Reliability in Power Electronic Systems: A Hierarchical Approach
Written by Bikash Sah, Sebastian Sprunck, and Marco Jung
The electric grid is transforming with the inclusion of a variety of generation sources, loads, and associated supporting systems. This transformation is made possible with the developments in the control and communication system technology, standards for operation and increased usage of controlled systems, such as power electronics converters (PEC).
Among these developments, PEC are a key component that helps bridge the flow of power between generation systems and loads. Hence, these PEC are required to be reliable and to be designed with due consideration of all possible issues which can occur during operation. The reliability of the PEC is not only influenced by its own state, but also impacted by the operational state of the connected systems and the environmental conditions. For example, a line-to-ground fault in the electric grid side will lead to potentially unsafe loading of the PEC. Although standards are defined to operate the PEC in case of a fault, the stress introduced in the PEC impacts the overall lifetime or increases the failure rate. Similar examples of failures at the system level, affecting the operation at the subsystem or component levels, are available in the literature [1, 2]. Hence, it is essential to relate the reliability testing of the PEC to the faults or disturbances in the connected system. In general, the reliability testing procedures of a subsystem in a system should be defined considering the overall behavior of the system.
Relating the Reliability at System Level Operation
Standards and test procedures are established in the literature to perform the reliability testing of a system and the related subsystems. However, defining a general approach to evaluate the reliability of the subsystems connected together inside a system is challenging. The cause, type, and behavior of faults or disturbances vary according to application, power level, environmental conditions, and operating condition. For instance, consider an inverter integrated into a powertrain, which feeds power to a motor, and a photovoltaic (PV) inverter that supplies the generated power to the electric grid. The faults in the inverter of a powertrain in the electric vehicle can occur e.g. due to changes in the acceleration profile of the driver or due to mechanical vibrations introduced while driving in off-road conditions; while in the case of the photovoltaic generation system, the possibility of fault or disturbance due to mechanical vibration is minimal. One solution to determine the reliability of PECs with respect to their application, power level, environmental conditions, and operating condition is the adoption of mission-based profile tests.
However, this solution is challenging to apply in a complex power systems scenario, since reliability testing in complex systems focuses more on system-level aspects than in less complex (sub-)systems, where individual components are much more relevant. When power system reliability is considered, the main concern lies not on the specific subsystems within, but on the overall capability to guarantee supply to the connected loads in the long term. Furthermore, for the short term, the reliability of the power system is specified by its capability to withstand any outages or contingencies. If the reliability of the overall power system is increasingly dependent on large numbers of lower-level systems or subsystems in contrast to large-scale power plants, they will have an increasing impact on the overall system reliability.
Today’s electric grids are increasingly incorporating PECs due to increasing renewable power generation. Apart from the renewables, the plan to integrate larger capacity electrolyzers in the multi-MW to GW range, large-scale energy storage systems and bidirectional electric vehicle chargers adds to the usage of PECs in a power system. Unfortunately, PECs are the most vulnerable component in the power system since they are highly prone to wear-out failures . In general, the manufacturer of a PEC defines the lifetime according to particular testing conditions. These conditions could–and most often will–vary in a real-time deployment scenario. The stresses due to a unique or novel disturbance in the power system introduce new fatigue in the PEC, which reduces the overall lifetime in an unfamiliar manner. Prognostics is a solution to the problem, but it requires detailed development of condition monitoring models that incorporate multiple physical, mechanical, or any other related parameters. Developing reliability-testing procedures by modifying the existing ones and identifying appropriate mission profiles is a possible solution. Additionally, the test procedures should also aim to characterize parameters that will influence the PEC’s operation based on the particular type of system or scenario in which it is to be deployed.
Table 1 lists a few common test procedures found in literature associated with standards, the parameter measured or observed, the suitability to use for testing a device or a PEC, and remarks while performing the test.
Table 1: List of standard test procedures widely used in testing PEC and devices
|Test Name||Related Standards||Measurements/Observations||
Suitable for Devices/Modules
|Suitable for PECs||Remarks||Refference|
|Thermo-sensitive electrical parameters (TSEP)||N/A||
||Yes||No||Many samples should be tested, and possible temperature variation of the oven can reach between 30°C to 170°C.||[4, 5]|
|Accelerated power cycling test (APC)||
Multiple strategies are defined:
|Solder constants for Garofalo equation.||Yes||Yes||Tests are carried out using multiple temperatures and pressure levels.|||
|High-temperature operating life test (HTOL)||JESD22-A108
|The breakdown of the device or the system is observed.||Yes||Yes||The device is subjected to high temperature using an oven; DC and RF stresses are applied for a fixed interval. The device/system is tested for the expected electrical properties after the test.||
|Accelerated corrosion test (similar to Autoclave/ Unbiased Highly Accelerated Stress Test (HAST))||ISO 9227
|Failure of the device/system is observed.||Yes||Yes||The temperature and humidity for conducting the test are to be considered based on the standards. Also, the testing time can vary.|||
|Highly Accelerated Life Testing (HALT)||N/A||Determine the weak links for failure in the product and the time of failure.||No||Yes||The test is conducted based on multiple temperatures and vibration profiles in the parallel application of current and voltage stress.|||
The tests listed in Table 1 are methods to investigate the possible causes of failures in a PEC. These tests help to get insights into the possible mechanism(s) of breakdown of the device or the entire PEC. However, these tests are limited to the failure modes they are designed for and are currently not extended for further reliability assessments of the system in which a PEC is deployed. The reliability evaluation of the PEC is largely reported to be performed based on the physics of failure, mission profile, multistage evaluation, and design review based on the failure mode. In general, the outcomes of the standard test procedures and the reliability assessment methods are not utilized in convergence with a common goal of reliability assessment. Hence, unique reliability assessment procedures based on application, power level, environmental conditions, and operating conditions for testing of devices and PEC are required to be developed. These procedures will pave the way to increased utilization of PECs to support the integration of renewables, electrolyzers, large-scale energy storage systems, and EVs.
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This article was edited by Ali Nabavi.
Bikash Sah (SM’14) received the B.Tech. degree in Electrical and Electronics Engineering from National Institute of Technology Arunachal Pradesh, India, in 2014, and the Ph.D. degree from the Department of Electronics and Electrical Engineering, Indian Institute of Technology Guwahati, Guwahati, India, in 2022. In 2015, he started working as a Research Fellow in the Electric Mobilty Lab, Indian Institute of Technology Guwahati where he was involved in the development of algorithms for vehicle to grid infrastructure, converters for chargers and powertrain, and charging techniques for reducing electrochemical degradation in Li-ion batteries. He is currently working as Wissenschaftlicher Mitarbeiter in Bonn Rhein-Sieg University of Applied Sciences in the R&D Group Power Electronics for Renewable Energies, Electric Vehicles and Sector Coupling. His current research interests include vehicle to grid systems, electric vehicles and their charging infrastructure, and converter design using wide bandgap devices.
Dr. Sah is a member of IEEE and European Center for Power Electronics e.V (ECPE). He is also an active reviewer in IEEE Access, IEEE Open Journal of Industrial Electronics Society, and IEEE Journal of Emerging and Selected topics in Industrial Electronics.
Sebastian Sprunk studied electrical engineering at the University of Kassel, Germany, from 2010 to 2016. He received his B.Sc., M.Sc., and Ph.D. degrees in 2014, 2016, and 2021, respectively.
In 2017, he started working as a research assistant at the University of Kassel's Centre of Competence for Distributed Electric Power Technology (KDEE) where he worked on the miniaturization of power electronic systems and on the application of wide band gap semiconductors in power electronic devices. Since 2020, he is working as a research assistant at the Fraunhofer IEE in Kassel, Germany, in the Converters and Drive Technology Department, where he is leading the Devices and Measurement Systems group. He is investigating the influences of individual technological advances, like wide band gap semiconductors, and of broader trends, like the German "Energiewende", onto power electronic components, circuits, and systems. His main interest focuses on the characterization and optimization of (WBG) semiconductor switching losses and their implementation in power electronic systems.
Dr. Sprunck is a member of the VDE Association for Electrical, Electronic & Information Technologies and of the VDI Association of German Engineers e.V.
Marco Jung (M’2020, SM’2021) completed an apprenticeship for communication electronics in 2003 and continued to study Electrical Engineering at the TH Mittelhessen University of Applied Sciences and at the University of Kassel, where he received his Diploma and M.Sc. degrees in 2008 and 2010, respectively. He continued his studies at the Leibniz University Hannover, where he received his Ph.D. degree in 2016.
Parallel to his Ph.D. studies, he started working at the Fraunhofer IEE in 2010. Since 2017, he is head of the Converters and Drive Technology Department. In 2019, he additionally became a full Professor at the Bonn-Rhein-Sieg University of Applied Sciences, Sankt Augustin, Germany. At the Institute of Technology, Resource and Energy-Efficient Engineering, he is responsible for power electronics for renewable energies and electric vehicles.
Mr. Jung is chairman of the IEEE Joint IES/IAS/PELS German Chapter since Jan. 1st, 2021. He is a member of the International Scientific Committee (ISC) of the European Power Electronics and Drives Association (EPE) and a member of the European Center for Power Electronics (ECPE).