.. _ltm_time: Time in the LTM-core ==================== This section covers the many concepts of time in the LTM-core the LtmApi. Many of these concepts *feel* natural, but some of the concepts are known to introduce confusion among the users, therefore we recommend dedicating time to understand these concepts. Introduction ------------ The Long-Term Models (LTM) build and solve **weekly** optimisation problems in sequence for a defined period, the period is normally from 1 to several years (modelling maximum is 25 years). The models take into account uncertainty in weather and prices in defined **scenarios**. The weather data is normally based on historical times series. .. image:: TimeLineLTM.PNG :alt: Illustration of period :scale: 50% :align: center Due to the week centered modelling philosophy of the LTM models, each year in the LTM-models are not defined by the earths time in orbit around the sun. But each year contains 52 weeks, 364 days or 8736 hours and is refered to as an **EFI year** and each week as an **EFI week**. The EFI year start in week 1 and ends after week 52. The EFI week's first day is Monday and the last day is Sunday, it contains 168 hours and a user defined number of time steps which is described here: :ref:`timesteps_per_week`. LTM-core long-term periods -------------------------- Below is the important long-term periods in the LTM-core: 1. **Simulation period** 2. **Historical period** 3. **Data period** 4. **Water value period** we recommend making an effort to understand and remember the concepts. Below is an illustration of the period "hierarchy" and how they relate to each other. .. image:: Period_hierachy.png :alt: Illustration of period hierarchy :scale: 15% :align: center where: .. centered:: *Data period* >= *Water value period* >= *Simulation period*. Ideally, these periods should overlap, but the reality is that this is not *always* possible depending on the start and end of the periods and the system modelling. .. _simulation_modes: Simulation modes ~~~~~~~~~~~~~~~~ Further there are two simulation modes available, for illustration of the modes a simple example is provided where the uncertainty is represented by three scenarios: 1. **Parallell simulation** - all scenarios starts from the same system state given as input boundary conditions. All scenarios start at the same time and are simulated for a specified **simulation period** (week 1 to week N). This mode is used for day-to-day operation. All scenarios are independent of the others and can be simulated in parallell using parallell processing to increase calculation speed. .. image:: Para_sim.png :alt: Illustration of para sim :scale: 15% :align: center 2. **Serial simulation** - the scenarios are simulated in series, only the first week of the first scenario use input boundary conditions to set the system state. The simulation period for each scenario is 52 weeks. Boundary conditions are passed on from the weekly solution as input to the next week to set the system state until all scenarios are solved. Simulation speed up using parallell processing is not possible. This mode is typically used for investment and system analysis. .. image:: ser_sim.png :alt: Illustration of ser sim :scale: 15% :align: center Simulation period ----------------- The simulation period is the period for which the power system is simulated using the LTM-models and power system results can be extracted. The period must be defined within the bounds of both the *data period* and the *water value period*. Historical period ----------------- Used to define the period of historical data input to the model, such as: - inflow - temperature - wind - solar - exogenous price Scenarios --------- The scenarios, representing the modelling uncertainty, are defined based on a combination of the mentioned periods. Where the simulation period defines the scenario start point and scenario length. For serial simulation the scenarios are always 52 weeks (1 EFI year). Data period ----------- The period where data is needed in order for the LTM-core to run successfully. It represents the outer limits of the timeline of the system modelling. The period must as a minimum start and end at the same time as the *simulation period* and the *water value period*, but may span a longer period in both directions. For example the data period can be 3 years, starting in 2024. All timeseries and other data must be complete for the 3 year period from 2024 - 2027. By default this period is calculated by the LtmApi based on the simulation period. **Important requirements**: - The data period starts at the beginning of an EFI-year (beginning of week 1) - The data period ends at the end of an EFI-year (end of week 52) See examples in :ref:`data_period` Water value period ------------------ Period defining the bounds for the strategy calculation (or water value calculations). The period must be shorter or equal to the *data period* and longer or equal to the *simulation period*. In some cases, the simulation may require data that extends both before and after the specified simulation period. For example in the strategy calculation it may be necessary to look sufficiently far ahead in the calculation process, requiring data to be available several weeks beyond the simulation period. .. _timesteps_per_week: LTM-core short-term periods --------------------------- .. include:: intra_week_time_LTM.rst