harmonics interaction between converters is a quite complex phenomenon. Despite the voltage interaction, the harmonics interaction may also cause commutation failure at a remote converter. The harmonics interaction reduces the voltage time area to remove the charges stored in forward conduction. The harmonics also distort the voltage waveform and shift the zero crossing to the left, providing less time to valves for ignition. The famous anomalous commutation failure happens due to this harmonics interaction. In an anomalous commutation failure, commutation failure occurs for faults of less severity. The postcommutation failure converter blocking scheme and strong AC system may result in less harmonics interaction between converters. It is reported that converters with a multi-infeed interaction factor (MIIF) of less than 0.1 virtually have no chance of harmonics interaction. The multi-infeed scenarios have made the commutation process more complex due to interconverter interactions. Commutation failure at one converter may cause commutation failure at other converters in proximity. This leads to a new concept of concurrent commutation failure. A concurrent commutation failure depends on the MIIF between any of two converters.