A Protection Scheme for Multi-Terminal VSC-HVDC Transmission Systems

High-voltage direct-current (HVDC) power transmission is becoming increasingly important due to steadily rising need for bulk power delivery and interconnected power transmission and distribution systems. DC grids are vulnerable to dc faults, which lead to a rapid rise in dc fault currents. The dc faults must be cleared within the timeframe of milliseconds to avoid the collapse of the HVDC system. In the event of primary protection (PP) failure, back-up protection (BP) must be applied to clear the fault. In this paper, a novel algorithm based on a Naïve Bayes classifier is proposed to determine threshold levels and operational time frames for primary and back-up protection in multi-terminal voltage source converter-based HVDC. Local voltage and currents are measured to detect and identify the kind of fault. A four-terminal HVDC transmission system is developed in PSCAD/EMTDC and is subjected to line-line faults at different locations and time, to assess the designed protection schemes. Results show that a relaying algorithm effectively detects the fault and expedite the primary protection operation. On malfunctioning of PP, BP is accelerated in a short delay of 0.2 ms. Furthermore, the relaying algorithm provides faster protection compared with techniques available in the literature. The resulting reduced fault clearance time truncates the maximum fault current and, inevitably, leads to reduced power ratings required for dc grid equipment.