A System-Level Suppression Method for DC Bias Based on Reverse Unbalanced Currents in the Same Transmission Section

Abstract

Conventional dc bias suppression strategies mainly rely on a single type of device, such as blocking capacitor or blocking resistance. However, these methods are passive palliatives and cannot restrain dc bias currents from the sources. In the light of this, a system-level suppression method for dc bias is proposed in this paper. The average value of currents flowing through the earth is offset by actively generating unbalanced current through other dc projects in the same transmission section, which will reduce the currents flowing into the neutral transformers and alleviate the negative impacts of dc bias. First, the feasibility of the same section dc continuous power support is analyzed. Then the dc bias suppression strategy based on reverse unbalanced currents is elaborated. By scanning all the theoretical operable solutions of unbalanced current generation modes and the possible switching combinations of blocking capacitors, the optimal dc bias suppression solution is output considering the number of adjusted dc projects, the overload margin of dc projects and neutral dc currents of transformers. The effectiveness and superiority of the proposed method are verified by comparative case studies of the East China power grid. INDEX TERMS DcBias; System-level Suppression Method; Reverse unbalanced currents.

Mitigation Strategy of Neutral-Point DC for Transformer Caused by Metro Stray Currents

“…Using the proposed method, with thresholds of indicators set as 0, through 20,000 samplings of dynamic metro operation scenarios, the installation probabilities of optimized placement results are shown in Figure 11. Substations 2,5,6,12,13,22,23,61,62,64,67,70, and 71 need to have BDs installed, and the BD installation probability exceeds 90 % with the train operations. Among them, the probability percentage of substations 7, 26, 72, and 73 is less than 20%.…”

“…To protect the transformer and urban power grid from transformer DC bias, one widely accepted existing method is to install a blocking device (BD) in series at the transformer neutral point [9][10][11]. Since the BD consists of capacitors, once installed, the BD truly and completely stops the flow of DC from the neutral point to the transformer windings [12]. Therefore, after the transformer neutral DC exceeds the threshold, the installation of BD can completely block neutral DC and mitigate the DC bias.…”

“…The most significant constraint is that all neutral DC magnitudes are smaller than the threshold. And the objective function is to minimize the BD installation [10,12]. The research for mitigating neutral DCs caused by metro stray currents is light, and the maximum value is used to determine whether the transformer works in the DC bias state [13].…”