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<JUN, Issue, 2012>
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Component & Power

Building Maximum Reliability in PV System

Reliability is key to the proliferation of any solar energy product, especially those involving sensitive electronics, such as solar power optimizers and micro inverters.
For solar energy to gain mainstream market penetration, distributed power electronic products must be durable enough to withstand harsh solar environments and last at least 25 years. However, many technologies on the market today simply have not been in standard use long enough for hard field data to prove their reliability. Fortunately, there are steps and considerations engineers can take to build an extreme level of reliability into a product. These steps translate into long-lasting, durable products with lower lifetime costs of ownership.
This, in turn, provides project developers and investors a higher level of confidence to fund projects today.

By Rob Dixon

 

 

Power Electronics Increase ROI

 

The success of a PV installation is largely gauged by how quickly it can reach return on investment and how much it can reduce energy costs. PV modules have made significant advances in the past thirty years, but it is the rise of solar power optimization electronics that are bringing the most dramatic developments to the industry. Solar power optimization electronics shorten return on investment and diversify suitable installation locations.

Power optimization electronics, such as DC to DC converters and micro-inverters, increase total energy harvest by reducing the impacts of shading, panel mismatch, and other obstructions to power generation. Power optimized electronics result in a faster return on investment for system owners because power that would otherwise have been lost is recovered and used. This accelerated ROI depends largely on how robust and reliable the arrays power electronics are. A typical solar array is expected to function for 20-25 years without requiring unscheduled maintenance. If power electronics are unreliable and fail to operate effectively, unforeseen maintenance and repair costs can reduce the systems overall energy production and delay ROI.

In general terms, reliability refers to the ability to perform and maintain full function in routine conditions as well as in hostile or unexpected circumstances. Depending on the manufacturer, this broad definition can hold a variety of interpretations.

 

Component Selection

 

In order to attain maximum reliability, it is essential to select components that have the capability to support long mission times (25+ years). If a single component has insufficient performance or contains technologies known to have long-term reliability risks, that component will limit, or reduce, the overall reliability of the product to that of the lowest performing element. As such, Azuray Technologies uses only Automotive/Military/Industrial-grade components. Azuray believes that this class of component is required to withstand the extreme demands of solar power generation.

The company excludes +85C rated components in mission critical circuits as well as components known to put a products reliability at risk, such as electrolytic capacitors.

 

Thermal Acceleration of Failure Rates

 

An accepted guideline for thermal acceleration of failure rate states that the failure rate for a given component doubles for every 10 degrees of temperature increase.1),2) In other words, for every 10 degrees of operational rating in a given environment, the failure rate for a given part at a specific temperature is cut in half. For example, a +105C rated part will have four times the reliability of a +85C rated part at a given temperature. Likewise, a +125C rated part will have sixteen times the reliability of a +85C rated part.

Employing a single +85C part limits an entire products ability to attain an extreme reliability profile. Power optimization modules, mounted on the backs of panels, commonly experience temperatures that exceed +85C. This significantly overstresses the power optimization components and puts the PV system at risk of failure. This is because component failure mechanisms are highly accelerated by temperature. Thus, a components temperature rating fundamentally affects the failure of not only a particular component but the entire product.

Surprisingly many power optimization manufacturers employ +85C rated parts in their DC to DC converters and micro-inverters.

 

Automotive/Military-Grade Components

 

In light of the thermal acceleration of failures in +85C-rated components listed above, Azuray Technologies uses only Industrial, Automotive (SAE) or Military/Aerospace-grade components in its AP product line. This is a step beyond competing products in the power optimization sector, where commercial-grade components (rated to +85C) are commonly used.

 

 

Technology Components Excluded

 

To ensure the extreme reliability of its AP product line, Azuray excludes certain components that are acknowledged by the company, industry manufacturers and reliability experts to put at risk high product reliability and long mission time. These exclusion in Azurays AP product line include, but are not limited to:

-Electrolytic capacitors3)

-Tantalum capacitors4)

-Large multilayer ceramic capacitors

-Optocouplers

-Powdered iron core inductors with organic binders5)

-Any component parts rated +85C or lower.

 

Component Derating

 

In the 1960s, NASA and military suppliers pioneered the use of component derating guidelines as one way to enhance the reliability of systems. Azuray agrees with experts in the field that employing Department of Defense-based derating in circuit construction makes them more reliable as well as improves circuit performance.6) Derating guidelines are not requirements within industry and are obviously not implemented when +85C components are being used in solar power applications with measured field conditions that meet or exceed that rating.

 

Reliability Modeling

 

Azuray subscribes to the Telcordia SR332 failure rate database which provides the very latest information on actual field failure rates for components. This information is used to build precise models of Azuray products and to run hundreds of what if scenarios. These scenarios precisely model actual mission profiles for specific locations worldwide and identify the highest contributors to product failure rates. It is this modeling and analysis that demonstrated devices containing electrolytic, tantalum and large multilayer ceramic capacitors would severely limit a products ability to reach the target reliability of a 25-year failure-free life.

Reliability testing at Azuray Technologies includes a wide range of tests including Highly Accelerated Life Test (HALT), environmental tests and reliability demonstration testing. Coupled with these tests is a grounded philosophy that Annualized Failure Rate is a more informative indication of reliability than Mean Time Between Failure (MTBF).

Other power optimization electronics manufacturers may publish their highest Mean Time Between Failure (MTBF) rate as a way to establish a perception of reliability. However, from the perspective of Azuray Technologies, using this metric as a determination of reliability is disingenuous and misleading. By contrast, Azuray takes this predicted lifetime from a simulation (MTBF) and generates an Annualized Failure Rate to estimate the reliability over the useful life of the product based on actual measured field conditions (mission profiling).

 

Understanding Mean Time Between Failure (MTBF)

 

Throughout the electronics and power conversion industry, the Mean Time Between Failure metric is widely misunderstood. People without experience in reliability engineering interpret the MTBF to be the useful life of the product. They think a product with a high MTBF rate will last longer than a similar product with a lower MTBF rate. This is not always the case since how the MTBF is generated determines not only the accuracy of the prediction but the validity.

An example of this misunderstanding would be if a product has a 5 million hour MTBF, people might think it should last over one hundred years. However, this is not what MTBF means. Instead, MTBF is an estimation of when 62% (the mean time to failure in a population) of the installed base will have failed. It is the mean of the distribution of failures. In other words, the MTBF indicates when more than half (62%) of the products in a given population will have failed. Stated another way, for any single product, there is a 62% chance the product will have failed by the MTBF estimate. This is a poor measurement of what a customer should expect because it is not intuitive how to relate MTBF to actual failures in time.

This is not to say MTBF as a metric is invalid. It is very useful in determining the steady state failure rate and for making comparisons between two items (if those items use the exact same methodology and the same exact failure rate database). This allows an A/B comparison that provides information to determine which product has the best design reliability for the cost. Unfortunately not all manufacturers invest the time and expertise to develop highly accurate reliability predictions and instead simply generate a high MTBF number that on the surface seems to indicate high reliability.

It is essential to also understand that a calculated MTBF is a poor predictor of wear-out in a product. A high MTBF figure may accurately reflect a low failure rate during the normal operating portion of a products life but completely fail to indicate a wear-out failure mechanism from overstressed parts. It is not only possible but entirely likely that using 85C parts in an application that exposes those parts to an 85C environment (100% stress ratio) would have a high calculated MTBF. This will appear in actual application as a low steady state failure rate that rapidly increases due to wear-out well before the required 25-year useful life.

 

Field Data

 

Unfortunately, the panel level solar power electronics sector is so new and its technology is so cutting-edge that no manufacturer has had enough units in the field for a long enough period of time to collect meaningful long-term reliability data.

We have seen manufacturers of solar power devices state multi-million hour accumulations of field data, but when one examines the data you find that the actual hours per unit deployed averages to less than one year in field per device. This is hardly acceptable data for demonstrating long-term reliability since the data contains no information of failures occurring during the critical time period of out 10+ years. That data simply does not exist today.

 

Highly Accelerated Life Test (HALT)

Another critical aspect of the extreme reliability of the AP product line is Azurays studious attention to margin. Margin is when a product is designed to handle stress above its operating environment. The higher the design margin the more robust the product will be. For example, if a product is specified to operate at 65C but it breaks at 66C, that product has very little if any margin. Compare that with a 65C-rated product that can operate to a temperature of 105C. The implication of having extreme design margin due to component selection and aggressive derating can be significant.

The large design margins of Azuray products are the result of multiple HALT tests during product development. This process identifies the weakest elements in a design. Once identified, those elements are improved, replaced or corrected until the HALT testing is able to take an Azuray product to the fundamental limit of technology. Through its HALT tests, Azuray is able to show the AP product line has the capability to operate well beyond the specified range of operation. During the final HALT series, the AP300 reached the thermal/vibration limits of the test chambers before identifying the next order failure mechanism. This represents a significant design safety margin beyond the specified product operating limits. It also demonstrates that when properly executed HALT can be used to drive product design to the fundamental limit of technology and therefore its highest reliability potential.

 

Environmental Test

In addition to HALT testing, complete environmental compliance testing is executed. This includes but is not limited to: temperature, humidity, thermal shock, random vibration, swept sine vibration, operating shock, dust/contaminant intrusion, corrosive atmospheres, fungal growth, altitude, and transportation shake/shock.

 

Reliability Demonstration Testing

The products also undergo long-term testing in order to prove to a statistical certainty that the test population does not exceed the target failure rate. This typically consists of calculating the required test sample size and test duration to demonstrate that the target failure rate is not exceeded. The test is monitored daily, and any failures are removed and analyzed for root cause. A formal corrective action process is followed to ensure any product improvement opportunities are not missed.

 

During the development process we drive the stress/strength relationships and therefore the design margin to the fundamental limits of technology. Our large margins ensure that the inherent strength of the design far exceeds the stress that the product experiences in the end-use environment. We employ derating guidelines developed and proven by other ultra long life and high reliability industries such as the automotive, military/aerospace and telecommunications industries. This level of innovation allows our product to have the highest performance envelope while avoiding known long-term reliability risks at both the component and product levels.

When system owners and integrators feel assured an array will generate solar power to the utmost of its ability, regardless of shading mismatch and other obstructions, the reliability of the products supports the financial models used to fund the arrays installation. Power optimization electronics emplyed to increase a systems energy harvest must be highly fuctional and extremely reliable to encourage their use in new and existing PV installations.

 

Rob Dixon is the Senior Reliability Engineer at Azuray Technologies (www.azuraytech.com), a Senior Member of the American Society for Quality and a Certified Quality Engineer. Dixon has over 25 years of reliability engineering experience focused on high reliability applications in medical, telecom, military/aerospace and power industries.

 

REFERENCES

1) Engineering Design News Europe, David Marsh, Thermal modeling heats up for the Mainstream, June 202 (http://www.edn.com/contents/images/220698.pdf)

2) Experts Exchange Acceptable Internal Temperature for an HP Server (http://www.experts-exchange.com/Hardware/Misc/Q_24903583.html)

3) Center for Advanced Life Cycle Engineering Substandard Electronic Parts, Whats New Archiver, 2002 (http://www.calce.umd.edu/whats_new/2002/China.pdf)

4) IEEE. Fresia, E. J. Eckfeldt, J.M. Failure Modes and Mechanisms in Solid Tantalum Capacitor Second Annual Symposium on the Physics of Failure in Electronics. September 1963. Current version 21 May 2007 (http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4207581%2F4207582%2F04207614.pdf%3Farnumber%3D4207614&authDecision=-203)

5) MMicroMetals, Thermal Aging: Cost Loss Increase Due to Thermal Aging in Iron Power Cores (http://www.micrometals.com/thermalaging_index.html)

6) IEEE, Jim McLinn, Derating Guidelines, IEEE Newsletter. 31 Oct. 2006. http://www.ieee.org/portal/cms_docs_relsoc/relsoc/Newsletters/Jan2007/DeratingArticle.pdf

 

 

For more information, please send your e-mails to pved@infothe.com.

2011 www.interpv.net All rights reserved.

 
 

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