Tansient fault analysis of a VSC-based multi-terminal HVDC scheme
Authors:   , Malanda Sindisiwe Cindy 
Published: August 2021

A multiterminal HVDC system includes the connection of different HVDC terminals to a common grid. Most of the MTDC networks are realized in voltage source converter (VSC) high voltage direct current (HVDC). Over long distances, HVDC transmission is preferred to high voltage direct current (HVAC). Furthermore, HVDC is subjected to minimal harmonics oscillation problems due to the absence of frequency. HVDC enables the interconnection of systems at different frequencies, and the system becomes free of angular stability problems. VSCs employ insulated gate bipolar transistors (IGBTs) switches, and High-frequency pulse width modulation is used to operate the IGBTs in order to achieve high-speed control of active and reactive power. The growth of MTDC networks may require a new type of VSCs topology, which is resilient and efficient to dc and ac network fault. This research investigation focuses on the transient dc-side fault analysis in a two-level Monopolar VSC- Based Multi-Terminal HVDC Scheme consisting of four asynchronous terminals sharing a rated 400kV DC-grid was carried out in PSCAD software. During dc-side fault analysis, a pole-to-ground fault was taken into consideration as it’s more likely to occur, although it is less severe compared to pole-to-pole. The converters are interconnected through 100 km dc cables placed 0.5 gm apart and at a depth of 1.5 m underground. It was observed that during the steady-state analysis, the dc voltage in the grid was maintained at the rated value 400 kV, the currents measured at the converters bus was 0.5 kA, and the current flowing through the cables was 0.25 kA. Under the fault condition, the dc voltage drop needs to be maintained to a closed range to avoid the grid to collapse. The voltage droop technique was incorporated in the dc voltage controller to keep the dc voltage at the narrow range. Depending on the value and nature of ground fault resistance, the fault current magnitude varies, and distance variation along the cable has a significant contribution in the fault current. It is observed that fault close to the converter (5 km’s measured 9 kA) results in high fault currents compared to fault away from the converter (50 km’s measured 7.8 kA). The protection design of the VSC needs to be able to detect whether its ground fault or short circuit since the location of the fault needs to be identified and repaired. Another observation made when the fault is inserted 50 kms away from the converter, meaning the fault is at the center of the two converters, the outcome results in high currents in both converters. The isolation of the fault should be fast and selective as the critical time is very short. The dc circuit breakers are mostly recommended to be used as primary protection; however, different protection techniques need to be incorporated with dc circuit breaker in order to quickly identify, select and reliable isolate the faulted line. Moreover, the protection should be able to isolate the line before the fault reaches the maximum fault current to avoid the damage in the converter components.