Drives, Harmonics and Gensets

While there are many factors that affect the proper operation of an emergency or standby generator set, one that is commonly overlooked is the impact of variable frequency drives (VFD).

As all building electrical system designers know, the push towards greater energy efficiency has encouraged the increased use of VFDs in facility applications. In fact, today one commonly sees VFDs used for supply and return fans, chillers, domestic water pressurization pumps, chilled and condenser water pumps, and many other pieces of motor-operated equipment in all types of facilities—you name it. VFDs are being used for almost everything.

And with increased use of VFDs, there is always the possibility that the level of harmonics generated by these systems will have an operational impact on facility equipment, but designers aren't always thinking about the effect of harmonics on the gensets.

The potential adverse effect of harmonics on the on-site generator set is caused by the interaction between the harmonics on the generator output wave form and the voltage regulator. For a typical, self-excited generator, the voltage regulator monitors the output voltage and makes adjustments in the generator field windings to maintain a constant voltage to the load.

If there are significant harmonics reflected from harmonic-rich loads, such as VFDs, UPS systems and fluorescent ballasts, the voltage regulator attempts to adjust the voltage up or down based on the output wave form that is distorted by the harmonics. Since the distortion gives false information to the voltage regulator, it tries to adjust the voltage to maintain voltage level at a constant level, but the distortion may make the voltage go up when it should stay constant, or down when it should be going up.

Because this voltage adjustment is a closed-loop control system, as the voltage is incorrectly changed, based on the distorted information, it is then even more incorrect and the output voltage finally gets so far off that the generator ceases to produce an output that is usable by the loads. For generators that utilize permanent magnets mounted around the generator shaft to generate power for the field windings, there is much less sensitivity to harmonics, since there is no direct connection between the voltage regulator and the output voltage.

While there are many factors that can affect the harmonic levels created by VFDs (see “Harmonics and the Variable Frequency Drive”), the use of harmonic filters and the level of available fault current have a direct impact on whether the harmonics affect generator operation.

For instance, utilization of a six-pulse VFD with a 5% total harmonic distortion (THD) passive harmonic filter will not create enough harmonics to affect generator operation. The absence of the filter, however, could allow enough harmonics into the system to disrupt the generator voltage regulator operation. In place of the passive harmonic filter, an active filter may provide a more satisfactory solution to system harmonics, but the engineer should analyze the various options before selecting the best alternative.

The location of filters can also affect how the generator reacts to VFDs in the system. Let's say, for example, a single harmonic filter that is intended to mitigate all harmonics in an installation, and which is sized to meet IEEE 519, is connected to the incoming service equipment. In this case, there will be adequate mitigation of the harmonics to satisfy the electrical utility, but the generator set will still be affected.

To assure that there are no interactions between the VFDs and the generator, the harmonic filters must be located at each of the VFDs or at the main distribution panel or motor control center feeding the VFDs. This will prevent the harmonics from affecting the generator operation, because they will not be visible to the generator voltage regulator.

In the past, some engineers have depended on drive-isolation transformers or series inductors to mitigate harmonics created by a VFD. While there is some small benefit for harmonic mitigation in the use of these inductive devices, dependence on them may not work for many systems since the harmonic attenuation for these systems is generally inadequate to reduce the harmonic level enough to prevent voltage regulator interference. This is not to say that the inductive systems are not, in some cases, beneficial in the installation of VFDs. They simply are not the option of choice for harmonic mitigation.

Therefore, when one finds that there is a single, large VFD or multiple, smaller VFDs in a distribution in which they are designing an emergency generator set, consider that the created harmonics may make the generator non-operational. Utilization of properly located harmonic filters, active harmonic filters, reduced harmonic VFDs and permanent magnetic generators can all mitigate the harmonic effects on the generator and it will operate properly.