Mga Isyu ug mga Kontra-Medida alang sa 10kV SF₆ Gas-Insulated Common Tank Ring Main Unit (European-Style) Cable Connections

Mga Isyu ug mga Kontra-Medida para sa 10kV SF₆ Gas-Insulated Common Tank Ring Main Unit (European-Style) Cable Connections​

Human sa ekstensibong paggamit sa cable lines sa urban distribution networks, ang 10kV SF₆ gas-insulated common tank Ring Main Units (RMUs) (European-style) gigamit kas network nodes tungod sa ilang karakteristikas nga full insulation, complete enclosure, maintenance-free operation, compact size, ug flexible installation. Ang mga European-style SF₆ common tank RMUs ang makapugos sa coastal areas nga may adlawon ug asinon nga environment ug naghatag og mataas nga operational reliability.

Ang mga recent operational failures sa RMUs nagsulti nga ang daghang isyu gikan sa mga problema sa connection points tali sa RMU bushings ug 10kV cables. Kini labi na importante sa indoor ug outdoor RMUs nga nag-handle sa dako nga current ug large-section cables. Kapag may failure, ang buong RMU kinahanglan i-de-energize ug i-replace, ug ang iyang cable T-body connector kinahanglan i-re-install. Kini nagpadayon sa significant impact sa power supply reliability ug nagresulta sa substantial economic losses.

Ang connection tali sa RMU bushings ug 10kV cables usa ka critical operational weak point. Ang maong article nagsusi sa existing problems ug nagproposa og mga kontra-medida.

​1. Mga Isyu sa Common Tank RMUs ug Three-Core Cable Connections​

Karon, ang 10kV SF₆ common tank RMUs (European-style) ug ang ilang associated cable T-body connectors dominante nga European brands. Kini primary designed para sa single-core cables, mas sayon mapirmahan ug install, walay torsional torque sa bushings, sigurado ang good contact tali sa terminal ug bushing, ug gitawag sa pagbawas sa likelihood sa thermal faults. Sa kabalaka, ang installation sa three-core cables labi na komplikado, resulta mao ang daghan kaayo nga issues wala sa single-core installations:

1. ​Three-core cable fixing point is the outer sheath:​​ Ang individual phases wala mahimo pirmahan independent. Kahit ania pa ang connection, ang weight sa cable o external forces mahimo mogamit og torsional torque sa bushing sections.
2. ​Phase sequence alignment requires torque:​​ Sa panahon sa installation sa three-core cables, ang phase sequence alignment karanihon magkinahanglan og torque bago matumanan. Human sa installation, ang internal stress gikan sa twisting gradual release, generating a restoring torque nga moact sa bushings.
3. ​Limited cable chamber height:​​ Ang compact cable chamber height sa RMUs (designed for single-core cables) limita ang available length sa each individual cable core phase.
4. ​Limited adjustment after termination:​​ Pagkahuman ang cable lug crimp, ang installation length fixed. Ug ang shorter individual core lengths (due to space constraints) wala easy to bend, forcing the T-body connector into position often requires applying excessive pushing, pulling, or levering forces. Kini risks damaging bushings or causing poor contact.

​2. Mga Kontra-Medida​

Arangkada sa pag-address sa mga isyu, ang mga kontra-medida mahimong implementar bahin sa RMU itself, ang T-body connectors, installation practices, ug ang civil foundation sa RMU.

​2.1 Ring Main Unit (RMU)​​

2.1.1 ​Adequately Increase Cable Chamber Height:​​
Ang SF₆ common tank RMU cable chambers typical small (approx. H: 600mm, W: 350mm). Kini suitable for single-core cables pero nagresulta sa difficulty sa installing T-body connectors, especially on large-section cables (240mm² or 300mm²), for three-core cables. Ang trifurcating sleeve sa T-body connector usab ang nanginahanglan og espasyo, nag-iwan lamang ~400mm for cable cores. Ang large-section cores stiff, ug combined with site constraints, achieving correct T-body positioning is challenging.

• ​Solution:​​ Although common tank RMUs are standardized, installation height can be increased using an extension base. Elevating the chamber height to ~800mm and ensuring the cable clamp's vertical distance from the HV bushing center point is ≥750mm allows core lengths of ~600mm. This facilitates correct T-body installation. Essentially, the extension base lengthens the separated single-phase cores after the three-core cable split, enabling connection similar to single-core cables.
• ​Benefits:​​ (1) Significantly reduces torsional torque on bushings; (2) Increases installation tolerance, minimizing need for force; lowers gas leakage risk; (3) Facilitates correct positioning of lugs and stress cones.

2.1.2 ​Consider Bushing Conductivity During RMU Selection:​​
Standard 630A RMUs often have bolt-type bushings with an outer copper tube diameter of 25mm and threaded inner hole for M16 bolts (conductive area ~289.6mm²). Actual contact area is often smaller due to fit tolerances. When stainless steel bolts are used (due to soft copper), conduction relies only on this end contact. Inside the sealed insulation, heat dissipation is poor. If the lug-to-bushing contact is poor under high currents (>400A), thermal faults occur.

• ​Solution:​​ For RMUs using 240mm² or 300mm² cables running >400A, select models with 800A-rated bushings (outer copper tube Ø 32mm) to reduce thermal fault risk.

2.1.3 ​Enhance RMU Bushing Temperature Monitoring:​​
Sealed common tank RMUs cannot be opened for inspection. Standard IR thermography cannot measure joint temperatures. Adding inspection ports compromises the IP rating.

• ​Solution:​​
• Routine checks: Manually feel cable chamber front panel temperature to detect T-body overheating.
• Critical units: De-energize periodically after initial high-current operation to inspect connections for overheating signs.
• ​Best practice (Technology):​​ Install temperature sensors directly on RMU bushings or T-body connectors for real-time temperature monitoring.

​2.2 Cable T-Body Connector​

2.2.1 ​Ensure Quality of Conductive Components:​​
Switching to stainless steel bolts makes conduction solely dependent on end contact, increasing demands on lug structure/material quality. Common issues found:

Lug contact surface too narrow/hole too large → reduced contact area.

Poor lug material quality, uneven plating.

Mismatch between lug hole taper and double-ended bolt → lug cannot contact bushing properly → conduction only via bolt.

Copper washer too thin/small → cannot ensure parallel lug-to-bushing contact.

All lead to reduced current capacity and thermal fault risk.

• ​Solution:​​ Specify T-body connector conductive components clearly:
• Lug contact surface width: 25mm or 32mm (match bushing conductive area).
• Lug material: T2 copper (>99.9% Cu, electrolytic, molded, annealed). Tin or silver plating.
• Washer: Large surface, ≥3mm thick to ensure good pressure contact.

2.2.2 ​Select Soft-Material T-Body Connectors to Ease Installation:​​
EPDM or rigid plastic/rubber T-bodies are hard/brittle, difficult to adjust during installation (especially large cores/stress cones/insulation), and hard to verify positioning. Poor elasticity/radial force risks long-term interface separation and tracking.

• ​Solution:​​ Choose ​Silicone Rubber​ T-body connectors for common tank RMUs. Benefits: Soft, elastic → easy positioning adjustment; Excellent radial force and uniformity → good sealing, prevents tracking; Sufficient mechanical strength for RMU chambers.

​2.3 Site Installation Practices​

2.3.1 ​Secure Cable Entry Point:​​
Secure the three-core cable entering the RMU ​directly below the HV bushings​ using a cable clamp. Avoid tilting or unsupported cable entry. Unsecured cables impose torsional/pulling forces, potentially compromising bushing/seal integrity → SF₆ leakage, bushing cracks, HV faults.

• Position cores vertically and symmetrically; minimize twisting.
• Place the branch glove and cable clamp as low as possible (≥750mm vertical distance from bushings).
• ​Site Process:​​ After pulling cable through foundation into chamber, cut off any damaged cable end. Verify phase sequence. Align cable entry angle so cores are straight towards bushings. If angle is excessive, retract cable to trench/pit, correct angle, then re-insert and clamp firmly. ​Double-fixing:​​ Where possible, add a second clamp point (e.g., fixing beam in cable pit below) to secure the outer sheath further.

2.3.2 ​Cable Phase Separation and Preparation:​​

1. Fix cable branch glove using clamp before trimming core lengths.
2. Align B phase with B bushing.
3. Slightly bend A/C phases outward at the root before vertically aligning them with their bushings.
4. Place termination bolt into bushing, hang lug loosely on it.
5. Cut core ends to exact required length after verifying alignment.

• ​Crucial:​​ ​Fix cable before final trimming.​​ Failure to do so results in inconsistent core lengths → bushing stress and poor contact.
• ​Peeling/Cleaning Process:​​
• Follow T-body manufacturer's peeling dimensions exactly.
• Avoid damaging inner layers while peeling outer layers.
• Absolutely prevent longitudinal scratches on core insulation → prevents internal tracking.
• Use manufacturer-supplied cleaning paper. Avoid other solvents like industrial alcohol.
• Use polyfluoroether-based lubricant (compatible with silicone rubber). Avoid silicone grease → mutual dissolution → interface drying → tracking risk.

2.3.3 ​Stress Cone Installation:​​

• Ensure stress cone matches cable size → correct interference fit. Too tight: hard install, risk splitting. Too loose: poor sealing, risk surface discharge.
• Position strictly per T-body manufacturer’s instructions (positions relative to insulation and cable core affect stress control/sealing). Minimal tolerance.
• Position stress cone on the vertical section of the cable if possible → ensures best seal.
• Prevent sharp objects from scratching silicone rubber surfaces.
• Apply uniform coating of compatible lubricant on interference fit surfaces.

2.3.4 ​Ensure Sufficient Conductor Contact Area:​​
Conductor connection inside the insulation sleeve is invisible/hard to check. Must ensure:

• Lug surface is parallel to bushing conductive surface → minimized stress on bushing.
• ​Excellent contact​ to prevent heating.
• ​Crimping:​​ Crimp lug to core per procedure. Ensure lug face orientation is parallel to bushing plane. After crimp dies close fully, hold pressure for 10-15 seconds. Deburr surfaces. Clean lug and core insulation.
• ​Connection:​​ Place lug onto bolt, push T-body into bushing → ensure parallel lug-to-bushing contact before tightening.

2.3.5 ​Ensure Reliable Grounding:​​
Shielded T-body connectors must be properly earthed using dedicated grounding rings/wires connected to the RMU ground grid. Failure risks:Static charge build-up on surface → shock hazard.

Surface discharge to nearby ground → material electrical erosion.

​2.4 Requirements for RMU Civil Foundation​

• RMU base typically 300-500mm above ground level.
• ​Cable pit depth below base should be ≥800mm; strive for ​1000mm​ if site permits.
• ​Purpose:​​ Provides adequate bending radius for cable entry (especially large sections), allowing near-vertical entry → reduces stress on cable/connection.