

Is reliability of your mission-critical machine your top priority?
Quite honestly, given the high cost of downtime and the reveue at stake, the answer is most likely yes. Chances are that your mission-critical machine is powered by a large Medium Voltage drive. And you are seeking independent insights into the reliability of these Medium Voltage drives?
Keep reading—you’re in the right place!
Different medium voltage drive topologies exhibit indeed varying levels of reliability based on their design, components, and operational characteristics. Expect to find here below quite some useful insights regarding reliability for most of the Medium Voltage drive topologies :
What is generally true for all Medium Voltage drive topologies?
Which Medium Voltage drive topology could meet my reliability-availability requirements?
Some reliability metrics explained
Key takeaways and conclusions
Some demystifcation of technical terms
No time to read all this, contact-us for a free video call.
General Factors Affecting Reliability
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Component Count:
- A lower number of active and passive components generally leads to higher reliability, as there are fewer parts that can fail. The silicon area is a good metric to use when comparing the reliability of semiconductors and, in general, the smaller the silicon area, the lower the failure rate. Therefore, IGBT-based systems often have an advantage.
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MTBF
- While MTBF (mean time between failures) provides a useful measure of a system’s reliability, it’s crucial to consider its limitations, understand the underlying assumptions and definitions, and supplement it with other metrics and considerations when assessing real-world system reliability.
- Practically all drive topologies have a comparable system MTBF (without redundancy). Due to the existing uncertainty of the MTBF data the small differences between the topologies are negligible.
- Regular maintenance and, if available, restoring lost redundancy have a significant impact on the MTBF and availability of the drive system.
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Semiconductor Devices:
The type of semiconductor device used significantly impacts reliability.
- IGBTs (Insulated Gate Bipolar Transistors) are commonly used in voltage source inverters (VSI) and have low gate power requirements, reducing the complexity of gate driver circuitry and improving reliability. The use of low-voltage IGBTs in medium-voltage drives ( applied for MMC and 2L SC-HB topologies) improves reliability through the use of well-established, high-volume components with fast innovation cycles, the ability to implement modular and redundant designs, the simplified gate driver requirements, reduced stress on components, lower failure rates, and distributed energy storage.
- SGCTs in current source inverters (PWM CSI) have higher current gate drivers with higher average power requirements, since the turn-off time is so short. Therefore the gate driver circuitry uses electrolytic capacitors, which are prone to failure and lead to a higher failure rate compared to IGBT.
- IGCTs (Integrated Gate Commutated Thyristors) are used in some high-power applications but may have higher failure rates due to more complex gate driver circuitry.
- Thyristors, particularly SCRs (Silicon-Controlled Rectifiers), are less reliable than IGBTs due to slower switching speeds and higher conduction losses.
- Capacitors:
- The type and arrangement of DC link capacitors affect reliability. Film capacitors are more reliable and can be used in medium voltage IGBT-based inverters.
- Cell-based topologies such as the modular multilevel converter (M2C=MMC) and the 2 Level Cascaded H-bridge toplogies (2L SC-HB) with distributed capacitors have lower fault energy in case of a short circuit thereby reducing the associated damage to the equipment.
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Redundancy:
- There are various redundancy strategies:
- A complete backup drive can take over the function of a failed drive at any time, providing the best fault coverage.
- Redundant drive subsystems such as a common spare converter/transformer package.
- Redundancy in the drive configuration by splitting the converter power part into two identical channels.
- Redundant assemblies such as redundant control, excitation systems, recooling units.
- Redundancy of individual drive devices/components such as the N+1 power cells redundancy in case of cell-based design (MMC and 2L SC-HB topologies) or implementing series redundancy in the thyristor stack for LCI (Load Commutated Inverter).
- N+1 redundancy is a strategy that uses one extra component as a backup to enhance system reliability and availability. In an N+1 configuration, ‘N’ represents the number of components needed for normal operation, and ‘+1’ signifies the additional redundant component. If a primary component fails, the redundant one takes over, allowing the system to continue functioning. Implementing redundancy, such as (N+1) configurations, significantly improves reliability, especially in cell-based designs. (MTBF * 1.4 to 1.7). In cell-based drive systems (MMC and 2L SC-HB topologies) the N+1 relates to the redundant power cells to ensure continuous operation in the event of a failure of one power cell.
- Restoring lost redundancy through maintenance is critical for maintaining high availability.
- There are various redundancy strategies:
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Cooling:
- Proper cooling and regular maintenance can improve the reliability of all drive topologies.
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Operating Environment:
- The operating environment, including temperature and humidity, has a direct impact on the reliability of the drive.
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Systematic Failures:
- The approach of using FIT rates to evaluate reliability is limited because it doesn’t include systematic failures, only random ones.
Specific Topologies and Their Reliability
The modular multilevel converter (M2C) and the 2 Level Cascaded H-bridge topologies (2L SC-HB):
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- The modular multilevel converter (M2C) and the Cascaded H-bridge toplogies (2L SC-HB) use multiple low-voltage cells connected in series to achieve medium-voltage levels.
- Provide for high reliability thanks to the use of well-established low-voltage (LV) IGBTs and film capacitors, which are produced in high volumes.
- In the event of a short circuit in one cell, the fault energy is very small. This reduces the severity of faults and associated damage to equipment, increasing safety and reliability.
- Reduced Stress on Components: In multilevel topologies using low-voltage IGBTs, each IGBT switches only a small fraction of the total output voltage. This reduces voltage stress on individual components and improves the overall reliability of the system.
- Excellent redundancy can be achieved by adding extra cells and bypass switches, allowing operation to continue even after a cell failure. With (n+1) redundancy the MTBF of an M2C drive increases by a factor of approximately 1.7.
- Is motor friendly thanks to its low output harmonics and low dV/dt.
- Neutral Point Clamped (NPC):
- The 3-level or 5-level topologies can have reliability issues due to complex control and higher component counts.
- Offers limited redundancy options and may require output filters for long cable applications.
- Requires inverter duty motor. This toplogy is not motor friendly: if used on conventional DOL motor a sine wave filter will have to be added.
- Current Source Inverters (CSI):
- Thyristors and GTOs have a lower reliability compared to IGBTs.
- Modern CSIs (PWM CSI) use SGCTs, which are more efficient but still have higher losses and more complex gate drivers compared to VSIs . The use of electrolytic capacitors in the gate driver circuits presents a potential reliability concern, due to the typical failure modes of this type of component. Electrolytic capacitors are less reliable than film capacitors.
- Are generally less reliable than VSI drives due to higher component counts and the complexity of the gate driver circuitry.
- Redundancy of power parts seems not to be documented for commercially available PWM CSI.
- Load Commutated Inverter (LCI):
- The LCI is a cost-effective solution for very large power applications where high reliability is important. LCI-based drives can approach the reliability of cycloconverter drives.
- Redundancy:
- Thyristor-based converters can achieve increased reliability through series redundancy in the thyristor stacks.
- Some large LCI drives utilize two independent channels, each with its own set of six-pulse SCR bridges. This dual-channel setup provides a degree of redundancy. If one channel experiences a fault, the other channel can still operate, allowing the drive to continue functioning, though potentially at a reduced capacity.
- The redundancy in LCI drives may not be as comprehensive as in modular multilevel converters (M2C) which can bypass an entire cell using contactors.
- Although LCI drives are a cost-effective solution for very large power applications and have high efficiency, they produce significant torque pulsations. A torsional rotordynamic analysis of the entire drive train to identify critical speeds that could be excited by torque pulsations will need to be performed.
Reliability Comparison Metrics
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- Mean Time Between Failures (MTBF): MTBF is a statistical average and does not mean a product will last exactly that long, only that a certain percentage of a population of similar systems will fail by that time.
- Failure in Time (FIT) rate: Failure In Time is a unit used to express the failure rate of components, particularly in the context of electronics and power systems. It is defined as the expected number of failures per one billion (10^9) hours.
- Mean Time To Repair (MTTR): Drives with modular designs and easily accessible components have a lower MTTR. Having a good stock of spare parts and a well trained service team will reduce the MTTR .
- Inherent availability: considers only MTBF and MTTR.
- Achieved availability accounts for preventative maintenance as well as corrective maintenance.
- Operational availability takes into account maintenance that is not instantaneous because of logistics or replacement parts not in stock.
Key Takeaways
- Cell-based topologies such as M2C and 2L SC-HB utilizing low voltage components, redundancy and film capacitors, often offer the highest reliability.
- VSIs generally have a higher reliability compared to PWM CSIs, due to simpler designs and less complex gate drivers.
- Redundancy is a crucial factor for improving reliability.
- Component selection plays a significant role; using high quality components with lower FIT rate and operating them below their nominal rating improves reliability.
- Importance of Comprehensive Evaluation
- Unbiased third-party evaluations can ensure an optimal product decision.
- Besides MBTF it makes sense to include the vendor’s quality control processes, modularity and flexibility of the product, Mean Time To Repair (MTTR), and total cost of ownership (TCO).
In conclusion,
while all MV drive topologies aim for high reliability, the choice of topology, components, and redundancy significantly impacts their performance and operational life. When selecting a drive system, it is crucial to consider these factors and evaluate the specific needs of your application.
In all cases above we would be glad to assist you. All it takes is that you contact us for a free consultation.
Further down the road, we will enable informed decision-making by offering you our independent advice and support for following tasks:
- Setting up the project plan presentation.
- Definition of the requirements of the power drive system.
- Evaluation of the composition of the power drive system (PDS).
- Internal and external pre-bid meetings.
- Setting up the invitation to bid to vfd and motor manufacturers and EPC’s.
- Screening of preferred vendors.
- Verification quotation and exceptions VS your specifications and requirements.
- Order clarification of the composition of your power control system.
- Factory acceptance tests (FAT) requirements of your power drive system.
- Witnessed and non-witnessed factory acceptance tests.
- Site acceptance tests requirements.
- Validation of the site acceptance tests (SAT) for your power drive system.
Glossary
Here is a glossary of technical terms used in our page and their relevance to reliability:
- Cascaded H-bridge (2L SC-HB): A type of multilevel converter topology that uses multiple H-bridge cells connected in series to achieve medium-voltage levels. It is noted for high reliability due to the use of low-voltage IGBTs and film capacitors.
- Cell-based Topologies: Refers to converter designs like the modular multilevel converter (M2C) and cascaded H-bridge (2L SC-HB), which use multiple low-voltage cells to achieve medium-voltage levels. These designs often offer higher reliability due to modularity, redundancy and use of well-established low voltage components.
- Current Source Inverters (CSI): A type of inverter where the DC link is a current source. Modern PWM CSIs use SGCTs, but are generally less reliable than VSIs due to higher component counts and complex gate driver circuits, and the use of electrolytic capacitors.
- Cycloconverter: A type of frequency converter that directly converts AC power at one frequency to AC power at another frequency without an intermediate DC stage. It is mentioned as having similar reliability to LCI drives.
- dV/dt: Refers to the rate of change of voltage with respect to time. Low dV/dt is a characteristic of M2C drives and cascaded H-bridge (2L SC-HB) converters making them motor friendly.
- Electrolytic Capacitors: A type of capacitor used in gate driver circuits, particularly in current source inverters (PWM CSI). These are prone to failure and less reliable than film capacitors.
- Failure in Time (FIT) Rate: A unit used to express the failure rate of components, defined as the expected number of failures per one billion hours.
- Film Capacitors: The type of capacitor that is more reliable than electrolytic capacitors. Used in medium voltage IGBT-based inverters.
- Gate Driver Circuitry: The electronic circuits that control the switching of power semiconductor devices such as IGBTs, SGCTs, and thyristors. Simpler gate driver circuitry can improve reliability.
- IGBT (Insulated Gate Bipolar Transistor): A type of semiconductor switch commonly used in voltage source inverters (VSI). They are noted for having low gate power requirements and high reliability.
- IGCT (Integrated Gate Commutated Thyristor): A type of thyristor used in high-power applications, but may have higher failure rates due to complex gate driver circuitry.
- Inverter Duty Motor: Motors designed to operate with variable frequency drives (VFDs) .Neutral Point Clamped (NPC) drives require an inverter duty motor or need to be equipped with a sine wave filter for use on a conventional DOL (Direct On Line) motor.
- LCI (Load Commutated Inverter): A type of inverter used for very large power applications. LCI-based drives can approach the reliability of cycloconverter drives. LCI drives can have series redundancy in the thyristor stacks.
- Mean Time Between Failures (MTBF): A statistical average of the time between failures of a system. It is important to consider the limitations of MTBF and supplement it with other metrics.
- Mean Time To Repair (MTTR): The average time it takes to repair a failed system. Drives with modular designs and easily accessible components have a lower MTTR.
- Modular Multilevel Converter (M2C): A type of multilevel converter topology known for high reliability due to its use of low-voltage IGBTs, film capacitors, and redundancy capabilities. With (n+1) redundancy the MTBF of an M2C drive increases by a factor of approximately 1.7.
- Neutral Point Clamped (NPC): A type of multilevel converter topology that can have reliability issues due to complex control and higher component counts. It also has limited redundancy options.
- PWM CSI (Pulse Width Modulation Current Source Inverter): A modern type of current source inverter that uses SGCTs.
- Redundancy: The implementation of backup components or systems to ensure continued operation in case of a failure. Redundancy, such as (n+1) configurations, significantly improves reliability, especially in cell-based designs.
- SGCT (Symmetrical Gate Commutated Thyristor): A type of thyristor used in modern current source inverters. They are more efficient but have higher losses compared to VSIs.
- Silicon Area: A metric used when comparing the reliability of semiconductors. Smaller silicon area generally means a lower failure rate.
- Systematic Failures: Failures that are caused by design flaws, incorrect usage, or other non-random factors, which are not captured by FIT rates.
- Thyristor: A type of semiconductor switch used in LCI and some CSI drives. Compared to IGBTs, they generally have lower reliability.
- Torsional Rotordynamic Analysis: Analysis of the entire drive train to identify critical speeds that could be excited by torque pulsations. LCI drives produce significant torque pulsations, necessitating this analysis.
- Voltage Source Inverters (VSI): A type of inverter where the DC link is a voltage source. VSIs commonly use IGBTs and generally have higher reliability than PWM CSIs, due to simpler designs and less complex gate drivers.
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