Debugging EPM Models in GAMS#

If your run didn’t converge, or worse, converged with unrealistic results, this guide provides tools and methods to investigate what went wrong.

1. Understanding the PA.gdx Output#

The PA.gdx file (often auto-generated) contains post-solve values for variables and equations. It’s critical for diagnosing model behavior.

Each variable includes:

Field

Meaning

Level

The actual solution value (e.g. power output, demand served, etc.)

Marginal

Dual value — often shadow price or reduced cost

Lower

Lower bound on the variable

Upper

Upper bound on the variable

Scale

Scaling factor applied (typically 1 unless specified otherwise)

What to Look for#

  • Zero levels: If all levels are zero, check if:

    • The variable was properly connected in constraints.

    • It had a feasible range (Lower, Upper) that allowed movement.

    • There’s a cost or penalty preventing it from being selected.

  • Marginals = EPS: GAMS uses EPS (epsilon) when dual values are effectively zero, i.e. the bound is not active. This is normal unless you expected that variable to be tightly constrained.

  • Very high or low levels: Could indicate:

    • Missing unit conversions (e.g. kW vs MW)

    • Mis-specified bounds or parameters

    • A cost that unintentionally encourages unrealistic dispatch

2. Reference Outputs for Debugging#

GAMS provides reference files to trace model structure, usage, and execution flow.

.log File — Execution Log#

Contains runtime logs and time stamps:

  • File inclusions

  • Model generation times

  • Solver startup and memory use

  • Useful for checking if the solver started, crashed, or timed out

.ref File — Model Symbol Reference#

Enable it within GAMS Studio using command parameters.

This produces <filename>.ref which contains:

  • All declared symbols (sets, parameters, variables, equations)

  • Where each symbol was:

    • Declared

    • Defined

    • Assigned

    • Referenced

  • Warnings for @declared but unused symbols

Why it’s useful: Quickly shows whether a variable like vPwrOut(g,f,q,d,t,y) is declared but never referenced in a constraint — a common source of “zero everywhere” issues.

.lst File — Execution Listing#

Automatically generated. Includes:

  • Compilation diagnostics

  • Code listing

  • Full equation listings (left-hand side, right-hand side, status)

  • Solver messages and summaries

Tips:

  • Set option limcol = 0; option limrow = 0; to limit variable/equation listings.

  • Use the “Variable Listing” and “Equation Listing” sections to examine values in context.