Isyu at mga Tugon para sa 10kV SF₆ Gas-Insulated Common Tank Ring Main Unit (European-Style) Cable Connections

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

Dahil sa malawakang paggamit ng mga cable line sa urban distribution networks, ang 10kV SF₆ gas-insulated common tank Ring Main Units (RMUs) (European-style) ay malawakang tinatanggap bilang mga network nodes dahil sa kanilang mga katangian na buong insulation, complete enclosure, maintenance-free operation, compact size, at flexible installation. Ang mga European-style SF₆ common tank RMUs na ito ay angkop para sa mga coastal areas na may maalat at maulap na kapaligiran at nagbibigay ng mataas na operational reliability.

Ang mga recent operational failures ng RMUs ay nagpapakita na karamihan sa mga isyu ay nagmumula sa mga connection points sa pagitan ng RMU bushings at 10kV cables. Ito ay lalo na totoo para sa mga indoor at outdoor RMUs na nagbabantog ng malaking current at malaking section ng cables. Kapag nangyari ang isang failure, ang buong RMU ay kailangan de-energizein at palitan, at ang cable T-body connector nito ay kailangan i-re-install. Ito ay nagbibigay ng malaking epekto sa power supply reliability at nagdudulot ng malaking economic losses.

Ang koneksyon sa pagitan ng RMU bushings at 10kV cables ay isang critical operational weak point. Ang artikulong ito ay nag-aanalisa ng mga umiiral na mga problema at nagpaparoon ng mga kontra-medida.

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

Kasalukuyan, ang 10kV SF₆ common tank RMUs (European-style) at ang kanilang associated cable T-body connectors ay pangunahing European brands. Ang mga ito ay pangunahing disenyo para sa single-core cables, na mas madali ilagay at i-install, hindi nagpapadala ng torsional torque sa mga bushings, nagse-set ng mahusay na contact sa pagitan ng terminal at bushing, at binabawasan ang likelihood ng thermal faults. Sa kabilang banda, ang pag-install ng three-core cables ay mas komplikado, na nagdudulot ng ilang mga isyu na wala sa single-core installations:

1. ​Ang fixing point ng three-core cable ay ang outer sheath:​​ Ang individual phases ay hindi maaaring independent na ilagay. Kahit matapos ang koneksyon, ang sariling bigat ng cable o external forces ay maaaring magpadala ng torsional torque sa mga bushing sections.
2. ​Kailangan ng torque para sa phase sequence alignment:​​ Sa panahon ng pag-install ng three-core cables, ang phase sequence alignment kadalasang nangangailangan ng torque bago ang fixation. Pagkatapos ng installation, ang internal stress mula sa pag-twist ay unti-unting lumalabas, nagpapabuo ng restoring torque na nag-act sa mga bushings.
3. ​Limited cable chamber height:​​ Ang compact cable chamber height ng RMUs (disenyo para sa single-core cables) ay nagpaparestrict sa available length ng bawat individual cable core phase.
4. ​Limited adjustment after termination:​​ Kapag ang cable lug ay crimped, ang installation length ay fixed. Dahil sa mas maikling individual core lengths (dahil sa space constraints) na mahirap ibend, ang pagsisikap na iposisyun ang T-body connector kadalasang nangangailangan ng excessive pushing, pulling, o levering forces. Ito ay nagdudulot ng panganib sa mga bushings o poor contact.

​2. Kontra-Medida​

Upang tugunan ang mga nabanggit na isyu, ang mga kontra-medida ay maaaring mapatupad sa pamamagitan ng RMU itself, T-body connectors, installation practices, at ang civil foundation ng RMU.

​2.1 Ring Main Unit (RMU)​​

2.1.1 ​Adequately Increase Cable Chamber Height:​​
Ang SF₆ common tank RMU cable chambers ay karaniwang maliit (approx. H: 600mm, W: 350mm). Ito ay angkop para sa single-core cables ngunit ginagawa ito ng mahirap ang pag-install ng T-body connectors, lalo na sa large-section cables (240mm² o 300mm²), para sa three-core cables. Ang trifurcating sleeve ng T-body connector din nangangailangan ng espasyo, nagiiwan lamang ng ~400mm para sa cable cores. Ang mga large-section cores ay stiff, at kasama ang site constraints, mahirap makamit ang tama na posisyon ng T-body.

• ​Solution:​​ Bagaman ang common tank RMUs ay standardized, ang installation height ay maaaring taas gamit ang extension base. Ang pagtaas ng chamber height sa ~800mm at siguraduhin na ang vertical distance ng cable clamp mula sa HV bushing center point ay ≥750mm ay nagbibigay ng core lengths ng ~600mm. Ito ay nagpapadali ng tama na installation ng T-body. Sa esensya, ang extension base ay nagpapahaba ng separated single-phase cores pagkatapos ng three-core cable split, nagpapadali ng koneksyon tulad ng 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:​​
Ang standard 630A RMUs kadalasang may bolt-type bushings na may outer copper tube diameter ng 25mm at threaded inner hole para sa M16 bolts (conductive area ~289.6mm²). Ang aktwal na contact area ay kadalasang mas maliit dahil sa fit tolerances. Kapag ang stainless steel bolts ang ginagamit (dahil sa soft copper), ang conduction ay nakadepende only sa end contact. Sa loob ng sealed insulation, ang heat dissipation ay mahina. Kung ang lug-to-bushing contact ay mahina sa high currents (>400A), ang thermal faults ay nangyayari.

• ​Solution:​​ Para sa RMUs na gumagamit ng 240mm² o 300mm² cables na nagbabantog ng >400A, pumili ng models na may 800A-rated bushings (outer copper tube Ø 32mm) upang mabawasan ang thermal fault risk.

2.1.3 ​Enhance RMU Bushing Temperature Monitoring:​​
Ang sealed common tank RMUs ay hindi maaaring buksan para sa inspection. Ang standard IR thermography ay hindi maaaring sukatin ang joint temperatures. Ang pagdaragdag ng inspection ports ay nagcompromise sa 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:​​
Ang paglipat sa stainless steel bolts ay nagpapadependi ng conduction solely sa end contact, nagpapataas ng demand sa quality ng lug structure/material. Ang mga karaniwang isyu na natatagpuan:

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:​​
Ang EPDM o rigid plastic/rubber T-bodies ay mahirap/brittle, mahirap i-adjust sa panahon ng installation (lalo na sa large cores/stress cones/insulation), at mahirap i-verify ang posisyon. Ang mahina na elasticity/radial force ay nagpapalubha ng long-term interface separation at 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.