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).

Introduction

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 [3]. 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	On-state voltage Threshold voltage Internal gate resistance Turn-on transient	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)	JESD22-A105C JESD22-A122A JESD22A104E JESD659B JESD91A	Bond wire fatigue Aluminum reconstruction and ratcheting Solder fatigue Delamination at the interface between Mold/Copper Chip cracking	Yes	No	Multiple strategies are defined: Constant Ton and Toff times. Constant heat sink temperature. Constant power loss. Constant junction temperature.	[6, 7]
Creep test	ASTM4 ASTM E139	Solder constants for Garofalo equation.	Yes	Yes	Tests are carried out using multiple temperatures and pressure levels.	[8]
High-temperature operating life test (HTOL)	JESD22-A108 JESD22-A108F	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.	[9, 10]
Accelerated corrosion test (similar to Autoclave/ Unbiased Highly Accelerated Stress Test (HAST))	ISO 9227 JESD22-A101-B	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.	[11]
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.	[12]

 

Outlook

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.

 

References

1. Y. Guo, H. Gao, and Q. Wu, “A Combined Reliability Model of VSC-HVDC Connected Offshore Wind Farms Considering Wind Speed Correlation,” IEEE Transactions on Sustainable Energy, vol. 8, no. 4, pp. 1637–1646, 2017, doi: 10.1109/TSTE.2017.2698442.
2. A. Kwasinski, “Quantitative Evaluation of DC Microgrids Availability: Effects of System Architecture and Converter Topology Design Choices,” IEEE Transactions on Power Electronics, vol. 26, no. 3, pp. 835–851, 2011, doi: 10.1109/TPEL.2010.2102774.
3. S. Peyghami, F. Blaabjerg, and P. Palensky, “Incorporating Power Electronic Converters Reliability Into Modern Power System Reliability Analysis,” IEEE J. Emerg. Sel. Topics Power Electron., vol. 9, no. 2, pp. 1668–1681, 2021, doi: 10.1109/JESTPE.2020.2967216.
4. A. Griffo, J. Wang, K. Colombage, and T. Kamel, “Real-Time Measurement of Temperature Sensitive Electrical Parameters in SiC Power MOSFETs,” IEEE Trans. Ind. Electron., vol. 65, no. 3, pp. 2663–2671, 2018, doi: 10.1109/TIE.2017.2739687.
5. P. Mawby, J. Hu, J. O. Gonzalez, L. Ran, and O. Alatise, “Temperature Sensitive Electrical Parameters for Condition Monitoring in SiC Power MOSFETs,” in 8th IET International Conference on Power Electronics, Machines and Drives (PEMD 2016), Glasgow, UK, 2016, p. 6.
6. C. Durand, M. Klingler, D. Coutellier, and H. Naceur, “Power Cycling Reliability of Power Module: A Survey,” IEEE Trans. Device Mater. Relib., vol. 16, no. 1, pp. 80–97, 2016, doi: 10.1109/TDMR.2016.2516044.
7. S. Narumanchi, “Reliability Aspects of Power-Dense Electric Drive Power Electronics,”, 2022, United States. https://www.osti.gov/servlets/purl/1841150
8. J. Pang, B. S. Xiong, C. C. Neo, X. R. Mang, and T. H. Low, “Bulk solder and solder joint properties for lead free 95.5Sn-3.8Ag-0.7Cu solder alloy,” in 53rd Electronic Components and Technology Conference, 2003. Proceedings, New Orleans, Louisiana, USA, 2003, pp. 673–679.
9. Renesas Electronics Corporation, “TB515: High Power High Temperature Operating Life (HTOL) at High Currents,”, 2018
10. M. Rzin, A. Curutchet, N. Labat, N. Malbert, L. Brunel, and B. Lambert, “Schottky gate of AlGaN/GaN HEMTs: Investigation with DC and low frequency noise measurements after 7000 hours HTOL test,” in 2015 International Conference on Noise and Fluctuations (ICNF), Xian, China, 2015, pp. 1–4.
11. J. Kiilunen and L. Frisk, “Reliability testing of frequency converters with salt spray and temperature humidity tests,”
12. R. Schmidt and C. Spindler, “Failure assessment and HALT test of electrical converters,” in 2012 Proceedings Annual Reliability and Maintainability Symposium, Reno, NV, USA, 2012, pp. 1–6.

 

This article was edited by Ali Nabavi.

To view all articles in this issue, please go to June 2022 eBulletin. For a downloadable copy, please visit the IEEE Smart Grid Resource Center.