BY J. Michael Fife
There are two main reasons that the inverter is the primary failure point. First, the inverter typically contains the only moving parts in a PV system. But the other reason for historically poor reliability performance has been the fact that inverters are typically installed in physical environments that are damaging for electronic components. Power electronics are typically housed in controlled environments, but solar inverters are usually placed near the arrays in environments that are prone to high temperature extremes and wide temperature fluctuations, humidity, corrosive elements, dust and other environmental stresses that are influenced by the geographical location of the solar plant. Mitigating the effects of these factors has required a complete rethinking of the fundamentals of grid-tied inverter design.
Improving Reliability: Redesign
Several techniques can be employed to improve the reliability of commercial and utility-scale solar inverters, but the most effective path is to throw out the designs of the past and completely redesign the inverter with the primary goal of achieving high-reliability. One way to do this is to use sophisticated software modeling and design techniques that have been employed in other mission-critical fields, such as extensive modeling of component failure modes.
To accurately predict component stresses and associated wear-out mechanisms that solar inverters experience due to natural cycles, a complex time-dependent modeling approach is required. Because temperature cycling contributes to device wear-out, simpler constant hazard rate and Mean-Time-Between-Failure (MTBF) calculations that might apply in other situations are not a reliable predictor of real-world performance.
For the case of thermal stress, many subsystem component temperatures do not necessarily track ambient conditions. Solar heating, self-heating, conduction, and convection (both passive and active) can influence component temperatures. Therefore, a comprehensive thermal model must be developed that produces realistic component temperatures as a function of time. This requires a complete time-dependent thermal simulation of the critical components. For example, if an inverter has an active cooling system, a thermal simulation may be constructed that takes into account forced convection and its effect on component temperatures given a time dependent ambient temperature profile. There are a number of methods that can be used to perform a multi-component time-dependent thermal simulation. The preferred method at PV Powered is use of a custom Matlab program that solves the heat transfer equations (convection, conduction, and heating rate) given an input file containing a set of component properties, thermal interaction parameters, and cooling control law parameters.
There is also an important secondary purpose for time-dependent thermal modeling. It allows the cooling control system of the equipment to be simulated during the design phase so that its performance can be assessed and optimized for a variety of geographic locations.
The second step of this analysis involves calculating subsystem reliability while taking into account the component stresses. Then, once the basic design and component selection are completed, rigorous stress-testing with root-cause analysis during the design phase can direct fine-tuning of the design that yields additional gains in system reliability.
The roots of this innovative approach at PV Powered go back a little over three years ago, when the company began working with Boeing in an intensive program to improve inverter reliability as part of a project funded by the U.S. Department of Energy (DOE) to pursue the government? Solar America Initiative (SAI) of making solar power cost-competitive with other forms of energy. As a result of this three-year program, the company has delivered a new grid-tied utility-grade solar inverter that is designed for a 20+ year operating life--something that was completely unheard of just a few years ago.
More recently, the company has been leading a team of partner companies in Stage 2 of the DOE-funded Solar Energy Grid Integration System (SEGIS) program to develop command and control technologies that will continue to improve the reliability and efficiency of connecting solar power systems to the grid.
Less Components, More Reliability
The work that the company has done has shown that inverter reliability can be significantly improved by reducing the number of components in the inverter design. By starting with a clean sheet of paper, the team took a fresh look at the components selection, and the resulting design employs approximately 30% to 50% fewer components compared to other inverter designs. With fewer components to fail, and more attention given to which ones are used, the projected uptime of the system can be significantly increased.
Another novel invention that contributes to the PV Powered inverters’ industry-leading reliability is a redundant cooling system called “Smart Air ManagementTM.” The design principle behind this is to cool each individual component based on its cooling needs versus doing a universal cooling of the system, and to use redundant fans that allow the inverter to continue to operate at full power if one should fail. Variable speed fan control is used to deliver cooling when and where it is needed. Fan speed, energy use and temperatures are remotely sensed, and alerts and faults are generated if problem conditions occur. Importantly, high capacity air intake filters are used to keep entire inverter clean and free of contaminants that could damage sensitive electronics.
Maintaining high reliability is a key component of managing overall life cycle costs, and improved inverter reliability is having a major impact on removing the economic barriers to proliferation of commercial and utility-scale solar systems. By reducing the frequency of service calls and repairs and producing the maximum amount of kWh’s day in and day out without fail, inverter manufacturers can significantly increase the value of the overall PV system.
Dr. J. Michael Fife is Director of Reliability for PV Powered. Dr. Fife brings more than 15 years of experience in technology development and failure analysis (http://www.pvpowered.com/).
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