What Would Cause a Capacitor to Fail?

Cause of damage of parallel power capacitor

1. When cutting off the capacitor bank, restriking overvoltage caused by breaker restrike leads to the insulation damage or breakdown between capacitors. Some capacitor banks don’t have any overvoltage protection measures nor series reactor, particularly, the frequently operated capacitors easily lead to insulation damage or even explosion.
2. Too large current over the capacitor and overcurrent caused by overcurrent harmonics in power grid make the capacitor overheated, insulation reduction or even damage.
3. The capacitor is not equipped with a single fuse or the fuse characteristics (characteristic aegis of ampere second) are too poor. When the internal components of the capacitor are seriously breakdown and the fault current is generated, the fuse can't fuse in time. Meanwhile, effective protective relaying measures fail to keep up, overcurrent leads to a sharp rise in the temperature inside the capacitor and thereof leads to capacitor expansion or explosion.
4. Poor product quality. The paper oil insulation fails to be dried or dipping treatment under a strict vacuum condition, the residual bubbles inside produce partial discharge under the long-term working voltage, while partial discharge further leads to insulation damage and aging. As a result, the temperature also rises, which eventually leads to electrochemical breakdown of components and the capacitor damages. 

Technical measures to prevent damage of the capacitor
The main cause of capacitor damage is restriking overvoltage and poor fuse quality. On this basis, following measures are recommended.

1. Protection of commonly used MOA and combined overvoltage protector. MOA can not protect the interphase overvoltage caused by the operation of vacuum circuit breakers while the general overvoltage protector adopts a composite structure of four-star connection. To prevent neutral drift, a neutral point arrester is generally added at the neutral point of capacitor bank. At this time, protection value of interpolar insulation of shunt capacitors equals to the sum of the residual pressure of the three lightning arrester units. The accumulated residual pressure is much higher than the insulation tolerance of the protected equipment. Once the interpole breakdown occurs, all the other protections are meaningless. Moreover, the capacity of the protector is small, which can not protect the insulation safety of capacitors.
2. Adopt the single fuse protection. It is an effective measure to prevent the tank explosion. Experience shows that the fuse can break the capacitor's fault current at 0.3ms. When using, the fusing current of fuse can be equipped in accordance with the rated current of the capacitor.
3. Current-limiting reactor must be installed for mutual switching between two or more groups of capacitors.
4. The capacitor bank had better adopt the double-star connection with neutral point ungrounded and double-star zero flow balance protection. Together with the single fuse protection, it can almost eliminate the explosion of capacitors.
5. Double-star zero flow balance protection has the characteristics of simple protection and strong anti-interference performance. Unbalance system voltage, single-phase grounding fault and higher harmonic current will not lead to the protect misaction, it’s the most effective measure to protect the internal faults of capacitors at present for cooperation between double-star zero flow balance protection and single fuse protection.
   Measure the capacitance of a capacitor regularly. Once a big change occurs, stop operation immediately and find out the causes.
6. Check the oil leakage of capacitor regularly. Once any oil leakage is found, stop running immediately.

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