Analisi ta' Aċċident tal-Esplożjoni ta' Trasformatur tal-Voltiġġ 35 kV

Il-voltajj transformers (PTs) jinkludu nukli tal-ħadid u spire tan-nuqas, li jagħmlu b'mod simili għal transformers imma bl-aċċess kapajsa żgħir. Huma jwassfu l-voltajj ta' l-alta għal voltajj ta' l-baża għall-protezzjoni, misurament, u misuratura, u huma mhuxxija ħafna fikar/stazzjonijiet. Iklassifikaw minnha l-għadd ta' isolament: dry - type (≤6 kV), cast - type (indoor 3 - 35 kV), oil - immersed (outdoor ≥35 kV), u gas SF₆ (għal appljanzi kombinati).

Waqt l-operazzjoni tal-stazzjonijiet, l-avvenimenti mill-resonanza elettromagnetika tal-PT jew l-invecchi tar-isolament jiġu mmexxa qishom. Pereżempju, fl-Marsa 2015, il-PT tal-35 kV tal-linja tal-ħalqa tal-pjanta tal-enerġija termika tneħħet minħabba l-invecchi tar-isolament, li saslu l-mitħla tal-35 kV Bus I & II. Analisi wara l-esplorazzjoni fuq is-sit:

1 Mod ta' Operazzjoni Qabel it-Telf

L-istat tas-sistema tal-pjanta qabel it-telf huwa mostrat f’Figura 1.

Il-stazzjonija tħallas minn żewġ linji tal-ħalqa tal-35 kV (Jingdian 390 Line, Jingre 391 Line). Il-switches tagħhom huma magħluqa, u huma mqassmin mal-35 kV Section I & II busbars. Dawn il-busbars jippermettu l-wiring ta' l-bus singoli. Il-parasturi tal-ħalqa protettivi jiservu l-ċap tal-ħalqa tas-silġ; mhux hemm protezzjoni tal-ħalqa fuq is-silġ tal-pjanta termika. Links tas-silġ tas-silġ:

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

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

• 10 kV Section I & II busbars jirrijnu fl-parallel.

2. Esplorazzjoni fuq is-sit & Ritratt tal-Avveniment

Il-personal tal-operazzjoni/manteniment assenjaw żewġ trazz ta' esplożjoni:

• 35 kV Jingdian 390 Line - side PT3: Jmonitorja l-voltajj tal-linja tal-Faza A/B. L-esplożjoni tneħħet tal-fond, tarka marki tal-ħolqa.

• 35 kV Jingdian 390 Line Incoming Switch: Il-kurrent tal-ħolqa qasira saslu l-esplożjoni. Il-bolti tal-kaċċa tal-kablu tneħħew; il-kontatti/dita tneħħew/kkawed.

2.1 Analisi ta' Data tal-Voltajj tal-35 kV Section II Busbar

Il-data tal-registrazzjoni tal-erġa tal-35 kV Section II busbar tneħħet biex tirristora l-formi tal-voltajj, kurrent, u parametri elettriku waqt l-avveniment. L-analisi akkar ta' data tassigura l-iżvilupp tal-erġa, li tafforna l-evidenza wkida għal determinazzjoni ta' l-kausa tal-avveniment.

2.2 Iżvilupp tal-Erġa & Analisi Elettrika
(1)Distorsjoni tal-Voltajj Qabel it-Telf

• 19.6ms qabel it-telf: Il-35kV Section II busbar għandu voltajjet tri-faża simmetriċi, voltajj ta' l-zero-sekwenz minimu → l-equipament normali.

• 13.6ms qabel it-telf: Il-voltajj tal-Faza A/B jidherġu għal 49.0V/43.1V; Il-Faza C tirkupra għal 71.8V; il-voltajj ta' l-zero-sekwenz jirkupru għal 22.4V → l-insolament tal-transformer tal-voltajj jkun damaġġjat.

• 1.6ms qabel it-telf: Il-voltajj tal-Faza A/B jidherġu għal 11.9V/7.4V; Il-Faza C tidherġu għal 44.5V; il-voltajj ta' l-zero-sekwenz jirkupru għal 23.5V → l-invecchi tal-insolament jikseb aktar.

 (2)Oċkor ta' l-Erġa & Risposta tal-Protezzjoni

Waqt l-erġa: Il-insolament tal-Faza A/B jikkollapsa (ħolqa qasira għal l-terra); Il-voltajj tal-Faza C jidherġu. 3ms wara, il-voltajjet tri-faża jirkupru għal zero; il-PT tneħħi → determinata bħala tri-faża qasira għal l-terra.

Konkluzzjoni: Il-voltajj pre-erġa tal-busbar kienu normali (mhux hawn xejk tal-ħalqa/misoperazzjoni → l-overvoltajj tal-resonanza eskludut). L-operazzjoni tal-lanġas taħdem fil-insolament tal-transformer tal-voltajj → l-damaġġ intern tal-insolament led għal inter-turn short circuit → evolvuta għal tri-faża insulation breakdown/short-circuit → l-linja tneħħet.

(3)Setup tal-Protezzjoni & Azzjoni

Il-switches tal-ħalqa tal-ħalqa (Jingdian 390, Jingre 391) mhumiex protezzjoni tal-ħalqa. Il-stazzjonija prinċipali għandha protezzjonijiet bl-istess settings:

• 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).

Waqt l-erġa, il-kurrent fit-tnejn linji jirkupru. Wara l-transients, ir-reġġivu l-steady-state:

• 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 tal-protezzjoni:

• 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 Analisi tal-Kausa & Misuri Preventivi
3.1 Kausi tal-Avveniment

Il-voltage transformer fully-insulated electromagnetic, commissioned in 2008, had no outage maintenance/electrical tests. Long-term operation caused internal insulation failure. Key causes:

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

• Environmental Contamination: Dirt on porcelain sleeves → sharp drop in insulation resistance 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 Tests tal-Damaġġ tal-Insolament

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.