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Greater levels of standardisation are substantially reducing the delivery times for most ‘standard’ MV drives.

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The highs and lows driving the voltage vs. current question


Until quite recently, most engineers faced with a high-power application would choose a low-voltage (LV) drive system working at very high current levels. The alternative – medium-voltage (MV) drives – have historically never been very attractive, due to the perceived high purchasing cost, physical size, requirement for custom engineering and high installation costs. Recently though, the MV drive, in particular Allen-Bradley’s PowerFlex 7000 series, has shed this image and is offering a real alternative to LV drives in the higher power sector. Jan van den Bossche, Hi-Power Drives business manager, Rockwell Automation explains.

The emergence and greater acceptance of MV drives should not imply that they are the solution to every high-power application.

A number of reasons are given for the heightened levels of interest in MV drives. These reasons include: increased selection; advances in power semiconductor switches; increase reliability; reduced overall drive cost; and standardisation. In the past most MV drives were highly engineered products with lead times of six months or more. Today, greater levels of standardisation are substantially reducing the delivery times for most ‘standard’ MV drives.

The list above can also be added to with the major advantages of the ‘transformerless’ design of Allen-Bradley’s PowerFlex 7000 MV drive. This Direct-To-Drive technology reduces drive system size by 30 to 50% and drive system weight by 50 to 70%. It also improves system efficiency, by eliminating transformer heat losses. In addition, air conditioning costs to cool transformers are avoided, enabling users to benefit from a lower total cost of ownership.

The emergence and greater acceptance of MV drives should not imply that they are the solution to every high-power application. No two applications are the same, so, in applications where there is more than one possible solution (LV or MV), then the most cost-effective strategy is to conduct an analysis of the benefits of the two systems. With different manufacturers’ products this can be difficult; however, with Rockwell Automation providing the PowerFlex series right across the power range, through LV up to 690V and 1,500kW and MV up to 6,600V, an effective comparison is achievable.


 

The move to more modular systems is simplifying MV structures, making their acceptance more widespread.

The first factor to be considered is cost. Even with recent developments, medium-voltage AC Drives are not inexpensive. Typically, the cost of a low-voltage AC Drive and an Output (Step-Up) Isolation Transformer is only 50 to 75% of the cost of a medium-voltage AC Drive.

As well as equipment costs, the cost of cabling the two competing systems must be considered. For LV drives at high power, cable and installation costs are high. Moreover, with LV systems, a transformer is always required, adding the cost of wiring between transformer and drive; and they also require shielded EMC cable, which is very expensive. In contrast, the expense for MV systems is much less, due largely to the lower current carried and the fact that they do not need to employ EMC shielded cables. In addition, by using a PowerFlex 7000 MV drive, cabling costs are cut further still, due to the non-requirement for a transformer (Direct-To-Drive Technology) and the saving in wiring and system complexity this delivers.

The ‘transformerless’ Direct-To-Drive technology of the PowerFlex 7000 also contrasts favourably with the requirement of LV systems for transformers to suppress harmonics. Direct-To-Drive provides the PowerFlex 7000 with a highly compact active front-end rectifier that reduces harmonics by active switching and selective harmonic elimination. The effectiveness of this technology contrasts well with common-mode voltage protection, but with no transformer or transformer protection relays required, offers users the substantial benefits of reduced equipment and cabling costs, reduced installation and savings in floor space.

As well as equipment costs, the cost of cabling the two competing systems must be considered.

The ability of Direct-To-Drive technology to achieve space saving also impacts upon two other areas where a direct comparison between LV and MV drives is usually essential: their relative size and weight. In general terms, a packaged LV system would definitely be smaller and lighter than an MV system based upon conventional architecture; but the margin between the systems would be much closer to call using the Direct-To-Drive technology of PowerFlex 7000.

Another major factor to be considered is communications. LV drives are more commonly associated with the communication interfaces that facilitate factory networks. Here, however, the PowerFlex LV and MV drives have taken a lead, by providing the same communication options, and, in addition, employing the same programming and software tools. Finally, there has to be the question of service and maintenance. MV applications have traditionally been regarded as complex, while most maintenance staff are comfortable with LV AC drive applications.

Custom engineered MV products of the past were certainly unwieldy and required specialist attention. However, the move to more modular MV systems, as epitomised by the PowerFlex 7000 family, is simplifying MV structures, making their acceptance more widespread and their application as understood as LV types.