The integration of renewable energy sources into existing electric power systems is connected with an increased interdependence between natural gas and electricity transmission networks. To analyse this interdependence and its impact on security of supply, we developed a novel quasi-dynamic simulation model and implemented it into the simulation tool SAInt (Scenario Analysis Interface for Energy Systems), the first published software application that allows the combined simulation of gas and electric power systems in a single time frame and simulation environment. The model is composed of a transient hydraulic simulation model for the gas system and an augmented AC-Optimal Power Flow model for the electric power system, which includes a model for dispatchable power system loads and considers time transitional constraints, such as the ramp rate and the start-up time of generation units. Both models take into account the control and constraints of the most relevant facilities present in both systems. The bidirectional interconnection between both systems is considered and established by coupling equations describing the fuel gas offtake for power generation in gas fired power plants, and the electric power supply to LNG terminals and electric driven compressors in gas compressor stations. The resulting system of equations for the combined model are solved in a single simulation time frame. In order to quantify the impact of different contingencies on the operation of the combined system, a number of security of supply parameters are proposed, which can be utilised to compare the impact of different contingencies on [GRAPHICS] security of supply and the effectiveness of countermeasures to mitigate this impact. The capabilities of the combined model and the functionality of the simulation tool SAInt are demonstrated in a case study of a sample gas and power transmission system. Results indicate how the combined simulation of gas and electric power systems can give insight into important and critical information, such as the timing and propagation of contingencies cascading from one system to the other or the grace period to react to these contingencies. Such information can contribute to improving the coordination between gas and power transmission system operators in the event of a disruption, thus, increasing the resilience and the level of security of supply in the combined energy system. The information provided by the combined model cannot be obtained by the traditional co-simulation approach, where both systems are solved in different time frames. Furthermore, the studies stress the importance of using transient gas simulation models for security of supply analysis instead of steady state models, where the time evolution of gas pressure and linepack are not reflected appropriately. (C) 2017 The Author(s). Published by Elsevier Ltd.