The aim of this paper is to present a simple model that describes the behavior of a gas transmission pipe to the start-up or shut-down of one or several combined cycle gas turbines (CCGT). Focusing on the evolution of pressure (in time and in space) in a pipeline with unbalanced in-flows and out-flows, this model provides explicit formulas to understand qualitatively, and to quantify the multiple phenomena at stake. Intensive comparisons with the commercial software SIMONE1 proved the accuracy of this approach.
GRTgaz develops, maintains and operates the main part of the French gas transmission system. For a long time, steady state models have provided reliable results to help dispatchers take relevant operational decisions. The recent and fast development of many CCGT changed the deal and implied a need for new landmarks and for a better understanding of the phenomena of gas transmission. Indeed, these industrial consumers are challenging for network dispatching : They tend to synchronize their production cycles, as they respond to the same signals : electricity need (regional) or gas/electricity arbitration (global).They can be used to balance electrical offer and demand and therefore, start under short notice. Their nominal consumption is high and obtained quickly, while gas entries are flat during the day.
Thus, CCGTs increase the reactivity required from the dispatchers and the probability of mismatch, both geographical and temporal, between demand and gas availability. As a consequence they deeply modify network operation management. Impact studies, based on local studies and simulations of particular cases, provide interesting results for dispatchers but they also put forward a need for a more generic and systematic approach to understand and quantify the changes in the network due to CCGTs.
The behavior of a gas transmission pipe in a dynamic environment has historically been widely studied. Different approaches have been developed so far. Doing a compromise on the sophistication of the model, they put the emphasis either on the physical understanding of phenomena (see [2]) or on the final accuracy of the results. Because of the complexity of the phenomena and of the mathematical partial differential equation from Navier-Stokes, accuracy and sophistication often imply using advanced numerical analysis techniques based on finite elements, loosing direct, explicit physical feeling of events. The model we present here is a compromise between these two extremes, trying to keep the multiple advantages of having explicit and "handy" formulas, while having the richest model possible.
In section 1, we will describe the frame of our study and how the Navier-Stokes equations are handled, so that an explicit solution can be found.
Section 2 will deal with the physical and mathematical validity of our approach, questioning the quantitative accuracy of the model, both theoretically and on a numerical point of view through comparisons with computations from SIMONE. Finally, we will present practical applications of the model in section 4. In particular, we will discuss how the explicit formulas provided by the model can be used for intraday decision-helping tools based on dynamic models.
