## 2 State of the Art
The problem of the design and expansion planning is addressed in [38; 39] for the integrated gas and power systems at the distribution level and the transmission level, respectively. Recently P2G has gained significant interest. A number of studies [4; 40] have investigated the interdependencies introduced by P2G units on the integrated gas and power system operation in GB. The application of P2G for seasonal storage in gas networks was investigated in [41]. The security perspective including the reliability and the adequacy of integrated gas and power systems has gained significant interest due to increasing dependencies among the systems. Such studies may include the cascading effects of contingencies where the performance of the networks is reduced [8; 42; 43; 44; 45]. In [8], an integrated simulation model that aims at reflecting the dynamics of the systems in case of disruptions is proposed. While developing the integrated model, first gas and power systems are modeled separately and then linked with an interface. Despite the growing interest in analyzing the integrated gas and power system in reliability aspects most of the studies on that area have used steady-state or successions of steady state formulations (i.e., supply and demand are assumed balanced at all time) to define each system in order to reduce the complexity of the problem [21; 46; 47]. However, these formulations could not reflect the different behavior of the two systems appropriately, since gas and power system dynamics evolve on very different timescales. Gas systems react slower to changes in the system, because of the larger system inertia, due to the quantity of gas accumulated in the pipelines, also referred to as linepack. Since steady state gas models do not account for the changes in linepack, these models are inadequate for describing the dynamic behavior of gas systems, when boundary conditions change over time (demand, supply, etc.) [48; 49]. Capturing the dynamic behavior of gas systems correctly requires the use of transient models. Nevertheless, few references can be found in literature considering the integration of gas dynamics with electricity systems [27; 49]. In [27], gas and electricity systems have been modeled in a coupled manner to assess the coordinated daily scheduling of interdependent gas and electricity transmission systems that are based on slow transient process of gas flow. However, the authors did not take into account the ability of GFPPs to change their output within the day. Moreover, the flexibility of the gas system to adapt itself to changing demands of GFPPs is not analyzed in the study. In [49] an integrated gas and electric flexibility model has been developed where a relevant flexibility metric is introduced to assess the ability of the gas transmission networks to react to changes in the power system, particularly, due to intermittent renewables. The proposed model uses both steady-state and transient gas analysis and electrical DC optimal power flow, where the bus voltages and reactive power balance are neglected. The simplification used in DC power model may provide too optimistic results, mainly because voltage profile of buses and reactive power has significant impacts on the system conditions when perturbed by failure events [8]. This study extends previous work in the area in several ways. First, to the best of our knowledge it is the first scenario-based integrated simulation tool to analyze the cascading effects of the contingencies for integrated gas and power systems in such detail. The proposed framework (referred to as SAInt) couples a transient gas hydraulic model, which considers sub-models of the most important facilities, such as compressor stations, LNG terminals and UGS facilities, with a steady state power model based on AC flow, where the transmission capacity, active-reactive generation and upper-lower limits on voltage magnitude are considered.