## 1 Introduction  
The first commercial high-voltage direct current (HVDC) power transmission using line-commutated converter (LCC) technology was commissioned in 1954 [1]. The initial projects were built using mercury arc rectifiers. Meanwhile, the advancement in semiconductor devices has made it possible to build thyristors of high power rating to replace traditional mercury arc rectifiers. The first HVDC with silicon-thyristor-based technology came into use in 1970 [2]. Since then, the use of HVDC in power transmission has become common due to its significant technical and economic benefits over conventional HVAC transmission. In modern power systems, HVDC is being adopted for long-distance bulk power transmission, interconnection of different asynchronous AC systems, underwater power transfer using submarine cables, integration of renewable energy resources, back-to-back power transfer, and control of power flow [3; 4]. LCC-based HVDC (classic HVDC) links have been operating with less maintenance, high reliability, and large power capacity in the last half century. The rapid evolution of HVDC links in a power system tends toward situations where two or more HVDC links feed to AC systems that lie in close electric proximity or a common AC system [5]. Such configurations are called multi-infeed HVDC systems. The first multi-infeed HVDC concept was reported in 1972, discussing the harmonic instability and power frequency phenomena of single and multiple infeed DC systems [6]. The interaction between converters in close electric proximity is illustrated in [7] for the first time. It is discussed that multiple DC infeeds potentially degrade the transient performance of the overall HVDC system. Converters in multiple DC infeeds may work as a rectifier/inverter or both (rectifier + inverter). The challenge in the operation of multi-infeed HVDC is the mutual interaction between converters, followed by a temporary disturbance or fault at a common bus or either of the commutating buses. This mutual interaction is one of the major concerns of an AC/DC interconnected system. The interaction between converters largely depends on the AC system strength. The stronger is the AC system, the higher is the ability to regulate voltage during disturbance [8; 9]. The AC system strength is location dependent; it means that the AC system strengths for different buses are different. The AC system strength plays a key role in the AC/DC interaction and mutual interaction between converters [10].  
Since the advent of HVDC transmission, researchers and the manufacturing industry have been involved in the evaluation of single infeed HVDC systems under normal and abnormal conditions. This is why most of the literature data deal with single infeed HVDC and its parameters. The serious challenge in the evolution of multi-infeed HVDC systems is lack of data and knowledge about the steady state and dynamic behavior of multiple DC infeed systems. The performance of multi-infeed HVDC is largely influenced by the interaction between DC converters and other allied phenomena, which are applicable for single infeed HVDC systems. The effect of an AC system strength on the performance of single infeed HVDC is also true for multi-infeed DC systems. Similarly, reactive power support, commutation failure, transient overvoltage, and fault recovery time are also valid for multi-infeed HVDC systems. Besides these phenomena, interconverter voltage interaction, voltage stability, local and concurrent commutation failure, interconverter direct current interaction, and discrete tap position problem are also new challenges for multi-infeed HVDC systems. These transient phenomena of HVDC are analyzed in PSCAD/EMTDC software. The results are equally acceptable for practical purposes as the software uses interpolation technique to estimate the data. The specific literature available on these topics is discussed in this paper.