Status and Needs of Power Electronics for Photovoltaic Inverters

Photovoltaics is the utility connected distributed energy resource (DER) that is in widespread use today. It has one element, the inverter, which is common with all DER sources except rotating generators. The inverter is required to transfer dc energy to ac energy. With all the DER technologies, (solar, wind, fuel cells, and microturbines) the inverter is still an immature product that will result in reliability problems in fielded systems. Today, the PV inverter is a costly and complex component of PV systems that produce ac power. Inverter MTFF (mean time to first failure) is currently unacceptable. Low inverter reliability contributes to unreliable fielded systems and a loss of confidence in renewable technology. The low volume of PV inverters produced restricts the manufacturing to small suppliers without sophisticated research and reliability programs or manufacturing methods. Thus, the present approach to PV inverter supply has low probability of meeting DOE reliability goals. DOE investments in power electronics are intended to address the reliability and cost of power electronics. This report details the progress of power electronics, identifies technologies that are in current use, and explores new approaches that can provide significant improvements in inverter reliability while leading to lower cost. A key element to improved inverter design is the systems approach to design. This approach includes a list of requirements for the product being designed and a preliminary requirements document is a part of this report. Finally, the design will be for a universal inverter that can be applied to several technologies. The objective of a universal inverter is to increase the quantity being manufactured so that mass-manufacturing techniques can be applied. The report includes the requirements and recommended design approaches for a new inverter with a ten-year mean time to first failure (MTFF) and with lower cost. This development will constitute a ''leap forward'' in capability that leverages emerging technologies and best manufacturing processes to produce a new, high reliability, inverter. The targeted inverter size is from two to ten kilowatts. The report is organized into four sections. A brief introduction by Sandia is followed by Section Two from Millennium Technologies (a company with UPS experience). Section Three is provided by Xantrex (a PV manufacturing company) and the University of Minnesota provided Section Four. This report is very detailed and provides inverter design information that is irrelevant to the layman. It is intended to be a comprehensive documentation of proven technology and the manufacturing skills required to produce a high reliability inverter. An accompanying report will provide a summary of the recommended approach for inverter development