Modeling of large fluid systems requires in-house (specialized) tools, since applicability of Modelica and existing environments is limited.Nevertheless Modelica is a very powerful and descriptive modeling language, which is best suited for physical modeling in a heterogeneous environment. Its object oriented approach, the built-in documentation and the availability of commercial and free libraries justifies the decision for Modelica as the preferred modeling language within Siemens Energy.For an appropriate analysis of transient power plant processes, there often are large fluid systems to be modeled, i.e. there can be several thousand states. For such plant models, we use our in-house tool Dynaplant (DP), which is specialized for large fluid systems. A comparison between DP and Dymola[1] reveals some deficiencies of the Modelica world concerning performance and plant model construction: Especially, successive initialization and sparse matrix solvers are important features in need.