LT Plan - Problem Size
Contents
1. Integer and Linear Decisions
The capacity 'sub-problem' of the LT Plan formulation uses annual build/retire decisions. Thus the number of these elements in the formulation is relatively small. No problem size reduction is required to these elements. However, if all decisions are integer (as controlled by the LT Plan Optimality option), then the problem can become very hard to solve. You can reduce the number of integers by selectively turning off integer optimization at the Generator level using the Generator Expansion Optimality property. You can also specify the integerization horizon using the LT Plan Integerization Horizon attribute. For example in a 30-year study you can choose to integerize only the first 10 years, leaving decisions beyond 10 years linear.
2. Steps and Chronology
LT Plan by default spans the whole simulation planning horizon in a single optimization 'step'. For realistically-sized power systems and long planning horizons this may lead to extremely big problems in the production cost 'sub-problem'. To make this problem tractable, two problem size reduction techniques can be used:
- The production cost sub-problem's chronology can be reduced by using less periods in the chronological steps or by using the load duration curve (LDC) methodology. This is controlled by the Chronology option, which is described in the MT Schedule article.
- The horizon can be either a) split into multiple steps using the LT Plan At a Time attribute (the steps can optionally Overlap a number of years thus avoiding 'end effects' between steps); or b) solved using Rolling Horizon (see LT Plan Overlap for details).
The key controlling parameters for the Chronology options are:
- LT Plan LDC Type
- LT Plan Block Count and Last Block Count
- LT Plan Sample Type
- LT Plan Reduced Sample Count
The equivalent parameters are described in detail in the MT Schedule article.
The level of detail used to model Generator heat rates can have a significant effect on the size and difficulty of the LT Plan formulation. The setting LT Plan Heat Rate Detail by default uses the "Simplest" possible form in order to reduce problem size.