To calculate harmonics of a system, the following data must be supplied:
  • Source transformer kVA, impedance, and short circuit capacity
  • Total quantity of VFDs, HP, and voltage fed from source.
  • Total HP of other non-VFD loads (ie: full voltage starters)
  • If multiple source transformers are used to feed groups of VFDs, then a breakdown of HP groupings is required.
  • An electrical one-line diagram of distribution system.

Before adding a VFD, determine the total harmonics generating loads on the unit substation transformer. The new harmonic generating loads shall not exceed 10 percent of the transformer base rating. Harmonic generating loads, as defined here, include electronic ballasts, computers and their peripherals, solid state power supplies, UPS systems, VFD drives, etc.
If the total load on the transformer, after the additions of the current project, exceeds 75% of its rating; and/or if the total harmonic generating loads on a substation will exceed 50%; the consultant generally prepares a study for review by the client showing that the transformer can safely carry those loads. If the study shows the transformer inadequate, the project shall either increase the transformer size, or add additional transformers. Before adding a VFD, evaluate the possible effects of the VFD on power factor correcting capacitors or harmonic sensitive equipment on the same bus. Avoid installing a VFD on the same bus with capacitors or 'sensitive' equipment. Sensitive equipment, as defined here, are loads adversely affected by harmonic voltage distortions. These include high sensitivity laboratory equipment, patient monitoring or treatment equipment, computers, etc. Provide calculations per IEEE Standard 519 showing the current and voltage total harmonic distortion (THD) that will be reflected into the existing client power system, for any load exceeding 10% of the rating of the transformer serving it. The VFD shall limit the THD to the values noted below when operating at any load from zero to 100 percent.
  • VFD input voltage waveform: less than 3 percent THD
  • VFD input current waveform: less than 100 percent THD

After startup of the VFD, the mechanical contractor generally provides, and the electrical contractor installs, any additional reactors or filters required to reduce the actual THD to the calculated THD.