General - Capacitors are commonly used as standalone devices in industrial, transmission, and distribution power systems to compensate for the inductive effects of loads and lines. This compensation is commonly referred to as "power factor correction", since the resulting power factor after inserting a capacitor bank into the system will be closer to unity than before.
The black and white cylinders on this circuit board are small capacitors. Larger banks are made up of smaller cylindrical capacitors.
Capacitors break the NEC's trend of requiring at least 125% of the continuous load and 100% of the noncontinuous load in conductor ampacity. Instead, conductors are required to carry 135% of the rated current. Why the difference? IEEE 18 actually defines standards for capacitor banks and requires that they be capable of loading to 135% of their rating. By sizing the branch conductors to the capacitor to 135% of the rating, we allow the capacitor to be fully utilized under overvoltage events.
Additional Design Consideration - There's more to consider with capacitors than just the ampacity problem. Some other issues that occur when inserting a capacitor bank into a system:
Harmonic resonance: An inductive system with a shunt or series capacitor bank will have a resonant frequency. If there are harmonic-generating devices nearby (like variable-frequency drives, motor soft starters, or inverters), the potential for distortion in power quality due to these harmonics must be considered.
Transient overvoltage: When a capacitor is switched into service, there may be a rapid inrush of current and a transient "ringing" effect where the voltage oscillates beyond its steady-state condition, potentially to a very large value. The transient overvoltage problem can be mitigated with synchronous closing of capacitors and/or pre-insertion impedances used to reduce the initial current flowing into a capacitor.
Steady-state voltage rise: Even if the transient overvoltage from a capacitor bank is resolved, this does not mean that the steady-state design conditions will be favorable. Capacitor banks can lead to voltage rise where they are installed because of the capacitances offset of the line inductance.
Discharge: Capacitor banks store energy in the form of an electric field. The residual voltage from this electric field will stay in place for a long period of time without a proper discharging resistor to drain the energy. The capacitor's stored energy presents a hazard for people in and around the equipment. Additionally, the leftover voltage on the bank can cause a phenomenon known as "re-striking", where the voltage between the capacitor and the line feeding it becomes high enough to ionize the air and bridge the open circuit.
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