Uchunguzi wa Ajali ya Kupanuliwa kwa Mabadiliko wa Umbo wa Volts 35 kV

Mfumo wa kubadilisha viti (PTs) unajumuisha magamba ya chuma na mitumizi ya mizigo, hufanya kazi kama muundo wa transformers lakini na uwezo mdogo. Wanabadilisha vitu vya juu kwa vitu vya chini kwa ajili ya ustawi, utathmini na mifano, inatumika sana katika viwanda/vituo. Kulingana na usafi: aina ya kiu (≤6 kV), aina iliyotokana na kuvunjika (ndani 3 - 35 kV), aina iliyofungwa na mafuta (nje ≥35 kV), na aina iliyofungwa na SF₆ gas (kwa zoezi zinazojulikana).

Wakati wa kudhibiti vituo, matukio kutokana na uwiano wa maumbo ya PT au ukosefu wa usafi bado yanatosha. Kwa mfano, Machi 2015, PT ya kuingiza 35 kV katika viwanda vya umeme ulivyofunguka kwa sababu ya ukosefu wa usafi, kusababisha upimaji wa 35 kV Bus I & II. Tathmini baada ya utafiti:

1 Njia ya Kudhibiti Kabla ya Hitimisho

Hali ya mfumo wa viwanda kabla ya hitimisho imeonyeshwa kwenye Ramani 1.

Vituo vinapopata umeme kutoka kwa mzunguko wa kuingiza 35 kV (Jingdian 390 Line, Jingre 391 Line). Vitufe vyao vilivyoimekundwa vinawasiliana na 35 kV Section I & II busbars. Busbars hizi hutumia mzunguko wa kiotomatiki. Mazingira ya kuambatana yanasimamia tovuti ya kupatikana; hakuna usalama wa kuingiza kwenye tovuti ya viwanda vya umeme. Uhusiano wa tovuti:

• 35 kV Section I busbar → 3# transformer mkuu → 10 kV Section I busbar.

• 35 kV Section II busbar → 4# transformer mkuu → 10 kV Section II busbar.

• 10 kV Section I & II busbars yanaruka pamoja.

2. Utafiti wa Mitaa & Mapendekezo ya Matukio

Walimu wa kudhibiti/kusimamia walipata alama mbili za ufunuli:

• 35 kV Jingdian 390 Line - side PT3: Inaangalia vitu vya Phase A/B. Ufunuli ulifunga chini lake, akalemeza nyama.

• 35 kV Jingdian 390 Line Incoming Switch: Umeme wa kushoto ulisababisha ufunuli. Viti vya kabeli vilivyoimekundwa vilivunjika; mawasiliano/mashahidi vilivunjika/kuvunjika.

2.1 Tathmini ya Data ya Umeme wa 35 kV Section II Busbar

Data ya kurekodi hitimisho ya 35 kV Section II busbar ilikuwa imepatikana ili kurudia vitu vya umeme, mzunguko wa umeme, na parameta ya umeme wakati wa hitimisho. Tathmini sahihi ya data hutumia njia ya hitimisho, kukupa ushahidi muhimu wa kuthibitisha sababu ya hitimisho.

2.2 Maendeleo ya Hitimisho & Tathmini ya Umeme
(1)Kuwa na Vitu vya Umeme Kabla ya Hitimisho

• 19.6ms kabla ya hitimisho: 35kV Section II busbar ana vitu vya umeme vyenye vipimo sawa, zero-sequence voltage ndogo → vifaa vyenye ustawi.

• 13.6ms kabla ya hitimisho: Vitu vya Phase A/B vinachukua 49.0V/43.1V; Phase C inaongezeka 71.8V; zero-sequence voltage inaongezeka 22.4V → insala ya usafi ya transformer ya umeme imeharibiwa.

• 1.6ms kabla ya hitimisho: Vitu vya Phase A/B vinachukua 11.9V/7.4V; Phase C inachukua 44.5V; zero-sequence voltage inaongezeka 23.5V → ukosefu wa usafi unaongezeka.

 (2)Ukweli wa Hitimisho & Jibu la Usalama

Wakati wa hitimisho: Insala ya Phase A/B imeharibiwa (kushoto kwa ardhi); Vitu vya Phase C vinachukua. Baada ya 3ms, vitu vya umeme vyote vinarejelea zero; PT inafunuli → imethibitishwa kama kushoto cha vitu vya umeme vyenye vipimo sawa kwa ardhi.

Mwisho: Vitu vya umeme kabla ya hitimisho vya busbar vya normal (bila lightning/misoperation → overvoltage ya resonance imeondolewa). Ustawi wa muda mrefu umesababisha ukosefu wa usafi wa transformer ya umeme → usafi wa ndani uliharibiwa → kushoto cha inter-turn → iliyohusika kwa three-phase insulation breakdown/short-circuit → mzunguko wa mshale.

(3)Usalama & Jibu

Vitufe vya kuingiza (Jingdian 390, Jingre 391) havijapatikana kwa usalama. Vituo vya muhimu vyanapatikana na maelezo sawa:

• Differential protection: 5A setting, 0s operation.

• Time-limited quick-break protection: 21.2A setting, 1.1s operation.

• Over-current protection: Further analysis needed (ref. Figure 2 for incoming current recording data, not provided).

Baada ya hitimisho, umeme wa mzunguko wa mshale uliongezeka. Baada ya transients, wamepata ustawi:

• 35 kV Jingdian 390 Line: 14,116 A (steady-state primary fault current);

• 35 kV Jingre 391 Line: 10,920 A (steady-state primary fault current).

Operations za usalama:

• Jingdian 390 Line (remote main station side): Differential protection tripped 268 ms post-explosion. Fault not isolated as 35 kV Sections I & II busbars were looped.

• Jingre 391 Line (remote main station side): Time-limited quick-break protection tripped 1,173 ms post-explosion, isolating the fault.

3 Tathmini ya Sababu & Hatua za Kuzuia
3.1 Sababu za Hitimisho

Transformer mzima wa umeme electromagnetic, uliyetengenezwa mwaka 2008, hakukuwa na huduma ya kudhibiti/maelezo ya umeme. Ustawi wa muda mrefu umesababisha harubifu la usafi wa ndani. Sababu muhimu:

• Product Defects: Substandard design → insufficient insulation, short service life.

• Environmental Contamination: Dirt on porcelain sleeves → sharp insulation resistance drop in rainy seasons, flashovers, and long-term insulation damage.

• Insulating Oil Deterioration: Poor sealing → moisture ingress, electric field distortion, reduced oil withstand voltage/dielectric properties.

• Aging & External Impacts: Thermal aging (ambient conditions, long-term use); mechanical aging (switching overvoltage, short-circuit currents damaging insulation).

3.2 Insulation Damage Tests

Regular insulation resistance tests prevent failures:

• Primary Winding: Use 2,500 V meter during handover/overhaul → insulation resistance ≥ 3,000 MΩ. In preventive tests, resistance drop ≤ 50% of initial value.

• Secondary Winding: Use 1,000 V meter during handover/overhaul → insulation resistance ≤ 10 MΩ.

3.3 Common Fault: Resonance Overvoltage
Conditions for Occurrence :

• Electromagnetic voltage transformers are nonlinear inductors. Excitation current increase causes ferromagnetic saturation → inductance drop (main resonance cause).

• Resonance requires matched capacitance/inductance (inductive reactance ≤ 100× capacitive reactance).

• Trigger conditions: no-load bus switching, sudden ground-fault clearance, lightning, switching overvoltage, etc.

Preventions: Ground voltage transformer neutrals via harmonic eliminators + small resistors; install harmonic elimination devices at bus voltage transformer open deltas.

4. Conclusion

Insulation aging in voltage transformers causes breakdowns and bus outages – common in grids. Strictly follow preventive test regulations, test/replace unqualified equipment. In this accident, unprotected thermal power plant incoming lines and failed #1 35 kV bus tie switch widened the fault. Regularly check protection configuration/reliability. Accident analysis helps quickly identify issues, take targeted actions, reduce fault risks, and boost substation reliability.