What are the reasons that can cause a capacitors to explode once it is powered on?

Background:

480V capacitor, reactor impedance 7% with a 1.35 times linear, THDU set at 12%, the reactor is connected above the capacitor, and the capacitor bursts or bulges directly from the cover. The AC contactor from Chint is also damaged. The project is located at the Beihai Chemical Plant in Guangxi. The power reactive power compensation capacitor installed exploded as soon as it was powered on, 12 units exploded and 6 units remained intact. What could be the reason? It is speculated that the minimum THDu is 8% and there is resonance.

 

I. Direct Cause: Harmonic Resonance Leading to Catastrophic Overcurrent

 

1.1 How did resonance occur?

The resonant frequency of LC branch (7% reactor + capacitor) is:

Corresponding to the 3.78th harmonic.

 

Typical harmonics from chemical plant sources (VFDs, rectifiers) include the 3rd (150Hz), 5th (250Hz), and 7th (350Hz). When the system impedance characteristics change, harmonics near 3.78 (possibly 3rd or 4th) can be severely amplified.

 

1.2 Physical process of "explosion upon energization"

Moment of capacitor energization → LC circuit formed → If harmonic components near 189Hz exist in the grid → Parallel or series resonance occurs → Harmonic current amplified 5~20 times

Capacitor experiences current far exceeding its rating within seconds → Internal metallized film overheats rapidly → Dielectric breakdown → Large amount of gas generated → Pressure bursts from the weakest point at the top cover → Explosion (or bulging if pressure is not released in time)

 

1.3 Why only 6 out of 12 exploded?

Uneven distribution of resonance energy among three phases (one phase happens to be close to resonance point)

Different switching sequence of capacitors (the first ones in bear the brunt)

Individual component variations (some capacitors have slightly lower tolerance)

After explosion, the circuit opens, protecting the remaining capacitors

 

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II. Key Contributing Factor: Insufficient Reactor Linearity (1.35x)

 

2.1 What does "1.35x" mean?

Industry standards (e.g., GB/T 1094.6) generally require reactors to maintain linearity (inductance change <5%) up to 1.8x rated current. 1.35x is a significantly low specification, meaning:

When current exceeds 1.35x rated value, the reactor begins to magnetize saturate

After saturation, inductance drops → actual reactance ratio drops from 7% to 5% or lower

Resonance frequency shifts upward (from 189Hz possibly to 200~250Hz)

 

2.2 Deadly consequences after saturation

Stage	Phenomenon	Consequence
Normal	7% reactance, resonance at 189Hz	Avoids main harmonics, safe
Overcurrent → saturation	Inductance drops, resonance point shifts up	May fall exactly near 5th harmonic (250Hz)
Resonance amplification	5th harmonic current amplified	Harmonic current on capacitor surges
Positive feedback	Higher current → deeper saturation → resonance closer to 5th → even higher current	Avalanche failure

 

2.3 High ambient temperature in chemical plant worsens saturation

High ambient temperature (summer in Beihai, Guangxi can reach 35°C+)

Reactor generates its own heat from copper and iron losses

Temperature rise reduces magnetic core permeability → lowers saturation threshold

 

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III. Basic Configuration Flaw: Insufficient Margin of 480V Capacitor with 7% Reactor

 

3.1 Voltage rise effect

With a 7% series reactor, the voltage across the capacitor becomes:

If the actual system operating voltage is 400V (common value), the capacitor experiences about 430V, which appears lower than its 480V rating. However:

 

3.2 Harmonic voltage superposition

On-site THDu ≥ 8%, voltage waveform is distorted

Harmonic voltage peaks can raise the actual peak voltage to 1.2~1.5 times the fundamental peak

The electric field stress on the capacitor dielectric approaches or exceeds its design limit

 

3.3 Safety selection comparison

Selection	Voltage withstand under fundamental	Safety margin under harmonic environment	Conclusion
480V cap + 7% reactor	~430V	Insufficient	Dangerous
525V cap + 7% reactor	~430V	Adequate	Industry recommended

The 480V selection was "critically tight" for your site, leading to immediate collapse when resonance occurred.

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IV. Triggering Factors: Inrush Current + Residual Charge

 

4.1 Inrush current

Capacitor energization produces an inrush current of 5~10 times rated current. With existing harmonic background, the inrush superimposes with harmonics, resulting in even higher peaks.

 

4.2 Residual charge

If the capacitor is not fully discharged after de-energization (requires >3 minutes)

Residual charge leaves voltage across the capacitor terminals

Upon re-energization, residual voltage adds to supply voltage → extremely high surge voltage and current → immediate dielectric breakdown

 

4.3 Evidence from contactor damage

The damaged CHINT AC contactor indicates: 

Inrush current or resonance current exceeded its rated making capacity

Contacts may have welded or burned out

This further confirms the severity of the overcurrent event

 

V. Complete Failure Chain (Chronological Order)

VI. Immediate Actions & Corrective Measures

 

⚠️ Immediate Execution (Must be done before re-energizing)

DO NOT re-energize: Do not replace capacitors and energize until the cause is identified

Measure power quality: Without capacitor banks in service, measure the harmonic spectrum at the main incoming feeder to identify dominant harmonic orders and magnitudes

Check discharge circuit: Verify discharge resistors are functional and controller discharge time setting is ≥3 minutes

 

🔧 Fundamental Corrective Actions

Issue	Corrective Measure	Priority
Insufficient capacitor voltage margin	Replace with 525V rated voltage capacitors	Mandatory
Poor reactor linearity	Replace with reactors having linearity ≥ 1.8x (still 7%)	Mandatory
Harmonic resonance risk	Install Active Power Filter (APF) for source-level harmonic mitigation	Strongly recommended
Inrush current impact	Replace AC contactor with Thyristor Switch (TSC) for zero-crossing switching	Recommended
Inadequate protection	Add fast-acting fuses per capacitor branch, enable controller overcurrent/overvoltage protection	Recommended

 

• Suggested Selection Parameters (After Correction)

Capacitor: 525V, 30 kvar (example, adjust according to actual required capacity)

Reactor: 7%, linearity ≥ 1.8x (or 2.0x), rated current selected as 1.3x capacitor rated current

Switching device: Thyristor switch (TSC), or dedicated contactor with pre-charge resistors

Discharge resistor: Ensure discharge to below 50V within 3 minutes

 

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VII. One-Sentence Summary

 

Insufficient margin of 480V capacitor with 7% reactor + low reactor linearity (1.35x, prone to saturation) + strong harmonic background in chemical plant → after saturation, resonant frequency shift triggers harmonic resonance → overcurrent causes 6 capacitors to burst from the top.

Core corrective actions: upgrade capacitors to 525V + use reactors with linearity ≥ 1.8x + install APF for source-level harmonic mitigation.

Safety first. Please complete power quality measurements before replacing equipment.