With the significant decrease in natural gas prices in many parts of the world, the employment of gas turbine (GT) units has increased steadily in recent years. The ever-increasing deployment of GT units is strengthening the interconnections between electric power and natural gas systems, which could provide a higher level of operational flexibility and reliability. As a result, the planning and operation issues in the interconnected electric power and natural gas systems have aroused concern.

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... n these circumstances, the impacts of increasing deployment of GT units in power system operation are studied and evaluated through well-being analysis (WBA). The fast responsive characteristics of GT units are analyzed first, and the definition and adaption of WBA in a power system with increasing deployment of GT units are addressed. Then the equivalent reserve capacity of GT units is estimated, taking demand fluctuations, commitment plans, and operational risks of GT units into account. The WBA of a power system with increasing deployment of GT units is conducted considering the uncertainties of system operation states and renewable energy sources. Finally, the proposed methods are validated through an integrated version of the IEEE 118-bus power system and a 10-bus natural gas system, and the impacts of GT units on power system security under various penetration levels are examined. Simulation results demonstrate that the role of a GT unit as a low-cost electricity producer may conflict with its role as a reserve provider, but through maintaining a proper proportion of idle GT capacities for reserve, the well-being performance of the power system concerned can be significantly improved. Energies 2017, 10, 955 2 of 18 frequency adjustment, and spinning and non-spinning reserves [3, 4] . However, the impacts of increasing GT capacity and the interactions between the natural gas and electric power systems on the secure operation of the concerned power system have not yet been given much attention. The increasing deployment of GT units and newly emerged power-to-gas (P2G) infrastructures has strengthened the interconnections between the electric power and natural gas systems more than ever before, which means the natural gas network will be under significant pressure when both GT's natural gas demands and direct natural gas consumption are high [5] , and supply limitations and element failures may have significant impacts on the security of the integrated energy system. Therefore, it is necessary to analyze the impacts of the ever-increasing deployment of GT units on the secure operation of energy systems. Coordination initiatives among multiple energy sectors have been emerging around the globe [6,7], and many existing publications are devoted to the planning and operation of the integrated energy system [8, 9] . An expansion co-planning framework is proposed in [10] to address the uncertainties introduced by the increasing utilization of natural gas in the power system, and a planning strategy with the highest benefit/cost ratio is developed to reduce the operational cost, the carbon emissions cost, and the unreliability cost. The optimal expansion planning of multiple energy infrastructures is studied in [11] from the perspective of an energy hub. A security-constrained co-optimization planning model that considers long-term interdependency and natural gas transportation planning is proposed in [12] . An integrated stochastic day-ahead scheduling model is proposed in [4] to explore flexible ramping so as to accommodate stochastic VRES. A mixed-integer linear programming (MILP) formulation that considers the gas traveling velocity and compressibility in the integrated electric power and natural gas system is developed in [13] , which attempts to assure energy adequacy in short-term operations. The P2G infrastructures are intensively studied in [14] to explore their transmission and utilization characteristics as well as their impacts on the natural gas network. Stochastic programming approaches are employed in the planning and operation of P2G energy hubs for evaluating the benefits of P2G storage with uncertainties of electricity price and hydrogen demand considered [15] . The dynamic modeling and interaction between two energy networks are studied in [16] under the microgrid frame, and the improved models of micro turbines are presented. However, the abovementioned publications generally focus on the modeling of integrated energy systems, while the operational risks are not systematically addressed. At the same time, the increasing popularity of natural gas usage, especially by GT units, will lead to more frequent occurrence of natural gas scarcity and inadequacy. Thus, the energy integration will eventually be hindered when the secure and reliable operation of the integrated electric power and natural gas system cannot be guaranteed. In addition, the dynamic performance of GT units may severely disturb power system stability even though they outweigh conventional generating units in terms of emissions reduction [17, 18] . Much research has been done on the state analysis of GT units. For instance, a detailed model for a heavy-duty GT unit that considers the ambient temperature and frequency dependence of the turbine power output is derived in [19] based on field testing, whereas [20] estimates the parameters of a heavy-duty GT unit model for dynamic studies with recourse to the available operational and performance data. In [18] , operation issues of a heavy-duty GT unit are addressed and some methods such as back-tracing and feed-forward employed to prevent the wind-up phenomenon and improve the stability and response time of a GT unit. So far, the well-being analysis (WBA) has been applied to generating systems [21] and large-scale power systems [22] through Monto Carlo simulations. The integrations of wind power generation [23, 24] , electric vehicles [25, 26] , and energy storage equipment [27] are also analyzed. The WBA of a power system is observed and analyzed from a global perspective, and the risks from the whole system are identified and evaluated. However, the impacts of the natural gas system on the well-being of the power system have been underestimated, and the integration of GT units has not been systematically addressed. As the share of GT units in total power generation is increasing, systematic investigations on the integrated system will not only help avoid potential risks, but also Energies 2017, 10, 955 3 of 18