Reliability evaluation of UHVDC systems based on fault tree analysis

The ultra high-voltage direct-current (UHVDC) transmission system has complex structure, multiple components and operation modes, especially with the derated capacities and redundant elements, which has several merits compared with the AC and HVDC transmissions. The fault tree analysis (FTA) is an analytical method for reliability analysis, which considers the basic failure incidents, discusses the source of system outage, and goes deeply into the failure groups. The fault trees with different available capacities are built and their availabilities are quantified. Besides the traditional indices, e.g. the energy availability (EA), et al, two sensitivities are defined, i.e. the absolute sensitivity of state probability to the transition parameters and the relative sensitivity. They help to quantify the vulnerable elements and find enhance measures. Numerical results on a typical UHVDC system are provided, which show: (i) the installation rate of the reparable elements has notable effect on the reliability of the UHVDC system; (ii) the redundant components improve the availability and the EA; (iii) the DC line has the most notable impact on the reliability of the UHVDC system, followed by the AC filter and the DC smoothing reactor.