Last of two parts.
Part One of this two-part series (Pumps & Systems, May 2014) explained how factors other than initial purchase price work together to generate the total cost of ownership (TCO) for medium-voltage variable frequency drives (VFDs). Part Two discusses what creates high reliability in a medium-voltage VFD.
Drive Innovations Improve Reliability
Some suppliers use a simplified, integrated design that results in a drive with a smaller footprint. A traditional drive can have up to five separate components: harmonic filter, power factor correction components, transformer, power converter and motor output filter (see Figure 1).
An integrated design drive, however, only has two components, an isolation transformer and a power converter. To achieve an integrated design, the drive must be high-performance so that, unlike a non-integrated design, additional components are not required. For example, the design for advanced drives has progressed to more levels at the output and these multilevel topologies extend motor life, which is stressed by the lower level counts that a less expensive drive may offer. Output filtering is not required. These filters add components and assemblies, but they are rarely considered when calculating the reliability of drives that focus on converter part count only. The simplicity of an integrated design results in a more straightforward system with fewer components, which increases reliability and reduces downtime.
Figure 1. Typical medium-voltage VFD
components—Note that the drives from some suppliers may incorporate a number of these components internally in an integrated design.
Specifically, an integrated design allows the entire drive system to be factory tested and then shipped to the site. Often, other designs are not tested as a system until all the components arrive onsite and are interconnected. Thorough factory testing of all the drive system’s components for an extended period of time increases initial reliability substantially and also contributes to better medium- and long-term reliability.
Because the integrated unit has been thoroughly tested and requires no interconnecting wiring onsite, the end user can rely on the drive to work as designed when it is delivered, reducing commissioning and startup time to the bare minimum. Factory testing of the complete integrated drive system also provides accurate efficiency measurements, which are important for sustainability—critical for almost every application.
Reliability is also increased by using a series of low-voltage cells together in a building block approach to create the medium-voltage power output required by the drive. If a drive uses this building-block approach, then it is possible to quickly bypass a failed cell while the drive continues to operate.
Fault tolerance, a third design feature that improves reliability, allows a drive to continue to operate in the event of a noncritical fault. This strategy ensures that the drive never trips on a single drive fault and waits for a second condition to confirm that a problem exists. This fault tolerance strategy also provides a hierarchical series of warnings of drive or component failure.
This fault tolerance allows significant time for an operator to review the situation and avoid a system shutdown. Because of their ability to stay online during a noncritical fault, drives with fault tolerance have been used in many critical industry applications in many facilities—including process plants, refineries, power plants and nuclear power plant systems.
High-availability, medium-voltage VFDs with fault tolerance and cell bypass are the only type certified for use in nuclear facilities because of their reliability in demanding applications.
A medium-voltage VFD with higher reliability may also require less maintenance, further contributing to a lower TCO.
Medium-voltage VFDs include complex components, so maintenance may require outside service personnel. This can be costly and time consuming, particularly if the supplier does not have a worldwide network of service personnel and access to a wide range of repair parts.
Service, Reputation & Longevity
Major suppliers generally have a worldwide network of applications engineers and service centers. Some suppliers have better support in some regions than others—a factor that should be evaluated by end users because faster local support can reduce downtime and contribute to a lower TCO.