Analysis of an Explosion Accident of a 35 kV Voltage Transformer

Voltage transformers (PTs) consist of iron cores and winding coils, working similarly to transformers but with small capacity. They convert high voltage to low voltage for protection, measurement and metering devices, widely used in plants/stations. Classified by insulation: dry - type (≤6 kV), cast - type (indoor 3 - 35 kV), oil - immersed (outdoor ≥35 kV), and SF₆ gas - filled (for combined appliances).

During substation operation, accidents from PT electromagnetic resonance or insulation aging still occur. For instance, in March 2015, a 35 kV incoming - line PT at a thermal power plant exploded due to insulation aging, causing 35 kV Bus I & II outage. Analysis after on - site investigation:

1 Operation Mode Before Fault

The plant’s system state before the fault is shown in Figure 1.

The substation gets power from two 35 kV incoming lines (Jingdian 390 Line, Jingre 391 Line). Their switches are closed, connecting to 35 kV Section I & II busbars. These busbars use single - bus sectioned wiring. Surge arresters protect the power supply side; no incoming line protection exists on the thermal plant side. Power supply links:

• 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

Operation/maintenance staff found two explosion traces:

• 35 kV Jingdian 390 Line - side PT3: Monitors Phase A/B line voltages. Explosion burst its bottom, leaving burn marks.

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

2.1 35 kV Section II Busbar Voltage Data Analysis

Fault recording data of the 35 kV Section II busbar was retrieved to restore voltage, current waveforms, and electrical parameters during the accident. Accurate data analysis traces the fault development, providing key evidence for determining the accident cause.

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

During 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.