Medium-Voltage AC Drives Shed Custom Image
Where do you find control solutions for the largest industrial- or power-plant-type loads that require multi-megawatt electric motors to power them? Today, one growing technology option is the medium-voltage (MV) ac drive. MV drives are similar to, but hundreds of times larger than, their more common low-voltage (LV) cousins.
Frank J. Bartos, CONTROL ENGINEERING
Where do you find control solutions for the largest industrial- or power-plant-type loads that require multi-megawatt electric motors to power them? Today, one growing technology option is the medium-voltage (MV) ac drive. MV drives are similar to, but hundreds of times larger than, their more common low-voltage (LV) cousins. Custom-engineered by tradition, MV drives now use design approaches and system tools no different from low-voltage drives.
Besides delivering the necessary power to an application, variable-frequency drives operating at medium voltages produce less losses than low-voltage drives, and allow smaller cable sizes from the motor and power source. This yields better overall drive efficiency and lower system costs.
And just what is "medium voltage?" The term is not uniformly defined; it varies by industry and application. For ac motor drives, the range from above 600 V up to 15 kV represents some consensus. In Europe, 1 kV is considered the MV threshold. More practically, existing MV drive products have a narrower range of 2.3 to 7.2 kV. However, that range can be expected to expand upward in the near future. Standard North American medium voltages are 2.3 and 4.16 kV, while Europe and the rest of the world prefer 3.3 and 6.6 kV.
MV drives on the move
Virtually all suppliers agree a major driving force behind recent activity for MV drives is savings in energy costs.
Rockwell Automation Canada Inc. (Cambridge, Ontario, Canada) notes substantial energy savings are possible in powering large fans, pumps and compressors—typical applications for MV drives. "In many cases, the lower cost of new MV drives allows users to enjoy a payback in less than one year," says Ralph M. Paling, manager of marketing communications, Medium Voltage Business at Rockwell Automation Canada. Mr. Paling cites other factors for renewed activity in MV drives:
Advances in power semiconductor switches—new devices such as SGCTs (see power switch sidebar) improve packaging, increase reliability, and reduce overall drive cost;
More competitors worldwide—most major drive manufacturers now have a MV product, raising market awareness and offering customers more choices than ever before; and
Standardization—historically, most MV drives were highly engineered products with lead times of 20-30 weeks. Standardized product lines and business system automation today can shrink delivery time for most "standard" MV drives to six to eight weeks.
ABB (New Berlin, Wis.; Turgi, Switzerland) is keen about market growth for ac variable-frequency drives (VFDs). It attributes the increased interest to improvements in drive performance, reliability, efficiency, and power quality over prior generation products.
Ram Bhatia, manager of Medium Voltage Drives at ABB Automation Inc., concurs about the importance of energy savings, especially when considering replacement of an existing fixed-speed drive with a VFD. "Savings are achieved via higher efficiency and power factor," he remarks. "Users are concerned about three important areas: total cost of a new installation (that is, need to replace existing fixed-speed motors), payback time, and reliability of VFDs." Mr. Bhatia estimates typical payback at between one and five years, depending on specifics of an installation.
ABB's answer to these user concerns is its ACS 1000 MV drive, launched in Europe in late 1997 and later in the U.S. The drive incorporates an advanced control method called Direct Torque Control (DTC) that allows accurate control of motor speed and torque without pulse encoder feedback from the motor shaft. DTC enables full-load torque at zero speed. It also meets IEEE 519-1992 and the U.K.'s G.5/3 specifications for harmonic distortion for virtually all installations, according to ABB. Power range is 315-5,000 kW at 2.3, 3.3, and 4.16 kV.
Retrofit and regional markets
ACS 1000 is optimized for the retrofit market, with no need to derate existing induction and wound-rotor ac motors. A built-in sinusoidal output filter minimizes motor insulation stress caused by common mode voltages (harmful voltage spikes arising from rapid switching of power semiconductors). "Simply enter the existing motor nameplate data into ACS 1000 software, and the drive does the rest," says Mr. Bhatia.
Jeff Mason, marketing manager, Large Drives at Siemens Energy & Automation (Alpharetta, Ga.) agrees that energy costs are fueling MV drive developments. "With deregulation, even utilities are not immune from energy efficiency concerns. Large fans and pumps in power plants make prime MV drive applications," he states. Mr. Mason further notes regional design differences for these drives. In Europe, an MV drive is typically installed in the middle of the plant floor. Power connection at the back (or bottom) of the unit is adequate with this more "open construction." North American installations are often against the wall or in more confined spaces, requiring front or side access for the power feed. Also, overhead cable trays and bus ducts are more common in the U.S., making power connection from the top a desirable feature.
Renewed activity in MV drives is also seen at Robicon (New Kensington, Pa.). Aside from the ubiquitous need to conserve power, impetus comes from new product introductions with superior capabilities. "These drives give customers new options without the drawbacks of previous MV products," says Richard Osman, Robicon's vp of technology.
New generation products focus on improving power quality at both the input and output of the MV drive. Earlier drives did not adequately address this need, and users got a taste of power quality problems. "Users realized that an MV drive had to deliver very low harmonics on the input to avoid upsetting nearby equipment, and good output waveforms to avoid damaging the motor," explains Mr. Osman.
Looking at benefits
Robicon believes that bringing variable-speed operation to processes with high power demand is the real efficiency gain (or benefit) of MV drives. Users find the ability to tune the process through speed control creates more flexibility to respond to changing production needs. "Operating at reduced speed also makes the rotating equipment last longer. Avoiding the need to start a motor on line, which causes a large inrush current, is another substantial benefit of MV drives," adds Mr. Osman.
The much larger MV motors represent a sizable investment to protect. Starting is not a routine step for these motors, and manufacturers limit the recommended number of starts over time. Soft start capability of MV drives provides a useful function.
MV drives offer benefits beyond improved power output and efficiency. New power switches—available up to 6.5 kV (peak inverse voltage)—enable drives with higher voltage ratings (up to 6.6 kV) and wider power ranges, without step-up transformers. They "reduce component count and cost at the higher voltage ratings. Historically, 6.6 kV drives were not competitive below 1,000 kW," says Rockwell Automation's Mr. Paling.
Variable-frequency soft starting of large MV motors and multimotor starting/speed control are further beneficial features of MV drives. In some cases, multiple motors can be started individually by one MV drive, then synchronized to the input line frequency as each motor comes up to full load and speed (see applications section in Online Extra at www.controleng.com ). The drive only needs to be sized for the largest motor, reports Rockwell Automation.
Power switches: the heart of the drive
GE Industrial Systems (Salem, Va.) considers the new high-voltage semiconductors at the forefront of its recent investments in MV drive products. "Better performance and new economics" of these devices—for example, ever higher voltages, fewer components, and faster switching times compared to older power switches—are vital to progress, according to Paul Bixel, MV product engineer. "In short, it's much easier to work with the new power devices," he says.
Compared to previous generations of MV drives, GE Industrial Systems sees customer benefits, such as unity power factor (PF=1), high output frequencies, lower harmonics, and lower installed cost coming from MV drives that use the latest power switches. Its regenerative PWM drives offer the added benefit of VAR (volt-amperes reactive) control. This type of drive can produce a leading or lagging VAR component to help offset opposing VARs produced by other ac equipment in a customer's plant—thus correcting the power factor.
Mr. Bixel notes growing awareness of MV drives in industry; and potential users now show more willingness to try out a newer technology. "As customers become more comfortable with MV drive technology they are applying them to an expanding scope of processes to reduce stress on their power systems, lower operating costs, and extend the life of their equipment."
Innovation Series MV drives from GE Industrial Systems offer non-regenerative units using IGBTs from 2.3-7.2 kV ac, up to 9 MW power. For regenerative drives, today's economics favors IGBTs below 6 MW. At power levels above 10 MW non-regenerative (6 MW regenerative), IGCTs are the device of choice for PWM drives, according to Mr. Bixel.
These drives control synchronous as well as induction motors. A common control platform consisting of operator interfaces, diagnostic tools, communication networks, training, etc., exists across all of the company's LV and MV drives. It simplifies users' or integrators' implementation tasks, regardless of the mix of drives they may have in their system.
Even with recent developments using new IGBT- and IGCT-based drives, GE Industrial Systems is not putting all of its eggs into one power switch technology basket. "We continue to follow trends in power devices, expect new developments, and will respond with new products as the situation warrants," he adds.
Siemens (Erlangen, Germany; Alpharetta, Ga.) focuses on HV-IGBTs as the power switch in its newer drives. Simovert MV drives—termed ac converters—operate between 2.3 and 6.6 kV, with a power range from 300 kW to 6.8 MW. These modular units reportedly offer precise speed control from 0-9,000 rpm and "one of the smallest footprints in the industry," which can open up wider applications. Both air- and water-cooled units are available; water cooling presently applies to drives above 4 MW.
Siemens E&A's Mr. Mason mentions even larger, special medium-voltage drives, such as cycloconverters and LCI (load-commutated inverter) units that range up to 100 MW!
Siemens claims several advantages of HV-IGBTs versus GTOs and IGCTs. For example, IGBTs provide full control of voltage and current transients via the IGBT gate; also, they need less gating power and use fewer gating parts, which implies higher reliability.
Simovert MV drives include an active front-end (AFE) or input converter that adjusts the power factor to essentially 1.0 and, if its power rating has adequate margin, can help correct for reactive load components (VARs). An AFE is also used when an application needs regenerative braking. An input filter on the AFE limits harmonics entering the voltage supply line.
The latest MV drive from Rockwell Automation is PowerFlex 7000 (announced in December 1999), with first-phase products in the 500-4,000 hp (375 kW-3 MW) range at 2.3-6.6 kV. Its power-switching devices are yet another variant of semiconductor technology—the SGCT (symmetrical gate-commutated thyristor). SGCTs are used in both the inverter and PWM rectifier sections, providing benefits of design efficiency and fewer spare parts because of this common power structure.
"The SGCT with integrated gate drive, high switching frequency, and double-sided cooling is an ideal power semiconductor switch for MV drive applications," explains Mr. Paling. "A pulse-width modulation (PWM) switching pattern is optimized for the lowest possible conduction and switching losses, resulting in a compact and efficient inverter design," he adds.
Large, yet modular equipment
A patent-pending Power Cage houses the main power components in Rockwell Automation's PowerFlex 7000 MV drive. This compact, modular package (see photo) includes advanced heat sink design that, combined with a "high-pressure air flow pattern," ensures efficient heat transfer and reduces thermal stresses. Power Cage components can be replaced on the spot without any special tools in less than five minutes, according to Rockwell.
Newer drives with PWM sinewave output also handle retrofit of existing motors or new standard ac motors without derating.
ABB's medium-voltage drive, ACS 1000, employs IGCT power semiconductors (see sidebar on power-switching devices). Actually, these are "in-house" power switches, developed by a division of the company, ABB Semiconductors (Lenzburg, Switzerland; U.S. headquarters, Pittsburgh, Pa.). The division also markets IGCTs to other customers, including drive manufacturers. "IGCT is an evolutionary technology that incorporates significant advantages of GTO and IGBT devices," adds Mr. Bhatia.
For more about related topics, refer to the following Control Engineering issues: ACS 1000 ( CE , Dec. 1997, p. 9); ABB's IGCTs ( CE , July 1997, p. 17); and "Power Modules and Devices Advance Motor Controls" ( CE , April 1998, pp. 91-101).
A different approach
Robicon takes a different approach in applying power semiconductors to MV drives. Its Perfect Harmony drive uses IGBTs in a new series-cell, multilevel circuit topology. It consists of standard IGBTs arranged in modular units called power cells. These standalone power-conversion modules, with single-phase output, are then connected in series to obtain the higher voltage requirements of the application.
For present MV products, Robicon relies entirely on IGBTs. It reports success with this design, based on standard IGBTs available in high-volumes and at favorable prices. However, the company is looking at 3.3 kV HV-IGBTs with the objective to build power cells for 13.2 kV drives, according to Mr. Osman.
As for alternative technologies, Robicon considers IGCTs a "substantial improvement over GTOs." Conduction losses favor IGCTs versus IGBTs, but the case is vice versa for switching losses. Mr. Osman also points to the disadvantage of few IGCT producers compared to the number of IGBT suppliers. [ABB Semiconductors, see above, and Powerex (Youngwood, Pa.) are two suppliers.] "At first, IGCTs had higher voltage ratings than IGBTs, but now IGBTs are available at 6.5 kV," he adds. See www.controleng.com for more details.
The new generation Perfect Harmony drive boasts much smaller footprint and lower cost. It covers the 300 kW to 3 MW power range at 2.3-6.6 kV. Robicon's MV drives, including water-cooled units, cover standard medium voltages of 2.4, 3.3, 4.0, 4.8, 6.0, and 7.2 kV, at 400 kW to 10 MW.
With power-switching technology developments and other design refinements continuing to evolve, medium-voltage ac drives are moving away from a custom-engineered image. They have an incentive to emulate the success of their low-voltage counterparts.
A Babel of power-switching devices
The heart of any medium-voltage ac drive is the type of power electronics device that handles the rapid and controlled switching of large voltages and currents. Translating the "Tower of Babel" of related power device acronyms:
GTO (gate turn-off) thyristor is a power semiconductor similar to an SCR (see below), but handles lower currents and can also be turned off by a negative gate terminal signal. Its switching frequency is higher than an SCR. Low power factor, low efficiency, and need for output filtering add to its application costs.
SCR (silicon-controlled rectifier) is a one-directional, solid-state switch offering high current handling capability that retains its usage in very large MV drives. Current going to the gate terminal controls breakover voltage , the point at which conduction starts. Turn-off occurs as current is reduced below a holding value. Drawbacks include relatively slow switching speed and large size of the resulting drive. GTOs and SCRs are mature power devices.
IGBT (insulated-gate bipolar transistor) combines best features of a MOSFET (metal-oxide semiconductor field-effect transistor) input and a bipolar transistor output in a newer power-switching device. Very rapid switching results, since no junction effect exists at the input. Power consumption is small due to the insulated gate. Standard IGBTs have voltage-switching limits and need to be connected in series for MV drive usage. A still newer high-voltage device, HV-IGBT , extends operating voltages to those required by MV drives. This eases the need to gang standard devices. See Online Extra at
IGCT (integrated gate-commutated thyristor) combines the high-switching frequency and low switching losses of IGBTs with the high voltage handling capability and low on-state (conduction) losses of GTOs. The integrated diode and gate unit lowers the parts count, resulting in increased reliability.
SGCT (symmetrical gate-commutated thyristor) is a modified version of a GTO device and similar to the IGCT. SGCTs block voltage in both directions while allowing only one-directional current to flow. However, SGCTs do not need a series diode or anti-parallel diode, as IGCTs do. This is said to result in the lowest possible component count. Presently, IGCTs and SGCTs have fewer suppliers than IGBTs.