With the increasing interactions between natural gas systems (NGS) and power systems

component failures in one system may propagate to the other one

threatening reliable operation of the whole system. Due to neglect of such cross-sectorial failure propagation in integrated electricity-gas systems (IEGSs)

traditional economy-oriented reserve expansion models may lead to unreasonable planning results. In order to address this

an innovative reserve expansion model is proposed to determine the allocation of energy production components through the harmonization between costs and reliability. First

novel multifactor-influenced reliability indices are defined considering synthetic effects of multiple uncertainties

including failure propagation

load uncertainties and generation failures. In reliability index formulation

contribution of failure propagation on system reliability is analytically expressed. To avoid high computational complexity

the fuzzy set theory is combined with conventional methods

e.g.

Monte-Carlo simulation technique to reduce numerous contingency states. Sampled contingency states are aggregated into several clusters represented by a fuzzy number. To effectively solve the planning model

a decomposition approach is introduced and applied to decompose the original problem into a master problem and two correlated reliability sub-problems. Numerical studies show the proposed model can plan reasonable reserves to guarantee reliability levels of IEGSs considering failure propagation.