Asymmetric Trap Control

Written by Josh Haase

Power factor and harmonics are difficult problems to control simultaneously. With the use of asymmetric trap control in passive harmonic filters, both problems are resolved.

Power factor is a measurement of the cosine of the angle between current and voltage. Ideally, we want the power factor to be near unity, PF=1, however, in many instances, power factor falls into the category of either leading or lagging. To achieve power factor correction, extra equipment must be installed to assist in bringing the voltage and current into phase.

When the power factor is leading, it begins to overexcite the electromagnetic field in a generator. The regulator must then reduce the excitation current in the electromagnet to compensate for this extra reactive current. This causes excess stress on a generator and reduces the lifespan.

When applying a passive harmonic filter to a variable frequency drive (VFD), simultaneously controlling harmonics and power factor can be challenging.

Asymmetric trap control is the process of operating multiple tuned trap circuits within a passive harmonic filter, to improve power factor and Total Demand Distortion (TDD) performance at the Point of Common Coupling (PCC).

This is a customizable option that allows the ability to lower the amount of kVAR present in a power circuit, while still filtering harmonics to meet requirements held by IEEE-519. By using an asymmetrical trap this will avoid leading power factor when power factor capacitor banks or generators are on the line. Asymmetric trap control would also allow for harmonic filtering on a multi-staged load application where low load points would be critical.

Why is Leading Power Factor Harmful?

Simulation results of 950 HP power factor with respect to load

A programmable logic controller, PLC, is a digital computer used for automation of industrial electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many machines, in many industries (“Introductory PLC Programming”).

PLCs pull in and out the differently tuned trap contactors based on preset load ratings per filter rating. This will ensure the power factor never goes leading and the TDD for the VFD is below IEEE-519.

How Does It Work?

Asymmetric traps can be used on applications such as HVAC Chillers, wastewater treatment plants, mining, or other machinery that use VFDs.By adding an
asymmetric trap, this will:

  • Remove harmful harmonic currents and prevent overheating of power equipment
  • Meet IEEE-519 at any load percent
  • Improve overall power factor
  • Based on load, decrease line capacitance in the presence of generators and cap banks, decreasing downtime on power equipment
  • Avoid leading power factor

Applications For Asymmetric Traps

As the leader in power quality products, TCI offers solutions for a wide range of applications. With TCI’s fast response time, it is now quicker than ever to ensure proper protection on any application.

Please visit transcoil.com or contact a TCI representative at 800-824-8282 for more information on the industry-leading HarmonicGuard® Active filters or HarmonicGuard® Passive filters.

Simulation based on a 950 HP VFD Load with asymmetric trap control
Example schematic of an asymmetric trap harmonic filter