Unsa ang mga dahon sa pag-abot ug pag-uban sa 35 kV GIS voltage transformer?

1. Overview sa Accident
1.1 Struktura ug Koneksyon sa 35kV GIS Switchgear Voltage Transformer

Ang ZX2 gas-insulated double-bus switchgear, gipangandoy sa Marso 2011 ug opisyal nga gibutang sa operasyon sa Hulyo 2012, nakonpigurado uban duha ka grupo sa bus voltage transformers (PTs) alang sa matag bus section. Ang duha ka PT groups sa parehas nga bus section gidisenyo isip usa ka switchgear cabinet nga may sukad 600 mm. Ang three-phase PTs gi-arrange isip usa ka triangular formation sa ilalum sa cabinet.

Ang PTs gikonekta sa disconnectors sa bus chamber sa PT switchgear pinaagi sa short cable plugs. Ang disconnectors gikonekta sa three-phase bus pinaagi sa moving contacts sa SF₆ fully-enclosed bus chamber. Ang fully-enclosed bus structure mogamit sa pagbawas sa failure rate, ug ang bus wala magdugay sa dedicated bus protection. Ang mga bus faults gitubdan pinaagi sa backup protection sa power incoming switch.

1.2 Operating Mode Bago ang Burnout

Bago ang accident, ang power grid nagoperasyon ngadto sa sumala:

• 220kV System: Ang Qiaoshi Line ug Huishi Line nag-run in parallel samtang ang bus tie switch nagsilbi.

• Main Transformer Load: Ang No.1 main transformer nagdala og 47 MW, ug ang No.2 nagdala og 14 MW.

• 35kV System: Ang Unit A nagoperasyon uban sa double buses sa split operation. Ang Generator No.2, nagdala og 30.5 MW, gikonekta sa Bus II sa Unit A pinaagi sa Bus 1 sa Unit E, ang hot oil interconnection line switchgears 361 ug 367, ug nag-operate in parallel sa No.2 main transformer.

1.3 Proseso sa Accident

• Fault Precursor

• Sumala sa Abril 19, 15:11:20.393, ang protection device sa switch 367 sa Unit E (Bus Unit for Generators 1 ug 2) mubo nga gihatag PT disconnection alarms, nga intermittent reset.

• Equipment Burnout

• Sa 15:12:59, ang usok ug arcing gihatagan sa PT cabinet sa Bus 1 sa Unit E. Ang zero-sequence overcurrent protection sa switches 361 ug 367 giatiman, tripping ang tanang switches.

• On-Site Inspection

• Ang cabinet door gibukas. Ang Phase A PT severe burned, ug ang plug sa Phase B fractured. Ang internal equipment charred.

• Ang secondary wires sa adjacent arrester cabinet damaged. Ang bus chamber pressure ug insulation tests normal.

2. Pagsusi sa Kasinatian
2.1 Kalidad sa Equipment ug Defects sa Installation

• Design ug Manufacturing Issues

• Poor insulation paint process leading to partial discharge.

• Loose lamination of iron cores causing eddy current heating.

• Irregular coil winding increasing the risk of inter-turn short circuits.

• Installation and Maintenance Defects

• Poor welding of grounding screws increasing contact resistance.

• Deformation of iron cores during transportation/installation.

• Transverse stress from short cable plugs causing epoxy cracking over time.

2.2 Abnormal Operating Conditions

• Secondary Circuit Faults

• Overloading in the secondary circuit due to excessive parallel loops, resulting in increased heat generation according to \(Q = I²rt\).

• Secondary short circuits triggering primary current surges and overheating.

• System Overvoltage

• Ferroresonance caused by switching operations or arcing grounding, generating overvoltages up to 2.5 times the rated value.

• Waveform distortion accelerating insulation aging.

• Three-Phase Imbalance

• High harmonic content (mainly odd harmonics) causing impedance imbalance.

• Neutral point displacement current leading to overheating in the zero-sequence circuit.

2.3 Manufacturer's Disassembly Analysis

• Fault Location

• Epoxy cracking at the flange mounting hole of Phase A PT led to intermittent grounding.

• Mechanical fracture of Phase B plug triggered phase-to-phase short circuit.

• Stress Analysis

• Non-flexible cable connections generated transverse stress concentrated at flange holes.

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

3. Retrofit Plan
3.1 Equipment Monitoring Optimization

• Implement online partial discharge monitoring for GIS switchgears of the same model and establish baseline data.

• Conduct periodic insulation resistance tests with a threshold of 200 MΩ.

3.2 Structural Design Improvement

• Cabinet Expansion: Increase cabinet width from 600 mm to 800 mm to improve heat dissipation.

• Connection Upgrade: Replace short cable plugs with direct connections to reduce stress.

• Modular Design: Adopt pluggable PTs/arresters to minimize maintenance time.

3.3 Protection System Enhancement

• Add dedicated circuit breakers for PT switchgears with overcurrent/overvoltage protection.

• Install dedicated bus protection devices for rapid fault isolation.

• Optimize zero-sequence circuit design to reduce resonance risk.

3.4 Operation and Maintenance Strategy Adjustment

• Establish full lifecycle management records for equipment, documenting installation and maintenance data.

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

• Conduct annual PT volt-ampere characteristic tests for comparison with 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.