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MV drives now come ‘transformer-free’

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Medium-voltage (MV) ac motors have long been the preferred motive source for high-power (larger than 300kW) rotating plant often found in Australia’s mining sector.

Because of transmission line and motor winding I2R efficiency, the MV motor is often the best option for larger conveyors, ball and SAG mills, high-pressure grind rollers, induced draft fans and so on. In Australia, such MV motors are usually rated at 3.3 or 6.6kV.

Rockwell Automation Australia product manager medium voltage Scott Southby says until around 10 years ago, Australian mines used a range of electro-mechanical solutions for MV drive starting and speed regulation.

“Traditional solutions included wound rotor motors with slip energy recovery systems, or fluid coupling transmission systems,” Southby says.

“These are complex, maintenance-intensive and provided fairly poor matching of the output power to the actual load.”

Advances in MV power electronics changed all this a decade ago, with the introduction of MV solid-state variable speed drives.

Today, MV variable speed drive systems support the motor starting and variable load/speed adjustment needs of these very large items of rotating plant.

To date, the downside of the conventional MV drive is the bulky and costly isolation transformer required to accompany such drives. However, recent technology advances are set to free the MV drive of its transformer legacy.

The need for an MV drive isolation transformer stems from the variable speed drive’s evolution. Two distinct breeds of solid-state variable speed drives have evolved over the past 20 years or so: the current-source inverter (CSI) and voltage-source inverter (VSI).

Both rectify the ac power to dc and then invert the dc back to ac at the required frequency (which, in turn, determines the motor speed). CSI drives are inherently current regulating and need a large internal dc link inductor to operate, as well as a motor load. VSI drives, by comparison, are voltage regulating and use capacitors in place of the inductor.

Because of the physical bulk of the dc link inductor, most manufacturers in the low-voltage (LV) drive sector have opted for the VSI topology.

For simple reasons of tradition, VSI topology has tended to jump the gap from LV into the MV drive world.

Southby says while VSI works well for the “less than 300kW” low-voltage motor market, it is not the most practical solution in MV systems.

“The MV drive is a completely different animal to the LV drive,” he says.

“While the dc link inductor of an MV CSI drive still represents a fairly large weight and size, the MV VSI topology requires a large and expensive multi-winding isolation transformer. So by opting for a VSI-type you’re really robbing Peter to pay Paul.”

Southby says a further disadvantage of VSI against CSI is that achieving regenerative braking with the former is impractical, complex and costly. With CSI, regenerative braking is inherent.

But the isolation transformer is the Achilles heel of the conventional MV drive: It protects the motor from the common-mode voltage (CMV) stress that occur as a result of the VSI’s design, along with providing some reduction of the harmonics generated by such large drives.

The isolation transformer is used to isolate the supply system earth from the drive system earth, which allows the motor neutral point to be earthed.

Although this method protects the motor from CMV stress, that same high-level CMV stress is instead imposed on the transformer and the cable insulation. This requires extra transformer insulation and cable insulation to withstand the CMV stress, adding engineering and extra costs.

As a result, the conventional MV drive isolation transformer is particularly bulky: an isolation transformer can represent 30 to 50% of a drive system’s size and 50 to 70% of the system’s weight. This extra size, weight and cost can make MV drive retrofits difficult and costly.

PowerFlex 7000 direct-to-drive technology, a new drive design from Rockwell Automation, combines CSI topology with advanced technologies to free the MV drive from the isolation transformer.

“While the CSI topology was one of the earliest variable speed drive architectures, advances in solid-state switching technology have allowed it to be ‘re-invented’,” Southby says.

“This is proving a real plus for MV drive applications.”

To achieve this, the direct-to-drive technology combines three innovations: new CMV stress protection, an active front end (AFE) rectifier and the symmetrical gate commutated thyristor (SGCT).

Instead of using the transformer method, the PowerFlex 7000 Direct-to-Drive technology uses zero sequence impedance to almost eliminate the CMV on the motor neutral. The MV drive can now use standard motor and cable designs with no isolation transformer.

Conventional MV drive manufacturers usually rely on the isolation transformer secondary winding phase-shift to help reduce harmonics.

“The more secondary windings, the higher the ‘pulse number’ and the better the harmonic elimination level. But with up to 15 sets of secondary windings, complexity and high component count are major disadvantages. In addition, this method requires a perfectly balanced three-phase distribution system for optimum harmonic elimination,” Southby says.

Instead of an isolation transformer, the PowerFlex 7000’s direct-to-drive AFE rectifier uses semiconductor switching to reduce line current harmonics to levels that comply with the world’s most accepted harmonic standards.

The AFE rectifier, also known as the pulse width modulated (PWM) rectifier, uses the SGCT to produce a PWM switching pattern that prevents the drive from producing high levels of line current harmonics while avoiding the use of a phase-shifting isolation transformer.

Importantly, the CSI topology allows the drive to provide 100% continuous full current regenerative braking without putting thermal stress on the motor. This ensures efficient and cost-effective braking, which is essential for mining applications such as large downhill conveyors.

Size and weight

The PowerFlex 7000 with direct-to-drive technology is usually smaller and lighter than drive technologies using isolation transformers.

The typical volume of space needed for a 950kW drive with isolation transformer is 13m3, and typical weight is 4200kg. The transformer-less 7000 of the same voltage and power is 60% smaller at 5.4m3, and 65% lighter at 1350kg.

A smaller and lighter drive system means no shipping splits to connect and no inter-cabling between drive and transformer.

As a result, the transformer-less MV drive is suitable for mining application retrofit projects where space in existing switch and control rooms is often limited or at a premium.

Reliability, one of the most important factors in the drive and motor system, is also improved using this transformer-less approach.

The CSI-based direct-to-drive system uses the dc link inductor and line reactor to limit fault currents to levels less likely to damage the drive system or connected equipment. It also eliminates the need for fuse protection in the drive.

By using SCGT components with high voltage ratings, the 7000 MV drive has a component count a fraction of conventional MV drives. As an example, in a usual 3.3kV, 750kW motor application, a legacy VSI-based MV drive can have up to 385 components in its power circuit – largely diodes, fuses, capacitors, IGBTs or IGCTs. By contrast, an equivalent PowerFlex 7000 with direct-to-drive technology has less than 30 components. Fewer components improves mean time between failure and total drive system reliability.

Aside from the direct cost saving achieved by removing the isolation transformer, the direct-to-drive design reduces the install spend in other areas: notably it eliminates the need for an isolation transformer protection relay, a dV/dt filter, sine filter or motor terminator and special cables.

There are also other cost savings with transformer-less MV drive installations. These include a reduction in cabling, air conditioning, civil engineering, concrete pad construction for outdoor transformers and overall installation.

By adopting a transformer-less drive, the cost of isolation transformer crating, handling and transportation are also eliminated.

“Shipping large transformers across Australia can prove very costly,” Southby says.

“An MV drive isolation transformer is usually too large for container storage and has to be specially crated and shipped on the deck.”

With most transformer makers on the east coast, freight costs to some of Australia’s major mining centres can be many thousands of dollars.

“Also a transformer-less drive can be built and shipped to site much quicker than transformer manufacturers can build and ship an isolation transformer.”

Southby says an example is the 12 PowerFlex 7000 direct-to-drive MV systems being supplied to Alcoa World Alumina’s Pinjarra refinery between February and July this year.

“This is a particularly tight delivery timeline. If isolation transformers had been involved, the delivery date simply couldn’t have been met. The isolation transformers couldn’t have been made and delivered in the time.”

Southby says the strategic advantage of the transform-less MV drive for the Australian mining industry is clear.

“Why buy an isolation transformer when you don’t need one?” he says.

“A transform-less direct-to-drive system reduces MV drive capital and install spend, cuts the build and delivery time, and is ultimately a simpler and more reliable piece of plant to own and maintain. This is really the MV drive of the future,” he says.

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