When a Capacitor Blows Up
Introduction: Why Use a Capacitor Behind a Transformer?
Reactive power compensation is an everyday part of electrical engineering. When a transformer runs under no-load conditions, it still draws reactive power—even without any real load. That’s where a compensation capacitor comes into play. Seems simple... until it explodes.
In this article, we’ll look at a real-world case where a poorly placed capacitor behind a transformer exploded. We’ll explore what happened, why it happened, and most importantly — how to avoid it.
Real-World Case: Capacitor Explosion Behind a Transformer
One of our customers operates a substation with a 22 kV / 0.4 kV transformer. They installed a compensation capacitor directly on the LV output of the transformer—with no disconnector, no thermal protection, and no isolation from the environment. And then it happened: the capacitor exploded.
“The capacitor blew up in our substation two years ago, right on the LV side behind the transformer. It was big. Luckily, we had a second transformer, which gave us time for repairs.”
Since then, the customer has taken several safety steps:
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Installed a disconnector switch for safe capacitor isolation,
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Placed the capacitor inside a metal enclosure,
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Planning to add a thermostat for temperature monitoring,
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Considering a contactor that could automatically disconnect the capacitor in case of overheating,
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Exploring an SMS communicator for remote fault alerts.
That’s the right direction. Here’s what a safe capacitor installation should include.
Technical Recommendations: What to (Not) Do When Compensating Transformer No-Load Current
1. Location: Never Directly Behind the Transformer Without Protection
Connecting the capacitor directly to the transformer output = high risk.
Better practice:
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Connect via a separate protected circuit or panel,
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Use a disconnector or breaker for safe servicing and emergency cut-off.
2. Protection: Absolutely Essential
At a minimum, the capacitor setup should include:
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Fuses or breakers at the input,
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A contactor for controlled disconnection during faults,
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A thermostat inside the enclosure—overheating is a top cause of capacitor failure,
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Surge protection to prevent switching transients.
3. Enclosure: Always Use One
Never leave a capacitor “naked” in an open cabinet. Place it in a metal box with proper ventilation or passive cooling if needed.
4. Maintenance: Capacitors Don’t Last Forever
Even top-tier capacitors age. Watch for:
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Manufacture date and runtime hours,
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Overheating, humming sounds, or changes in network behavior,
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Visual signs of wear or loose connections.
What Standards and Literature Say
According to EN 61921, capacitors should include systems for safe disconnection in case of overheating or internal fault. Experts frequently warn about risks from switching surges, harmonics, and thermal buildup.
International manufacturers like ABB and Siemens note that placing capacitors too close to transformers can cause resonance, leading to overvoltage or overcurrent—major factors in capacitor damage or explosions.
Summary: Be Smarter Than the Blast
| ✘ Bad Practice | ✔ Good Practice |
|---|---|
| Capacitor directly behind transformer with no protection | Capacitor isolated via disconnector, contactor, thermostat |
| No thermal control | Thermostat + SMS alerts |
| No regular maintenance | Preventive checks and monitoring |
Conclusion: Don’t Wait for the Bang
Capacitors aren’t “just small passive components.” They’re active devices with the potential to cause serious damage—unless given the right care and installation. While they serve a useful purpose in no-load compensation, improper setup can do more harm than good.
Our advice? Treat a capacitor like a volatile friend: give it space, monitor it, and always listen when it sends signals.
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