Polarity reversal stress simulation in high voltage direct current mass-impregnated submarine cables: Simulating electric field during polarity reversal in high voltage direct current mass-impregnated submarine cables
Ristiniemi, Jussi (2024)
2024
Sähkötekniikan DI-ohjelma - Master's Programme in Electrical Engineering
Informaatioteknologian ja viestinnän tiedekunta - Faculty of Information Technology and Communication Sciences
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Hyväksymispäivämäärä
2024-11-26
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-2024111910280
https://urn.fi/URN:NBN:fi:tuni-2024111910280
Tiivistelmä
High voltage direct current (HVDC) interconnections have an important role in the electricity transmission systems and electricity markets. They help maintain the security of supply of electricity during planned and unplanned outages, and they make the electricity market more efficient by connecting non-synchronized power grids and areas of power grids separated by sea, facilitating better the power transmission from the areas of power surplus to the areas of power deficit. The introduction of the 15-minute market time unit in the common European electricity market in March 2025 could possibly increase the number of power flow reversals in the HVDC links, thus the number of polarity reversals in the line commutated converter (LCC) high voltage direct current submarine cable systems could increase. It is generally accepted that polarity reversals create electric field enhancement in the cable insulation, especially near the conductor. For this reason, the electric stress induced by polarity reversals must be assessed in order to safely operate the submarine mass-impregnated non-draining cables according to the needs of the electricity market.
This thesis was done for Fingrid Oyj, which is the Finnish transmission system operator. Fingrid operates and maintains not only the overhead transmission lines and substations in Finland, but also the HVDC interconnections to the neighboring countries in cooperation with the relevant counterparties. The aim of this thesis was to evaluate the difference in the electric stress at the conductor at a polarity reversal in a mass-impregnated cable with different time intervals between the polarity reversals. Electric field distribution in the mass-impregnated insulation was simulated with a COMSOL Multiphysics model. The model simulated the macroscopic electric field, temperature and internal pressure distribution in the cable insulation. The simulations were performed with different load scenarios and time intervals between the polarity reversals. The insulation ageing caused by the electric stress at the conductor was evaluated with inverse power model (IPM) that is widely used in the designing of high voltage power transmission systems.
The simulations yielded similar results regardless of the different load scenarios used in the simulations. In four out of four load scenarios, shortening the polarity reversal time interval decreased the electric stress at the conductor. Additionally, in three out of four scenarios, shortening the polarity reversal time interval decreased also the maximum electric field magnitude at the polarity reversals.
Based on the results of the simulations in this thesis, in which only the macroscopic electric field at the conductor is considered, there is no need to avoid polarity reversals with 15-minute interval. However, it must be noted that the number of reversals per time unit should be kept at a reasonable level. If total number of reversals per time unit increases after the introduction of the 15-minute market time unit, accelerates the ageing of the insulation because of the high partial discharge activity caused by the fast-changing voltage during a polarity reversal. However, evaluation of the ageing caused by these partial discharges was outside of the scope of this thesis.
This thesis was done for Fingrid Oyj, which is the Finnish transmission system operator. Fingrid operates and maintains not only the overhead transmission lines and substations in Finland, but also the HVDC interconnections to the neighboring countries in cooperation with the relevant counterparties. The aim of this thesis was to evaluate the difference in the electric stress at the conductor at a polarity reversal in a mass-impregnated cable with different time intervals between the polarity reversals. Electric field distribution in the mass-impregnated insulation was simulated with a COMSOL Multiphysics model. The model simulated the macroscopic electric field, temperature and internal pressure distribution in the cable insulation. The simulations were performed with different load scenarios and time intervals between the polarity reversals. The insulation ageing caused by the electric stress at the conductor was evaluated with inverse power model (IPM) that is widely used in the designing of high voltage power transmission systems.
The simulations yielded similar results regardless of the different load scenarios used in the simulations. In four out of four load scenarios, shortening the polarity reversal time interval decreased the electric stress at the conductor. Additionally, in three out of four scenarios, shortening the polarity reversal time interval decreased also the maximum electric field magnitude at the polarity reversals.
Based on the results of the simulations in this thesis, in which only the macroscopic electric field at the conductor is considered, there is no need to avoid polarity reversals with 15-minute interval. However, it must be noted that the number of reversals per time unit should be kept at a reasonable level. If total number of reversals per time unit increases after the introduction of the 15-minute market time unit, accelerates the ageing of the insulation because of the high partial discharge activity caused by the fast-changing voltage during a polarity reversal. However, evaluation of the ageing caused by these partial discharges was outside of the scope of this thesis.