Operation of medium voltage drives: A service cost of ownership
This post provides important considerations about the lifecycle cost of a medium voltage variable frequency drive (MV VFD) from the service perspective. The “end of life” and “life extension” options are described. It is a relevant topic for every owner of VFD when the product reaches certain age.
Introduction
After successfully purchasing a medium voltage drive system, the original project setup is transferred into operation and maintenance ownership. The maintenance recommendations of the respective original equipment manufacturers (OEM) meet with the internal maintenance strategies of the plant asset own ers, and maintenance personnel. Those can differ quite heavily and lead to misunderstandings in terms of periodicity, actions and lifetime expectations of the respective equipment. While this article is not meant to set a generally applicable maintenance strategy for medium voltage (MV) drive assets as such, it aims to build common understanding in the underlying reasons of maintenance actions applicable to of those assets.
Failure rates of medium voltage drives over time
Figure 1 shows typical development of failures of medium voltage drives over time.
Phase 1 – Installation, commissioning, baby sitting
In the initial phase, and in most cases, the original manufacturer of the drive is responsible for commissioning the MV drive equipment itself. This can be performed through a separate commissioning contract or be part of the original project delivery scope of supply. The critical phase is reached when several different parts of the MV drivetrain are put into operation. As for example, transformers, medium MV drive, motor, prime mover of the end-user’s equipment. Coordination and overriding scheduling responsibility is needed, since all parties rely on each other’s input to move forward in their respective workflow. As waiting times are often excluded from the respective commissioning parties’ contracts, the cost of commissioning is heavily dependent on the readiness of interconnected drivetrain equipment. A delay causes involved parties to stay onsite for additional time versus initially scheduled. In the worst case the commissioning is delayed, and parties need to reschedule further dates and organize additional trips to and from end user sites.
The bathtub curve starts its first decrease during hot commissioning of the drivetrain equipment. The adjustment in drive parameters, with real operation conditions, helps to set a stable production. It is usually recommended to conclude hot commissioning with a baby-sitting phase, where the commissioning engineer or commissioning supervisor remains onsite to correct any behavior outside of normal operation. Until the drivetrain has reached point B, several weeks can typically be needed. Depending on the
readiness of interconnected drivetrain equipment, depending on the operational material readiness from end user such as production material (steel, minerals, water, oil, gas, petrochemical mixtures, air) or application readiness (hoists, test stands, propellers).
From a cost of ownership perspective, Phase 1 of the medium voltage drive cost of ownership (COO) represents between 5 – 20 % of the initial purchasing price of the drive. With a tendency to be reduced thanks to standardizations and faster parameter settings onsite. A badly timed, coordinated or scheduled commissioning can lead to dramatic unforeseen cost increases up to 50% of the initial drive value.
Phase 2 – Standard life cycle phase – Normal operations, periodic maintenance, periodic troubleshooting
Once the medium voltage drive has reached a stable production phase, the interventions can be scheduled periodically, jointly between the end user and the OEM. Some basic maintenance actions can be performed by end user’s site personnel directly; it is recommended to countercheck the skill level of operation personnel before any interventions.
During this phase, or also during phase 1 or even earlier in the greenfield project phase, the future operation personnel of the end user can be trained and certified by the OEM to perform certain maintenance actions. As for example this link will lead you to a training portfolio.
The cost of a classroom training is usually less than 1 – 3% of the original MV drive purchasing price and can prevent several failures, save time and money for end users.
Some other service interventions would need presence of OEM’s personnel onsite. This is usually provided by a local representative of the OEM in the country. If the end user country has no local OEM presence, it can be supported through neighbouring countries in a joint framework. A maintenance and response time agreement can be made between the end user and the OEM to agree on response times, hourly rates for regular work, or even complete maintenance contracts.
The typical service interventions and recommended maintenance actions to be performed can be found in end-user project documentation or even in publicly available information to end users.
Here are some examples of preventive maintenance schedules of medium voltage drives shared publicly:
Depending on the environmental conditions and effective load rating point, the schedules can be adapted to the real operating conditions. For example, gas turbine starters, soft starters in hydro power plants, blowers, fans, auxiliary drives only used in the ramp up or ramp down phases are less exposed to aging factors than continuously operated drives.
Phase 3 – End of life and opportunities
Once the MV Drive converter enters Phase 3, and the original recommended lifetime of components is exceeded, failure typically occurs to happen. Most of the time the end user operating the MV drive is also facing life cycle issues on certain components of the MV drive that can no longer be produced or supplied from the OEM. Ultimately the support and field service knowledge are also weakening from organizations that have moved to another technology. The cost of failures and cost of non-production can add up and put pressure on the end user operation teams, which need to justify losses of production to their plant management. It is therefore recommended to never enter Phase 3 and look earlier at opportunities such as control upgrades or replacement of the MV Drive asset. This can also be aligned with the overall lifetime planification of the end user operations, similarly other equipment purchased at the same time will require overhaul and major service interventions to keep running as it was initially after the Phase 1.
Lifecycle upgrades
This section reveals the options to extend VFD lifetime, preserve the reliability and maintain lifecycle cost at reasonable level.
Due to the shorter life cycle of the control hardware used in medium voltage drives, it is recommended to perform a control upgrade after typically 10-15 years of operation. While the innovation cycles of control hardware are much longer in industrial equipment than in consumer products, it is generally safe to consider at least one control upgrade in the lifetime of the medium voltage drive asset. The control upgrade will mainly focus on the obsolescence of printed circuit board assemblies (PCBAs), or other components installed in the drive. The target is to keep remaining most of the components which are not subject to
lifecycle changes or aging and renew the lifecycle of the MV drive with new, up to date control hardware.
The scope of supply of the control upgrade is dependent on the technology change that has been witnessed by the MV drive lifetime. Typically, the longer a drive is in operation, the larger the scope of thecontrol upgrade gets. Upgrading medium voltage drive from the late 1980’s to an up to date – 2025 –control hardware platform takes several challenges in terms of engineering, environmental and standardization constraints. From a cost perspective, the control upgrade ranges from 20% up to 70-80% of the purchasing price of a new drive, with a tendency to be around 50% less cost intensive than a replacement with a new MV drive of the same size. The variation in costs depends heavily on the type, size of the MV Drive and the age of the installation. Some other environmental constraints, mechanical space for example, may influence end users on the control upgrade versus replacement decision process.
After a control upgrade, the original components are still supported by the OEM. Typically, those parts are not aging as fast as the control hardware, they can withstand another cycle of control hardware lifetime and remain present in the MV Drive from the original manufacturing date until typically 40-50 years. In some cases a warranty can cover the parts that have not been upgraded. In this manner, an MV drive after a control upgrade can be supported in the same manner as a replacement drive from the OEM. The slight difference might happen with some additional replacements of single components to be made during lifetime after control upgrade. For this reason, the difference in cost of ownership between replacement and control upgrade is reducing over lifetime. Based on an assessment onsite, a second control upgrade is sometimes applied on MV Drives, giving another cycle of life to the asset. LCI Drives from the 1980’s are still in operation today thanks to two generations of control hardware upgrades that can support end user operation above the initial design expectations.
Figure 4 above shows the evolution of the cost of ownership versus time. The initial purchase of the MV Drive is not included in the cost of ownership. As the service cost of ownership of the initially purchased MV Drive shows a steady increasing cost of ownership until 10 – 15 years where decision between control upgrade or replacement is made. Case B shows how the cost of ownership is reduced with a control upgrade, by saving investments in keeping existing components of the MV drive installed and running. Case C shows the cost of ownership after a full MV drive replacement, an additional CAPEX investment is made and is pushing the COO higher. Over time, the difference in COO between control upgrade and replacement is reducing slightly. Time period 15 – 30 years. After which the decision can be made to pursue a second control upgrade, or with a replacement of the MV drive. Typically, after 30 – 40 years of operations.
Replacement
Based on end user wishes or specific internal operation policies of the end users, the alternative to a control upgrade can be to replace the entire MV Drive with a new MV Drive, also called replacement. The original design, electrical and mechanical layouts of the MV Drive can be reproduced if still available. If not, larger site works are to be planned to fit the new MV Drive installation into the existing mechanical space.
Typically, erection works, power cables routing, construction work needs to be undertaken. These costs add to the purchasing price of the new MV Drive.
The key elements to analyze before taking a replacement decision are the following:
- Is the OEM still able to supply the same MV Drive?
- Is there no alternative with control upgrade?
- What is the price difference, control upgrade vs. replacement?
- Analysis with 3rd party drive, assessment of risks of replacement
- Mechanical, space constraints of the existing installation, cost analysis of site modifications for power cables, ventilation, water/air cooling
- Mechanical layout of the new MV Drive matching the existing MV Drive?
- Certifications
Third party upgrades, replacements
An alternative to both cases (Control upgrade, replacement) presented above could be to apply a control upgrade from a third-party manufacturer on the originally supplied MV Drive. Also called 3rd party control upgrade. In this case, the MV drive after control upgrade becomes a hybrid between originally supplied parts from the OEM, and new control parts supplied by the 3rd party company.
Usually, 3rd party control upgrades can be found when:
- The OEM of the originally supplied MV Drive no longer supports the delivered technology
- The end user is searching for better support capabilities with another supplier of MV Drive technology
- The 3rd party control upgrade is cheaper than the OEM proposed control upgrade. However, in this case it is always recommended to countercheck the scope of the control upgrade and the service promise after the control upgrade. To avoid the “cheap and dirty” executed control upgrade which will not be supported in time by the 3rd party supplier.
Challenges for end users operating 3rd party control upgraded MV Drives:
- Warranty/support on originally supplied parts can be void
- Responsibilities shifted to supplier of control upgrade for the power parts, remaining components unchanged with control upgrade
- Safety and reliability of hybrid equipment, root cause analysis for failures is complicated and the respective manufacturers of control upgrades and OEM will reject responsibility to each other.
Summary
Options and activities in the phase 3 of MV drive lifecycle are decisive to keep the performance of the drive, preserve high reliability, ensure compatibility with other subsystems and keep predictable service cost. Upgrade program can significantly boost the effective drive lifetime. The upgrade can be performed by the OEM, if such services are available. Alternatively, a third party upgrade can be done.
Acknowledgement
We would like to thank Mr. Leo Lefret, Area Sales Manager of ABB Motion – Service (MOSE), for authoring this article and sharing his valuable professional experience in supporting customers’ VFD fleets over the entire lifecycle and beyond.
Email: leo.lefret@ch.abb.com
References
[1] Modernization and performance improvement for drives, https://new.abb.com/service/motion/modernization-and-performance-improvement-services/drives
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