When should power capacitors be replaced based on capacity degradation?

What are the recommended and mandatory thresholds?

Regarding the extent to which power capacitor capacity degradation necessitates replacement, there are different reference standards within the industry. Based on a comprehensive view, the following two key thresholds can guide maintenance and replacement decisions:

• Recommended Replacement Threshold (Warning Line) : When the measured capacity drops to below 80% to 85% of its rated value, a planned replacement is recommended.

    At this point, the capacitor's compensation capability has significantly decreased, but it does not necessarily require immediate shutdown. It can be      included in spare parts procurement and maintenance plans for scheduled replacement.

• Mandatory Replacement Threshold (Action Line) : When the measured capacity drops to below 90% of its rated value, some stringent standards (such as those from China Southern Power Grid and certain industry specifications) mandate immediate replacement. 

• This is primarily based on safety considerations, as capacity degradation not only affects compensation effectiveness but can also alter system parameters, potentially causing resonance overvoltage and jeopardizing the safety of other equipment. Some perspectives also hold that replacement is mandatory when the capacity degradation exceeds 20% (i.e., below 80%), and if it reaches 50% (below 50%), it constitutes a severe failure.

Below is the summary of judgment criteria based on varying levels of strictness: