Goal 1: Degradation Study of Components and Subsystems

Thermal Management Subsystem

Thermal management of inverter systems and internal components is critical for ensuring lasting performance reliability. Research is currently being carried out to more accurately characterize inverter components and system topologies of varying scales. This work includes:

1. Thermal/fluid phenomena of internal inverter components, and their impact on electrical performance and reliability.
   • Finite Element Analysis (FEA)/Computational Fluid Dynamics (CFD) and analytical modeling
2. Power semiconductor switch (e.g. IGBT) and capacitor thermal cycle modeling and empirical characterization
3. Inverter solar gain impact on inverter reliability

As an example, Figure 1 shows a simulation of the cooling subsystem of an inverter.

Figure 1: Thermal/fluid modeling of the cooling subsystem.

For more information, contact Jennifer Granata.

Inverters

The inverter reliability program includes efforts to develop a reliability model for inverters based on data from the laboratory and the field. A model for the entire inverter will complement the reliability characterization efforts of critical subcomponents such as capacitors and switching devices. Many of these subcomponent characterization studies consist of accelerated testing in a laboratory environment while inverter data is usually taken from the field. Therefore the inverter reliability model being developed in this thrust will serve as a link between field data and laboratory data for the subcomponent reliability results. Figure 1 is an example of a reliability model for inverter control boards based on field measurements [1].

Figure 1: Reliability models for three control boards in inverters located in the Southeast based on field measurements applied to an Arrhenius formulation.

[1] N. R. Sorensen, E. V. Thomas, M. A. Quintana, S. Barkaszi, and A. Rosenthal, “Thermal study of inverter components,” presented at the IEEE Photovoltaics Specialists Conference, 2012.

For more information, contact Jennifer Granata.

Capacitors

The capacitors in an inverter system are typically borrowed from other industries. As such, they are often the first components to fail as they are not designed for the environmental and operational rigor associated with photovoltaic applications, particularly the thermal environment. While thin-film capacitors hold promise for better reliability over their electrolytic counterparts, actual data to substantiate such claims are limited [1].

The inverter reliability program has constructed a dedicated testing chamber to study the degradation of electrolytic and thin film capacitors, shown in the schematic in Figure 1. A photograph of the test chamber is shown in Figure 2. This system enables the study of capacitor degradation through a variety of characterization and failure analysis techniques. In addition, residential-scale inverters and micro-inverters can be tested in this chamber.

The results of this study will provide information to develop best practices in testing and operation of the inverter for the PV industry. This chamber may also be used to validate the test procedures in the upcoming new version of IEC 62093: balance-of-systems components for photovoltaic systems – design qualification natural environments. The characterization techniques will also provide promising approaches to develop a PHM system to monitor the life of the capacitor or other components during operation.

Figure 1: Schematic of the capacitor accelerated testing chamber.

Figure 2: Photograph of the capacitor accelerated testing chamber.

[1] J. D. Flicker, M. Marinella, R. Kaplar, and J. Granata, “PV inverter performance and reliability: what is the role of the bus capacitor?,” presented at the IEEE Photovoltaics Specialists Conference, 2012.

For more information, contact Jennifer Granata.

Switches

The switches are the key component that enables the inverter to function. Like capacitors, they are also prone to failure due to harsher environmental conditions and a far greater number of required lifetime switching operations than other high-power counterparts in other industries. Wide bandgap switching devices promise better performance than traditional insulated gate bipolar transistors (IGBTs) but are still in relatively early stages of maturity. As such, their long-term reliability is not well understood.

The inverter reliability program is studying the degradation mechanisms of both IGBTs and wide bandgap switching devices [1]. One important tool is the development of a custom-built H-bridge with interchangeable parts housed in a test chamber, as illustrated in the schematic shown in Figure 1. This setup allows performance characterization and controlled stressing of different switching technologies. It also enables us to study the degradation of power semiconductor switches in an environment very similar to what would be experienced by the switch in a fielded inverter. This new capability is expected to be operational by mid-2013.

Figure 1: Schematic for the inverter switch testing chamber under construction.

[1] R. Kaplar, R. Brock, S. DasGupta, M. Marinella, A. Starbuck, A. Fresquez, S. Gonzalez, J. Granata, M. Quintana, M. Smith, and S. Atcitty, “PV inverter performance and reliability: What is the role of the IGBT?,” in 2011 36th IEEE Photovoltaic Specialists Conference (PVSC), 2011.

For more information, contact Jennifer Granata.