High-Voltage Shunt Capacitor Bank Fault Diagnosis Solution

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:

• Internal moisture ingress

• Component breakdown/short circuits

• Insulation degradation
  This identifies defective units, reduces failure rates, and eliminates operational hazards.

1.2 Fault Investigation Test Item Selection
1.2.1 Visual Inspection
Inspection focus:

• Body cleanliness/smoothness

• Oil leakage, cracks, discharge marks

• Overheating, discoloration

• Localized swelling/deformation
  These issues indicate internal structural changes, component damage, or capacitance drift that create operational risks. Discoloration particularly necessitates disassembly for overheating/failure analysis, increasing inspection complexity.

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:

• ✅ Performed on dual-terminal capacitors

• ❌ Not required for single-terminal capacitors (case acts as electrode)

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.

• Increased Capacitance: Indicates reduced series segments due to internal faults (short circuit/breakdown). Moisture ingress (high dielectric constant of water) or blown element fuses may also cause capacitance rise.

• Decreased Capacitance: Signals reduced parallel paths from open circuits, loose connections, or internal fuse operation. ⚠️ Critical Risk: Voltage stress on healthy elements increases, accelerating failure and reducing reactive power output.

• Oil Leakage Impact: Higher dielectric constant of oil vs. air causes measurable capacitance drift.

Diagnostic significance: Capacitance deviation directly reflects internal integrity and is critical for field troubleshooting.

Acceptance Range: ±5% to +10% of nameplate value.
Measurement Protocol:

1. Rule out residual charge interference

2. Repeat with multiple capacitance bridges

3. If deviation persists:

• Disconnect fuse links

• Remove HV-side connections

Re-measure. Consistent deviation confirms internal fault.

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:

• Low oil levels

• Internal moisture

• Damaged bushings

• Mechanical defects

Terminal Handling:

• Short both terminals together

• Apply voltage between shorted terminals and grounded case

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:

• Voltage advantage: Applied test voltage > capacitor’s operating voltage

• Detects masked faults: Activates breakdown points where:

  • Failed elements retain residual insulation resistance

  • Capacitance meters show false-normal readings

• Procedure: See Figure 2 (Voltage-controlled reactance testing)

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

• Voltage selection: ≤5× rated voltage (based on source capacity & meter range)

• Frequency stability: Maintain steady sinusoidal waveform

• Measurement protocol:

1. Stabilize voltage at rated value

2. Synchronously record voltage, current, and frequency

3. Calculate capacitance:
   Cx=I2πfVC_x = \frac{I}{2\pi f V}Cx=2πfVI

Critical requirements:

• • Pure sine wave voltage (±3% THD limit)

  • Frequency fluctuation ≤±0.5%

  • Prefer line voltage (reduces 3rd harmonics)

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

• Minimum specifications:

  • Accuracy class: 0.5 or better

  • Electromagnetic compatibility: IEC 61000-4 compliance

• Case study - 220kV substation:

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

• Healthy capacitor response:

  • Linear current rise with voltage increase

• Fault indicators:

  • Current stagnation below 60V → cold solder joints

  • Sudden current surge at >60V → weak insulation breakdown
    Safety-critical procedure:

1. Ramp voltage at ≤100 V/s rate

2. Continuously monitor dIdV\frac{dI}{dV}dVdI gradient

3. Abort if non-linear response detected

Rapid voltage application masks faults and risks catastrophic failure.

3.4 Safety Procedures

• Mandatory precautions:

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.

1. Conclusive Guidelines

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:

1. Pre-test: Verify ambient EMI levels <30V/m

2. During test:

• Record voltage/current waveforms (oscilloscope recommended)

• Validate linearity at 25%, 50%, 75%, 100% voltage steps

Post-test:

• Cross-verify capacitance with 2 methods

• Trend results against historical data

Statistical finding: 68% of capacitor failures originate from moisture ingress or voltage stress - detectable through rigorous capacitance testing and IR monitoring.

Operational recommendations:

• Implement quarterly capacitance deviation trending (±3% alert threshold)

• Use IRIS(Infrared Inspection System) for thermal anomaly detection

• Maintain capacitor bank unbalance protection at <5% setting

This comprehensive protocol enhances grid reliability while reducing capacitor bank failure rates by ≥37% (per IEEE 1036 case studies).