Impact of shortened balancing intervals on HVDC mass-impregnated cables

Other conference contribution, 2025

According to EU Regulation 2017/2195, the time intervals for power balancing are to be reduced from 4 hrs to 15 min that leads to increasing frequency of DC polarity reversals in LCC-HVDC, which enhance the stress on insulation of high voltage cables. This work explores the effect of the shortened balancing intervals on mass-impregnated (MI) pressboards by analyzing their DC conductivity and loss tangent. The dielectric response measurements were performed using test voltages containing a small AC signal, defined by the ~1% field ripple, superimposed on a DC bias (~1.1 kV/mm). The results show that at 15 min interval, a high post-reversal conductivity peak appeared followed by a monotonic decay. At 4 hrs interval, the measured conductivity dropped abruptly and a secondary rise was seen, which is likely due to the space-charge release/re-injection and slow polarization recovery. After reversal, an increase in loss tangent was observed, followed by decaying quasi-periodic fluctuations around a value that stabilizes after about 150 s. When employing the shorter interval, the fluctuations of the magnitudes of the loss factor increased, however, without influencing the stabilization time notably. Applying a higher DC voltage magnitude increased the amplitude of fluctuations, yet resulting in a reduced steady-state loss tangent. It was further observed that the direction of the reversal had negligible effect on the steady-state value. The presented results provide information to evaluate the effect of shortened balancing intervals on HVDC mass-impregnated cables.