Isang Aksidente sa Pagpapadala sa GIS Equipment Dulay sa mga Error sa PT Secondary Wiring

1. Pananaw sa Accident

Ang bag-ong gipangutana nga 110kV substation’s GIS miexplosa samtang gikomisyon dahil sa short-circuit sa PT secondary circuit. Bisag simple ra ang hinungdan, severe ang mga resulta, na nagpuyo sa pag-usab.

2. Proseso sa Accident

Sa adlaw sa power-transmission:

• Ang upper-level power supply naka-charge sa 110kV GIS (isang combined appliance).

• 20s human sa pag-close sa incoming switch ug unang live-impacting sa 110kV bus, mibuntog ang puti nga usok tali sa PT compartment ug control cabinet.

• Sa loob sa sampung segundo, miexplosa ang bus PT’s GIS compartment. Nag-trip ang upper power; ang disc insulator sa PT compartment naburst, ngadto sa GIS room ang mga fragmento sa insulator ug SF₆ combustion by-products.

3. Pagsusi sa Hinungdan
3.1 On-site Investigation

Ang 110kV PT (Shanghai MWB, electromagnetic type) adunay:

• 1 basic secondary winding: 100/√3 V (150VA, 0.2 class).

• 1 auxiliary secondary winding: 100V (150VA, 3P class), wala gamiton batas sa disenyo (nag-uli sa control cabinet terminal block, walay external connection).

Pangunhing natuklasan:

• Phase C: Severe PT secondary winding insulation burns; basic winding conductor charred. Auxiliary winding conductor insulation totally burned; insulation cylinder between auxiliary winding & iron core charred. Insulation resistance between basic & auxiliary windings = 0.

• Phase B: Basic secondary winding outer insulation had burning marks; internal paint film intact. Auxiliary winding surface insulation showed overheating charring.

• Phase A: Slight basic secondary winding surface insulation burn; other conductors/insulation intact.

Preliminary conclusion: PT secondary circuit short-circuit, likely in B/C phase auxiliary windings.

Further control cabinet inspection: B/C phase auxiliary windings were shorted inside. Design-intended connections (C-ncf to terminal 11, B-nbf to terminal 15) were misrouted (C-ncf to 12, shorting to 14-cf; B-nbf to 16, shorting to 18-bf).

3.2 Pagpuyo sa Accident

• Short-circuit start: After primary system closure, B/C phase auxiliary windings (cf, ncf; bf, nbf) shorted → B/C voltage drop.

• B phase short-circuit end: After 20s, B phase short-circuit disconnected (primary lead burned by arc). Short time caused only B auxiliary winding overheating.

• C phase consequence: 43s short-circuit (per accident recording) overheated C phase secondary conductor → insulation melt/char. White smoke came from this.

• Insulation failure: PT auxiliary winding insulation melt degraded SF₆ gas in the PT chamber → lower insulation level.

• Discharges: Reduced insulation + close phase-to-ground distances → A/B/C phases discharged to GIS shell.

• Explosion: Arc energy raised gas chamber pressure → PT disc insulator burst.

3.3 Human Error

During commissioning:

• Technicians rechecked PT polarity/insulation.

• For convenience, they removed the PT secondary circuit from the control cabinet terminal block.

• Mistakenly shorted B/C phase auxiliary winding ends; failed to verify → B/C auxiliary windings shorted.

4. Preventive Measures
4.1 PT Secondary Circuit Safety

Short-circuits in PT secondary circuits damage components or burn PTs. Installed in a closed GIS chamber, PT faults cause explosions (risking injury, delaying repairs). Thus, GIS PT installation/wiring demands strict attention.

4.2 Secondary Circuit Protocols

“Electrical Safety Work Regulations” and “Relay Protection On-site Work Security Regulations” mandate secondary work safety tickets for disassembly/wiring. Using these (and integrating risk control) could have prevented wrong wiring amid workload/time pressures.

4.3 Pre-energization Testing

Strengthen pre-energization PT tests (standardized, documented). Enforce strict test execution to avoid errors from careless work.

4.4 Organizational & Protection Measures

Protect commissioned equipment (lock cabinets, use seals). Modify only post-approval; have supervised restoration.

4.5 Unused Circuit Removal

Remove unused secondary circuits (reduce error risks). Here, the unused auxiliary winding (routed to the control cabinet) caused the accident via miswiring.

4.6 PT Body Air Switch

Install secondary air switches in PT body wiring boxes (current setup in control cabinets can’t protect GIS-to-PT circuits). This isolates faults below the PT secondary outlet.By reconstructing the accident, analyzing causes, and proposing 6 preventive measures, a roadmap for GIS secondary circuit safety is established.