Multi-terminal HVDC grids facilitate the integration of various renewable resources from distant locations; in addition, they enhance the reliability and stability of the grid. Protection is one of the major obstacles in realizing reliable and secure multi-terminal HVDC grids. This paper presents a comprehensive review of the existing protection schemes for multi-terminal HVDC grids. First, DC fault current stages are demonstrated; in addition, fault analysis studies and the existing fault current calculation methods are reviewed. Then, HVDC grid protection requirements including multi-vendor interoperability conditions are extensively discussed. Furthermore, primary protection algorithms are classified into single- and double-ended schemes, and a detailed comparison between each category is presented such that the distinctive algorithms from each group are highlighted. Moreover, the recent DC reclosing schemes are reviewed highlighting their role in enhancing grid stability and ensuring supply continuity. Finally, available standards for HVDC protection systems alongside their design considerations and procedures are thoroughly outlined. This paper focuses on the recently proposed methods to design reliable protection schemes for multi-terminal HVDC grids and highlights the main advantages and disadvantages associated with them; thus, it offers a beneficial guide for researchers in the HVDC protection field.

Protection of High Voltage Direct Current (HVDC) grids is a crucial step that requires further advancements to ensure the secure integration of green renewable energy sources (RESs). This paper proposes a fast, reliable, and selective singleended primary protection scheme for multi-terminal HVDC grids. In the proposed scheme, Hilbert-Huang Transform (HHT) is applied to local voltage measurements to extract the instantaneous frequency and energy features. Abrupt changes in these features during internal faults are detected as outliers. Simultaneous outliers in the extracted features correspond to internal faults, which offers a setting-less fault detection criterion; thus, eliminating the need for simulation-based and grid-specific thresholds. The proposed scheme can detect internal faults with high fault resistances up to 1000 Ω within 1 ms. In addition, the proposed scheme can reliably distinguish between internal and external faults even when the boundary reactor size is as small as 10 mH. A four-terminal HVDC grid is simulated in PSCAD/EMTDC software, and various fault scenarios are investigated to verify the effectiveness of the proposed scheme in detecting and discriminating between internal and external faults under severe fault conditions.

Breaker failure backup protection schemes are essential for the reliable operation of high voltage direct current (HVDC) grids and to prevent damages to power system equipment due to sustained faults. This paper proposes a rapid, reliable, and setting-less breaker failure backup protection scheme for multi-terminal HVDC grids. The proposed scheme employs Hilbert-Huang Transform (HHT) to extract two features from local voltage measurements, namely the instantaneous frequency and energy. Based on detected outliers in the extracted instantaneous features, breaker failure events are rapidly detected. The proposed setting-less outlier-based criterion can be applied to HVDC grids with various configurations, parameters, and breaker technologies. The proposed scheme depends only on local voltage measurements to detect breaker failure events within 1 ms from the intended breaker trip time without requiring breaker voltage or current measurements. In addition, the proposed scheme can successfully detect breaker failure events during high-resistance faults (1500 Ω) and in grids with small boundary reactors (10 mH). Numerous simulations in PSCAD/EMTDC software environment for a four-terminal HVDC grid are executed to demonstrate the rapid and reliable performance of the proposed breaker failure backup protection scheme under severe fault conditions