Offshore wind power has been attracting significant attention toward achieving carbon neutrality by 2050 in Japan. While offshore wind farms (OWFs) allow for large-scale integration, they are often located far from load centers. high-voltage DC transmission (HVDC) systems using submarine cables are therefore expected to be a viable solution for long-distance, high-power transmission because they do not generate reactive power. Multi-terminal HVDC systems are being considered as a means to efficiently distribute generated power to multiple load centers, and the effectiveness of power sharing between AC grids interconnected via the HVDC system has also been demonstrated.

This paper investigates a method for the rapid black start of an AC grid with small demand (small-scale grid) by supplying power from an AC grid with large demand (large-scale grid) in a three-terminal HVDC system connected to an OWF. constant voltage constant frequency (CVCF) control is applied to the onshore terminal connected to the small-scale grid. During a black start, the grid voltage must be ramped up from zero to the rated value; however, this process can generate transformer inrush current within the small-scale grid, potentially leading to an overload condition of the voltage-sourced converter (VSC) of the onshore terminal. This study aims to clarify the effects of the voltage ramp rate (dV/dt) and the short-circuit ratio (SCR) measured from the onshore terminal on the overcurrent of the VSC caused by this inrush current through numerical simulations.

Simulation studies were conducted using PSCAD v5.0, where a model was developed by connecting a transformer and a resistive load to the AC bus of the onshore terminal for the small-scale grid. The VSC AC current was evaluated in per-unit (pu) values based on a rated power of 500 MW per pole within a bipolar HVDC configuration and a rated AC voltage of 250 kV; accordingly, 1.0 pu corresponds to the total rated operation of 1000 MW for the bipolar system. Initially, the threshold for the allowable ramp-up rate (dV/dt) was identified by fixing the SCR at the point of common coupling to 10—equivalent to a transmission line impedance of 0.1 pu on a 500 MVA base—to ensure that the VSC current limit was not exceeded. Subsequently, a sensitivity analysis was performed by varying the SCR while maintaining a constant ramp-up rate near the identified threshold, aiming to investigate the specific impact of grid conditions on the allowable dV/dt.

Simulation results confirmed that a fast dV/dt led to VSC overload due to transformer inrush currents. It was demonstrated that this effect becomes more pronounced as SCR decreases. Based on these findings, the allowable ramp-up rate to avoid VSC overcurrent was quantified as a function of SCR. These results are useful as soft-start design guidelines for CVCF control in VSC-HVDC-based black starts. Future work will investigate appropriate voltage ramp-up methods for CVCF control in weak grids with an SCR of 10 or less, considering load conditions where induction motors (dynamic loads) and resistor-inductor pairs (static loads) are connected in parallel.