Page 478 - ExtendSim User Guide

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452 Advanced Topics
LP technology
• If a rate section has a critical constraint, it places an upper limit to the effective rate within that section. Since effective rates are decision variables in the LP’s objective function calcu- lation, critical constraints place an upper bound on some of the equation’s variables.
• Relational constraints describe the dependencies between different rate sections. In some blocks, relational constraints can vary depending on the state sensitivity of the block; in oth- ers they are permanently active.
• AnexampleofastatesensitiverelationalconstraintcanbefoundinaTankblock. (Tanks are always within two rate sections; the input side and the output side define the sections.) As long as a Tank is empty, its relational constraint is defined as “effective outflow rate is less than or equal to effective inflow rate”. Once the Tank becomes “not empty”, the relational constraint doesn't apply.
• AnexampleofapermanentrelationalconstraintcanbefoundinaChangeUnits block where a conversion factor defines the relationship of the inflow effective rate to the outflow effective rate. If the factor varies over time, the relational constraint may also vary. But the dependency between the inflow and outflow effective rates is active for each calculation which includes the two rate sections.
Bias information
When the LP area is created, the Executive ranks all Bias blocks and any Merge or Diverge blocks with a bias order in a list. The Executive considers the top bias from that list as part of the objective function and instructs the Solver to perform an intermediate LP calculation using that function and the current critical and relational constraints. Then the Executive takes the next bias order into consideration, and so forth.
Since bias affects critical and relational constraints, each succeeding bias order means a new objective function will be determined and a new set of critical and relational constraints will be added to the previous ones. The results from the previous LP are used as inputs to the next LP. This results in multiple intermediate LP calculations – one for each bias order in the list – until the final result.
Most blocks with bias order supply:
• Flowrules(criticalandrelationalconstraints)whichapplytoalltheLPcalculations.(This information is not supplied by a Bias block.)
• AsetofcoefficientsthattheExecutivewillusetobuildtheobjectivefunctioncorresponding to this bias order. Depending on the bias information received from the blocks in the LP area, the intermediate objective function can include coefficients that are other than 1. (A coefficient of 0, for instance, indicates that a particular effective rate does not need to be maximized; it causes that effective rate to not be directly affected by the maximization.)
• Flowrulesforthatbiasorderwhichusetheresultsoftheintermediatecalculation.(Afterthe intermediate calculation, the results of the calculation are used to add new flow rules to the succeeding calculations.)
Some additional situations that enter into the calculation include:
• If a Bias block has a bias order that is Blank or is less than or equal to 0, the block does not express any preference.
Discrete Rate
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