Power Quality Concerns: Chasing Down the Ghosts

As energy efficiency and process controls continue to push for the use of power factor correction and VFDs in industrial power systems, the regularity of power quality issues also grows. There are many types of power quality issues, including blackouts, brownouts, and voltage surges; however, not all issues come from the power grid. Instead, the problems experienced in many plants are caused by the interaction between items on the plant’s distribution system.

One increasingly common power quality problem is the growth of harmonics from VFDs and other non-linear loads. In short, any electrical item that has a ballast or is converting power will contribute harmonics into the power system. Harmonics are distortions to the voltage and current characteristics of the system that can interact in negative ways with other power elements. For example, a system with high harmonic content can have strange readings on transmitters due to interaction between the signal and the harmonics present.

A system with over 20% harmonic content should be reviewed and mitigation steps pursued. Here are a few options for mitigating harmonic content on the power system:

  1. Specify VFDs to include line reactors, and use shielded VFD cable to limit the harmonic content that is leaving the drive-to-motor system.
  2. Add isolation transformers in the power system to fully isolate a drive from the system or protect a piece of
    sensitive equipment from the effects of harmonics.
  3. Add active filters to the power system. These devices monitor the harmonic content of the system and inject
    a signal that counteracts and reduces the overall harmonic content.

Power factor correction capacitors also contribute to power quality. These units serve the important purpose of improving the power factor of the electrical system and reducing electrical bills. The challenge with capacitors is that harmonic content in the power distribution system will interact with the capacitor and may cause a condition called resonance. Resonance results in many negative system impacts, including trip-out of VFDs.

To avoid these resonance conditions and protect the capacitors, it is critical to plan for the possibility of interaction with the harmonics on the system. Some steps to take include:

  1. Perform harmonic analysis to understand the power system parameters that will be acting on the unit.
  2. Check the capacitor size and step sizes against harmonic resonance points. If resonance points from
    the capacitor align with the system measurements, a detuned capacitor system should be installed.
  3. For large capacitor banks, review the size of the bank against the size of the service transformer. Ideally, the
    total bank size (in kVAR) should be less than 25% of the size of the transformer (in kVA). Step sizes for
    automatic banks should also be limited to avoid large transients.

Though addressing power quality issues can often feel like chasing ghosts, awareness of these potential problems
during project planning is critical to avoiding operational issues.

Click here if you are interested in learning more about harmonics or suspect there may be a power quality issue at your facility.


Sam Fopma, Interstates Electrical Engineer



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