What could be the reason for the frequent burning out of capacitors?

Frequent capacitor failures are usually not caused by a single factor alone, but rather result from the cumulative effect of multiple issues, including electrical stress, thermal stress, environmental conditions, and the degree of compatibility between the capacitor’s specifications and its application. The following is a systematic analysis of the underlying causes, particularly relevant for capacitors used in applications such as metal-halide lamps, industrial lighting, and power compensation:

1. Overvoltage operation

One of the most common causes of capacitor failure is prolonged exposure to voltages exceeding the rated value.

• The grid voltage is excessively high (e.g., in a system nominally rated at 220V, the actual voltage reaches over 240V);

• The system experiences operational overvoltages or lightning surges due to the absence of protective devices.

• Under light-load or no-load conditions, the metal-halide lamp circuit may experience resonant overvoltage.
  Since the power loss in a capacitor is proportional to the square of the voltage ((P ∝ U²)), even a small overvoltage can significantly increase heat generation, accelerate aging, and even lead to dielectric breakdown.

2. Excessive harmonic current

Modern power grids extensively use nonlinear loads such as variable-frequency drives, switch-mode power supplies, and LED drivers, which generate abundant higher-order harmonics (such as the 3rd, 5th, and 7th harmonics).

• When harmonic currents flow through a capacitor, their capacitive reactance decreases as the frequency increases ((X_C = 1/(2πfC))), causing high-frequency currents to be significantly larger than the fundamental-frequency current.

• The internal loss of the capacitor (P = I² × ESR) rises sharply, causing the temperature to increase rapidly.

• In severe cases, it can trigger resonance (especially in reactive power compensation systems), causing the current to amplify several times and instantly burning out the capacitor.

3. Capacitor mismatch with the system

• Incorrect capacity selection: If the compensation capacity is too large, it can lead to overcompensation, causing voltage rise; if the capacity is too small, it will fail to effectively correct the power factor, resulting in low system efficiency and increased current.

• Lamp characteristics were not taken into account: Capacitors specifically designed for metal-halide lamps must withstand ignition pulses (several thousand volts) and high-frequency operating currents. If ordinary power capacitors are used by mistake, they can easily break down due to excessively high dv/dt.

• Three-phase imbalance: In a three-phase system, if the capacitance of each phase is inconsistent, it will lead to uneven current distribution and cause one phase capacitor to be overloaded over the long term.

4. Poor heat dissipation or excessively high ambient temperature

• The capacitor is installed inside a sealed luminaire, closely adjacent to heat-generating components such as the ballast and igniter.

• The ventilation holes are blocked and severely dusty, hindering natural convection.

• High-temperature environments (such as summer factory buildings or outdoor exposure to direct sunlight) can cause the capacitor’s外壳 temperature to exceed the allowable value (typically ≤70℃).
  High temperatures not only accelerate the aging of electrolytes or impregnating agents but also reduce dielectric strength, creating a vicious cycle of “heating → aging → increased susceptibility to heating.”

5. Product quality or manufacturing defects

• The thin-film dielectric contains impurities, pinholes, or uneven thickness;

• The metallized coating has poor adhesion and weak self-healing ability.

• Insufficient impregnation leaves internal air gaps, leading to partial discharge under high voltage.

• Poor sealing leads to moisture ingress, resulting in performance degradation.
  Even if the parameters of low-quality capacitors meet the nominal specifications, they still struggle to withstand the long-term stresses of actual operating conditions.

6. Frequent switching or inrush current

Each time a metal-halide lamp is started, the igniter generates a high-voltage pulse of 3–5 kV to ionize the gas inside the lamp tube.

• If the luminaire is frequently switched on and off (e.g., via motion sensing or fault recovery), the capacitor will repeatedly endure high-voltage surges.

• Ordinary capacitors have insufficient insulation margins; after repeated impacts, the dielectric material becomes fatigued and eventually breaks down, leading to a short circuit.

7. Lack of protective measures

• No fuse or overcurrent protection is configured; a single capacitor short circuit can trigger a cascading failure.

• Without an explosion-proof structure or pressure relief valve, energy cannot be safely released in the event of an internal failure, leading to rupture and fire.

• Without installing a series reactor (such as one with a 7% reactance rate) in a harmonic environment, resonance cannot be suppressed.

How to troubleshoot and resolve?

1. Measure the actual operating voltage and harmonic content: Use a power quality analyzer to determine whether overvoltage or excessive THD (Total Harmonic Distortion) is present (>5% warrants caution).

2. Check capacitor temperature rise: Use infrared thermometry; if the surface temperature exceeds 65℃, it is considered abnormal.

3. Verify the selection parameters: Confirm whether the capacitor is specifically designed for metal-halide lamps, whether its voltage rating is sufficient, and whether its capacitance matches.

4. Improve the installation environment: Increase thermal clearance and avoid direct exposure to heat sources.

5. Install protective and filtering devices: Add a fast-blow fuse or a series reactor at the front end of the capacitor, or install an active filter on the system side.

6. Switch to high-quality, certified products: Choose brand-name capacitors that are certified by CQC, UL, Ex explosion-proof, and other relevant standards.