Multi-time-step and multi-core simulations in EMTP-RV using the new FMI options




EMTP is an advanced computer program for the simulation of electromagnetic and electromechanical transients in multiphase electric power systems. Its unique numerical methods are capable of performing load-flow, steady-state and time-domain simulations in an advanced graphical user interface.

EMTP is recognized to be the perfect tool for the simulation of large and complex power systems with increasing numbers of renewable energy sources and power electronics based devices. EMTP also provides an advanced library of protection components for the simulation and analysis of protection systems.

The simulation of EMTP power components requires numerical integration time-steps that are much smaller than the sampling rates of related control systems. Therefore, controller models are often simulated with unnecessarily short time-steps leading to large computational cost. This article explains how the new FMI capabilities introduced in EMTP 3.4 allow to implement multi-time-step and multi-core simulation of control systems in EMTP to significantly speed-up computations.



Functional Mock-Up Interface (FMI)



The Functional Mock-Up Interface (FMI) is a tool-independent standard for model exchange and co-simulation. The primary goal of the FMI is to standardize data exchange between different simulation tools. The FMI is based on a master-slave concept where the slaves simulate sub-models whereas the master is responsible for both coordinating the overall simulation process as well as transferring data.

The compatibility with the FMI standard was introduced with EMTP 3.4.


Two modes are available:

    - Synchronous: Master and slaves perform their calculations sequentially. They can have different time-steps.
    - Asynchronous: Master and slave models are solved in parallel. They must have the same time-step.

Therefore, it is possible to use the FMI options to run multi-time-step and multi-core simulations. A test case is used in this article for demonstration purposes. It is based on the DFIG wind park model available in the EMTP examples. 


Presentation of the test-Case

The simplified single-line diagram of the studied wind park is presented below. The wind park collector grid is composed of one radial 34.5 kV feeders and is connected to a 120 kV grid through a 120 MVA step-up transformer. 






Fgure 1: EMTP diagram, wind park simulation model and wind park Subcircuit (No Aggregation)



The simulation is automatically initialized from the load-flow results and a three-phase fault is applied on the high-voltage side of the 120 MVA substation at t=1 s. The total simulation time is 10 s.

Each wind turbine controller is placed in a slave model using the FMI, to allow multi-time-step and multi-core simulations. It is verified that the controller model can run with a 200 µs time-step without compromising simulation accuracy.  



Figure 2: Original Wind Turbine model (left). The controller model is placed in a slave model using the FMI (right).





Figure 3: Wind turbine controller details (Slave model).



Three cases are analysed:

    - Base case (No FMI)
    - Synchronous Mode: The Master model time-step is 50 µs. The slave time-step is 200 µs. Calculations are performed sequentially. 
    - Asynchronous Mode: The master and the slave have the same time-steps (50 µs). Calculations are performed in parallel.


Calculations are repeated with different numbers of aggregated wind turbines to evaluate the impact of the number of slave models on the total simulation performance. The Windows Task Manager is shown below for a simulation with 10 wind turbines in asynchronous mode. Total CPU usage of EMTP is around 60% on a 4-core computer.



Simulation performance

The simulations have been performed on a regular i7 computer with 4 cores. 





Using the FMI approach, the wind turbine controllers can be simulated on different computer cores with different time-steps and without any loss of accuracy. The gains in CPU usage are considerable (15% - 35%). This approach can be extended to any EMTP design with control systems.

The implementation of the FMI is easy (no coding is required) and is explained in the help document available in the “FMI Toolbox”. 
The FMI Toolbox is available in the “FMI” Library of EMTP.


WIND-TURBINE-ACTIVE-POWERFigure 4: Wind turbine active power. LLLG fault on the HV side at 1s and cleared at 1.15s. No FMI (red). FMI in synchronous mode (blue)


Do not hesitate to contact us our support team (support@emtp-software.com) to get a copy of the EMTP simulation files or for any questions related to EMTP and/or the FMI.


Any need to accelerate your simulations? 

Engineering services are offered to speed up your EMTP simulation. With a proper design, simulation can be 4 times faster! Our experts will analyse your design and use the FMI multi-time step mesh/multi core features to significantly improve the simulation speed. Contact us at engineering@powersys-solutions.com for more information.