With the current trends in renewable energy integration, the concept of a 100% inverter-based power system is becoming more of a reality. However, the existing Voltage Source Converter (VSC) control schemes for such systems focus mostly on the operation of low-voltage microgrids, which have different requirements from the transmission system perspective. This paper proposes a new classification of VSC control strategies depending on their mode of operation. Then, the concept of partial grid forming VSC is introduced and it is shown that a system with zero rotational inertia can operate without a dedicated grid-forming VSC unit, but rather with partial forming of key system characteristics distributed across different VSC units. The performance of this approach is tested on detailed VSC models developed in both MATLAB Simulink and virtual Hardware-In-the-Loop (vHIL) platforms. Furthermore, an investigation towards necessary converter and network criteria for providing a stable system under the proposed control concepts is presented.
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