‘Real’ High-Voltage Motors Are Here

AT A GLANCE

High-voltage motors

Large synchronous motors

Connect motors directly to grid

Variable-frequency drives

Sidebars: What actually is ‘high voltage’?

Imagine electric motors operating with input directly from a utility distribution line. Just think about the energy savings, drastic reduction of heat and power losses, and elimination of the costly, maintenance-prone transformer plus related switching equipment. Higher voltage also allows operation at lower current for the same power output.

A distant dream, you say? Actually, one manufacturer has already produced such motors and has two installations in place in rugged industrial settings. Other motor manufacturers also may be considering high voltage offerings.

Based on earlier work that led to development of a generator product for producing high voltage for direct transmission to the grid, ABB Automation Technologies AB (Sweden) has introduced a novel ‘very high voltage’ (VHV) ac synchronous motor product able to operate with inputs in the 20-70 kV range. VHV ‘Motorformer,’ an ABB-trademarked name, combines motor and transformer functions, eliminating the need for an intermediate transformer. The design applies to both 4- and 6-pole machines. When not speed regulated, the four-pole motor has synchronous speed of 1,500/1,800 rpm at 50/60 Hz operation. (Presently, ABB’s generator technology, named Powerformer, is licensed to Alstom. That company has built and installed several generator units.)

First application of Motorformer was to drive a compressor at an air-separation plant on the west coast of Sweden. The synchronous machine outputs 6.5 MW of active power, connects directly to a 42-kV bus, and cuts plant energy losses by about 25%, according to ABB. In operation since late 2001, no unplanned outages associated with the VHV motor have been reported as of this writing.

A second Motorformer application involves two 40-MW units set to run on 56-kV supply in a compressor module at Norway’s Statoil ‘Troll A’ gas production platform in the North Sea. This application includes variable-speed control of the motors. Spin tests are scheduled for fall 2004 and winter ’04/’05, with production startup expected later in 2005.

ABB’s VHV motor appears to be a unique development, with no other comparable product presently on the market. Large conventional ac machines (synchronous and induction) are available from a number of manufacturers, including ABB, but top out at about 15 kV input.

Synchronous, not induction

Motorformer’s development favored the synchronous rather than induction motor type because of higher power levels available (>100 MW vs. up to 20 MW); higher inherent efficiency; wider air gap that eases design; and ability to control reactive power, explains Johannes Ahlinder, business development, AC-Machines at ABB. ‘An induction motor always consumes reactive power,’ he says. Control of reactive power is a significant issue to stabilize and protect the electrical network where numerous large motors are being started during the work cycle of major industrial areas.

It may be surprising that Motorformer’s design is based on conventional synchronous motor technology. Included are many proven parts, such as an identical salient-pole rotor and conventional bearings (see cutaway diagram). This approach provided a lot of experience and confidence in the product, according to Ahlinder. The main differentiator is the stator.

Winding design is key

ABB incorporated a unique design into Motorformer’s stator windings and stator core slots. Extensive analysis, simulation, and testing have gone into cable design, including electromagnetic FEA, thermal simulation, and computational fluid dynamics modeling. Yet, criteria used for stator core magnetic flux density is the same as in a conventional motor, thus limiting uncertainty in product development.

Ahlinder notes that the typical insulation system used on a conventional motor has a voltage limit around 15 kV, extendable to a 22-25 kV range with special design techniques. ‘Above this point it is not possible to make a stator winding with conventional techniques,’ he continues. ‘Cables [for Motorformer] have a cylindrical shape which produce a homogenous electrical field strength and make it possible to increase voltage levels compared to the conventional rectangular-shaped winding.’ The shape of conventional windings results in uneven electrical fields, for example, concentrating the field at the conductor’s corners under high voltage.

The cylindrical cable incorporates a solid dielectric layer of cross-linked polyethylene (XLPE) insulation, but uses no metallic shielding. At present, the stator can handle voltage supply as high as 70 kV. However, the cable design can work up to a 150-kV limit. ‘Actually, the practical lower end of the voltage from an economical point of view would be at approximately 20 kV, depending also on the load,’ states Ahlinder.

Physical size of Motorformer varies substantially depending on its power output. Even such parameters as motor shaft height are highly application dependent. While the motor itself is larger than a conventional unit of the same power rating, total installation space for a VHV system—motor and circuit breaker—is substantially less than for a conventional system with transformer and related equipment.

Design challenges

Starting of large HV motors can consume enough energy to temporarily disturb the grid and the power supply to nearby utility customers. ABB has developed special delayed starting methods to avoid supply disruption. It employs reactor equipment to prolong starting time to about 20 seconds and also adds capacitors to strengthen the grid during start up.

Ability to handle higher voltages also means that temperature limit of the XLPE-clad, cable-wound stator has to be met via cooling—air-cooling at lower power levels and water-cooling at higher power. Motorformer’s stator construction accommodates both cooling methods.

ABB sees very few limitations to HV motors. ‘[Any] limits are linked to the development of conventional products, for example the rotor design,’ says Ahlinder. However, cable temperature capacity is noted as a limiting factor, particularly in hot environment applications, where water-cooling may not be available.

Control Engineering asked several major motor manufacturers if they have investigated HV motors. Some offered no reply. Another said it evaluated HV motor technology and determined it was not a ‘viable option for our business.’ TECO-Westinghouse and Emerson Motor Technologies provided useful perspectives, included in ‘Other views’ sidebar (see online article).

Applications today, tomorrow

Most current interest in HV motors comes from offshore industries, where the trend is to replace gas turbine-driven equipment with electrical drives, explains Ahlinder. Specific targets include offshore compressor drives as well as generators for floating production storage and offloading (FPSO) systems and other vessels useable as offshore power plants. These vessels could supply several surrounding offshore platforms with electric power, linked via an appropriate transmission system.

In a larger view, high-voltage motor ‘technology would be suitable for any application where you find conventional synchronous machines today,’ he says. Examples cited include pump motors, refiner motors for pulp and paper production, compressor motors for air separation, fans, blowers, extruders, and steam/gas turbine-driven generators.

Of course, HV motors are not meant for all applications, but wider usage likely lies ahead. New developments to extend the cable winding temperature class are also ongoing. ‘This technology has been shown able to lead to new solutions, as in the case of Statoil’s Troll application, where VHV motors are under variable-speed-drive control,’ adds Ahlinder.

ABB has taken on a true forward-looking role to promote and champion this exciting new technology. Perhaps a North American application and introductions from other manufacturers are not too far in the future.

For related products visit www.controleng.com/buyersguide; for system integrators help, go to www.controleng.com/ integrators; also visit:

www.abb.com/motors&drives
www.powerconv.alstom.com

Online Extra

Other views about HV motors Insulating high-voltage motors is among its technology challenges.

Cross-linked polyethylene (XLPE)-wrapped cables also have been used in stator windings of generators in very high-voltage machines (136 kV), says Dr. George Gao, vice president of TECO-Westinghouse Motor Co . These cables offer very high dielectric strength, compared to mica tape, and the extrusion of cable insulation technology also allows for a much higher and more consistent quality of cable insulation, he explains.

“With additional additives blended into XLPE materials—to improve thermal conductivity, dielectric strength, mechanical strength—the cable-wound stator concept for VHV (very high voltage) machines should have very bright future in the industry.

No doubts,” Dr. Gao says.

As for costs, cable winding versus winding with mica-taped coils can offer up to 20% savings in process and labor, according to TECO-Westinghouse.

Dr. Gao does express other concerns that could block application of HV motors:

Cost of cable installation (additional tools required); and

Cable size selection/delivery limit (most generator/motor manufacturers don’t have cable production capability).

TECO-Westinghouse is currently reviewing the VHV concept. “We believe that it will take another 10 years to allow industry to accept this concept. It will come,” adds Dr. Gao.

For special situations Emerson Motor Technologies views motors working with 40 kV input as “only viable in very unique situations,” involving extremely large motors of 10,000 hp [7.5 MW] or above to ease the winding design. “Even in that situation, the issue of motor control is significant,” says Tim Albers, director of marketing & product planning for Emerson’s Industrial Motors Division. “The motor control component cost [for 40 kV and higher input] would be extremely high.”

Among issues posed by Emerson for HV motors are the need to greatly increase the amount of insulation materials, leading to reduced stator slot fill and larger motor size. The need for larger thermal clearances and larger air gaps is also mentioned.

Other comments offered by Emerson MT technical personnel include:

A large synchronous motor design could make sense because power factor is controlled.

“Direct off the utility” makes sense, but an electric utility may not allow a motor to run directly from a distribution line without anything in between.

“Overall, there could be some unique applications, such as a large synchronous motor running direct off an electric utility,” adds Albers.

What actually is ‘high voltage’?

What could be simpler than defining a voltage range for motors? Well, think again. High voltage is far from uniformly defined.

High voltage means different things to different users—depending on industry, global location, application, standards-making bodies, and associations. For example, at least one manufacturer refers to its small 100-240 V ac units as ‘high voltage motors.’ Possibly this relates to a safety point, in line with the European Union’s Low-Voltage Directive, where the threshold is 75 V. Similarly, some servo motor suppliers proudly claim their 460-V offerings as ‘high voltage.’

In the U.S., the National Electric Code defines HV as 6,001 V and up—with a lower 601-6,000 V range designated as ‘medium voltage.’ Europe uses different designations. Institution of Electrical and Electronic Engineers’ (IEEE) Std. 100 defines HV for electric power systems as 100 kV or greater, ranging up to 230 kV. An extra-high voltage(EHV) class follows above 230 kV. For power cables, still other ranges are specified in IEEE Std. 100.

High voltages find application in the world of electrical power transmission and interconnect circuits. These can range up to 500 kV and more recently 765 kV. Also, ultra-high voltage (UHV) lines are now being explored for 1.1 MV transmission.

ABB has designated its Motorformer product with inputs of 20-70 kV as a very-high-voltage (VHV) motor and is what this article defines as VHV.

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