## Introduction

A small-scale model of an actual AC cable-connected offshore wind power plant (PP) is proposed to compare different stability analysis methods and strategies for mitigating instability in converter-based power systems. This post provides a more detailed description of the benchmark system proposed by CIGRE WG C4.49. The system features power generation units (PGUs) connected to an AC grid via an extensive offshore 66-kV array cable system, offshore step-up transformers, both offshore and onshore HVAC transmission cables, and an onshore step-up transformer.

The primary goal of this benchmark power system is to offer a reference model where interactions between converters, as well as between converters and the grid, can be studied. It is designed to support small-scale, easy-to-model system studies and serve as a foundation for evaluating various instability mitigation methods introduced in CIGRE 928 technical brochure.

The benchmark system includes either aggregated converters or a collection of individual converters linked by a complex MV cable network. The model is formulated in the dq-reference frame to facilitate stability analysis using both impedance and modal approaches, allowing for a direct comparison of their results.

## Study cases

### Case 1: Aggregated grid following converters connected to a Thevenin equivalent

The transmission system is modeled as a long HVAC cable connected to a simplified Thevenin grid equivalent. The 420 MW power plant is represented by two aggregated power generation units (PGUs) of 180 MW and 240 MW.

The parameters for this aggregated system were derived from a detailed system representation, ensuring that the dynamics of the converters and their interaction with the grid remain consistent between the two. While the original benchmark configuration falls well within the capabilities of the modal and impedance analysis techniques discussed in CIGRE technical brochure 928, a simplified reduced-order model was developed to make the benchmark more accessible for researchers exploring new stability analysis methods, as well as to present results more concisely and clearly.

In this reduced version, each group of 5 PGUs, totaling 20 or 15 PGUs per group, is simplified into an equivalent cable segment and a single PGU. While the reduced model aligns well with the detailed model in the low-frequency range, significant deviations appear at higher frequencies, particularly beyond 500 Hz, as the Nyquist frequency of 1475 Hz, associated with the PGU control system, is approached. Consequently, the reduced-order model is not recommended for studying high-frequency resonance phenomena, but it is suitable for analyzing control interactions in the lower frequency range.

### Case 2: Aggregated grid forming converters connected to a Thevenin equivalent

The transmission system is modeled as a long HVAC cable connected to a simplified Thévenin grid equivalent. The 420 MW power plant is represented by two aggregated power generation units of 180 MW and 240 MW. In this scenario, the converters operate in grid-forming control mode.

## Download benchmark system

### In Matlab

### In PSCAD

### In PowerFactory

To be ready soon...

## References

[1] Ł. Kocewiak, Ch. Buchhagen, R. Blasco-gimenez, J. B. Kwon, M. Larsson, Y. Sun, X. Wang et al., “Multi-frequency stability of converter-based modern power systems,” *Technical Brochure 928*, Page(s) 1-147, CIGRE, March 2024.

[2] Ł. Kocewiak, R. Blasco-Gimenez, C. Buchhagen, J. B. Kwon, M. Larsson, Y. Sun, X. Wang, “Practical Aspects of Small-signal Stability Analysis and Instability Mitigation,” in *Proc. The 21 ^{st} Wind & Solar Integration Workshop*, 12-14 October 2022, The Hauge, The Netherlands.