Choosing the right voltage

MODERN variable speed drives (VSD) have evolved into reliable controllers, commonly used across manufacturing sectors.

Traditionally, VSDs have been used for process control of pumps, fans and 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 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 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 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 to 75% of the initial cost of an MV drive.

However, to gain a true cost of an 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 standards such as IEEE 519, EN61000-2-4 and G5/4. There are several methods to reduce harmonics on large installations and the best solution is usually a mix 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 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 transformer’ 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 integration 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 compensated centrally at the medium-voltage level via an auto-transformer.

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 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 undertaken.
The distance between the drive and the motor is a critical factor when determining the correct drive technology and configuration. As cable distances increase 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 3% loss at 1MW is 30kW, 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 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 effectively removes about 90% of the heat generated by the VSD losses out of a control enclosure, but involves additional costs for pumping cabinets and heat exchangers, if not already available on the 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.