How Do Grid Simulator Approximate Real-World Grid Impedance with FPGA-Based RLC Modeling?

Real-world power grid panorama is never static yet suffer from shifting and complex electrical conditions due to its frequency-dependent and time-varying system property influenced by network topology, loading conditions and DER interactions. An actual grid’s impedance is thus far more complex than a fixed lumped parameter that proposes, in turn, technically more challenges for laboratory testing scenarios where controlled impedance is required for reproducible evaluation.

Fig 1. Grid Impedance Modelling with ActionPower Grid Simulator – Credit: ActionPower

This article will explain from an engineering perspective about how grid simulators approximate real-world conditions of grid impedance through software and FPGA-based hardware for purpose of laboratory validation use cases, which streamlines pre-certification testing for those grid-connected products. ActionPower employs high-performance FPGA platforms to deliver fast, high-precision grid waveform synthesis, supporting up to 750 kVA per unit and 10MVA by parallation with our TITAN High-Power Regenerative Grid Simulator and enabling accurate emulation of grid faults such as voltage sags, flicker, and short interruptions for megawatt testing scenarios such as PV, wind, and energy storage converter validation.

Introduction to Grid Impedance Modelling with FPAG-Based RLC Circuit

Something to underline in the very first place is that the impedance from a grid simulator’s output bus with RLC modelling is NOT of in nature a 100% similar and fully-mathching complexity and pattern as that of real-world conditions, however, an approach through artificial circuit and control methods to approximate realistic grid impedance. Note that we use the verb “approximate” here instead of “replicate” or “simulate”. It is rather, a deliberately constructed circuit with equivalent and controllable network.

As a source-side impedance model, grid simulator defines its effective output impedance \(Z(\omega)\), short-circuit stiffness, voltage sag behaviour, current–voltage coupling, and dynamic response under disturbances.

From a mathematical perspective, it is a frequency-domain abstraction expressed as below:

$$ Z(\omega) = R + j \omega L + \frac{1}{J\omega C} \qquad(1) $$

One fact further to take not is that this model represents a local and source-side impedance abstraction for testing purposes only, rather than and far from the distributed, time-varying, system-level impedance of a real power grid.

In modern grid simulators, the RLC model is implemented digitally on an FPGA platform, where real-time current feedback is used to enforce a programmable source-side impedance through voltage reference synthesis. A detailed introduction to what is FPGA (Field Programmable Gate Array) technology can be found in this article. [1]

Fig 2. FPGA (Field Programmable Gate Array) Circuit Example

Engineering Implication of RLC Modelling: \(Z(\omega)\) Fundamentals and Real-World Approximation

This section focuses on the implications of and engineering-abstraction carried by the replicated impedance through RLC modelling compared to that of real-world power grid scenario. As we review the previously stated definition that “real-world impedance is frequency-dependent and time-varying system property influenced by network topology, loading conditions and DER interactions/Point of Common Coupling“, we draw and proceed with a mathematic definition as below for the same:

which is more accurately but far more complex compared with the in-lab simulation approach defined by the below equation as per (1):

$$ Z(\omega) = R + j \omega L + \frac{1}{J\omega C} \qquad(1) $$

The implication drawn from such inherent complexity and difference are that the purpose of RLC modelling in a grid simulator is therefore not to replicate a specific physical grid, but to provide an engineering abstraction that captures the dominant impedance characteristics relevant within a defined test bandwidth and application scope. From an engineering perspective, the RLC elements carry explicit physical interpretations as below which maps directly to grid strength, damping, and dynamic coupling seen at the PCC (Point of Common Coupling):

Table 1. RLC Implications and Test Impacts

When it is implemented on an FPGA-based control platform, the RLC is digitally controlled through real-time changing impedance enforcement \(Z_\text{sim}(\omega)\) instead of being passive components, with the measured output current \( I_\text{out}(\omega) \) processed within the FPGA to dynamically synthesized \(V_\text{ref} (\omega)\) for predefining frequency-dependent output impedance, expressed by equation as below (derived from Thevenin’s Equivalent Circuit) [2]:

This approach enables high precision, fast dynamic response, and deterministic behaviour under transient conditions, while maintaining repeatability and stability required for standardized testing – ideally for converter validation, weak-grid assessment, and compliance-oriented evaluation.

Applications and Use Cases with Simulated Grid Impedance in Testing Scenarios

By integrating programmable grid impedance into testing workflows, grid simulators enable realistic validation of converter behaviour under non-ideal grid conditions that cannot be revealed with stiff, low-impedance voltage sources.

Weak-Grid and Low-SCR Validation

Simulated grid impedance allows accurate emulation of weak-grid environments, enabling early identification of stability risks, control interaction issues, and oscillatory behaviour in PV, wind, and energy storage converters before field deployment.

SCR and Grid Robustness Assessment

Through digital adjustment of resistive and inductive parameters, engineers can evaluate converter robustness across a wide range of grid strengths and damping conditions, supporting standardized benchmarking and design margin verification.

Grid Fault and Ride-Through Testing

When combined with programmable fault waveforms, simulated impedance ensures that voltage sags, short interruptions, and flicker are reproduced with realistic source behaviour, enabling reliable LVRT/HVRT validation and grid-code compliance testing.

Harmonic and Resonance Analysis

Configurable impedance characteristics support targeted evaluation of harmonic interaction and resonance risks, particularly for systems with LCL filters or high-bandwidth current control, improving confidence in grid compatibility.

PHIL and Advanced System Validation

In PHIL (Power Hardware-in-the-Loop) configurations, simulated grid impedance provides a stable and repeatable physical interface between real-time grid models and power hardware, enhancing test reliability while reducing integration risk.

Discover ActionPower’s Grid Emulation Testing Solutions

Simulated grid impedance transforms abstract grid strength and stability challenges into practical, repeatable test conditions, enabling faster development cycles, reduced field risk, and higher confidence in grid-connected power electronics.

Building on this foundation, ActionPower’s grid emulation testing solutions combine FPGA-based control, high-power scalability, and Programmable impedance modeling to deliver realistic, deterministic grid conditions for PV, wind, and energy storage converter validation across a wide range of operating scenarios. Contact us to explore how advanced grid emulation can enhance the reliability and readiness of your grid-connected power electronics.

Bibliography

• [1] What is a field programmable gate array (FPGA)? IBM Think, by Josh Schneider and Ian Smalley, Link
• [2] How to Use Thevenin’s Theorem, All About Circuits, Link
• [3] Gul, U., Raza Ur Rehman, H.M., Gul, M.J. et al. Enhanced FPGA-based smart power grid simulation using Heun and Piecewise analytic method. Sci Rep 15, 32996 (2025). https://doi.org/10.1038/s41598-025-18105-8, Link