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.

06/01/2007


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.

Guarding Against Harmonics

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.

What to Look For

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.



Harmonics and the Variable Frequency Drive

To define the differences among the various variable frequency drives, and their potential impact on generator set operation, one must first examine the available equipment from the various manufacturers.

The most common and least expensive VFDs are the six-pulse drives. These have a single pair of electronic switches for each of the three phases, thus resulting in six pulses of harmonics within each full electrical cycle. With increasing prices, there are 12- and 18-pulse drives that, respectively, have two and three pairs of electronic switches for each phase, with the resultant 12 and 18 pulses of harmonics within each electrical cycle. With the increase of the number of pulses in a drive, there is a proportional decrease in the harmonic content reflected into the power distribution system. When the six-pulse VFDs are utilized in a facility, mitigation of the harmonics may require the addition of either active or passive harmonic filters for the facility to meet IEEE 519. For the majority of facilities, smaller or no harmonic filters are required when the 12- and 18-pulse drives are utilized.

The type of harmonics will also affect the impact on the generator operation. If the harmonics are limited to current harmonics, there will be some conductor heating and potential neutral problems, but these will not affect the generator operation. Current harmonics normally occur in systems with more limited quantities of VFDs and other harmonic producing loads and higher levels of available fault duty. Voltage harmonics, on the other hand, occur where there are many harmonic loads and limited available fault duty. Without some type of harmonic mitigation, there will be problems with the generator operation.

Low Harmonic

Every year, CSE magazine sponsors a “Product of the Year” competition, and the winners appear in the November issue. In 2006, the grand winner of this competition was ABB's ultra-low-harmonic drive.

“Improving line voltage quality by reducing harmonics is a primary concern for many variable-speed drive users,” said Ari Hedemaki, production line manager, ACS800 Industrial Drives, ABB, Low Voltage Drives. “Harmonics can disturb equipment of other power consumers, cause additional losses in power distribution and, in some cases, can even damage equipment connected to the same network.”

These drives, available as wall-mounted and cabinet-built units, use an active converter controlled with direct torque control to eliminate low-order harmonics and an active front-end line filter to combat high-frequency harmonics. With total distortion reduced to about 4%, it meets IEEE 519 and G5/4 requirements.

The 18-pulse Drive

For municipal and industrial pumping applications, Square D offers another version of low-harmonic drive. The Powergard 18-Pulse Series C enclosed drive controller features adjustable frequency drives/power converters. The result is said to be a packaged, adjustable speed drive with a clean power solution.

The series is claimed to be suitable for installations specifying “clean power” low-harmonic content in compliance with IEEE 519 guidelines for harmonic mitigation. The drives are also UL 508 listed.

In addition, they feature a heavy-duty industrial disconnect handle with lock-out/tag-out provisions. The drives are available in two application ratings: constant torque/heavy duty (40 hp to 450 hp at 460 volts) and variable torque/light duty (50 hp to 500 hp at 460 volts).

What can the 18-Pulse controller do for you? Several capabilities are offered by this type of drive. For example, it offers clean power—low harmonic content—for highhorsepower ratings.

Moreover, these drives can be customized for specific applications with an advanced technology platform that increases reliability and uptime.

Some other features of these drives are: an intuitive door-mounted graphic display screen in plain text; programming that is clear, precise and illustrated; and diagnostics that are intuitive.

Finally, operators mounted on enclosure doors are pre-programmed to save set-up time and expense.