FMU Method

FMU Integration Method

This directory documents the methodology for integrating Marine Hydrokinetic (MHK) device models into power system analysis tools using Functional Mock-up Units (FMUs).

Overview

The FMU (Functional Mock-up Unit) method enables co-simulation between detailed device models (typically developed in MATLAB/Simulink) and power system analysis software (such as PowerFactory). This approach allows:

• Detailed device modeling in specialized simulation environments
• System-level analysis in power system tools
• Standardized interface through the FMI (Functional Mock-up Interface) standard
• Efficient co-simulation without requiring full model ports

Why Use FMUs for MHK Integration?

Benefits

• ✅ Preserve proprietary models: Device models remain encapsulated
• ✅ Leverage specialized tools: Use best tool for each component
• ✅ Standard interface: FMI is an open standard supported by many platforms
• ✅ Computational efficiency: Optimized for co-simulation
• ✅ Version control: FMUs are portable across different platforms

Use Cases

• Grid impact studies with detailed MHK device models
• Fault ride-through analysis
• Power quality assessment
• Integration planning and feasibility studies
• Control strategy evaluation

FMU Integration Workflow

Phase 1: Prepare Simulink Model for FMU Export

1. Define Interface Variables

   • Inputs from grid: Voltage (3-phase), frequency, grid events
   • Outputs to grid: Active power, reactive power, current

2. Configure Model Settings

   • Set fixed-step solver (required for FMU export)
   • Choose appropriate time step (typically 10-100 μs for power electronics)
   • Ensure all blocks are compatible with code generation

3. Add FMU Export Blocks

   • Use Simulink's "FMU Export" capability
   • Map input/output signals to FMU interface
   • Configure variable names and units

4. Test in Simulink First

   • Verify model runs correctly standalone
   • Check output signal ranges and behavior
   • Validate control system response

Phase 2: Export FMU from Simulink

1. Configure FMU Export Settings

   % In Simulink model configuration
   % Code Generation -> Interface -> Advanced parameters
   % Set "Support: Model reference" to FMU

2. Build the FMU

   • Simulink Coder -> Build Model
   • Select FMU export type (typically Co-Simulation FMU)
   • Choose appropriate solver settings

3. Verify FMU Package

   • Check that .fmu file is generated
   • Verify modelDescription.xml contains correct interface
   • Test FMU with FMU Checker tool (optional but recommended)

Phase 3: Import FMU into PowerFactory

Using the PowerFactory Template

We provide a template PowerFactory model that uses an AC Current Source as a proxy for the MHK device. This approach allows the FMU to control the current injection into the grid, providing a flexible and stable interface for co-simulation.

PowerFactory template showing AC Current Source proxy for FMU integration

Template Structure

The PowerFactory template includes:

1. AC Current Source (ElmIac)

   • Acts as the proxy element for the MHK device
   • Controlled by the FMU outputs (active and reactive current)
   • Connected at the point of common coupling (PCC)

2. FMU Controller (ElmFmu)

   • Contains the MHK device model (WEC or CEC)
   • Receives voltage measurements from the grid
   • Outputs current commands to the AC current source

3. Measurement Elements

   • Voltage measurement (StaVmea) - reads 3-phase voltage at PCC
   • Current measurement (StaImea) - monitors injected current (optional)
   • Frequency measurement - provides grid frequency to FMU

4. Control Connections

   • Voltage signals from PCC → FMU inputs
   • FMU current outputs → AC Current Source setpoints
   • Proper signal scaling and unit conversion

Integration Steps

1. Load Template Project

   • Open the PowerFactory template file
   • Identify the placeholder AC current source location
   • Note the existing measurement and control connections

2. Import Your FMU

   • Navigate to Library → FMU Frames
   • Create new FMU frame element
   • Load your .fmu file (WEC or CEC model)
   • Verify FMU interface variables in the properties

3. Configure FMU Interface

   Inputs (Grid → FMU):

   • Three-phase voltage magnitude and angle at PCC
   • Grid frequency
   • Any additional grid status signals (optional)

   Outputs (FMU → Grid):

   • Active current component (Id or Ip)
   • Reactive current component (Iq)
   • Device status signals (optional)

4. Connect FMU to AC Current Source

   • Map FMU current outputs to the AC current source control inputs
   • Set proper scaling factors (ensure units match: A or pu)
   • Configure control block connections using composite model frame

5. Configure Measurement Elements

   • Link voltage measurement at PCC to FMU voltage inputs
   • Set measurement element parameters (voltage level, nominal values)
   • Ensure proper coordinate transformations (abc → dq if needed)

6. Set Co-simulation Parameters

   • Communication time step: Match Simulink FMU time step
   • Solver: Select RMS or EMT (EMT recommended for detailed dynamics)
   • Interpolation method: Zero-order hold or linear
   • Maximum iterations: 20-50 for convergence

Why AC Current Source?

Using an AC current source as a proxy offers several advantages:

• ✅ Numerical Stability: Current sources are more stable than power sources in power flow calculations
• ✅ Direct Control: FMU directly specifies current injection, avoiding additional conversion steps
• ✅ Flexibility: Easy to switch between grid-feeding and grid-forming modes
• ✅ Convergence: Better convergence properties in iterative co-simulation
• ✅ Standard Practice: Widely used in power system analysis for converter modeling

Alternative: Controlled Voltage Source

For grid-forming applications, you can alternatively use a controlled voltage source (ElmVac), where the FMU specifies voltage magnitude and angle. However, the current source approach is more commonly used and more stable for grid-following inverters, which is typical for MHK devices.

Phase 4: Run and Validate Co-simulation

1. Initial Testing

   • Start with steady-state conditions
   • Verify power flow convergence
   • Check variable magnitudes are reasonable

2. Dynamic Simulation

   • Run time-domain simulation
   • Monitor FMU status and errors
   • Analyze power flows and device behavior

3. Validation

   • Compare with standalone Simulink results
   • Verify energy conservation
   • Check numerical stability

Best Practices

Model Design

• ⚡ Keep FMU interface simple (minimize number of variables)
• ⚡ Use consistent units (SI units recommended)
• ⚡ Document all interface variables clearly
• ⚡ Include saturation limits on outputs to prevent numerical issues

Solver Configuration

• ⚡ Match time steps between Simulink and PowerFactory
• ⚡ Use fixed-step solvers in both environments
• ⚡ Choose appropriate stiffness for solver selection
• ⚡ Start with larger time steps, reduce if needed for stability

Debugging

• ⚡ Test Simulink model thoroughly before FMU export
• ⚡ Use signal logging to compare standalone vs. co-simulation
• ⚡ Check FMU modelDescription.xml for correct interface
• ⚡ Verify units and scaling factors match between tools

Performance

• ⚡ Simplify models where possible without losing fidelity
• ⚡ Use lookup tables for complex calculations when appropriate
• ⚡ Minimize algebraic loops
• ⚡ Balance accuracy with computational efficiency

Common Issues and Troubleshooting

Issue: FMU Export Fails

Solutions:

• Ensure all Simulink blocks support code generation
• Check that fixed-step solver is selected
• Verify Simulink Coder license is available
• Remove incompatible blocks (Scope, Display, etc.)

Issue: Co-simulation Diverges

Solutions:

• Reduce communication time step
• Check initial conditions match between models
• Verify scaling of input/output variables
• Add damping or filtering to outputs

Issue: Incorrect Power Exchange

Solutions:

• Verify sign conventions match (generation vs. consumption)
• Check per-unit vs. actual unit conversions
• Validate measurement point locations
• Ensure proper phase angle references

Issue: Slow Co-simulation

Solutions:

• Increase communication time step (if stable)
• Simplify FMU model complexity
• Use more efficient numerical methods
• Consider model order reduction

Templates and Examples

See the Synthetic Grid Example directory for complete working examples of:

• PowerFactory projects with FMUs integrated
• Pre-compiled FMU files
• Configured interface connections