10kV SF6 Ring Main Units: Structure, Faults & Cable Termination Quality

1. Introduction to 10kV SF6 Ring Main Units (RMUs)

A 10kV SF6 ring main unit typically consists of three main parts: the gas compartment (tank), the operating mechanism compartment, and the cable connection compartment.

• The gas compartment is the core component of the RMU. It is filled with SF6 gas and houses critical elements such as the load switch, busbars, and switch shaft. The load switch features a three-position design—comprising closing, opening, and grounding functions—and is primarily constructed with a blade switch and arc extinguishing chamber, utilizing SF6 gas to achieve excellent insulation and arc-quenching performance.

• Inside the operating mechanism compartment, the operating mechanism is connected via a switch shaft to both the load switch and the earthing switch. Operators insert a manual operating rod into the operating hole to perform closing, opening, and grounding operations. Since the switch contacts are enclosed within the sealed gas tank and not visible, a position indicator directly linked to the switch shaft is provided on the operating mechanism, clearly showing the current status of the load switch and earthing switch. Mechanical interlocks are installed between the load switch, earthing switch, and front cover to satisfy the "five-prevention" requirements, ensuring operational safety.

• The cable connection compartment is located at the front of the RMU, facilitating cable connection. The connection between the cable and the RMU's insulating bushing can use either touchable or non-touchable silicone rubber cable accessories, accommodating different safety requirements in various operating environments.

2. Analysis of Two Fault Incidents

2.1 SF6 Gas Leakage Fault

At 21:47 on March 31, 2015, a 10kV line experienced a fault outage. During inspection along the line, smoke was observed emanating from the Yangmeikeng RMU. Upon opening the cabinet door, it was found that the terminal post of switch #2 had fractured and the gas tank was leaking. Further inspection after removing the elbow connector revealed that the double-headed bolt used to install the bushing was misaligned with the center of the lug hole, causing the bushing to be subjected to continuous downward tension from the cable. This led to cracking at the upper end of the bushing base, resulting in SF6 gas leakage. This type of RMU (model: GAK4, manufacturer: Shenzhen Minyuanshun, i.e., Ormazabal) has experienced similar failures multiple times, indicating a familial design or manufacturing defect.

Such faults commonly occur at the cable terminal post. The main causes include improper cable installation leading to long-term mechanical stress on the terminal post, or inherent manufacturing issues in the RMU itself—such as inadequate sealing at certain points—both of which can lead to SF6 gas leakage.

2.2 Cable Terminal Fault in RMU

In December 2014, during routine patrol, blackening was observed on the cabinet door of a 10kV RMU, suggesting possible electrical discharge. The RMU was a four-compartment unit, with the fourth compartment unused and kept as a spare. After power shutdown and cabinet inspection, obvious signs of discharge were found in the second and third compartments. In the second compartment, phase C showed clear evidence of discharge from the stress cone to the cabinet body.

The stress cone had been installed too low, entirely positioned below the semiconducting layer cut-back point of the cable. Its lower end failed to overlap with the semiconducting cut-back, and its upper end did not contact the inner semiconducting layer of the elbow connector. This resulted in electric field concentration at the upper edge of the stress cone, leading to insulation breakdown over time and subsequent discharge to the cabinet wall. In the third compartment, phase B’s elbow connector showed visible signs of arcing damage.

Upon disassembly, it was found that the terminal lug used was designed for outdoor applications, not the original specified type. Due to dimensional differences, the outdoor-type lug had a smaller inner diameter, preventing it from fully seating onto the bottom of the terminal stud. To compensate, a washer was improperly added between the lug and the bushing conductor, resulting in poor contact, increased resistance, and overheating. Additionally, the elbow connector used in this compartment was oversized and mismatched with the stress cone, failing to tightly seal the cable termination. This compromised the full insulation integrity of the RMU, allowing moisture to condense on the surface of the cable insulation and support insulators, reducing insulation performance and creating tracking paths.

In conclusion, the quality of cable termination fabrication and the connection between the cable and the RMU are critically important. Given the compact structure and limited internal space of RMUs, high precision in cable joint workmanship is required. Improper handling of the conductor, shield, or semiconducting layer—leading to insufficient creepage distance—can easily result in insulation failure. Strict quality control during cable termination installation is essential to prevent faults at the source and reduce the likelihood of outages.