Developer Documentation
Variable, constraint and parameter naming scheme
Suffixes
_ne: used to denote a concept specific to network expansion problems_ls: used to denote a concept specific to implementations where production and consumption are variables._on_off: used to denote a concept where there are either-or choices, such is valve operations or binary direction choices.
Pressure
p: pressure squaredpd_min: minimum pressure squared difference (computed value)pd_max: maximum pressure squared difference (computed value)
Flow
f: mass flowfl: mass flow consumptionfg: mass flow production
Developing steady-state problems and formulations
In the current version of GasModels, the supported variable space is pressure (p) and mass flow (f) for steady-state modeling. Long term there are plans to support problems specified in the variable space of density $\rho$ and mass flux $\phi$. Generally speaking, most steady-state models use/assume the single network formulation that is not discretized in time or space. Thus, most natural gas network models are read in and directly used by steady-state specifications.
A long term development plan is to largely unify the declaration of variables and constraints which are common to both steady-state and transient model (he only difference being single vs. multi-network representations).
Developing transient problems and formulations
A basic version of the transient optimal gas flow problem is supported with the current version of GasModels. It uses the multi-network feature to formulate the problem. The entry point to create and prepare the transient data is the function parse_files() given below:
GasModels.parse_files — FunctionParses two files - a static file and a transient csv file and prepares the data object. The static file is the .m file and the transient file is a .csv file that contains the time-series data information. The function takes in the following keyword arguments: (i) total_time (defaults to 86400 seconds or 24 hours) - this is the total time for which transient optimization needs to be solved (ii) time_step (defaults to 3600 seconds or 1 hours) - this argument specifies the time discretization step (iii) spatial_discretization (defaults to 10000 m or 10 km) - this argument specifies the spatial discretization step (iv) additional_time (defaults to 21600 seconds or 6 hours) - this argument decides the time horizon that needs to be padded to the total time to in case the user wishes to perform a moving horizon transient optimization.
The function, in addition to taking as input the static matgas file and time-series csv files (the formats are discussed in File IO section), can take additional keyword arguments that concern a transient formulation. They are: (1) total_time which is the total time duration of the transient formulation. It defaults to the value of 86400.0. Note that when this is the case the .csv file should contain time series data for the full 24 hours or else trying to construct a transient formulation would result in an error. The other keyword arguments include (2) time_step, (3) spatial_discretization, and (4) additional_time. As mentioned in the Transient Problem Specifications section the transient Optimal Gas Flow problem currently can work with only time-periodic time-series data. If the data is not time-periodic, then GasModels will try to fit a time-periodic spline to the data and then run the transient optimal gas flow problem. Notice that fitting a time-periodic spline for a non-time-periodic time series data would result in deviations from the given profiles and hence the argument additional_time can be used to pad the total_time with additional time to circumvent this issue. The parse_files function results in a multinetwork data model with the appropriate number of time discretizations that can then be used to formulate any transient optimization problem. The details of the transient formulation can be found in the section Transient Problem Specifications.