1. Jack Flicker, Robert Kaplar, Matt Marinella, and Jennifer Granata Sandia National Laboratories Acknowledgements Contact Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04- 94AL85000. Jack Flicker PO Box 5800 Mail Stop 1084 Albuquerque, NM 87185-1084 jdflick@sandia.gov (505) 284-6810 Capacitor Reliability in Photovoltaic Inverters Introduction and Objectives Conclusions Sandia National Labs PV Reliability group has built test setup to perform accelerated lifetime testing of PV capacitors We have started testing metallized thin film capacitors to failure We have seen both capacitance and resistance degradation and catastrophic failure Future work will consist of testing a larger statistical sampling of capacitors and adding ripple current acceleration Capacitor electrical capacitance over time during an accelerated lifetime test (900V, 80 o C) of a 600  F, 800V metallized thin film capacitor. The grey box indicates faulty reading due to a loose connection. Prior to end of test (catastrophic failure) the capacitor demonstrated degradation in the form of an capacitance decrease. (Left) Circuit schematic of capacitor accelerated test setup. The user is protected with a dual Plexiglas box system with automatic shunt and both electrical and mechanical interlocks. The capacitor can be accelerated by voltage and temperature. (Right) Photograph of testing setup. Experimental Setup and Results PV inverter reliability increasingly important to overall PV system cost Capacitors one of most common reasons for inverter failure Inverter manufacturers moving from electrolytic capacitors (known unreliability, high resistance) to metallized thin film capacitors Metallized thin film capacitors thought to have much higher lifetimes and no catastrophic failure Experimental data on metallized thin film capacitors under realistic PV usage conditions is lacking Capacitor electrical equivalent series resistance (ESR) over time during an accelerated lifetime test (900V, 80 o C) of a 600  F, 800V metallized thin film capacitor. The grey box indicates faulty reading due to a loose connection. Prior to end of test (catastrophic failure) the capacitor demonstrated degradation in the form of an ESR increase. This indicates capacitor contributes to reduced PV efficiency over unit lifetime. Metallized Thin Film Capacitors Voltage ripple on the DC bus causes the operating point of the PV array to move off of the maximum power producing point (MPP) and produce less power. Connectors, 3% Others, 7% (Top left) Capacitor before and after accelerated test. (top right) Capacitor encapsulant burst packaging due to internal heating (Right) (Left) Circuit diagram of a single-phase PV inverter showing bus capacitor to reduce voltage ripple on the DC bus. (Right) Failure of these bus capacitors are a significant cause of inverter failures (~30%). However, even greater energy loss is experienced due to capacitor degradation, which increased voltage ripple. Accelerated test of metallized thin film capacitors under DC voltage and temperature stress Capacitor under test = 600  F, 800V rating; held at 900V and 80 o C Measure capacitance and resistance every 30 min over lifetime of capacitor Higher temperature and voltage acceleration factors than electrolytic capacitors Increase in cost exponentially with capacitance BUT…. 10x lower equivalent series resistance (ESR) means capacitor can handle higher ripple voltage with less internal heating Thought to be inherently safer because only degradation failure, no catastrophic failure