Experiences from the Exploitation of High-Voltage SF6 Circuit Breakers and SF6 Gas-Insulated Switchgear (GIS) in the Power Utilities of Croatia and Serbia

Serbia. The study is based on selected empirical data collected by the authors for voltage levels of 110 kV, 220 kV, and 400 kV.

In the first part, a comparative review of acceptance testing, operation, and maintenance practices in both countries is provided. Differences in national acceptance protocols are outlined, including inspection and testing procedures performed during factory acceptance, onsite acceptance, and prior to commissioning. Operational data collected between 2020 and 2025 by the Croatian Transmission System Operator (HOPS) and the Serbian Transmission System

Operator (EMS) are analyzed, with particular focus on SF₆ leakage rates, gas losses recorded during annual maintenance, and observed equipment failures. As the majority of installed circuit breakers and GIS are still relatively new, the paper also presents early experiences related to rare maintenance interventions involving the opening of interrupting chambers, handling, control, and disposal of SF₆ gas. These maintenance activities are expected to increase significantly over the next decade as the equipment ages.

The paper further discusses greenhouse gas (GHG) emissions related to SF₆ according to the

GHG Protocol and provides an illustrative example of emissions calculation based on measured gas losses. The findings confirm that while SF₆-based equipment continues to demonstrate high operational reliability, aging infrastructure will lead to increased maintenance requirements and growing volumes of SF₆ requiring recovery and disposal.

In the second part, two representative failure cases are presented: a 110 kV SF₆ circuit breaker explosion in Serbia and a 400 kV SF₆ circuit breaker failure in Croatia caused by seismic activity. In both cases, the incidents were not related to insufficient SF₆ gas pressure; however, they demonstrate that catastrophic events can result in sudden atmospheric releases of SF₆.

The paper concludes that notable differences exist in acceptance testing and maintenance practices even between neighboring transmission systems. It emphasizes the need for TSOs to acknowledge that aging high-voltage equipment will require more frequent maintenance and eventual disposal of larger quantities of SF₆. Furthermore, the study highlights the urgency for

TSOs in EU and EU-adjacent countries to acquire practical experience with SF₆-free and alternative-gas technologies. This is particularly relevant in light of Regulation (EU) 2024/573, which restricts the future use of SF₆, and underscores the importance of timely procurement and deployment of high-voltage equipment with alternative gases to ensure regulatory compliance and long-term sustainability.