# Fault Detection Algorithms in Medium-Voltage Direct-Current (MVDC) Grids
## 1 Introduction  
MV distribution grids play a critical role in power systems. Traditionally, MV distribution systems have been based on AC systems. However, MVDC systems present several advantages in contrast to AC distribution systems, facilitating versatile and regulated operation. They are especially well suited for transmitting power from renewable energy sources to energy storage facilities, hydrogen infrastructure, or other DC loads [1]. Nevertheless, the development of MVDC distribution systems requires both research and industrial efforts. This paper deals with the protection system for MVDC grids, which is one of the major challenges. These algorithms can be divided into local-measurement-based and communication-based algorithms [2]. Local-measurement-based algorithms use DC current and voltage measurements to process data. These algorithms use local measurements or other variables that are derivedfrom them as a fault detection criterion [3]. The main feature of these methods is the high-speed detection, but they usually lack selectivity [4]. Signal-processing algorithms, on the other hand, are commonly based on traveling wave analysis and employ various transforms, such as wavelet [5] or Fourier transform [6], to extract the required features of the signal. However, there are certain drawbacks, like the dependence on the sampling frequency value or the difficulties in the detection of the wave front due to the interference of signals [2]. Additionally, algorithms based on communication channels handle data from both ends of the protection zone. Although they are more selective than local algorithms [7], they are dependent on the time delay imposed by the communication channel. In this sense, MVDC grids without remarkably long lines will not be affected by such large delays, unlike HVDC systems with longer links. In the literature, there are myriad fault detection and location algorithms for HVDC systems, based on different operating principles that use local- or communication-based methods, as aforementioned. Nevertheless, the performance of these algorithms might present some limitations when applied to MVDC grids and might fail to fulfill their intended objectives adequately. In this paper, the performance of certain protection algorithms is analyzed in an MVDC grid, which includes renewable energy resources and green hydrogen production, in order to validate their use in MVDC grids. This study serves as a foundation for selecting the most suitable fault detection algorithms for MVDC grids. As far as the authors are aware, there is no publication that provides a comparison of the algorithms in MVDC grids. A prior paper [8] reviews and compares the protection challenges of HVDC and MVDC systems. While HVDC technology is well established, with several commercial installations, MVDC is still in the research and development phase, with limited commercial applications. In [9], the study proposes protection coordination methods for a 5 kV MVDC grid with distributed generators and models the system using PSCAD/EMTDC. The results of the simulations demonstrate that the selected protection devices function effectively, ensuring proper protection coordination for various fault conditions in the MVDC off-grid microgrid. The paper [10] discusses the development of China's first medium-voltage DC distribution network in Zhuhai, Guangdong, which was aimed at supporting the city's energy internet. It addresses DC control and protection systems, overvoltage protection, and transient behavior analysis. This fault detection study provides a key contribution by comparing multiple fault detection algorithms under specific fault conditions in MVDC networks, considering both the detection speed and accuracy across various scenarios. This enables the identification of the most suitable fault detection methods, laying the groundwork for optimized protection strategies in future MVDC grids. This article consists of five sections, staring with this introduction.