## 4 Commutation Failure (CF) in a Multi-Infeed LCC-HVDC System
The Thevenin impedance has a great impact on the overvoltage and commutation failure of an HVDC system [35]. The Thevenin impedance is calculated by looking into an AC system from a converter bus. If the MIIF between converters is less than 0.15, then each HVDC system in a multi-infeed scenario can be considered a single infeed system. In order to calculate the ESCR at a converter, all other converters in proximity are assumed to be blocked. This assumption leads to optimistic results because even the converter is blocked, but the nearby AC system is still intact with a local HVDC system. The drawbacks of a maximum permissible voltage drop formula are also discussed because the formula uses a steady-state calculation method that ignores the transient behavior of an HVDC system. This is why the maximum permissible voltage drop method for judging commutation failure sometimes gives a wrong measure of commutation failure susceptibility. The CFII approach described in [13] is used to assess the immunity of a converter to commutation failure. The most severe fault (i.e., three-phase inductive fault) is considered in the derivation of an immunity index. Commutation failure depends on the fault level and the time at which the fault is applied. It is reported in [36] that commutation failure depends not only on fault severity but also on the time instant at which the fault is induced. The CFII is independent of the time instant on the wave at which the fault occurs. The CIGRE benchmark model is investigated to have a commutation failure immunity of 13.3% at the short circuit level (SCL). PSCAD/EMTDC (an electromagnetic transient program) uses an interpolation technique between samples, and hence, the output results are no longer sensitive to a simulation time step [37]. Commutation failure mainly occurs due to AC side disturbance, but DC side inductance (smoothing reactor) also influences commutation failure. For the CIGRE benchmark model, the DC side inductance is 1.194 H cumulatively. The change in DC side inductance affects the CFII. In multi-infeed HVDC systems, generally the source impedance and coupling impedance are varied to get the desired MIIF and AC system strength in EMT simulations. The local CFII is reported to be independent of the MIIF and remote HVDC converter parameters [13]. This results in favorable values of the local CFII. Several scenarios are discussed in relation to concurrent commutation failure in the literature. The first scenario is analysis of concurrent commutation failure with a local converter permanently blocked. This scenario gives the impact of an AC network on concurrent commutation failure, as the local converter is blocked, so it does not have an effect on a remote converter. However, the coupling impedance and AC system strength have a significant part in concurrent commutation failure. The second scenario investigates concurrent commutation failure when a local converter is blocked after local commutation failure. This avoids post-local commutation failure interaction between converters. As a result, the chances of concurrent commutation failure are reduced. The third scenario estimates concurrent commutation failure when there is no scheme to block a local converter. This means that both converters remain in operation after commutation failure. This leads to the most severe interaction between converters, and hence, the chances of concurrent commutation failure increase. The CFII in case 3 is much less than in case 2. This is due to the fact that after commutation failure, a local converter disturbs the commutating voltage of a remote converter. Another phenomenon is reported, termed anomalous commutation failure. The commutation failure for faults of low severity instead of high severity due to distortion of a commutating voltage waveform is anomalous commutation failure. In ref. [34], it is also discussed that for faults of less severity, the zero crossing of a commutating voltage moves left, resulting in a decreased commutation margin.