The power semiconductors, today mostly Insulated Gate Bipolar Transistors (IGBTs) are the key components, and at the same time the most vulnerable components in the high power, high voltage converters, such as for the renewable energy sector and for HVDC transmission systems.
One major cause for lifetime reduction in power semiconductors is the so-called Power Cycling (PC) stress, where fluctuations in power lead to temperature fluctuations and then cause mechanical wear and damage of soldering joints and electrical contacts. The ambition for ReliPE is to provide methodologies for enabling real-time condition monitoring and estimation of time to failure or to repair for the components in operation, given the accumulated operating history for the components. For approaching this ambitious goal, the project work is processed by the following Work Packages:
In WP1 an MMC-type HVDC converter was subjected to thermal-electrical simulation for mapping the operational dependent IGBT stress conditions. The results were presented in a conference paper. Moreover, in WP1, a method was demonstrated for real-time estimation of the IGBT hot-spot temperature, knowing the ambient cooling conditions and the electrical stress for the component.. Simulation of hot-spot temperature dynamics confirms that the MMC converter in normal operation is subjected to moderate thermal stress. Then a furnace flicker compensator, which represents a converter application with harder stress conditions, was decided for the continued study. This was thoroughly investigated and demonstrated by real time operation as part of WP5.
PC lifetime models for IGBTs are generally established by running accelerated lifetime experiments on test objects. In WP2 such models have been assessed for their possible applicability for representing lifetime under real operating conditions. Two models have been investigated with inputs from results from the PC test program in WP4. The main conclusion is that one model is applicable as tool for real-time lifetime estimation. The results were presented in a journal paper and two conference papers.
WP3 has the main focus on Short-Circuit (SC) ruggedness of IGBT modules, especially highlighting the possible advantages gained by new technologies such as increased thickness of the emitter metallization. The major part of this WP was carried out by the PhD scholarship and additional staff at TU Chemnitz. Interim results from thermal-mechanical FE simulations were presented in a journal publication. The PhD student successfully defended his thesis in May 2018. Further in WP3, the impact from PC ageing on the SC ruggedness has been investigated by SC testing of aged test objects from the WP4 experiments. These results are presented in a conference paper.
The main goal for WP4 is the long-term PC test program for providing lifetime data for IGBT modules subjected to cyclic thermal stress representing the real operating conditions. The test objects are relevant components for multi-MW power converters. 5 specific test runs were accomplished for providing the sufficient statistical support for the WP2 lifetime model assessment. Additionally, two test runs have been accomplished for investigating to which extent the lifetime models are valid for combined high and low stress cycles. Such validity was confirmed. Finally, a test run was accomplished for investigating possible impact from previous hard short-circuit tests on the PC lifetime.
WP5 is combining findings from the other works packages for providing algorithms for online real-time condition monitoring of the converters IGBTs, and for estimating time to end-of-life. A prototype for real-time online estimation of hotspot temperature was implemented, whereby very good agreement between computer simulations (ref. WP1) and the real-time simulator was demonstrated for an MMC laboratory converter. Further in WP5, one laboratory converters emulating a furnace flicker load and one emulating a flicker compensator was implemented, whereby lifetimes with hard PC stress situations were estimated. The results from WP5 are presented in one journal paper and one conference paper. WP5 will provide valuable inputs to a proposal for a new innovative project highlighting online condition monitoring of IGBTs by "Digital Twin" concepts.
WP6 has the focus on strengthening the education of NTNU students on topics related to power semiconductor reliability. Especially mentioned is the contribution from the 1 ½ -year guest researcher working on topics related to real-time estimation of IGBT lifetime. Also mentioned is the almost continuous student works closely connected to the project WP tasks. For example, experimental work on PC lifetime of Silicon Carbide devices, with results presented in a conference paper.
All WPs of ReliPE have now been accomplished according to plans.
In ReliPE, methodologies for enabling real-time condition monitoring and estimation of time to failure or to repair for the IGBT components under live operation has been experimental validated. By that, important steps are made towards the realization of methodologies for real time condition monitoring and lifetime estimation of IGBTs under live converter operation. For the project partners, the industrialization of the results will contribute to more reliable converter product. Moreover, it is anticipated that the implementation of the proposed methodologies for online PC lifetime estimation will provide significant cost savings related to service and repair work. Especially this will have a positive impact on converters operating in the renewable energy sectors, such as offshore wind farms which are characterized by limited weather windows for service and repair. In that respect, the outcome from the project is assumed to contribute to enhancement of offshore wind power.
Conversion of electric energy by power electronic converters has an increasingly important role in all parts of the power system. A common factor for many applications is high reliability requirements caused by maintenance challenges and/or high cost of downtime. The complexity of converter systems calls for high R&D-efforts in order to improve component designs and to increase the ability to identify and predict faults.
The project aims to improve the cost-efficiency of high power converter systems through increased reliability, better understanding of failure mechanisms and tools for lifetime estimation.
The initial phase of the project will focus on developing a set of reference circuits and components to be used for detailed analysis. A thorough review and comparison of existing lifetime testing methods will also be performed. A contribution towards standardisation of test methods is the expected outcome of this work.
Lifetime analysis of last generation IGBTs (Insulated Gate Bipolar Transistors) will be performed through a combination of power cycling tests and Finite Element analysis based on the proposed test schemes. The objective is to develop an improved and accurate lifetime model.
The project will also contribute towards more robust IGBT devices. A particular focus is the behavior during and after fault conditions. The task will be based on a combination of theoretical and experimental work.
The project has ambitious targets in the field of real-time condition monitoring and remaining lifetime estimation. The objective is to develop a model with the ability to online determine the remaining lifetime based on the accumulated stress in a device.
Finally, the project has a dedicated work package devoted to education. The objective is to bring recent research into education in the field of physics, reliability and design of power electronics. A close co-operation between SINTEF, NTNU and Technical University of Chemnitz is planned for this task.