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Given the current evolution of Voltage Source Converters (VSC), Multi-Terminal High Voltage Direct Current (MTDC) grids are now becoming a real possibility. Still, some technical issues have to be addressed. The protection of High Voltage Direct Current (HVDC) grids is the main technical challenge that is slowing down the development of MTDC grids. Hence, this paper focuses on protection systems. Thus, protection devices, fault-clearing strategies and protection system requirements are considered. The main topic of this paper is the review of different types of protection methods for MTDC systems that are shown in the literature. They can be classified depending on the use of local measurements or a communication channel in their operation. The protection systems reviewed in this paper include protection systems based on current measurements, voltage measurement, traveling wave analysis and artificial intelligence. A protection system can employ only one of these methods or a combination of them. Finally, the main characteristics of the reviewed protection algorithms are compared, highlighting the system configuration, the converter technology, the adopted fault-clearing strategy, the implemented circuit breakers and the size of the limiting inductors. From the work presented in this paper, it is concluded that the actual tendency in MTDC protection systems is predominantly full-selective fault-clearing strategies combined with hybrid HVDC circuit breakers in series with limiting inductors. In addition, most protection methods are based on current measurement algorithms and a considerably high number of the reviewed protection systems employ a combination of several methods benefiting from their combined characteristics. • Review of protection systems for multi-terminal HVDC grids. • Classification into local-measurement-based and communication-based methods. • Current-based methods are more usually employed than voltage-based methods. • Combination of half-bridge MMCs and hybrid circuit breakers is predominant. • The preferred system configuration varies between symmetric-monopole and bipole. [ABSTRACT FROM AUTHOR]

• A fault detection scheme is proposed for MTDC by using local transient information. • Classify internal and external fault by tracking pulse count in a specific window. • The concept of protection unit is employed for rapid isolation of faulty part. • Results provide evidence for protection scheme's key role in handling MTDC faults. DC faults render hindrance in the flourishment of multi-terminal flexible high voltage DC (MTDC). The fault current rises rapidly, making a sharp peak, however, under a specific fault instance, isolation of the whole network is an impractical solution. For effective reliability of the whole network, rapid and accurate fault detection and selection of faulty pole is mandatory. A fault detection scheme is proposed in this paper to draw transient current harmonics from initial fault current by utilizing Discrete Fourier transform (DFT). Current harmonics employed the fault detection promptly at the converter stations without communication among the MTDC networks. Additionally, the proposed scheme can discriminate internal from external fault by tracking the number of pulse count in a specific interval of the harmonics of the voltage. In this way, the reliability of the protection scheme enhances and escape from the mal-operation of the circuit breaker. For four terminal MTDC transmission network, the simulations are carried out in PSCAD to authenticate the implementation of the proposed scheme and protection units (PU) are utilized for the isolation of faulty part with the combination of hybrid circuit breaker (HCB). A keen analysis of test results authenticated the ability of the proposed scheme to rapidly detect the faults with high precision, along with efficient classification of the nature of faults. Furthermore, consequential results provide a strong evidence for its key role in handling a multitude of MTDC faults. [ABSTRACT FROM AUTHOR]

This paper presents a protection scheme to protect multi-terminal high voltage dc (MTDC) networks for interconnection of renewable energy sources. The proposed scheme detects faults by using the Consecutive Data Window Method (CDWM) and harmonics of the system voltage to avoid maloperation of breakers without communication. Fault location is classified and faulty parts is isolated from the rest of the healthier network by using hybrid circuit breakers (HCBs). Moreover; it also categorises the fault with the help of the voltage drop. The rapid response to isolate the faulty portion in a few milliseconds is primarily considered to enhance the reliability and security of the network. The results of simulations verify the efficient fault detection and isolation for different DC faults and the investigation for the impact of significant parameters of the proposed scheme has been considered. The simulations are performed in PSCAD for four terminal MTDC networks to validate the proposed scheme. The performance is also verified under different fault conditions by using Matlab after computing the data from simulations. [ABSTRACT FROM AUTHOR]