Selecting the right high-powered variable frequency drives

Modern variable speed drives (VSDs) have evolved into reliable controllers that are commonly used across most industries, including manufacturing, mining and water. Traditionally, VSDs have been used for the control of pumps, fans, conveyors etc, but with the rising cost of power and the increased efficiency and reliability of modern VSDs, many applications can be found on the basis of power savings alone.

The development of VSDs has been dramatic. VSDs can now be applied in low voltages (LV) in applications above 2000 kW and medium voltage (MV) drives down to as low as 400 kW. This large power range overlap can lead to uncertainty as to what factors should be accounted for when considering what type to use for a particular installation.

Today, advances in drive development give 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 best 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 developments, MV drives can be relatively expensive. Typically, the cost of an LV VSD and an output step-up transformer are much lower at only 50-75% of the initial cost of an MV drive. However, to calculate the true cost of an MV drive over a five-year operation period, many other factors need to be taken into consideration, including harmonics, cabling, cooling costs and maintenance.

Harmonics

As more applications convert to VSD control, the need to comply with the requirements of supply authorities is increasing. This has resulted in a greater emphasis by utilities on compliance with harmonics standards such as IEEE 519, EN61000-2-4 and G5/4.

There are several methods to reduce harmonics in large installations and the best solution is usually a mixture of 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 operating with a greatly reduced harmonic draw on the supply.

Although AFE drives do have use for harmonics mitigation, they are unfortunately often used 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 multi-pulse variable speed drives, which can reduce the lifetime cost of the drive system.

For higher powered drives, a multi-pulse 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 transformer will supply six phases to the drives. The actual cost of the drive is not significantly affected by this design. Typically, a 400 kW VSD has parallel rectifiers, allowing simple integration into either a standard six-pulse (three-phase) or a twelve-pulse (six-phase) supply.

Figure 1: Typical 12-pulse low voltage configuration.

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 harmonics 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 11 kV. This can remove the need for an intermediary transformer, reducing the losses and simplifying the installation.

Figure 2: Medium-voltage 36-pulse drive structure.

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 compensated centrally at the medium-voltage level via an autotransformer.

With the increased use of power electronics in most of our everyday products, harmonics are here to stay. It is important to understand their content and effects, as well as to actively implement solutions to reduce or manage harmonics in installations to avoid faster ageing of equipment and unforeseen outages resulting in loss of productivity and efficiency. To determine the correct harmonic mitigation equipment required, a simulation should be undertaken.

Cabling

The distance between the drive and the motor is a critical factor when determining the correct drive technology and configuration. As cable distance increases between the drive and the motor, more consideration is required to ensure both the drive and motor are adequately protected from overvoltage 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.

Cooling costs

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 3% loss at 1 MW is 30 kW, a figure which would well justify the use of alternate cooling methods for VSDs and associated components.

An alternative to traditional cooling is the use of a separate airflow 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 for the switchroom that need to be dealt with.

Alternatively, liquid cooling effectively removes about 90% of the heat generated by the VSD out of a control enclosure, but involves additional costs for pumping cabinets and heat exchangers, if they are not already available on site. The decision between air conditioning or liquid cooling is usually application 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 planning, 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.

Maintenance

Finally, the question of service and maintenance needs to be considered. Many plant maintenance engineers are comfortable with LV AC drive applications, but have concerns regarding medium-voltage applications, which they may see as very complex solutions with additional safety concerns.

With modern MV drives, the maintenance requirements have been simplified and customers have the option to have their staff trained to undertake the maintenance. The move to modular MV systems is simplifying MV drive structures, making their acceptance more widespread and their application understood in the same way as LV drive structures.

The decision to re-evaluate preconceived perceptions when looking at both solutions and to take into account the points discussed above may result in a change in solution or may reaffirm the choice of solution you adopt.

Craig Southwell

Southwell is Drive Systems Manager at Schneider Electric and has over 30 years' experience in motor control and automation. His experience spans design, maintenance and commissioning through to project management, across a variety of industries including mining, infrastructure, power generation, chemicals, water and wastewater treatment, food and beverage and building utilities.

He joined Schneider in 2005 and has worked in various roles supporting VSDs in both New Zealand and Australia. He is also active in the industry and is a committee member of the Institute of Instrumentation Control and Automation (IICA) in NSW.