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All hardware-based, “brute-force” methods of harmonic reduction are affected negatively by input-power-system voltage imbalances. Most VFD manufacturers have computer programs that can be used to estimate harmonic distortion from VFDs.

The greater the base load on a substation transformer, the lesser the current distortion at a PCC. Because harmonic-current distortion causes additional transformer heating, utilities often oversize substation transformers relative to the loading expected from a facility. As a result, having the correct maximum transformer load (estimated or measured) is vital. Otherwise, maximum transformer I_L must be assumed.

THE DIRTY LITTLE SECRET

Most harmonic-analysis programs assume available power is a balanced voltage — for example, 480 v each on Phase A, Phase B, and Phase C. In the real world, however, no matter how well-designed a building distribution system is, perfect balance is unobtainable. The best one can hope for is a slight imbalance, such as 478:480:482 v. Most utilities allow power-voltage imbalances of up to 3 percent.

Many years ago, at a large university in the Midwest, the VFDs provided in an energy-saving retrofit project were being blamed for buildings exceeding the distortion levels recommended in ANSI/IEEE Standard 519. Harmonic analysis showed substantial third-harmonic content. In a perfect world, VFDs do not create third harmonics, as third and other triplen harmonics cancel because of the three-phase nature of VFDs. If, however, the voltage relationship between phases A, B, and C is unbalanced, cancelation cannot occur completely, and VFDs can create triplen harmonics. In this case, Phase A was approximately 450 v, while phases B and C were close to 480 v. The university was asked to move loads to get the input voltage to a more balanced condition. Once that was done, the VFDs stopped causing elevated levels of harmonic distortion.

During the mid-1990s, the Power Electronics Applications Center, a subsidiary of the Electric Power Research Institute, tested the drives of 17 manufacturers.1 A 0.2-percent voltage imbalance at the input lugs of a VFD with no input line reactor or DC-bus choke was found to cause up to a 17-percent current imbalance.

With an unbalanced input-power system, all hardware-based harmonic-mitigation technologies are subject to detrimental harmonic-cancelation effects. For example, a 12-pulse phase-shifting transformer has three input leads and six output leads and two components: a delta/delta winding set and a delta-wye winding set (Figure 10). This configuration causes a 30-degree electrical-phase shift in the power being fed into one of the drive's two diode bridges, causing, in a perfect world, fifth and seventh harmonics to be canceled. If input power is unbalanced, however, cancelation will not occur completely.

Some VFD manufacturers supply 18-pulse drives with an additional 5-percent impedance reactor in front of the auto transformer. This helps balance the current draw into the auto transformer's three sets of windings and helps to minimize the effects of unbalanced voltage and source feeds.

SO IT IS NOT A PERFECT WORLD — WHAT NOW?

The most effective means of obtaining ultralow harmonics at VFD inputs is an active filter or an active front end. An active filter works like an active noise-reduction headset. If, for example, it detects a 30-amp fifth harmonic in Phase A of a power supply, it injects 30-amp fifth harmonic 180 degrees out of phase with the VFD-created harmonic, creating a cancelation effect. This technology is less susceptible to incoming-voltage imbalances because it measures and injects corrective harmonic content automatically.