Medium Voltage VFD

Fundamentals and Working Principle

A Medium Voltage Variable Frequency Drive is a power electronic converter that changes a fixed-frequency, fixed-voltage medium-voltage AC input into a variable-frequency, variable-voltage AC output suitable for controlling the speed and torque of an AC motor. The Benshaw MVH2 Series specification treats the MV VFD as a complete motor controller, while the Siemens air-cooled specification defines a system that can include an incoming harmonic filter or power factor correction unit, an input isolation transformer, a VFD converter, a DC link, an inverter, and an output filter. Eaton's medium voltage VFD documentation likewise lists input voltage ratings from 2400 to 13800 V and output voltage ratings of 0 to 2400 V, 0 to 3300 V, or 0 to 4160 V, with higher output voltage available on contact. The frequency and amplitude of the output waveform determine motor speed under the standard V/Hz relationship used by AC induction machines.

The most common power conversion path is AC to DC to AC, implemented as a voltage source inverter topology. In the cascaded H-bridge arrangement described in EMA Inc.'s overview of the Siemens Perfect Harmony drive, single phase output VFDs are cascaded to create the final output waveform, a topology that EMA notes has the largest installed base of any medium voltage VFD and whose basic design has changed little in 17 years. The Benshaw specification further states that the drive manufacturer shall supply the drive and all necessary controls, and that any third-party or brand-labeled drive is not acceptable, ensuring that the converter, controls, and protection functions originate from a single engineering responsibility.

At the system level, the MV VFD controls motor speed by varying the frequency of the inverter output while adjusting the voltage in proportion, which keeps the magnetic flux in the motor near its design value. The Eaton ratings list defines a standard variable torque overload of 110 percent for 1 minute out of 10 minutes, with a constant torque overload also listed as an option. The drive accomplishes this through a control section that monitors current, voltage, and speed feedback, and through a power section built from medium-voltage switching devices, typically integrated gate-commutated thyristors or high-voltage IGBTs arranged in series or in H-bridge cells depending on topology. The Benshaw specification requires that proposal submittal documents cover the drive's electrical, mechanical and performance specifications, including current and voltage rating, applicable deratings, efficiency, power factor and harmonics, which makes those operating characteristics part of the formal procurement record.

• AC to DC to AC conversion chain with intermediate DC link
• Voltage source inverter topologies including cascaded H-bridge
• Integrated isolation transformer and optional harmonic filter
• Coordinated control of output voltage and frequency to regulate motor flux and speed

Specifications and Key Parameters

Specification sheets for medium voltage VFDs define a common envelope of ratings that buyers compare directly. The Eaton medium-voltage VFD ratings table lists a power rating of 300 to 12000 hp, with 6500 to 12000 hp called out separately, an input voltage rating of 2400 to 13800 V, output voltage ratings of 0 to 2400 V, 0 to 3300 V, and 0 to 4160 V with a note to contact Eaton if higher output voltage is required, a basic impulse level of 60 kV BIL, an interrupting rating of 50 kA rms symmetrical, an ambient temperature rating of 0 to 40 degrees C without derating, a standard variable torque overload of 110 percent for 1 minute out of 10 minutes, and an optional constant torque overload. These values appear verbatim in the source material and are reproduced in the spec table that accompanies this chapter.

Benshaw's MVH2 specification adds a different layer of detail focused on the procurement and quality system. The drive shall be manufactured by the MV VFD supplier at its own facility which has a quality assurance program that is certified in conformance with ISO Standard 9001, and the drive manufacturer shall have at least 20 years of experience in manufacturing medium voltage products. The manufacturer or representative must also have service, repair and technical support available on a 24/7 basis, and the drive shall be the MVH2 Series or approved equal meeting the exact requirements of the specification. Any proposed exceptions must be stated at bid time citing the reason for non-compliance, and a drive manufactured by a third party or brand labeled is not acceptable.

The Siemens air-cooled specification layers in additional requirements about integration and certification. The VFD system as defined in that document may include the incoming harmonic filter or power factor correction unit, input isolation transformer, VFD converter, DC link, inverter, and output filter, and the VFD system must represent a fully integrated and serviceable package that includes all material necessary to interconnect any VFD system elements, even if shipped separately. Modifications to a standard product to meet the specification shall be performed by the VFD manufacturer only, and the system shall be completely factory pre-wired, assembled, and tested as a complete package by the manufacturer before delivery. Any third-party certification, safety, or protection requirements shall be applied to the VFD system as a whole, and certification of individual components by themselves is not acceptable.

The Siemens specification further requires that the VFD supplier operate a facility with an Integrated Management System certified in accordance with ISO Standard 9001, 14001, and OHSAS 18001. The same document requires at least 15 years of experience in manufacturing medium voltage VFDs and a user list of similar design equipment, and it states that all equipment furnished under the contract must have documentation showing proof of actual operation for a minimum of 15 years in similar service. Proposal submittal documents per the Benshaw specification shall include a certification of compliance, the drive's electrical, mechanical and performance specifications, a preliminary spare parts list, a warranty, an external interconnection one-line diagram, and the required clearances, cooling requirements, and environmental considerations.

Types and Configurations

Medium voltage VFDs are categorized first by their inverter topology and second by their cooling method. EMA Inc. describes the Siemens Perfect Harmony medium voltage VFD as using a cascaded H-bridge topology that is a voltage source VFD, in which single phase output VFDs are cascaded to create the final output waveform. The same source notes that this topology was patented and that Siemens/Robicon enjoyed a competitive advantage as a result, and that the basic design has changed little in 17 years even as newer generations have enhanced electronics. The Siemens air-cooled specification labels its product as the SINAMICS GH180 Series, a medium voltage air-cooled variable frequency drive designed for any type of medium voltage AC motor application.

Cooling method distinguishes a separate product family. The Siemens medium voltage air-cooled VFD specification addresses a system that may be applied to any type of medium voltage AC motor application and that is fully factory pre-wired, assembled, and tested as a complete package. Cooling options appear in the SIMOVERT MV medium-voltage drives literature, which lists a degree of protection IP 54 cabinet with internal air-to-water cooler under option M57, cabinet doors without air intake openings with air intake from below through floor panels with cut-outs under option M59, locks for cabinet doors under option M60, wall mounting of auxiliary devices in a 600 mm wide supplementary cabinet under option M61, and a connecting panel for cable entry in a 600 mm wide supplementary cabinet under option M63. These option codes show how a base drive is reconfigured for different plant layouts and ambient conditions.

Input and output connection arrangements provide another axis of variation. The Allen-Bradley PowerFlex 7000 technical data excerpt references 18-pulse drives designated R18TX, noting that 18 pulse drives have nine line-side connections from the secondary windings of the isolation transformer entering the VFD, with lug pads provided for each connection, and that the lug pad and enclosure opening can generally accommodate two cables per connection, for a total of 18 cables. This shows how multi-pulse rectifier arrangements, which are a standard means of reducing line-side harmonics, are reflected in the physical interface of the drive. Product listings on VFDs.com further illustrate the range of catalog offerings, including a Galt Electric G5000 VFD listed at 750 HP, 99 A, 4160 V, and a TECO VersaBridge Medium Voltage VFD listed at 30000 HP, 13800 V in a NEMA 3R enclosure, demonstrating that catalog ratings span from sub-megawatt units up to very large horsepower classes.

Enclosure ratings are part of the configuration picture as well. The NEMA 3R rating on the TECO VersaBridge listing above indicates an enclosure designed for outdoor use, and the SIMOVERT MV options for IP 54 cabinets with internal air-to-water coolers indicate a different environmental target. The Eaton ratings list covers a power range of 300 to 12000 hp, with 6500 to 12000 hp called out as a distinct sub-range, indicating that manufacturers segment their medium voltage VFD lines by power class as well as by topology and cooling. The Benshaw specification also requires that any modifications to a standard product to meet the specification be performed by the VFD manufacturer only, which constrains custom configurations to the original manufacturer's engineering and quality system.

• Voltage source inverter with cascaded H-bridge cells
• Air-cooled and liquid-cooled cabinet configurations
• Multi-pulse rectifier input arrangements such as 18-pulse
• NEMA 3R and IP 54 enclosure options for indoor and outdoor sites

Selection Criteria for Procurement

Procurement specifications narrow the field of acceptable suppliers and products with explicit qualifying criteria. The Benshaw MVH2 specification requires that the drive shall be manufactured by the MV VFD supplier at its own facility with an ISO 9001 certified quality assurance program, that the drive manufacturer shall have at least 20 years of experience in manufacturing medium voltage products, and that the manufacturer or representative shall have service, repair and technical support available on a 24/7 basis. The drive shall be MVH2 Series or approved equal, and a third-party or brand-labeled drive is not acceptable. The Siemens air-cooled specification requires at least 15 years of medium voltage VFD manufacturing experience, a user list of similar design equipment available on request, and proof of actual operation for a minimum of 15 years in similar service for all equipment furnished.

Electrical matching is the first technical step. The Eaton ratings list provides the candidate envelope: power rating of 300 to 12000 hp, with 6500 to 12000 hp called out separately, input voltage of 2400 to 13800 V, output voltage of 0 to 2400 V, 0 to 3300 V, or 0 to 4160 V, a 60 kV BIL rating, a 50 kA rms symmetrical interrupting rating, and operation at 0 to 40 degrees C ambient without derating. The standard variable torque overload is 110 percent for 1 minute out of 10 minutes, and a constant torque overload is offered as an option. The motor's nameplate voltage, full load current, service factor, torque profile, and the driven load's duty cycle must be matched against this envelope before specific candidates are evaluated.

Mechanical and environmental fit is the second technical step. Proposal submittal documents per the Benshaw specification must include preliminary dimensions and weight estimation, required clearances, cooling requirements, and environmental considerations, plus an external interconnection one-line diagram showing all power, control, signal, monitoring, protection, and communication connections. Catalog entries on VFDs.com show the breadth of mechanical options, from a 750 HP, 99 A, 4160 V Galt Electric G5000 VFD to a 30000 HP, 13800 V TECO VersaBridge in a NEMA 3R enclosure. The 18-pulse R18TX configuration described in the PowerFlex 7000 technical data has nine line-side connections from the isolation transformer secondary windings, with lug pads for each connection, and the lug pad and enclosure opening can generally accommodate two cables per connection. This level of mechanical detail is necessary to verify cable entry, transformer interface, and physical footprint at the installation site.

Quality and lifecycle support complete the selection profile. The Siemens specification requires the manufacturer's facility to be certified to ISO 9001, ISO 14001, and OHSAS 18001, and Benshaw requires ISO 9001 plus a 24/7 service and technical support commitment. The Benshaw specification also requires a preliminary spare parts list and a warranty as part of the bid submittal. The Siemens specification requires that the VFD system be completely factory pre-wired, assembled, and tested as a complete package by the manufacturer before delivery, that modifications to a standard product to meet the specification be performed by the VFD manufacturer only, and that any third-party certification, safety, or protection requirements be applied to the VFD system as a whole rather than to individual components.

• Manufacturer experience in medium voltage products and 24/7 support
• Electrical matching of motor voltage, current, and torque profile to the drive ratings
• Mechanical envelope, cooling, and enclosure rating for the installation site
• Quality system certification and factory pre-wire, assembly, and test requirements
• Spare parts list, warranty, and interconnection one-line diagram in the bid submittal

Standards, Compliance and Testing

Both Benshaw and Siemens open their standard lists with American and international bodies. Benshaw requires that the MV drive be provided in accordance with the latest applicable rules, regulations, codes and standards of the International Organization for Standardization (ISO), the Occupational Safety and Health Act (OSHA), the American National Standards Institute (ANSI), the Institute of Electrical and Electronics Engineers (IEEE), the National Electrical Manufacturers Association (NEMA), and a North American third-party Nationally Recognized Test Lab, with Underwriters' Laboratories (UL/cUL) given as an example. Siemens requires compliance with ANSI, NEMA, IEEE, the International Electrotechnical Commission (IEC), and a North American third-party Nationally Recognized Test Lab, with Underwriters' Laboratories (UL) and the Canadian Standards Association (CSA) given as examples, and requires that VFDs be manufactured, assembled, tested and designed in accordance with UL, CSA, or CE standards as applicable.

Quality management system standards are explicitly invoked. Benshaw requires the manufacturing facility to have a quality assurance program certified in conformance with ISO Standard 9001, while the Siemens air-cooled specification requires the facility's Integrated Management System to be certified in accordance with ISO Standard 9001, ISO 14001, and OHSAS 18001. The OHSAS 18001 reference is a direct citation from the source; subsequent revisions of that occupational health and safety scheme have been published as ISO 45001, but the source document is the controlling reference for the values reproduced here. Compliance with these standards is part of the formal bid submittal, since Benshaw requires a certification of compliance with the specification as part of the proposal documents.

Testing and certification scope is defined at the system level. The Siemens specification states that the VFD system shall be completely factory pre-wired, assembled, and then tested as a complete package by the VFD manufacturer before delivery, and that any third-party certification, safety, or protection requirements shall be applied to the VFD system as a whole, with certification or protection of system elements or individual components by themselves declared not acceptable. Any modifications to a standard product provided to meet the specification shall be performed by the VFD manufacturer only, which keeps the certification chain inside a single engineering organization. The Eaton ratings list further establishes a baseline impulse and short-circuit withstand level of 60 kV BIL and 50 kA rms symmetrical, which are the values that the drive and its protective devices must be tested against in the relevant ANSI and IEC standards.

Documentation requirements are the practical bridge between standards and the buyer's file. Benshaw requires that documentation be custom prepared for the application and cover sufficient information to determine compliance with the project specifications and contract documents, that deviations to the specification be indicated within the submittal documents with a detailed written justification, and that proposal submittal documents include a certification of compliance, the drive's electrical, mechanical and performance specifications, a preliminary spare parts list, a warranty, an external interconnection one-line diagram, and mechanical data. The Siemens specification reinforces the testing intent by requiring that the VFD system represent a fully integrated and serviceable package, and that the manufacturer demonstrate at least 15 years of experience in manufacturing VFDs at medium voltage and their capability to provide parts and service support.

• ANSI, IEEE, NEMA, IEC, and ISO referenced in both Benshaw and Siemens specifications
• UL/cUL or CSA third-party Nationally Recognized Test Lab certification
• ISO 9001 quality system required by Benshaw; ISO 9001, ISO 14001, and OHSAS 18001 required by Siemens
• Factory pre-wire, assembly, and test of the complete VFD system before delivery
• Eaton ratings baseline of 60 kV BIL and 50 kA rms symmetrical interrupting rating

Market Landscape and Buying Process

The market for medium voltage VFDs is supplied by a small set of established manufacturers and their authorized channels. The Benshaw specification names a list of acceptable manufacturers and states that a drive that is manufactured by a third party or brand labeled shall not be acceptable, requiring direct accountability to the original manufacturer. The same specification requires the drive manufacturer to have at least 20 years of experience in manufacturing medium voltage products and to maintain service, repair and technical support on a 24/7 basis, either directly or through a representative. The Siemens air-cooled specification sets the SINAMICS GH180 Series as the named product and requires the VFD manufacturer to demonstrate at least 15 years of experience in manufacturing medium voltage VFDs, with similar design user references available on request, and disqualifies third-party or brand-labeled products to ensure timely onsite support, spare parts availability, and product life cycle support.

Product differentiation is visible in published catalog data. EMA Inc. describes the Siemens Perfect Harmony medium voltage VFD as having the largest installed base of any medium voltage VFD, with newer generations using enhanced electronics but the same cascaded H-bridge topology that EMA notes was patented and that allowed Siemens/Robicon to enjoy a competitive advantage. The Allen-Bradley PowerFlex 7000 technical data excerpt refers to an 18-pulse rectifier configuration designated R18TX that has nine line-side connections entering the VFD from the secondary windings of the isolation transformer, with lug pads for each connection and space for two cables per connection. Eaton publishes a rating set of 300 to 12000 hp, with 6500 to 12000 hp called out separately, and VFDs.com lists catalog entries including a 750 HP, 99 A, 4160 V Galt Electric G5000 VFD and a 30000 HP, 13800 V TECO VersaBridge in a NEMA 3R enclosure, showing that resellers stock and quote a wide range of medium voltage units from multiple vendors.

The buying process is documentation-driven. Benshaw's bid submittal list begins with a certification of compliance with the specification, followed by the drive's electrical, mechanical and performance specifications including current and voltage rating, applicable deratings, efficiency, power factor and harmonics, the drive's mechanical specifications including preliminary dimensions and weight estimation, required clearances, cooling requirements and environmental considerations, an external interconnection one-line diagram showing all power, control, signal, monitoring, protection, and communication connections, a warranty, and a preliminary spare parts list. Deviations to the specification must be indicated within the submittal documents and accompanied by a detailed written justification, and any proposed exceptions must clearly be stated at bid time citing the reason for non-compliance. Documentation shall be custom prepared for the application and shall cover sufficient information to determine compliance with the project specifications and contract documents.

After the order is placed, the same documentation chain continues. The Benshaw specification describes an after-order submittal stage that follows the purchase order and precedes delivery, although the visible portion of the source document is truncated at that point. What is clear from the surrounding context is that the buying process is designed to keep the technical, quality, and lifecycle records inside a single manufacturer's quality system, which is consistent with the requirement that any modifications to a standard product be performed by the VFD manufacturer only and that the system be factory pre-wired, assembled, and tested as a complete package before delivery. The combination of named acceptable manufacturers, experience requirements, ISO-certified quality systems, factory pre-test, and a structured submittal package defines the procurement landscape for medium voltage VFDs in the sources reviewed.

• Named acceptable manufacturers required by both Benshaw and Siemens specifications
• Experience threshold of 20 years for Benshaw and 15 years for Siemens
• Cascaded H-bridge topology with largest installed base per EMA Inc.
• Catalog offerings spanning 750 HP to 30000 HP across multiple vendors
• Bid submittal package of compliance certification, ratings, drawings, warranty, and spare parts list