## 3 Security of Supply in Integrated Gas and Power Systems  
The interactions between gas and electric systems make it increasingly difficult to separate security of gas supply from security of electricity supply. The changes in the overall system due to all type of incidents affect the dynamic behavior and vulnerability of the integrated gas/electricity system. The degree of integrated power and gas system vulnerability depends on some external conditions like the level of power system dependency on GFPs, power generation mixture of the region, weather conditions, natural disaster probabilities of the region, and failure probability of facilities in either of the systems, among other factors. Generally speaking, large disruptions in gas systems affecting both power and non-power consumers are not so common. The gas system is well known as reliable and safe. However, there could be incidents resulting in curtailment of gas in some conditions which can immediately cause problems in the power system such as, unexpected increase in demand, freezing of wellheads and disruption of pipelines among others. In such cases, the delivery pressure needed by the facilities has to be taken into account. This is particularly important in recently deployed GFPPs using modern combustion turbines, which need higher gas pressure to operate compared to conventional combustion turbines. It should be noted that, even if the gas system had enough capacity to deliver gas to GFPPs at peak demand, the coincidence of peak demand for GFPPs and for conventional use (household, commercial, industrial) may result in a significant diminished pressure in pipelines, which eventually may produce interruptions in the electricity generation because of insufficient pressure. In case of lack of gas supply in a GFPP, the possible solutions that may help bridge the gap of gas availability could be dual fuel capabilities or/and a variety of storage options (line-pack and UGS facilities close to consumption areas). However, the costs and feasibility of storage and fuel switching has to be analyzed in detail since sometimes they cannot be used as a solution in practice. In fact, quite frequently because of the cost of fuel-oil storage a dual fuel GFPP cannot switch to the alternative fuel due to lack of fuel stored on-site. When the consequences and cascading effects of a disruption originating in one system and propagating to the other system are compared, the gas system is more resilient to local and short-term disruptions compared to the electricity system. The main reason for this is that, in addition to the existence of the linepack as short-term storage, the majority of compressor stations are still powered by gas turbines, which keeps the pressure profile within limits, allowing continued operation. Furthermore, in case electric driven compressors are installed, a back-up power system (typically diesel) is usually available to protect the system from power outages. A massive power failure would generally have no serious effect on the physical pipeline facilities, provided that it does not last too long. Compressor stations that utilize electric drivers would be the most affected and have to be analyzed carefully. When analyzing and modelling integrated gas and electricity systems, there are several issues that have to be addressed mainly due to the differences in the structure of the systems. For instance, the failure response of the power and gas system infrastructures is significantly different. A technical failure in the power system infrastructure can result in an immediate loss of service from a generating unit or a transmission line, that can, under some extreme conditions, propagate loss of service to the electric customers due to cascading effects. On the contrary, most technical failures in gas systems (e.g., pipeline rupture, failure in compressor station or storage facility etc.) result in a locally or regionally reduced network capacity rather than an entire loss of service to the gas consumers [1].