Fault Analysis of High-Voltage DC Transfomers for 100-MW-Scale All-DC Renewable Energy Transmission

To address the challenges posed by the integration of large-scale new energy into the power grid, this paper conducts a fault analysis of a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 0 0 M W}$</tex>-scale high-voltage direct current transformers (HDCTs) applicable to all-DC transmission renewable energy systems. Focusing on faults such as highvoltage side DC bipolar short-circuit through a smoothing reactor, high-voltage side single bridge arm short-circuit through a bridge reactor, and high-voltage side AC outlet short-circuit. The dynamic characteristics of the fault were theoretically analyzed and verified through PSCAD simulation. The main findings indicate that the protection setting value (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda$</tex>) and the blocking delay time (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\triangle t_{\text{delay }}$</tex>) directly determine the magnitude of the bridge arm sub-module blocking current. Longer delay or higher protection setting values result in larger blocking currents, which may exceed the turn-off capability of power devices (e.g., the IGBT turn-off peak current is 1.6 kA). In the case of single bridge arm short-circuit through a bridge reactor, due to the direct injection of current from the bridge arm into the fault point and complex current paths, the maximum blocking current reaches 1.972 kA (when the protection setting value is 1.2 pu and the delay is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$100 \mu \mathrm{s}$</tex>), posing the greatest challenge to the IGBT turn-off capability. It is recommended to enhance the withstand capability through valve tower structure optimization (such as layout and heat dissipation design).