Transformerless medium-voltage drives perspective

Some drive manufacturers do not agree on the merits of medium-voltage (MV) drive operation without an input transformer. The following pro and con comments come from companies with and without a transformerless MV drive on the market.


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).

Also read: Inverter topologies: Voltage- source or current-source.

And: What is medium voltage?

Why Choose Medium-Voltage Drives

Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Choosing controllers: PLCs, PACs, IPCs, DCS? What's best for your application?; Wireless trends; Design, integration; Manufacturing Day; Product Exclusive
Variable speed drives: Smooth, efficient, electrically quite motion control; Process control upgrades; Mobile intelligence; Product finalists: Vote now; Product Exclusives
Machine design tips: Pneumatic or electric; Software upgrades; Ethernet advantages; Additive manufacturing; Engineering Leaders; Product exclusives: PLC, HMI, IO
This article collection contains the 5 most referenced articles on improving the use of PID.
Learn how Industry 4.0 adds supply chain efficiency, optimizes pricing, improves quality, and more.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security

(copy 5)