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Selecting the right voltage for high powered variable frequency drives

Modern variable speed drives (VSD) have evolved into a reliable controller commonly used across most industries including manufacturing, mining, and water. Traditionally, VSDs have been used for process control of pumps, fans, conveyors, but with the rising costs of power and the increased efficiency and reliability of modern VSDs, many applications can be warranted on power savings alone.

The development of VSDs has been dramatic. VSDs can now be applied in low voltages (LV) in applications above 2,000kW in low voltage, and medium voltage (MV) drives down as low as 400kW. This large power range overlap can lead to uncertainty on what factors should be accounted for when considering what voltage to use for a particular installation.

Today advances in drive development gives users many more options when selecting drive technology and voltage. No two applications are the same, and this means that where there is more than one possible solution, the most cost-effective strategy is to conduct a cost/benefit analysis of the various options.

The first and often only consideration in choosing a drive is the initial capital cost. Even with recent develop ments, MV drives can be relatively expensive. Typically the cost of an LV VSD and an output step-up transformer are much lower at only 50 to 75 per cent of the initial cost of an MV drive. However, to gain a true cost of a MV drive over a five year operation period, many other factors need to be included, including harmonics, cabling, cooling costs and maintenance.

Authority requirements
As more applications convert to VSD control, the need to comply with Supply Authority requirements is increasing. This has seen a greater emphasis by utilities to comply with harmonic stan dards such as IEEE 519, EN61000-2-4 and G5/4. There are several methods to reduce harmonics on a large installations and the best solution is usually a mix of the methods.

For applications such as decline conveyors, where the drives are required to regenerate excess energy back into the supply, active front end (AFE) drives are available in both MV and LV. An AFE drive also has the added benefit of oper ating with a greatly reduced harmonic draw on the supply.

Although AFE drives do have warranted use for harmonic mitigation, they are unfortunately often overused at the risk of a reduced mean time between failure (MTBF) and reduced efficiency. Where harmonics are the prime concern and regeneration is not a factor, alternate solutions should be investigated. The most efficient and robust solutions can be found with active harmonic filters or multipulse variable speed drives, which can reduce the lifetime cost of the drive system.

Total harmonics
For higher powered drives, a multipulse supply transformer is often the best solution – these transformers effectively increase the number of phases to be rectified, thus reducing the total harmonics produced at the standard three phase supply.

For example, a “12 pulse trans former” will supply six phases to the drives. The actual cost of the drive is not significantly affected by this design. Typically a 400kW VSD has parallel rectifiers, allowing simple inte gration into either a standard six pulse (three phase) or a twelve pulse (six phase) supply.

Integrated input
With higher-pulse number MV drives, one common method of achieving harmonic reduction is through an integrated input transformer with multiple phase-shifted secondary windings. These work on the principle of ‘the higher the pulse number, the greater the degree of harmonic reduction.’

For example, with the use of a 36 pulse MV drive, compliance to these standards is usually automatic, with the added benefit of higher overall efficiency and reliability.

These drives can be supplied with the primary winding made to suit the existing supply voltage, for example 11kV. This can remove the need for an intermediary transformer, reducing the losses and simplifying the installation.

Drive installation
Another solution is the use of active harmonic filtering. This technology corrects the harmonics on the supply bus and is fitted in parallel to the drive installation. This makes it easy to retrofit to an existing installation and also means that it can compensate for several VSDs and other harmonic loads at the same time. Large plants can also be compen sated centrally at the medium-voltage level via an auto-transformer.

With the increased use of power elec tronics in most of our everyday prod ucts, harmonics are here to stay. It is important to understand their content and effects, as well as to actively imple ment solutions to reduce or manage harmonics in installations to avoid faster aging of equipment and unforeseen outages resulting in loss of productivity and efficiency. To determine the correct harmonic mitigation equipment required, a simulation should be under taken. This service is available from Schneider Electric.

The distance between the drive and the motor is a critical factor when deter mining the correct drive technology and configuration. As cable distances increases between the drive and the motor, more consideration is required to ensure both the drive and motor are adequately protected from over voltage spikes as well rapid voltage changes (dV/dt). The cable costs and increased losses can be significantly higher with LV applications. Therefore, the longer the cable length the more weighting needs to be given to a medium voltage solution.

Traditional cooling
In addition to cabling, another cost that must be considered when comparing VSD solutions is cooling. All equipment that uses or handles power generates heat. With high-power drive systems in enclosures, this heat needs to be dealt with.

Most drives are air cooled, and when operating at high power, heat losses become significant. For example: a three percent loss at 1MW is 30kW, a figure which would well justify the use of alter nate cooling methods for VSDs and associated components.

An alternative to traditional cooling is the use of a separate air flow system. With this method, the option of using outside air via ducting to cool the heatsink is simplified. This results in only the control losses that need to be dealt with for the switchroom.

Heat exchangers
The alternative of liquid cooling effec tively removes about 90 per cent of the heat generated by the VSD losses out of a control enclosure, but involves addi tional costs for pumping cabinets and heat exchangers, if not already available on the site.

The decision between air condi tioning or liquid cooling is usually appli cation-based, and best made after assessing the availability of either option at the site. The cooling concept needs to be considered at the beginning of the plan ning, as changing an existing cooling concept ranges from uneconomical to technically impossible.

For example, in the water industry the availability of liquids and pumping equipment may offer distinct benefits when selecting liquid cooling solutions. Schneider Electric offers a variety of intelligent cooling solutions that are factory tested and approved.

The decision to re-evaluate pre-conceived perceptions when looking at both solutions and to take into account the discussed points may result in a change in solution or may re-affirm your choice in the solution you currently adopt.

[Craig Southwell is Drive Systems Manager at Schneider Electric.]

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