Medium-Voltage AC Drives: Addendum and Applications

M edium-voltage (MV) ac drives take over the scene at voltage and power levels beyond the capability of more familiar low-voltage (LV) drives. These larger MV drives-with power ratings to 100 MW and over-bring the benefits of adjustable-speed operation to large ac motors. A further benefit they offer is soft-starting of such large, expensive motors (see application section, below). At the same time, medium-voltage drives are starting to shed their ‘highly engineered product’ image by adopting modern design approaches and modular control structures of their low-voltage cousins. See main article for discussion of applicable voltage ranges, power switching devices, available products, and other MV drive benefits,

Additional manufacturer comments

Rockwell Automation Canada Inc. (Cambridge, Ontario, Canada) concentrates on improving reliability, ease of use, and total cost of ownership for MV drives. These are the three main sources of benefits to users, according to Chris Jouppi, vice president and general manager of Medium Voltage Business and Canadian Operations. An example of ease of set up via software in Rockwell’s new PowerFlex 7000 drive is a ‘start up wizard’ that’s coupled with system information entered into the drive software at time of order.

Mr. Jouppi also mentions the useful role of MV drives in starting large motors. This is a benefit, especially in developing countries, with weak power supply systems.

Robicon (New Kensington, Pa.) sees recent oil price increases as another reminder to industrial users of the importance of conserving energy. ‘MV drives are an important tool in energy conservation. In developing countries, the rate of industrialization is outstripping the increase in electrical power. So, it is imperative that they conserve energy in large-scale industrial uses,’ states Richard Osman, vice president of technology.

Due to the quality of output power, Robicon claims its MV drives can be used with ac motors (induction or synchronous) from any manufacturer-including retrofits of old motors. In addition to comments made in the main article on relative merits of IGBT (insulated-gate bipolar transistor) versus IGCT (integrated gate-commutated thyristor) power-switching devices, Mr. Osman adds, ‘the IGCT turns on so rapidly that it is harder to protect in a short circuit, and difficult to apply [with] free-wheeling diodes.’ The diodes are very necessary components of the drive, providing a parallel path for energy flow during the transistor’s switched-off phase. A more gradual turn off with the IGBT gives the free-wheeling diode a chance to recover. ‘Furthermore, the gate drive circuit for the IGCT is much more complex and power intensive than that of the IGBT,’ he states.

Robicon has an ongoing strategic partnership with Yaskawa Electric Corp. (Kitakyushu, Japan) for medium-voltage drive technology. The partnership involves the supply of Perfect Harmony Series MV drives for exclusive resale in Japan and other Asia/Pacific markets and for integration into Yaskawa Electric systems worldwide.

GE Industrial Systems (Salem, Va.) notes competition between LV and MV drives in new plants. ‘Some customers choose the MV drives over LV drives in high-power applications to reduce their cabling and facilities costs,’ says Paul Bixel, MV product engineer. ‘In retrofit situations there may be an additional requirement to interface to existing cables and motors or to control output harmonics to very low levels.’

MV drives are available in both air-cooled and water-cooled versions. GE’s Innovation Series MV drives, in the air-cooled version, employ ‘heat-pipe’ technology for efficient cooling of the power-switching IGBTs ( CE , June 1997, pp. 24-25). Historically, water-cooling has been more complex, costly, and required more maintenance.

ABB (New Berlin, Wis.; Turgi, Switzerland) incorporates extensive factory testing into its development cycle to reduce on-site testing and commissioning time of these drives. This includes full-load tests at ABB’s manufacturing plants. Most medium-voltage drives work with various motor types. ACS 1000 from ABB is designed for MV induction motors, but is applicable to adjustable-speed control of wound-rotor motors as well. Most manufacturers include induction, wound-rotor, and synchronous ac motors under MV drive control.

Design improvements are also at work to change water-cooled MV drives’ complex image. For example, simpler maintainability and servicing has drawn the designer’s attention in ABB’s ACS 1000 water-cooled drive (see photo in main article).

Siemens (Erlangen, Germany) sees the retrofit market-single-speed control going to adjustable-speed-as a prime area for MV drives. Higher efficiency operation and less maintenance are typical goals of this upgrading. Jeff Mason, marketing manager of Large Drives at Siemens Energy & Automation (Alpharetta, Ga.), notes the large size and power of these units is not compatible with high volume growth. ‘While medium-voltage drives are a lot less costly than 10 years ago, they still represent a substantial cost today,’ he says. New, higher efficiency MV motors complete the picture. Units in the 700-800 hp (525-600 kW) range are common. Motor size is slimming down, with units down to 200-300 hp available in above NEMA frame size at 2.3 kV. However, motors this small are not yet considered cost-competitive, according to Mr. Mason.

High-voltage IGBTs add to choices

Insulated-gate bipolar transistors are the unquestioned power-switch of choice today for low-voltage ac drives. Popularity and availability of IGBTs in production quantities is enticing for MV drives as well. But until recently, use of standard IGBTs in MV drives meant a more complex design due to voltage handling limitations of these devices. IGBTs had to be combined in series or ganged to obtain the voltage output needed. Direct MV capability in one device was missing.

Some recent developments are likely to change this. In July 1999, Powerex (Youngwood, Pa.) announced its new series of 4,500-volt, HV-IGBTs for use on 2.3 kV ac lines, including MV drives.

Powerex was founded in 1986 from a combination of General Electric Co.’s Power Semiconductor Div. and Westinghouse Electric Corp.-two pioneering companies in the power semiconductor industry. In 1994, Westinghouse sold its ownership shares to GE and Mitsubishi Electric Corp. of Japan. Powerex is now co-owned by these two companies.

Powerex is a leading supplier of discrete and modular high-power semiconductors worldwide. It has chip design/engineering and wafer production facilities on site, as well as access to advanced circuit and chip design capabilities of Mitsubishi Electric. Powerex specializes in the multi-element packaging of semiconductor chips (see CE , April 1998, pp. 91-101).

At about the same time-actually in June 1999-‘eupec GmbH ‘ (Warstein, Germany) further elevated the status of IGBTs by the announcement of its 6 kV IGBTs. These power devices represent the highest blocking voltage presently available from IGBTs, says eupec. They are intended for rigorous industrial and traction applications that experience large voltage fluctuations and overload conditions. The new 6.5 kV IGBT modules are based NPT (non-punch through) standards for ruggedness and lower conduction losses. They will be configured in a single-switch topology, with three package-widths (73/140/190 x 130 mm), ranging from 100 to 600 Amp.

One of the Infineon Technologies companies, eupec, just celebrated its 10thanniversary. The name change-from Siemens Semiconductors in April 1999-also represents a change of company image with a more entrepreneurial outlook. It has ‘given the company creative and financial independence to make decisions faster and more efficiently, [and] to react much faster to market fluctuations.’ A subsidiary of the company, eupec Inc. (Lebanon, N.J.), markets various power semiconductors to North America.

While it may be a bit early to tell, the new breed of HV-IGBTs can have a major impact on MV drives, along with other medium-voltage applications. And progress toward still higher voltage handling capability will continue.

Applications

Medium-voltage drives are perhaps best known in applications such as large pumps and fans in power plants. Giant fan systems in wind tunnels are also powered by MV drives. Siemens adds compressors, extruders, and conveyor systems to the list of MV drive applications in such industries as water; wastewater; petrochemical; steel, cement, and paper production; and machine and ship building. GE Industrial Systems also mentions large-load applications in steel manufacturing-specifically in main the drives for hot- and cold-rolling of steel. The following are summaries of some specific MV drive applications from around the world.

Kraftwerke Zervreila hydroelectric power generating station near Luzern, Switzerland replenishes its water supply for peak time usage by pumping water back into a reservoir. The station uses two 3.5-MW (4,700-hp) synchronous motors to power pumps that take water from a lake in the valley below. However, when the massive motors were started, businesses and residential customers experienced line voltage drops of up to 20% for as long as 10 seconds.

A Rockwell Automation (Milwaukee, Wis.) 18-pulse MV drive is helping to solve this serious voltage-dip problem originating at the local utility. The Bulletin 1557 MV drive provides several noteworthy benefits. It supplies reduced voltage (soft) motor starting to mitigate the voltage dip, yet delivers the high starting torque required. It also provides motor control and synchronous transfer capabilities that lets the utility continue using its existing pump motors with only one drive to start both of them. As each motor reaches full load and speed, the drive’s synchronous transfer feature synchronizes it to the input line frequency at Kraftwerke Zervreila. The MV drive proved to be a cost-effective solution as well, since it could be sized just for the load seen by the motors during starting, rather than full motor power.

The drive’s output waveforms are said to be smooth, producing very little stress on the motors, while input waveforms from the 18-pulse rectifier ‘all but eliminate line-side harmonics,’ according to Ralph M. Paling, manager of marketing communications at Rockwell Automation Canada. ‘The drive’s 18-pulse rectifier provided further savings, since no additional harmonic mitigation devices were required to meet the strict harmonic guidelines in Switzerland,’ he adds.

Mining for opportunities

Minera Escondida copper mine operates in one of the world’s harshest environments at an elevation of 3,150 m (10,335 ft) in Chile’s Atacama high desert. Located over a 5,000-hectare (12,355-acre) site, 200 km east from the nearest utility generator at Antofagasta (and 1,500 km from Santiago, the capital), the open-cast mine is plagued by high impedance electric supply due to the long power lines. Yet, the mine is a heavy user of electric power for transporting and processing large quantities of ore. Copper ore moves on conveyors as solid material and in slurry form via pumps. A round-the-clock, 364-day per year operation puts further stress on equipment availability.

Any adjustable-speed drive solution had to provide power quality (power factor correction close to unity), minimize harmonic disturbances on sophisticated equipment and instrumentation-increasingly deployed even at such remote sites-and be compatible with existing motors.

In 1996 Minera Escondida installed three 675 kW, 4.16 kV slurry pumps to transport copper ore suspended in water, from regrinding mills to regrinding ‘cyclones.’ Robicon supplied the Perfect Harmony MV drives that power the cyclone pumps. Experience has shown that the MV drives delivered on the above requirements. According to Robicon’s Mr. Osman, the MV drives have accounted for more than 15,000 hours of service with only two failures, since their installation in January 1997.

The MV drives’ underlying benefit has been process improvement at the copper mine, due to well controlled adjustable-speed operation of cyclone pumps. The original ‘direct, on-line constant-speed pump operation did not provide the right grinding quality of the copper ore. Proper separation of the fine particles from the cyclones is crucial for efficient ore separation,’ explains Mr. Osman.

Robicon mentions two other applications of its MV drive. These are applications in the U.S. and involve constant-torque output. One is a screw conveyor with 1,000 ft (305 m) of cable between the motor and 600 hp (450 k W), 4.16 kV MV drive; the other applies two 300 kW, 4.16 kV drives operating a paconite kiln in a steel mill

An open-cast mine is likewise the application locale for Siemens’ Simovert MV drives with active front-end (AFE). This installation, at the Laubag lignite coal mine (Welzow, Germany), is a first for Simovert MV in belt-conveyor operations, and involves four MV drives generating 2,000 kW each. Dispersion of the mine’s power network over a wide area, tending to make it unstable, and concerns for potential line reactance problems coming from the drive system had to be resolved.

Goals for the MV drive included eliminating the need for harmonics filters and other compensation equipment. AFE technology was up to doing the job with its ability to minimize harmonics, according to Siemens. In Simovert MV, both the input (line-side) and output (load-side) sections of the drive have identical construction, with IGBTs serving as the power-switching devices. See main article for more on Siemens MV drives with AFE technology.

Motor types involved in this application are induction motors for belt conveyor speed regulation and ac wound-rotor motors that run the upper and lower belt pulleys. The MV drive system was successfully integrated into the existing plant and began operating in September 1999.

With new developments and technologies continuing to flow into medium-voltage ac drives, their market and applications are certain to widen.

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