Historically, the task for low-voltage (LV) users was relatively straightforward: choose from among the many suppliers based on price, reliability, functionality and communications.
However, with drive control applications for the larger loads found in the Australian mining sector, the situation is no longer so clear-cut.
Until recently, most engineers faced with a high-power application would choose a LV drive system working at very high levels of current. The alternative of a medium-voltage (MV) drive was not very attractive because of perceived high cost, physical size, need for custom engineering and high installation costs.
Now, the MV drive is offering a real alternative to LV drives in the higher power sector; especially where installations are judged across a wide range of criteria, rather than just initial drive cost.
There are a number of reasons for this interest in MV drives: increased selection as most major manufacturers now have an MV product; advances in power semiconductor switches with new devices such as symmetrical gate-commutated thyristors that improve packaging, increase reliability and reduce overall drive cost; and greater levels of standardisation are substantially reducing delivery times for most standard MV drives (see Figure 1).
The emergence of MV drives and their increased acceptance in industry should not imply they offer the solution to every high-power drive application.
In applications where there is more than one possible solution, the most cost-effective strategy is to conduct an analysis of the benefits of the two drive systems. With different manufacturers products this can be difficult. However, with Rockwell Automation providing the Allen-Bradley PowerFlex series across a broad power range, LV up to 690V and 1500kW and MV up to 6600V and 12MW, an effective comparison is achievable.
In some cases comparisons are unnecessary, as the decision to go LV or MV is simply a question of the amount of power available in the distribution network at each voltage.
If adequate power is available at more than one voltage rating, then the choice of selecting LV or MV has to be based on another factor.
Even with recent developments, MV ac drives are expensive. Usually the cost of a LV ac drive and an output (step-up) isolation transformer is only 50% to 75% of the initial cost of a MV ac drive. However, as the application current increases, the system fault current also increases, which may have safety repercussions and can affect the cost of the drive system.
Furthermore, savings in additional cabling and installation costs can balance the initial expense of the drive system.
Technical issues such as drive architecture and topology best suited for high-power applications needs considering. At high power, the demands of the application and equipment investment are greater.
Variable frequency drive (VFD) output power quality is an extremely important consideration for protecting the motor investment and ensuring its continued performance. With LV architectures, the most common is pulse width modulation voltage source inverter (VSI) technology. This offers reliability, but loses efficiency with each on/off switching transition.
In addition, the true levels of voltage and dv/dt rise produced by the pulse-width modulation of voltage operation may cause reflected waves, motor insulation stresses and ground fault currents.
In contrast, the symmetrical gate-commutated thyristors (SGCT) in the PowerFlex 7000 MV drives are designed for high-voltage operation and ensure the lowest switching and conduction losses, while maintaining a high switching frequency.
The SGCTs also offer a simplified power structure, with fewer components to optimise reliability.
The simplified power structure of the PowerFlex 7000 also provides benefits in harmonics suppression.
As more applications convert to VFD control, there is a greater emphasis by utilities to comply with local relevant harmonic standards. Therefore, VFD makers have incorporated methods to reduce harmonics in their VFD designs.
The problems of cost, complexity and added weight, size and heat that transformers bring to LV and most MV systems have led Rockwell Automation to design a series of MV drives that do not require them.
The technology is known as direct-to-drive. It allows utility power to be connected directly to the drive without an isolation transformer.
New or existing motors can be connected directly to the drive, which eliminates unnecessary motor filtering.
The direct-to-drive functionality of the PowerFlex 7000 is achieved by using a compact active front-end rectifier in place of a traditional transformer. The rectifier reduces harmonics by active switching and selective harmonic elimination.
Direct-to-drive technology also reduces common mode voltage to levels suitable for existing motors. With no transformer or transformer protection relays needed, users benefit from lower equipment and cabling costs, simplified installation and savings in floor space.
The ability of direct-to-drive technology to save space is illustrated with a comparison between LV and MV. When comparing the size and weight of different drive solutions it is important to include all components: the VFD, the input impedance, output impedance, input disconnect device, and input and output filters. MV drive systems are traditionally supplied as totally integrated and cabled solutions, which usually means they are bulkier and heavier.
However, by using the direct-to-drive transformerless technology, the MV drive package becomes competitive in size (reductions of up to 50% compared with conventional systems) and weight (reductions between 50% and 70%).
Based on conventional architecture, a packaged LV drive system is generally smaller and lighter than an MV drive system. However, PowerFlex 7000’s direct-to-drive technology reduces the margin between the two drive systems.
Cable cost must be determined as part of the VFD solution because of its impact on total cost of ownership.
Cable size and cost vary with the level of current they conduct: the higher the current, the larger the cable, and the greater its cost.
This means cable and installation costs are high for LV drives at high power and LV installations need expensive shielded electro-mechanical compatibility (EMC) cable.
The contrasting expense for MV systems is much less, due largely to the lower current their cables carry and because they do not need EMC shielded cables. By using a transformerless PowerFlex 7000 MV drive, cabling and wiring costs are cut further.
The example shown in Figure 2 demonstrates that the cable cost difference between the 480V and the 4160V VFD solutions grows disproportionately as the motor current increases. Also, the distances between the source, the VFD and the motor have a big impact on cable cost, especially at high power.
All equipment that uses or handles power generates a certain amount of heat. With high-power drive systems in enclosures, this heat can be a particular problem.
Most LV drives are air-cooled and at high power heat losses can become significant. A three per cent loss at 2MW is 60kW; a figure that justifies using air conditioning to cool the drive and its components.
While many plant maintenance engineers are comfortable with LV ac drive applications, they often have concerns about MV drive applications. MV drive solutions are thought complex and are almost always installed, commissioned and maintained by the drive supplier. The concerns are expense and dependence on the manufacturer.
How relevant these concerns are today is debatable. Certainly custom-engineered products of the past were unwieldy and needed specialised attention, usually above and beyond that available from internal plant personnel.
However, the widespread move toward modular MV systems is simplifying MV structures and making their application better understood in the same way as LV types.
The PowerFlex 7000 range offers good performance in MV speed control applications and challenges pre-conceived perceptions about MV and LV drive solutions.
The MV drive’s compact design lowers the overall cost of ownership and its adaptability allows users to re-evaluate their existing drive applications in order to make an informed drive system choice.
*Marjan Sprajc is MV drives product manager with Rockwell Automation Australia.