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STEPSS

Overview

STEPSS (Static and Transient Electric Power Systems Simulation) is a power system simulation tool for dynamic studies of electrical grids. It performs power flow computations and simulates the dynamic response of power systems to disturbances under the phasor approximation.

STEPSS has been developed by Dr. Petros Aristidou (Cyprus University of Technology) and Dr. Thierry Van Cutsem (University of Liège).

For more information, visit the STEPSS project page and Thierry Van Cutsem’s software page.

The Three Modules

Section titled “The Three Modules”

STEPSS includes three tightly integrated modules:

The three modules of STEPSS

In the figure above, files shown in blue are provided by the user; those in black are produced internally.

ModuleFull NameDescription
PFCPower Flow ComputationDetermines the initial operating point using the Newton-Raphson method in polar coordinates. Computes bus voltage magnitudes and phase angles, with optional transformer ratio adjustment.
RAMSESRApid Multiprocessor Simulation of Electric power SystemsSimulates the dynamic evolution of the power system in response to disturbances. Supports Backward Euler, Trapezoidal, and BDF2 integration methods. Exploits OpenMP parallelism.
CODEGENCODE GENeratorTranslates user-defined models from text descriptions into Fortran 2003 code for compilation and linking with RAMSES. Supports excitation controllers, torque controllers, injectors, and two-port components.

Each module can be used independently:

  • PFC alone: Run a power flow computation to inspect the system state and/or save the solution for RAMSES
  • PFC alone: Run a sequence of power flow computations until obtaining the desired system state, then save the solution for RAMSES
  • RAMSES alone: With a pre-computed power flow solution, run multiple dynamic simulations from the same initial state
  • CODEGEN alone: Build and save models for future incorporation into a user-defined version of RAMSES

PFC Module

Section titled “PFC Module”

The power flow computation uses the Newton-Raphson method in polar coordinates. Input data consists of:

  • Network data (buses, lines, transformers, etc.)
  • Power flow data specified at PV, PQ, and slack buses
  • PFC control parameters (tolerances, reactive power limits, etc.) — optional, defaults are used if not provided

PFC can optionally adjust transformer ratios to:

  • Bring voltage magnitudes inside specified deadbands (in-phase transformers)
  • Bring active power flows inside specified deadbands (phase-shifting transformers)

PFC produces an output file including:

  • The voltage magnitudes and phase angles at all buses of the network
  • The adjustable transformer data with updated values of their ratios

RAMSES Module

Section titled “RAMSES Module”

RAMSES simulates the dynamic response of power system models under the phasor (RMS) approximation. It takes as input:

  • Network data (shared with PFC, with a few exceptions detailed in this documentation)
  • Dynamic component data
  • Solver control parameters (tolerances, time steps, reference speed, etc.)
  • Sequence of disturbances and actions

Integration Methods

Section titled “Integration Methods”

Three algebraization methods are available:

  • Backward Euler: xk+1=xk+hx˙k+1x_{k+1} = x_k + h \dot{x}_{k+1}
  • Trapezoidal: xk+1=xk+h2(x˙k+1+x˙k)x_{k+1} = x_k + \frac{h}{2}(\dot{x}_{k+1} + \dot{x}_k)
  • BDF2: xk+1=43xk−13xk−1+2h3x˙k+1x_{k+1} = \frac{4}{3}x_k - \frac{1}{3}x_{k-1} + \frac{2h}{3}\dot{x}_{k+1}

All three methods are implicit, ensuring numerical robustness. BDF2 is an L1L_1-stable scheme allowing larger time steps when fast transients are not of interest.

Solver Acceleration

Section titled “Solver Acceleration”

The solver was developed in response to the growing demand for simulations that last longer (e.g. long-term stability studies) or involve larger models (e.g. to account for the impact of active distribution networks).

The solver achieves high computational efficiency through two techniques:

Parallel Processing: The power system model is decomposed into the network, injectors, and two-ports. A Schur-complement approach for network equations ensures the exact same solution as a non-decomposed scheme. Tasks distributed among threads include:

  • Update and factorization of injector and two-port Jacobians
  • Computation of the mismatch vector of Newton method
  • Computation of injector contributions to the Schur-complement matrix
  • Solution of local linear systems

The implementation is general: there is no hand-crafted optimization particular to the computer system, the power system, or the disturbance.

Localization: After a disturbance, components exhibit different levels of dynamic activity. This is exploited at each time step to:

  • Skip Newton iterations on converged injectors/two-ports
  • Replace latent (inactive) injectors with sensitivity-based models

A fast-to-compute metric is used to classify injectors, which seamlessly switch between categories according to their activity.

Key References

Section titled “Key References”
  • D. Fabozzi, A. Chieh, B. Haut, and T. Van Cutsem, “Accelerated and localized Newton schemes for faster dynamic simulation of large power systems,” IEEE Trans. on Power Systems, Vol. 28, No. 4, pp. 4936-4947, Dec. 2013. doi: 10.1109/TPWRS.2013.2251915
  • P. Aristidou, D. Fabozzi, and T. Van Cutsem, “Dynamic simulation of large-scale power systems using a parallel Schur-complement-based decomposition method,” IEEE Trans. on Parallel and Distributed Systems, Vol. 25, No. 10, pp. 2561-2570, Sept. 2014. doi: 10.1109/TPDS.2013.252

CODEGEN Module

Section titled “CODEGEN Module”

CODEGEN allows incorporating user-defined models in RAMSES. The user describes a model in a text file, and CODEGEN translates it into Fortran 2003 code for compilation and linking.

Four types of user-defined models are supported:

  • Excitation controllers (EXC): excitation system and automatic voltage regulator
  • Torque controllers (TOR): turbine and speed governor
  • Injectors (INJ): components connected to a single AC bus
  • Two-ports (TWOP): components connecting two buses

The user model is compiled, not interpreted — resulting in efficient number-crunching code. While the solver code is proprietary, the models are designed to be freely shared, making STEPSS an open-source simulation software for the modeling part.

CODEGEN Studio

Section titled “CODEGEN Studio”

CODEGEN Studio is a browser-based visual editor for building CODEGEN models. Instead of writing DSL files by hand, you drag blocks onto a canvas, connect them, and export a valid model file. It can also import existing DSL files for visual inspection and editing.

Simulation Interfaces

Section titled “Simulation Interfaces”

STEPSS modules can be run through three interfaces:

InterfaceRAMSES (Dynamic)PFC (Static)CODEGEN
Command Lineramses -t cmd.txtpfc -t cmd.txtcodegen model.txt
GUI (Java)Full supportFull supportFull support
Python (PyRAMSES)Full support——

See the Quick Start for details on each interface.

Platform Support

Section titled “Platform Support”
FeatureDetails
STEPSS GUIWindows and Linux, Java 20
PyRAMSESWindows and Linux, Python 3.x
Command-line executablesWindows and Linux (ramses, pfc)
CODEGEN compilationVisual Studio 2022 + Intel oneAPI Fortran
Free version limits1000 buses max, 2 OpenMP cores

Next Steps

Section titled “Next Steps”