Assessment of Real and Emulated Grid-Impedance Ratio in PHIL Testing of Grid-Connected Converters

Power-Hardware-in-the-Loop (PHIL) offers a flexible environment for the dynamic validation of power electronic systems under realistic conditions. The interaction between the real hardware and the emulated network is strongly affected by the properties of the signal paths, such as inherent latencies and frequency-dependent attenuation introduced by digital and analog filtering. While certain system components (e.g. lines, transformers) are fully emulated, a defined proportion can also be introduced on the hardware side - e.g. via passive components. The aim of this work is to systematically investigate how the ratio of real to emulated impedance affects the stability, reproducibility and significance of the overall system, especially for transient processes. Such investigations serve to assess the suitability and capability of the PHIL setup itself, thereby ensuring the validity and reliability of subsequent analyses.

The methodology for investigation is as follows: A basic grid model with a voltage source behind an impedance is implemented in the PHIL system. Two fundamental operating scenarios are then analysed with the PHIL system is connected to a real converter with:

1) voltage source behaviour (e.g. typical grid-forming operation)

2) current source behaviour (e.g. feed-in by grid-following operation)

The overall setup allows the targeted variation of the hardware to software impedance ratio from 0 % hardware and 100 % emulated impedance to 100 % hardware and 0 % simulation in discrete increments. While keeping the impedance between the two sources consistent. Frequency-dependent effects, oscillation tendency, damping, possible instabilities and feedback loop parameters (e.g. low pass filtering of current signals) are analysed using transient test sequences. Disturbances such as gird voltage phase and amplitude jumps are applied.

Observations show that certain configurations - especially in voltage source operation with a high level of impedance emulation - can lead to oscillatory behaviour or even system instability. However, even low real impedance components in the 10-20 % range have a stabilising effect. Converters with current source behaviour are more robust against completely virtual network configurations.

The work provides a methodological basis for the design of stable PHIL test setups and aims to provide a well-founded assessment of the extent to which physical hardware can be replaced by simulation. The results are directly applicable to the validation of grid-forming and grid-following converters, emergency power supply scenarios and regulatory relevant test setups.