# Frequently Asked Questions¶

## Program Startup¶

Q: Why do I get an "ImportError: DLL load failed" when running ANDES?

Platform: Windows, error message:

ImportError: DLL load failed: The specified module could not be found.

This usually happens when andes is not installed in a Conda environment but instead in a system-wide Python whose library path was not correctly set in environment variables.

The easiest fix is to install andes in a Conda environment.

## General¶

Q: What is the Hybrid Symbolic-Numeric Framework in ANDES?

A: It is a modeling and simulation framework that uses symbolic computation for descriptive modeling and code generation for fast numerical simulation. The goal of the framework is to reduce the programming efforts associated with implementing complex models and automate the research workflow of modeling, simulation, and documentation.

The framework reduces the modeling efforts from two aspects: (1) allowing modeling by typing in equations, and (2) allowing modeling using modularized control blocks and discontinuous components. One only needs to describe the model using equations and blocks without having to write the numerical code to implement the computation. The framework automatically generate symbolic expressions, computes partial derivatives, and generates vectorized numerical code.

## Modeling¶

Q: Where to find the line admittance matrix?

A: ANDES does not build line admittance matrix for computing line power injections. Instead, line power injections are computed as vectors on the two line terminal. This approach generalizes line as a power injection model.

Q: Without admittance matrix, how to switch out lines?

A: Lines can be switched out and in by using Toggler. See the example in cases/kundur/kundur_full.xlsx. One does not need to manually trigger a Jacobian matrix rebuild because Toggler automatically triggers it using the new connectivity status.

### Reference of the existing model¶

Q: Is there any further reference of the existing model?

A: Most of them can be found online, such as ESIG and PowerWorld.