## 4 Commutation Failure (CF) in a Multi-Infeed LCC-HVDC System
If the interaction is strong, then there is a vast possibility that a local commutation failure may cause commutation failure at a remote converter. Thus, a fault on a local bus causing commutation failure at a local converter is termed local commutation failure, while a fault on a local bus causing commutation failure at a remote converter is termed concurrent commutation failure. In a multi-infeed HVDC system, remote commutation failure by a fault at a local bus is also caused by post commutation failure converter interaction. It means that a fault does not directly cause commutation failure at a remote converter; rather, it is the interaction between converters that distorts the voltage waveform, reduces the voltage, and shifts the zero crossing. Therefore, the harmonic content in a commutating voltage is also a major reason causing concurrent commutation failure. The two important aspects of commutation failure are commutation failure susceptibility of any HVDC link and recovery from commutation failure. Both these parameters depend on AC/DC system strength and control. It might happen that a system is more prone to commutation failure, but it is capable of recovering fast. At the same time, a system might be less susceptible to commutation failure but does not have the ability to recover fast from failure. Initially, commutation failure was assessed by the stability limits of the HVDC system. For multi-infeed DC systems, the transient behavior affects the occurrence of commutation failure. Thus, transient programs with a detailed modeling of generators, HVDC converters, valves, transformers, and control are encouraged to measure the susceptibility of commutation failure. The indices MIIF, MIESCR, and allied norms are a great reflection of commutation failure immunity. A few methods for detecting commutation failure are:  
Calculating the maximum acceptable voltage drop;  
Electromagnetic transient (EMT) program;  
Examining the valve conduction status;  
Comparing the valve current with a DC current;  
Estimating the AC current of all phases of a transformer;  
Inspecting the DC current of a converter;  
Observing the extinction angle The analytical method for computing commutation failure was first discussed in detail in [32]. During the conduction of a converter valve, the valve stores changes. These charges need a proportion of time to remove before going to a blocking period. Therefore, a significant voltage time area is required, named commutation margin angle. As commutation failure often occurs due to AC voltage reduction because of a fault or disturbance, which is unavoidable, the magnitude of a sudden voltage drop needs to be evaluated, which can initiate commutation failure. It is assumed that the DC current and extinction angle remain constant when a commutating voltage suddenly collapses. This leads to the same voltage time area. However, the overlap angle increases, reducing the extinction angle. This means that the commutation process extends to a new extinction angle. The formula developed calculates the maximum acceptable voltage drop that does not cause commutation failure. If a fault produced a voltage drop higher than the permissible limit, then commutation failure was supposed to occur. The formula was derived using steady-state equations of an HVDC converter. It is assumed that a voltage source is ideal, and a control system does not act during disturbance. This formula yields poor calculation of commutation failure susceptibility. An electromagnetic transient (EMT) simulation-based technique is reported in [13] to calculate commutation failure immunity. Commutation failure in HVDC is detected with several techniques. Some techniques even directly check the valve conduction status. A few methods detect commutation failure by examining the current flow in valves. A method comparing the valve current with the DC current is also reported in the literature. The AC current of all phases of a transformer drops to zero during commutation failure, and the DC current exceeds the threshold.