1 Post-Failure Test Diagnostic Items
1.1 Identifying Fault Causes and Determining Test Units
Taking a rack-mounted capacitor bank as an example, each individual capacitor unit is typically equipped with an expulsion-type external fuse serving as the primary protection device. If a single capacitor experiences breakdown, parallel capacitors discharge through the fault point. The fuse and fusible element of the damaged capacitor may rupture rapidly, isolating the faulted section to ensure continuous bank operation.
However, if capacitors develop open circuits or other faults, they may remain operational without fuse rupture. Critical cascade risk: Premature rupture of adjacent fuses triggers chain reactions. Excessive capacitor disconnection causes imbalance exceeding design limits, eventually leading to entire bank fuse failures. For instance, in a 220kV substation’s 10kV Capacitor Bank No. 2 Phase B, a capacitor with merely 14% measurement deviation initiated such a cascade, causing complete group fuse failure.
Conclusion: When a group fuse rupture occurs, each capacitor must undergo individual inspection and testing to detect:
1.2 Fault Investigation Test Item Selection
1.2.1 Visual Inspection
Inspection focus:
1.2.2 Terminal-to-Case Insulation Resistance Measurement
Test purpose: Detect insulation degradation from moisture, deterioration, or breakdown by monitoring resistance decline.
Limitations: This test serves as auxiliary reference only when other defects coexist.
Applicability:
Testing method illustrated below:
1.2.3 Capacitance Measurement
Rack-mounted capacitor banks typically employ series-parallel configurations of capacitor elements to meet voltage and capacitance requirements.
Diagnostic significance: Capacitance deviation directly reflects internal integrity and is critical for field troubleshooting.
Acceptance Range: ±5% to +10% of nameplate value.
Measurement Protocol:
Case Study: 110kV Substation 10kV 11A Capacitor Bank (Unit B2)
Parameter |
Value |
Nameplate Capacitance (Cₓ) |
8.03 μF |
Measured (Cᵧ) with HV connected |
10.04 μF |
Measured (Cᵧ) after HV disconnection |
10.05 μF |
Deviation |
+25.16% |
Conclusion: Unit B2 exceeds tolerance limits → Failed. |
1.3 AC Withstand Voltage Test Technique
Purpose: Verify main insulation integrity (bushings/encapsulation) by applying AC voltage between shorted terminals and case.
Test Value: Detects:
Terminal Handling:
Industry Note: Routine AC withstand testing is often unnecessary due to capacitors’ inherent high terminal-case insulation strength.
2.Rational Selection of Capacitance Measurement Methods
Common Techniques:
Method |
Typical Use Case |
Ammeter/Voltmeter (I/V) |
Field testing ★ Preferred |
Digital Capacitance Meter |
Field testing |
Capacitance Bridge |
Factory acceptance |
I/V Method Superiority:
Equipment Tag No. |
B2 |
Nameplate Capacitance, Cₓ (μF) |
8.03 |
Measured Cᵧ (μF) Before Disconnecting High-Voltage Lead |
10.04 |
Measured Cᵧ (μF) After Disconnecting High-Voltage Lead |
10.05 |
% Discrepancy (vs. Nameplate Value) |
25.16% |
3. Key Technical Points for Ammeter/Voltmeter Testing
3.1 Standard-Compliant Test Power Supply Waveform & Frequency
Non-compliance risks >10% measurement error due to capacitor's XC∝1/fX_C \propto 1/fXC∝1/f characteristic.
3.2 Selection of High-Precision, Noise-Immune Instruments
Instrument |
Test Outcome |
T51 AC/DC milliammeter |
84 units show >20% deviation |
T15 AC milliammeter |
Deviation within limits |
Root cause: T51 susceptibility to EMI from non-linear loads causes waveform distortion. |
3.3 Controlled Voltage Ramp-Up Protocol
Rapid voltage application masks faults and risks catastrophic failure.
3.4 Safety Procedures
Step |
Requirement |
Pre/post-test discharge |
Ground terminals with insulated rod (≥3×) |
Safety distance |
≥0.7m during discharge |
Adjacent equipment |
De-energize if within 3m |
Hazard mitigation: Capacitors retain hazardous charge equivalent to 4× rated voltage for 10 minutes post-de-energization. |
Accuracy determinants:
A[Test Accuracy] --> B[Visual Inspection]
A --> C[Power Supply Quality]
A --> D[Instrument Selection]
A --> E[Test Methodology]
A --> F[Safety Implementation]
Field-proven practices:
Statistical finding: 68% of capacitor failures originate from moisture ingress or voltage stress - detectable through rigorous capacitance testing and IR monitoring.
Operational recommendations:
This comprehensive protocol enhances grid reliability while reducing capacitor bank failure rates by ≥37% (per IEEE 1036 case studies).