Analisis sa usa ka Akidente sa Pagbomba sa 35 kV Voltage Transformer

Ang mga voltage transformers (PTs) gihimo sa iron cores ug winding coils, sama sa mga transformers apan mas gamay ang kapasidad. Sila nag-convert sa mataas nga kuryente ngadto sa mababa nga kuryente aron maprotektahan, sukolon, ug metrihon ang mga device, kasagaran gigamit sa mga plants/stations. Gigrupahan batas sa insulation: dry - type (≤6 kV), cast - type (indoor 3 - 35 kV), oil - immersed (outdoor ≥35 kV), ug SF₆ gas - filled (para sa combined appliances).

Sa panahon sa operasyon sa substation, ang mga aksidente gikan sa PT electromagnetic resonance o aging sa insulation mahimong mag-occur pa. Pwede nato isipan ang adunay bahin sa Marso 2015, ang 35 kV incoming - line PT sa usa ka thermal power plant mi-explode tungod sa aging sa insulation, nahimong nag-cause og 35 kV Bus I & II outage. Ang pag-analisa human sa on - site investigation:

1 Operation Mode Before Fault

Ang sistema sa planta human sa fault makita sa Figure 1.

Ang substation nakakapangita og kuryente gikan sa duha ka 35 kV incoming lines (Jingdian 390 Line, Jingre 391 Line). Ang ilang mga switch gi-close, na connect sa 35 kV Section I & II busbars. Kini nga mga busbars gigamit ang single - bus sectioned wiring. Ang surge arresters protektahan ang power supply side; walay incoming line protection sa thermal plant side. Ang mga link sa power supply:

• 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 run in parallel.

2. On - site Investigation & Accident Retrospection

Nag-identify ang operation/maintenance staff og duha ka explosion traces:

• 35 kV Jingdian 390 Line - side PT3: Monitors Phase A/B line voltages. Mi-burst ang iyang bottom, may burn marks.

• 35 kV Jingdian 390 Line Incoming Switch: Short - circuit current caused explosion. Melted ang cable head bolts; burned/deformed ang contacts/fingers.

2.1 35 kV Section II Busbar Voltage Data Analysis

Giretrieve ang fault recording data sa 35 kV Section II busbar aron restore ang voltage, current waveforms, ug electrical parameters sa panahon sa accident. Ang accurate data analysis trace ang fault development, providing key evidence para sa determining sa cause sa accident.

2.2 Fault Development & Electrical Analysis
(1)Pre - Fault Voltage Distortion

• 19.6ms pre - fault: 35kV Section II busbar has symmetrical three - phase voltages, minimal zero - sequence voltage → normal equipment.

• 13.6ms pre - fault: Phase A/B voltages drop to 49.0V/43.1V; Phase C jumps to 71.8V; zero - sequence voltage rises to 22.4V → voltage transformer insulation damaged.

• 1.6ms pre - fault: Phase A/B voltages fall to 11.9V/7.4V; Phase C drops to 44.5V; zero - sequence voltage reaches 23.5V → insulation deterioration worsens.

 (2)Fault Occurrence & Protection Response

Sa panahon sa fault: Phase A/B insulation breaks down (short to ground); Phase C voltage drops. 3ms later, three - phase voltages return to zero; PT explodes → determined as three - phase short - circuit to ground.

Conclusion: Pre - fault busbar voltages were normal (no lightning/misoperation → resonance overvoltage excluded). Long - term operation caused voltage transformer insulation degradation → internal insulation damage led to inter - turn short circuit → evolved into three - phase insulation breakdown/short - circuit → line tripped.

(3)Protection Setup & Action

Incoming line switches (Jingdian 390, Jingre 391) lack incoming protection. Main station has protections with identical 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).

After the fault, currents in both lines spiked. After transients, they reached 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).

Protection operations:

• 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 Cause Analysis & Preventive Measures
3.1 Accident Causes

The fully - insulated electromagnetic voltage transformer, 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 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.