Harmonics Mitigation

Adjustable speed drives, rectifiers, uninterruptable power supplies, DC motor drives and many other non-linear devices contribute to harmonic distortion of utility and industrial power systems. Harmonics can contribute to overheating of electrical equipment (motors, transformers, cables, etc.), generator instability, overloading of UPS or emergency generation equipment, erratic behavior of protective relays, torque pulsations on rotating equipment, interface with communications equipment, and other problems.

The severity of the impact on power quality of harmonic current is a function of several factors, including: the size of the non-linear load relative to the total connected load, the available back power (i.e.: the short circuit levels available from the utility or site generator), the point of common coupling (where, on the system, is power quality a particular concern), the presence of power factor correction capacitors, the location and effectiveness of any existing installed harmonic filters, etc.

Although certain loads, like those described above, inherently cause harmonic currents to be drawn from the power supply, the presence of harmonic currents does not necessarily constitute a problem. Only when the characteristics of the power supply and the harmonic loads meet certain criteria will a harmonic problem result.

IEEE Stdd. 519-1992 addresses harmonic limits for industrial power systems and is used as a guide by most utilities for power quality requirements.

Typical harmonic mitigation techniques include:

• Phase Multiplication (12p, 18p, etc. achieved by multiple winding transformers)
• Active Filters
• Passive Filters
• Addition of Input Impedance
  • Input Transformers
  • AC Line Reactors
  • DC Reactors