Scenarios and Hub Systems

A "combined scenario" for a hub system has many properties in common with scenarios for single energy carrier networks. But, it also has specific requirements defining the way SAInt carries out the combined simulation.

SAInt does not generate a dedicated *.*sce file for a combined scenario, like in the case of an electric scenario with a *.esce file or a gas scenario with a *.gsce file. But it requires the user to specify and load a scenario for each type of energy network contributing to the hub system and to specify events for the objects coupling the different systems. Altogether the contributing scenarios and the coupling events constitute a "combined scenario".

A combined scenario describes the operation of two or more networks of different types according to conditions specified in each participating scenario and to the type and properties of the coupling objects. Similarly to any other scenario, the conditions at which the system’s variables are evaluated are specified by the events of all participating scenarios. Also, the properties and settings of a combined scenario define its time span and resolution, as well as the mathematical equations and numerical tolerances adopted for the solution. But contrary to the flexibility offered in scenarios with single energy carrier networks, a combined scenario requires the settings of all participating scenarios to be the same or "aligned". The contributing scenarios are aligned when specific time-related and solver-related options have the same values. For time-related options, the contributing scenarios must have the same StartTime, EndTime, and TimeStep.

In a steady state combined scenario, the user may specify a different StartTime between the contributing scenarios.

However, aligning the StartTime of the contributing scenarios is recommended for consistency and to ensure the user selects the correct initial conditions when used with dynamic scenarios.

For solver-related options, the contributing scenarios must have the same values for MaxIterationSteps and ResidualTolerance. The user must ensure these options are the same for all contributing scenarios.

Furthermore, a combined scenario must match the scenario types for the contributing scenarios. The user can match any steady state scenario type or dynamic scenario type together. But steady state and dynamic scenarios cannot be mixed. The button Combined from the menu Simulation Execute is greyed out when the active scenario types or settings are unsuitable for the simulation.

In a combined scenario, the user must specify an initialization state for a fluid network whenever using a dynamic simulation case. This is also required when using an electric network when a quasi-dynamic scenario is selected. This last requirement is unnecessary when only electric networks are considered.

Finally, the user should review the Profile settings for each scenario to properly handle the profile start times. SAInt allows to user the options UseGlobalProfileStartTime and GlobalProfileStartTime to select and define a global start time for all profiles. In this way, profiles will be synchronized with the global start time even if they start at a different time in the contributing scenarios.

1. Coupling and control mode

In a combined network, the coupling among systems is achieved by setting specific objects as "links". A detailed description of such links is provided in the section "hub objects". From a modeling point of view, the coupling is expressed using a dedicated equation combining a variable from each system. When the two networks are independent, each has a control equation. When they are coupled, one variable controls both networks, and the control equation of the other is replaced by a coupling equation.In this way, one of the two systems is determining the control mode of the combined scenario.

Table 1 describes the set of coupled gas-electric objects available in the present release of SAInt. The table also reports the direction of the flow of power between the two sides of the coupled systems and which system is leading in terms of control mode. For example, the power is flowing from the electric system to the gas system in an electric-driven compressor station (EDGCS), but the control mode is determined but the gas system. This means that the amount of gas pushed by the compressor station is the variable defining the demand for electricity, and not the other way round.

Table 1. List of the coupled gas-electric objects, with the description of the power flow direction and the default control mode for a combined scenario. The letter "G" stands for the gas system. The letter "E" stands for the electric system. The arrow indicates the direction of flow, while the asterisk is where the control mode is.
Type of facility	Power "from to" (➞)
Gas-Fired Generator (GFG)	G ➞ E^*
Power-To-Gas (`P2G`)	E^* ➞ G
Electric-Driven Gas Compressor (EDGCS)	E ➞ G^*
Electric-Driven Gas Storage (EDGSTR)	E ➞ G^*
Electric-Driven LNG (EDLNG)	E ➞ G^*