Security-Constrained AC/DC Grid Optimal Power Flow Considering Asymmetrical HVDC Grid Operation using Sparse Tableau Formulation

TL;DR

The paper tackles secure post-contingency operation of meshed HVDC grids with bipolar converters, focusing on asymmetrical operation after single-pole outages. It develops a security-constrained OPF built on Sparse Tableau Formulation (STF) that explicitly models asymmetrical converter operation and a DC neutral-line switching (NLS) action. Key contributions are the DC-STF extension for HVDC, a converter-station selection mechanism via binary variables to choose which stations run asymmetrically (with a cap $N_b$ on symmetric bipolar stations), and the NLS strategy to limit DC neutral voltage offsets; validated on a modified CIGRE B4 DC grid. Results show that increasing asymmetrical operation can reduce operational costs but may raise neutral-voltage offsets, which NLS can mitigate, offering a practical tool for offshore HVDC network planning.

Abstract

This paper presents a security-constrained optimal power flow (SCOPF) model for HVDC grids that optimizes the asymmetrical operation of bipolar converter stations, i.e., different current injections of the positive and negative converter poles, to minimize operational costs under post-contingency conditions caused by single converter pole outages. The optimization model allows the selection of the number of converter stations that operate asymmetrically. The results indicate that increasing the number of asymmetrical stations lowers operational costs. The analysis also provides insight into the sensitivity of these costs to the level of asymmetrical operation. However, increased asymmetrical operation leads to higher DC neutral voltage offsets that can rise to undesired levels. Imposing limits on these offsets can, in turn, increase operational costs. To mitigate these effects, a neutral line switching (NLS) strategy is proposed for the post-contingency state.

Figures (11)

• Figure 1: Two-port network representation of DC lines.
• Figure 2: Two-port network representation of DC switches.
• Figure 3: Schematic of a bipolar converter station with DMR.
• Figure 4: Schematic of a symmetrical monopolar converter station.
• Figure 5: Schematic of a DC-DC converter station.