Analysis of reliability of Nordic HVDC interconnectors based on DISTAC data from 2016-2022

Commutated Converters (LCC) based on operational data reported by the Nordic and Baltic

Transmission System Operators (TSOs) to the Disturbance Statistics and Classification

(DISTAC), a subgroup of the European Network of Transmission System Operators for

Electricity (ENTSO-E). Using data from 2016-2022, the study focuses on converter station reliability, excluding disturbances related to cables, transmission lines, external alternating current (AC) grids and control centres. This approach provides a clearer insight of the converter station alone. The analysis is based on statistical comparison of outage data, combined with operational experience and technical expertise of the authors.

The operational statistics from DISTAC was chosen as the basis for this analysis due the possibility of extracting higher level of detail from this dataset, compared to other sources such as the surveys of HVDC reliability published by CIGRE [1]. This is mainly due to the authors' access to the complete dataset reported to DISTAC, as well as their in-depth knowledge of all systems included in the DISTAC reporting. However, where comparisons are possible, the

CIGRE statistics has been used to compare and benchmark results from the DISTAC data analysis. Both datasets do however have certain limitations, which are assessed where relevant.

The analysis examines overall converter performance to reveal an indication of lower availability in VSC systems than in LCC systems. For the two technologies, comparisons are made with respect to occurrence of maintenance and disturbance outages, as well as the duration of each outage. While the frequency of events is relatively similar for VSC and LCC systems, outage duration is significantly longer for VSC systems, prompting further investigation into fault categories and their location within VSC systems.

The findings confirm a notable difference in reliability between LCC and VSC systems. The paper provides a discussion of possible reasons for this, including a general higher complexity in VSC systems, less experience with this more recent technology, and some technical aspects especially influencing outage duration. Furthermore, the paper also highlights the importance of detailed and standardized data collection to DISTAC and CIGRE, to improve reliability assessments and support future studies.