Transformerless medium-voltage drives perspective
Not all drive manufacturers agree on the merits of medium-voltage (MV) drive operation without an input transformer. (Also see: Why Choose Medium-Voltage Drives). The following pro and con comments come from companies with and without a transformerless MV drive on the market.
“Eliminating the transformer is often driven by a first-cost criterion and may not provide the best overall drive solution,” says Tim Russell, senior system engineer at TM GE Automation Systems (TMEIC GE). He sees a greater role for drive transformers beyond just providing isolation. “They also provide voltage matching to the utility, phase shifting for harmonic reduction, and impedance for fault current limitation—as elements of total product installation criteria.”
Older generation current-source inverter (CSI) drives offered this capability for some time; and several modern voltage-source inverter (VSI) drives can be provided with active component rectifiers for transformerless operation, notes TMEIC GE. (See separate online coverage of CSI- and VSI-type drive configurations.)
TMEIC GE designs and develops advanced automation, large ac machines, and variable-frequency drives based on the combined heritage of Toshiba, Mitsubishi Electric, and General Electric.
“Replacing transformers with silicon in the form of power switches in multi-level configurations reduces the drive’s overall size and weight while providing the same key benefits,” says Paul Nolden, program manager, MV drives at ABB Inc. “Movement to transformerless MV drives is helping to reduce the cost/power ratio.” Nolden emphasizes that requirements for harmonic compliance (IEEE 519) and common-mode filtering—normally provided by the transformer—must still be met when selecting a transformerless drive. The latter design typically relies on active front-end circuitry to provide those functions. ABB manufactures MV drives with traditional as well as transformerless designs.
As mentioned in the main article, transformerless MV drives still require the proper input voltage. In a number of large industrial facilities, such as aluminum or steel processing plants, a distribution transformer may exist to provide the “correct MV bus” supply, which also would feed various other plant equipment. For example, 13.8 kV distribution voltage could be stepped down to 4.16 kV. Then, any 4.16 kV drive on site could connect directly to the bus, explains Nolden.
Rockwell Automation, an enthusiastic advocate of transformerless MV drives, suggests that a similar MV bus is available at oil drilling platforms and onboard ships. The substantially more compact transformerless MV drive offers special advantages in these limited space applications.
Scott Conner—manager, large drives sales applications engineering at Siemens Industry Inc.—agrees that some of the earliest variable frequency drives (VFDs) operated without a transformer. However, he believes there are good reasons for having a transformer; for example, to mitigate common voltage disturbance by grounding via the transformer and ability to buffer the VFD’s power electronics from the line (and vice versa), which helps minimize harmonics. “Not having a transformer creates issues that must be solved with a complicated active front end (AFE) or rectifier arrangement,” says Conner. “The drive’s AFE must handle harmonics and power factor which—at some speeds and loads—are more difficult to optimize at the same time than in a MV drive with a transformer.” It then becomes a choice as to which attribute to optimize. “Also, in many cases a distribution transformer must be added to the system to achieve a practical input voltage to the VFD,” he states.
On the benefits side, transformerless MV drives offer much smaller footprint and plug-and-play features. “In some retrofit cases with limited space for a transformer, a smaller footprint can be a benefit and in this case makes sense,” adds Conner.
Siemens MV drives include the following product lines: Robicon Perfect Harmony and Sinamics GM 150 (single-motor usage), SM 150 (single- and multi-motor usage), and GL 150 (for single, synchronous motors up to 75 MW power).
Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at email@example.com.