Ni ni sababu za kuzama na kuchomoka kwa msemaji wa voliti wa 35 kV GIS?

1. Mwambao wa Ajali
1.1 Muundo na Uhusiano wa Transformer wa Umeme wa GIS wa 35kV

ZX2 ya gas-insulated double-bus switchgear, iliyotengenezwa mnamo Machi 2011 na iliyorudi kazi rasmi mnamo Julai 2012, imeundwa na viwanja vya bus voltage transformers (PTs) vya mbili kwa kila sehemu ya bus. Viwanja vya PT vya sehemu moja ya bus vilivyovunjika vimeundwa katika mfuko mmoja wa switchgear una upana wa 600 mm. PTs za tatu zimeelekezwa kama muundo wa penzi chini ya mfuko.

PTs zimeunganishwa na disconnectors katika chumba cha bus cha PT switchgear kupitia cables ngumu nyingi. Disconnectors zimeunganishwa na bus tatu kupitia moving contacts katika SF₆ fully-enclosed bus chamber. Muundo mzima wa bus unapunguza kiwango cha matatizo, na bus haijafikia na bus protection maalum. Matatizo ya bus yanatathmini kwa njia ya backup protection ya power incoming switch.

1.2 Nyanja ya Kazi Kabla ya Kubakisha

Kabla ya ajali, grid ya umeme ilifanya kazi kama ifuatavyo:

• Mfumo wa 220kV: Qiaoshi Line na Huishi Line zilikuwa zinazozingatia kwa bus tie switch iliyofungwa.

• Mchuzi wa Main Transformer: No.1 main transformer alikuwa na 47 MW, na No.2 alikuwa na 14 MW.

• Mfumo wa 35kV: Unit A ilikuwa inafanya kazi na double buses kwa split operation. Generator No.2, anayezingatia 30.5 MW, ilikuwa imeunganishwa na Bus II ya Unit A kupitia Bus 1 ya Unit E, hot oil interconnection line switchgears 361 na 367, na ilikuwa inafanya kazi kwa pamoja na No.2 main transformer.

1.3 Mzunguko wa Ajali

• Unganisho wa Matatizo

• Tangu saa 15:11:20.393 tarehe 19 April, kifaa cha protection cha switch 367 katika Unit E (Bus Unit for Generators 1 and 2) kilipewa mara kwa mara PT disconnection alarms, ambayo zilikuwa zinachukua muda kwa muda.

• Kubakisha Vifaa

• Saa 15:12:59, ushoga na arcing ziliona katika mfuko wa PT wa Bus 1 wa Unit E. Zero-sequence overcurrent protection ya switches 361 na 367 ilianza, ikizimia switches zote mbili.

• Utambuzi wa Mtaani

• Lango la mfuko lilikuwa limelianguliwa. Phase A PT ilikuwa imebakishwa sana, na plug ya Phase B ilikuwa imeshindwa. Vifaa vya ndani vilikuwa vimechomoka.

• Secondary wires za mfuko wa arrester wa karibu walikuwa wameharibika. Chambani ya bus pressure na insulation tests zilikuwa safi.

2. Tathmini ya Sababu
2.1 Ubora wa Vifaa na Matatizo ya Upatikanaji

• Matatizo ya Unda na Ustawi

• Mfano wa paint wa insulation uliyobakisha partial discharge.

• Core za iron zilikuwa zimefunguka na kutokata, kutokumbusha eddy current heating.

• Coil winding isiyotumaini kijana ikijongea risasi ya inter-turn short circuits.

• Matatizo ya Upatikanaji na Huduma

• Welding duni ya grounding screws iliyongeza resistance ya contact.

• Core za iron zilikuwa zimevunjika wakati wa transportation/installation.

• Stress transverse kutokana na short cable plugs iliyokata epoxy kwa muda.

2.2 Nyanja Zisizotumaini

• Matatizo ya Secondary Circuit

• Overloading katika secondary circuit kutokana na parallel loops mingi, kutokumbusha heat generation kutokana na \(Q = I²rt\).

• Secondary short circuits yakitokana na primary current surges na overheating.

• System Overvoltage

• Ferroresonance kutokana na switching operations au arcing grounding, kutokumbusha overvoltages hadi 2.5 mara ya rated value.

• Waveform distortion yaliyongeza aging ya insulation.

• Three-Phase Imbalance

• High harmonic content (kutokana na odd harmonics) kutokumbusha impedance imbalance.

• Neutral point displacement current kutokumbusha overheating katika zero-sequence circuit.

2.3 Tathmini ya Manufacturer's Disassembly

• Eneo la Matatizo

• Epoxy cracking katika flange mounting hole ya Phase A PT iliyotokana na intermittent grounding.

• Mechanical fracture ya Phase B plug iliyotokana na phase-to-phase short circuit.

• Tathmini ya Stress

• Cable connections isiyotumaini yalitokana na transverse stress concentrated at flange holes.

• Fault progression: Intermittent grounding → Aluminum coating ablation → Fault reset → Final breakdown.

3. Mpango wa Kutenganisha
3.1 Usimamizi wa Monitoring wa Vifaa

• Implement online partial discharge monitoring kwa GIS switchgears sawa na kuunda baseline data.

• Conduct periodic insulation resistance tests na threshold ya 200 MΩ.

3.2 Maendeleo ya Structural Design

• Cabinet Expansion: Increase cabinet width kutoka 600 mm hadi 800 mm ili kutumainisha heat dissipation.

• Connection Upgrade: Replace short cable plugs na direct connections ili kurudisha stress.

• Modular Design: Adopt pluggable PTs/arresters ili kukurudisha muda wa huduma.

3.3 Enhancement ya Protection System

• Add dedicated circuit breakers kwa PT switchgears na overcurrent/overvoltage protection.

• Install dedicated bus protection devices kwa rapid fault isolation.

• Optimize zero-sequence circuit design ili kurudisha resonance risk.

3.4 Strategy Adjustment ya Operation and Maintenance

• Establish full lifecycle management records kwa vifaa, kudokumenta installation na maintenance data.

• Perform quarterly SF₆ moisture content tests na threshold ≤300 ppm.

• Conduct annual PT volt-ampere characteristic tests kwa comparison na factory data.

4. Lessons Learned and Preventive Measures
4.1 Key Lessons

• Design Flaw: Co-location of PTs increased fault propagation risk.

• Maintenance Gap: Failure to detect cumulative stress damage.

• Protection Deficiency: Reliance on backup protection delayed fault clearance.

4.2 Preventive Measures

• Strengthen equipment manufacturing supervision, focusing on insulation processes and structural integrity.

• Promote condition-based maintenance using vibration monitoring to assess stress levels.

• Revise design specifications to mandate flexible connections between PTs and buses.

• Conduct anti-accident drills to standardize emergency response procedures for PT faults.

4.3 Implementation Results

Post-retrofit data shows:

• Partial discharge reduced from 80 pC to 15 pC.

• Temperature rise under full load decreased by 12°C.

• Fault response time shortened from 600 ms to 40 ms.

5. Conclusion

This accident revealed multiple hidden risks in GIS equipment design, installation, and maintenance. Through structural optimization, protection system upgrade, and management enhancement, a comprehensive risk prevention system has been established. Continuous monitoring of equipment performance will provide replicable retrofit experience for similar substations.