Analysis of Common Problems in 10kV Gas-Insulated Ring Main Units (RMUs)
Introduction:
10kV gas-insulated RMUs are widely used due to their numerous advantages, such as being fully enclosed, possessing high insulation performance, requiring no maintenance, having a compact size, and offering flexible and convenient installation. At this stage, they have gradually become a critical node in urban distribution network ring-main power supply and play a significant role in the power distribution system. Problems within gas-insulated RMUs can severely impact the entire distribution network. To ensure power supply reliability, it is essential to seriously address the issues present in 10kV gas-insulated RMUs and implement appropriate measures for resolution, thereby enhancing power supply reliability.
I. Introduction to 10kV Gas-Insulated Multi-Compartment RMUs
The 10kV gas-insulated RMU series (often called: multi-compartment RMU, fully insulated cabinet) features compact mechanical structure, small volume, light weight, all-weather capability, excellent expandability, lightweight and flexible modular installation, easy maintenance without disassembly, and simple upkeep. Their technical parameters are reasonably divided based on design and functional features, and they can be broadly categorized by size into electrical cabinet types such as High-Voltage Non-Load Switching Units, Circuit Breaker Units, Load Switch-Fuse Combination Units, Cable Connection Units, and High-Voltage Metering Cabinets.
The term "multi-compartment RMU" primarily refers to its direct use of harmful gas media like SF₆ as the main electrical insulation and contact medium. This endows them with significantly superior electrical insulation and protective performance compared to ordinary air-insulated cable cabinets. The electrical clearance in fully insulated cabinets is also generally narrower, often much narrower, than in equivalent semi-insulated cabinets at the same voltage rating. Consequently, they occupy less space. The switch housing and all live copper connection components are permanently sealed within a stainless-steel shell filled with SF₆ gas, maintaining constant pressure unaffected by altitude. They are also suitable for basements and other relatively humid, cold environments. Precisely because of these characteristics, gas-insulated RMUs are extensively used in outdoor cable branch boxes and prefabricated substations.
II. Common Problems in 10kV Gas-Insulated Multi-Compartment RMUs
Currently, 10kV gas-insulated RMUs constitute a higher proportion in urban distribution networks than other types of RMUs. Regionally, with the exception of a few early-installed semi-insulated cabinets, all outdoor switching stations commissioned after 2007 are gas-insulated RMUs. Their vital role is self-evident, as their safe and reliable operation directly affects the reliability of power supply to users. Common operational and maintenance issues encountered with 10kV gas-insulated RMUs include problems with RMU bushings and cable connections, impacts from humid operating environments on the RMU, and SF₆ air tank failures.
2.1 Bushing and Cable Connection Problems
The 10kV multi-compartment RMU design originated in the US and Europe, where single-core cables are predominantly used. Consequently, the cable compartments of gas-insulated RMUs are relatively low. Single-core cables facilitate easier fixing and installation, and their terminations closely match the RMU bushing terminals, ensuring no thermal overload faults occur and providing resistance to torsional stress on the bushings.
In China, three-core cables are widely used. Compared to single-core cables, installing three-core cables is significantly more complex. Typically, clamps or bolts are used to secure the outer cable sheath, making it difficult to simultaneously ensure absolute fixation of all three cores. Due to their generally shorter and thicker diameter, even when the three-core cable itself is securely connected and fixed, torsional moments generated under the cable's own weight or any applied mechanical force are directly transmitted to the cable bushings. Analysis of recent RMU failures in some regions indicates that faults caused by inadequate connections between RMU bushings and cables are not uncommon.
2.2 Problems in Humid Environments
Although 10kV gas-insulated RMUs are designed for all-weather operation, prolonged exposure to humid environments, especially in coastal areas, can still lead to corrosion of the cabinet body and other metal components. If the sealing plates on the RMU cover panel or at the cable trench entry point are not tightly secured, moisture from the cable trench easily infiltrates the RMU cover and interior. This can lead to significant water evaporation forming condensation. This condensation is highly prone to causing faults in indicators, voltage indicators, and internal operating and control mechanisms. These components and the panel itself easily become damp, rusted, blocked, or corroded. Moist air can directly enter the RMU interior, causing chemical corrosion and rusting of the cabinet body and baseplate, drastically shortening the RMU's service life [1].
2.3 Air Tank Failures
The main cause of SF₆ switch air tank short-circuit failures is the sudden leakage of gas within the sealed SF₆ tank cavity due to various physical reasons. This compromises the integrity and density of the SF₆ gas seal, ultimately resulting in a short-circuit failure between the moving and fixed contacts under normal operating conditions, leading to an accident.
Leakage points are mostly found on cable terminals (cable ends inside the tank). This is primarily due to non-standardized cable installation, causing excessive stress on the terminals and leading to cracks at the junction between the air tank and the cable terminal, resulting in SF₆ gas leakage. Thus, air tank failures also reflect poor construction practices. Secondly, inherent manufacturing process issues in some RMUs can cause inadequate sealing in certain areas, leading to leakage.
2.4 Cable Termination Faults
Currently, one of the most common causes of failure in 10kV gas-insulated RMUs is the external flashover of cable terminations, caused by poor workmanship or substandard materials during their fabrication. This leads to RMU failure.
III. Countermeasures and Preventive Measures
The aforementioned problems can be addressed and prevented through the following approaches:
3.1 Increase the Height of the 10kV RMU Cable Compartment
To tackle the issue of narrow, low-height cable compartments in multi-compartment RMUs, most manufacturers opt to increase the compartment height. When a European brand entered the Chinese market, it adapted by changing the terminal block layout from stepped to horizontal, following Chinese preferences, thus improving cable compartment space utilization. During installation, the indoor RMU can be elevated using an appropriately sized box-type base. For outdoor cable branch boxes, the foundation height can be raised.
3.2 Employ Natural Ventilation or Install Dehumidification Devices
Currently, a direct, effective, and reliable method to prevent harmful moisture ingress is to completely seal the RMU cable compartment floor using sealed fireproof structural materials. This effectively blocks outdoor humid air, prevents small animal intrusion, and helps prevent vegetation growth – achieving multiple benefits. Indoor RMUs must utilize dedicated dehumidification equipment or automatic humidity control systems. Outdoor cable branch boxes can have their foundations raised and an appropriate number of ventilation/heat dissipation openings arranged around the bottom to facilitate moisture dispersion from cable trenches/pits. However, openings should not be excessive, and measures to prevent small animal hazards must be considered. Additionally, if a cable branch box contains a Potential Transformer (PT) and ventilation allows, a small air conditioning dehumidifier can be installed.
3.3 Strengthen Operation and Maintenance Management
SF₆ tank failures can cause major accidents. Therefore, selecting devices with stable quality is crucial, and management during construction must be strengthened. Relevant technical specifications from China Southern Power Grid explicitly require that air tanks be made of stainless steel >2mm thick, capable of withstanding normal and transient pressures encountered during use, and equipped with SF₆ pressure gauges and filling ports. Operators must regularly monitor tank pressure during routine maintenance. Especially before switching operations, confirming normal internal pressure is vital to avoid accidents due to leakage. Any detected leaks must be addressed promptly.
3.4 Ensure Strict Control of Cable Termination Fabrication and Daily Management
To prevent RMU failures caused by defective cable terminations, it is essential to carefully select qualified cable terminations meeting quality requirements. Installation personnel must strictly adhere to national standards during termination fabrication. Inspectors must rigorously enforce quality acceptance checks to prevent substandard terminations from being put into service. Furthermore, operation and maintenance units must diligently maintain cable termination records and categorize/analyze terminations exhibiting potential problems encountered during routine operation. This ensures timely identification of family defects and helps avoid the recurrence of similar quality issues and accidents [2].
IV. Conclusion
Due to their numerous advantages, 10kV gas-insulated RMUs have progressively become a critical node in the ring-main power supply of urban distribution networks, playing a vital role in the power distribution system. This article illustrates several prevalent problems in 10kV gas-insulated multi-compartment RMUs and proposes corresponding solutions and preventive measures. The goal is to reduce the failure rate of 10kV gas-insulated RMUs and enhance power supply reliability. Having evolved over many years, 10kV gas-insulated RMUs are now the primary nodes in urban ring-main distribution networks. Their safe and reliable operation directly impacts the reliability of power supply provided by electric utilities. In actual operation, various fault types can occur in 10kV gas-insulated RMUs due to environmental factors or quality issues, leading to a decline in power supply reliability.
