UTILITY-SCALE PV INVERTERS
More solar power means more photovoltaic (PV) inverters and a growing dependence of the grid on inverter performance and reliability. Roughly 70 gigawatts (GW) of grid-connected solar PV generation capacity were cumulatively deployed worldwide at the end of 2011. And looking ahead, policy, regulatory, and economic factors are expected to further drive rapid and sustained growth in PV for the foreseeable future.
Nearly all future energy scenarios include a significant amount of solar. In fact, most projections of future electricity generation indicate brisk solar growth, resulting in the resource comprising approximately 1% of worldwide capacity in less than 5 years, and double-digit percentages within the next couple of decades.
The increasing share of distributed solar PV is requiring greater attention be paid to PV plant components to assure that PV systems meet availability and performance expectations.
Inverter reliability is among the solar industry’s biggest concerns. Several early PV system performance studies have shown that inverter related issues caused considerable plant downtime. More recently, SunEdison has analyzed trouble calls from its fleet of 500 PV plants with an accumulated 300 MW of capacity. The latest investigation considered ~70% of the company’s fleet over an 18-month period—between January 1, 2010 and June 30, 2011—and evaluated about 1,000 failure incidents over ~6,000 systemmonths. The average plant size for this analysis was 311 kW (plants ranged between 7.2 kW and 9 MW) while the average PV plant age was 19 months, with the oldest systems entering Year 6 of operation. With more than two-thirds of identified failures, inverters represent by far the most likely PV plant component to cause trouble. A further investigation into the reliability of an inverter’s individual subcomponents revealed control software as the main cause behind inverter failure and, to a lesser extent, card/board failures (PCBs) as the second most frequent failure area. Software bugs were found to be the primary root cause behind most of the control software issues, while hardware component failures (e.g., power semiconductor components and communications circuit boards) represented the preponderance of hardware failures. Meanwhile, support structures, DC subsystem, and AC subsystem collectively contributed 20% of SunEdison’s identified system failures. Parts/materials and construction defects were generally found to be the primary root causes behind the failures in these subsystems. Overall, SunEdison’s analysis exposed an underlying connection between PV plant downtime and quality assurance and control in the inverters.
With increased PV inverter deployment there is an increased need to share selection and qualification and documentation best practices that lead to improved plant reliability and uptime. Presently there are no complete performance criteria for grid-connected inverters. IEEE 1547 provides guidelines for interconnect behavior, but this represents only a fraction of the performance concerns harbored by PV system owners. As penetration levels rise, utilities will increasingly need clear, complete specifications that can be easily referenced and broadly supported. Considering the interconnection resemblance and maturity, electricity metering industry specifications can be closely studied to provide insight into the nature and extent of criteria needed for PV inverters.
The full Electric Power Research Institute (EPRI) report this article is based on addresses the need for improved inverter specifications and better developed industry practices for selection, qualification, and product options. Included are samples of technical interconnection requirements used by utilities, a description of the needs for inverter product standards, and an account of industry practices and specification applied to utility grade electricity metering that can provide insight into the criteria needed for PV inverters. Other sections provide an overview and comparison of specifications for utility-scale inverter models, as well as conclusions and recommendations for future work. An appendix includes the various standards and regulations that current utility-scale inverter manufactures typically refer to in their product specifications.
UTILITY PRACTICES FOR QUALIFYING AND
Electric utilities have focused on the development of distributed generation (DG) interconnection standards that can work for low penetration deployments and also have flexibility for future higher penetration scenarios. These standards have evolved over 25 years and provide specific technical requirements for paralleling DG, including PV and storage, with the utility system. Today we have IEEE standard 1547-2003 and UL 1741. First and foremost interconnection requirements have addressed safety for lineman and the public, equipment protection, and system reliability and power quality. Specifically interconnection standards developed to date include: ensuring safety, reliability, and power quality; applying consistent requirements in an affordable way; streamlining review and approval process; and allowing higher level of DG penetration. Utility interconnection requirements for PV systems, especially for the utility-scale systems, are continuing to evolve with greater penetration of DG into the distributed grid. IEEE standard 1547-2003 stipulates the basic interconnection rules and several other standards are moving to align more closely to it. However, 1547 is intentionally general, leaving considerable negotiating room for establishing interconnection agreements. In the future EPRI expects that IEEE will develop new requirements specific to high penetration PV in IEEE P1547.8. This new standard will address changes in the operating requirement from low penetration to high penetration
as DG evolves from very small affect, to more significant impacts on voltage and eventually on energy balance in the electric system. In the meantime, a number of utilities are preparing for higher penetration of PV and wind energy in their systems. Examples of the diversity in technical requirements and interconnection practices used by a few utilities in U.S are included in the EPRI report.
PERFORMANCE SPECIFICATION S FOR PV INVERTERS
Utilities need more standard practices for specifying and selecting PV inverters for grid-connected applications. The PV inverter plays an important role as a power conversion device for PV and as a grid-connected generator that makes the handshake with the public power supply. As more of these devices are grid connected, clear requirements and products specified to meet these requirements will be needed. This evolution is also needed to speed up processing of interconnection requests and simplify system acceptance and testing. There are currently no industry accepted performance specifications for a “utility grade” grid-connected PV inverter product. The concept is well developed for other power conversion equipment such as adjustable speed drives and UPS inverters. Other gridconnected equipment such as revenue meters, relays, transformers, and lightning arresters have well developed product standards. Usually the product rating, functional options and performance expectations are covered in the U.S. by an ANSI or NEMA standard and in Europe as well as other parts of the world by an EU or IEC product standard. As PV penetration levels rise, utilities will increasingly need this kind of clear, widely accepted and easily referenced specification to be broadly supported in available products. The typical requirements being used by utilities for connecting PV inverters are described in greater detail in the full EPRI report. Referenced standards in specifying PV inverters used is both hardware and power purchase agreements are listed and described in the Appendix. These typically address the interfaces and safety, such as interconnection requirements in IEEE 1547, and premises installation requirements in NFPA NEC Article 690. However, they do not usually address the plant related daily functions, the energy performance, or the life-cycle economics of the inverter. Today utilities tend to specify inverters as if they are third party equipment,
owned and operated by someone other than themselves. Typically there are no PV inverter energy performance criteria that are a well defined industry practice. Usually these performance parameters are either not covered, or left to the system designer or a third party. We expect the mindset to change when the inverters and PV plants become owned and operated utility assets. With this transition, the criteria will change and requirements are expected to include a lot more on functions, energy performance, and life. Specifications will include more standardization of products and ratings for easier operations and maintenance, focus on energy conversion efficiency, maximum power tracking accuracy, and surge-withstand capability.
PV is the fastest growing technology in the electricity generation portfolio, and the PV inverter has proven to be the most critical component in determining plant availability. As PV penetration levels rise, utilities will increasingly need clear, complete inverter specifications that can be easily referenced and broadly supported. EPRI has published a report that examines current industry practices for specifying PV inverters in grid connected and large scale applications. This report identifies several opportunities to further develop performance and acceptance criteria for PV inverters. In particular, the inverter’s energy conversion performance, grid support functionality, and long term durability are all candidates for more detailed specification in utility procurements. Several examples are also provided where utilities have begun developing grid performance requirements. As a future utility plant asset, the inverter will need to be specified at a level of detail such as revenue meters or transformers.
About the Author
Aminul Huque is a senior project engineer for the integration of distributed renewables group at the Electric Power Research Institute (EPRI). At EPRI he is involved in the photovoltaic (PV) power conditioning system research including grid interconnection requirements, smart-grid functionality, and product reliability. Aminul received a Ph.D. in hightemperature electronics from the University of Tennessee in Knoxville, TN in 2010 and M.S. from the Imperial College London, UK, in 2003.