When we talk about power factor correction capacitors (PFCC), the term 'floating capacitance' sometimes comes up. What is 'floating capacitance'? What are its effects on an electrical distribution system and what can we do about it?
When sizing PFCC for an application, the amount of correction necessary is based on the inductive loads (typically motors) in the system. With PFCC in place, reactive current flows between the motor and the capacitor system rather than the motor and the utility company. If the capacitor remains connected to the distribution system when the motor is no longer running, it is said to be 'floating' on the line. The reactive current from the capacitor is still there but there is no motor to absorb it.
Effects of excess capacitance on an electrical distribution system include:
- A rise in line voltage
- A leading power factor
- The possibility of harmonic resonance at a specific frequency determined by the capacitor (kVAr) and the supply transformer values (kVA and %Z)
Typically there are no adverse effects on the electrical distribution system as long as the kVAr value of floating capacitance is less than 20% of the kVA rating of the transformer.
Alternatives to 'floating capacitance' include:
- Employing a 'Capacitor At Load' configuration using a Calmount® series PFCC, which is energized and de-energized with the motor
- In larger systems, installing an Autocapacibank™ or Autocapacitrap™ with an automatic controller which switches capacitance in steps to maintain a desired power factor in the presence of changing loads