Power Supply Obsolescence Replacement/Refurbishment Process Guideline
(Prepared for review by NUOG Members)
Prepared by:
Power Supply Task Team
February 06, 2003 Draft
Acknowledgements
This guideline has been developed through NUOG members, whose support is gratefully acknowledged. The following task team members assisted in development of this guide:
Allen Davidson / Scientech, IncBenjie Beck / Entergy Nuclear, South
Craig Irish / Nuclear Logistics, Inc
Mark C. Simmons, Sr. / Progress Energy
Stan Zabaglo / Modumend, Inc.
Fred Constance / Constellation Generation
Vijay Shertukde / Entergy Nuclear, North
Ujjal Mondal / CANDU Owners Group
Table of Contents
Power Supply Obsolescence Replacement/Refurbishment Process Guideline
1.0Purpose:
2.0Definitions
3.0 Maintaining Configuration Control:
4.0Critical Characteristics and Aging Mechanisms:
5.0Equipment Reliability and Maintenance Activities:
6.0Replacement Alternatives:
6.1Repair:
6.2 Refurbishment:
6.3Simple Equivalency Evaluation:
6.4Reverse Engineering
6.5Design Modification:
7.0Dedication of Power Supplies and subcomponents:
8.0Shelf Life and In Storage Maintenance Requirements
9.0Recommendations
10.0References
ATTACHMENT 1
Executive Summary:
Many nuclear plants are experiencing increasing problems associated with the aging and obsolescence of Power Supplies. In the decades since the plants’ original design, it is becoming more essential to use engineering resources to support old and disappearing critical components. Corporate acquisitions and takeovers, as well as changes in the manufacturing base in the last twenty years has left the nuclear utilities with substantial and increasingly more complex operational problems concerning the replacement of obsolete power supplies.
Power Supply obsolescence is an on-going concern in the power industry. In the past, many companies built power supplies for GE and Westinghouse and in turn they were qualified as part of the equipment where they were installed. Since then, many of these companies do not manufacture these power supplies or they are no longer in business. Other companies that manufactured and provided power supplies in the 70’s and 80’s are no longer in business. These power supplies are aging and require replacement. Utilities are faced with the problem of replacing or repairing these obsolete power supplies.
This guideline provides recommendations for addressing power supply obsolescence issues in the industry today.
1.0Purpose:
Industry guidance exists for many areas associated with maintenance and replacement of power supplies. This document assembles much of this guidance to provide a basis for the development and implementation of programs to assist in minimizing the risks associated with aging and obsolete power supplies. The following areas are addressed through this guide:
Understanding Power Supply Critical Characteristics and Aging Mechanisms: This section discusses the function characteristics of Power Supplies and the subcomponents that are subject to age related failures.
Maintenance Activities to Assure Power Supply Reliability: This section of the guide addresses proactive methods for the identification of power supplies in the plant and risk based assessments for the prioritization of power supply maintenance activities.
Repair and/or Replacement: A discussion is provided on the various options to address the aging and obsolescence of power supplies. These options include: Refurbishment; Repair; Reverse Engineering; and/or Replacement (cases where the power supply is no longer available as safety related, and an equivalent replacement is available but requires dedication activities for acceptability). A flowchart is provided to assist in the decision process.
Shelf Life and In Storage Maintenance: Recommendations are provided to ensure power supply inventory is maintained and operability is verified.
2.0Definitions
Power Supply Obsolescence:
A power supply is considered obsolete when it is no longer supported to the component level by the original manufacturer (OEM), or a third party supplier in its original form.
Black Box Approach:
Black Box approach constitutes the replacement of subcomponents (such as capacitors, resistors, transistors, etc.) without in-depth equivalency evaluation or alternate replacement evaluation. In a Black Box approach, the critical characteristics of the power supply (form, fit and function) are maintained and used as a basis for determining acceptability of the replaced subcomponents.
Component/Subcomponent:
For the purpose of this guide, component is used to refer to the Power Supply itself and the term subcomponent, is used to refer the pieces/parts that make up the component.
Obsolescence Inventory Replacement Database (OIRD):
The NUOG database used for documenting obsolete items and potential replacement solutions.
LINEAR POWER SUPPLY
This is the simplest form of power supply. The AC voltage (usually 60 hertz) is stepped-down from the 110 or 220 input line voltage to the desired output level. The AC Voltage is passed through a bridge rectifier to create the DC voltage. Filters and regulators are used to achieve the desired characteristics of the supply line/load regulation, output ripple, and noise. Linear supplies are very stable. The outputs are easily regulated and filtered resulting in good regulation and low noise.
The drawback of a linear supply is the physical limitation and weight of the transformer required to achieve higher power outputs (high weight to power output ratio). The efficiency of the linear is usually less than 50% requiring twice the input power to develop the desired output power.
SWITCHING POWER SUPPLY:
The purpose of a switching supply is to generate higher output power in a smaller lighter physical package. The supply takes the AC line input and rectifies it to a 300 volt DC level. This voltage is pulse modulated (switched) at a higher frequency than the 60 hertz line voltage, usually around 50 kilohertz creating a square wave. This signal is stepped-down through a transformer to the desired output level and rectified again to create the DC voltage. Filters and regulators are used to achieve the desired characteristics of the supply line/load regulation, output ripple, and noise.
The high frequency of the square wave requires a lot less iron in the transformer to develop higher power outputs. Since the transformer is smaller, multiple windings on the same core are used to develop multiple outputs in a smaller physical package (lower weight to power output ratio in comparison to linear power supply). The switching supply is significantly more efficient than the linear usually approaching 90% so less input power is required to develop the desired result.
The switching power supply’s drawback is the output is usually not as tightly regulated as the linear. Since it is more difficult to filter a square wave than a sine wave, the ripple and noise on the output voltage is also generally greater. Tighter regulation and lower ripple/noise characteristics can be achieved, but it is usually not required or cost effective for most applications.
3.0 Maintaining Configuration Control:
While approaching solutions for Power Supply obsolescence one must consider plant requirements for configuration management. “Creep” is used in this discussion to describe the change of a power supply due to design changes of the power supply subcomponents over time.
Throughout this guideline, recommendations for repair, refurbishment and/or reverse engineering of power supplies are based, in some cases, on “as found” conditions. The recommendation may involve the replacement of power supply subcomponent, such as a capacitor or board. In some cases, where unidentified in design documentation, the subcomponent is identified through visual inspection. The major concern with this approach revolves around the potential “creep” of the subcomponent specifications.
For example, an evaluation to replace an obsolete capacitor may select a higher rated capacitor as a replacement and the acceptability of the replacement is based on the higher rating, thus, de-rating the new capacitor. While the new capacitor may restore the power supply to acceptable ratings, future evaluations should consider the original capacitor and the original specifications instead of the “as found” configuration with the replacement capacitor.
For these reasons, when replacing power supply subcomponents with currently available parts, it is essential the change is accurately documented and the power supply’s configuration is maintained to its initial specifications.
4.0Critical Characteristics and Aging Mechanisms:
The critical characteristics of the power supply for our present purposes are:
- Dimensions
- Mechanical Characteristics, e.g., seismic qualification requirements
- Electrical Characteristics:
- Output Voltage
- Output Current
- Output Voltage ripple
- Output Voltage regulation
- Response time
- Operating Temperature range
- Over-Voltage and Over-Current capability
- Input Voltage Quality
- Environmental Qualification Requirements
When evaluating a power supply’s performance for a specific application, the a review of the above characteristics to
The life of the power supply is based on the performance of subcomponents associated with the power supply. Overtime, age sensitive subcomponents go through changes that result in variations in the subcomponents performance. These changes are the results of various factors, such as operating temperatures, ambient temperatures, radiation exposure, vibration, etc. Also, aging of one subcomponent, could lead to additional stresses for another subcomponent in the power supply.
Historically, the solution to power supply aging and failures has been to replace the capacitors without regard for the other associated subcomponents. Some of the common subcomponents that have age related failure mechanisms are:
- Resistors are used in power supplies to drop the voltage to a level that can be used by other subcomponents in the power supply. Over time the resistor can age causing variations to the voltage and current supplied to other parts of the power supply, resulting in further stresses to the electronics that use the voltage.
Given the simplicity of the resistors design, age related failures are not anticipated over the life of most plants.
- Transformers constructed of windings and a core. The heat generated from the current flow through the windings, induces age related failures to the core and/or windings. Transformers have performed well over the years and there is no data to suggest aging of transformers can be used as a predictive measure for determining the life power supply.
- Semi-conductors (diodes, transistors, etc.) exhibit age related failure through excessive leakage current, open circuits and/or short circuits. While the semi-conductors construction is such that the life of the item is anticipated to exceed the life of the plant, slight variations in the performance of the semi-conductor due to aging mechanisms will go unnoticed unless the performance of the power supply is monitored over time.
- Capacitors are more prone to age related failures than any other power supply subcomponent. As discussed in the shelf life section of this guide, Aluminum Electrolytic Capacitors (AECs) have a design which is more sensitive to age related failures. For this reason, the AECs are generally held as the problem when a power supply fails.
- Cable and wiring has been qualified to exceed the life of the plant and does not pose an age related failure mechanism for power supplies.
- Melamine fuses are used in some power supplies. The life of melamine is generally established at less than 40 years.
Given the preceding, most power supply maintenance activities will continue to revolve around the life of the Aluminum Electrolytic Capacitors. When establishing maintenance intervals, shelf life or refurbishment criteria, each power supply should be evaluated to determine the age sensitive components. When refurbishing the power supply all age sensitive materials should be considered.
5.0Equipment Reliability and Maintenance Activities:
With power plants aging, Power Supply aging and obsolescence concerns increase every day. The industry continues to struggle to find the right balance between equipment reliability and the resources to dedicate to preventative maintenance activities/programs. The following provides a discussion of issues related to establishing a maintenance program for power supplies.
Understanding power supply degradation mechanisms and critical characteristics provide a better understanding and the basis for selecting a maintenance option. For the utility engineer, the decision making process is based on various factors:
- Interface, application, and function of the power supply
- Availability of the equivalent replacement power supply
- Availability of the subcomponents for the power supply that needs to be replaced
- Availability of technical skills and test equipment in-house
- Availability of the approved qualified supplier who can perform repair/refurbish and dedication of the unit for the utility company
Each plant is faced with the task of determining the level of resources to expend on maintenance programs to ensure equipment reliability and continued successful operation. The following factors should be considered when devising a preventative maintenance plan:
- Power Supply Identification:
Each plant should determine all power supplies installed in the plant. This may be done using existing Bill Of Materials data, Component Data and material issue history. For some plants, these sources may produce a relevant list, however many power supplies were supplied as a subcomponent of another piece of equipment, system or skid. System Engineering involvement is usually required to identify these “hidden” power supplies.
The list should, as a minimum, identify the power supply nameplate data and the age of the power supply. If walkdowns are used to obtain this data, the walkdowns should identify the date codes of any Aluminum Electrolytic Capacitor contained on the power supply.
- Criticality of the Power Supply:
Once the power supplies have been identified, the program should then review the applications to establish how critical the power supplies are to the safe operation of the plant. The program should categorize the power supplies, based on the criticality. An example of categories may use maintenance rule classifications:
- Not in scope of the Maintenance Rule
- Low Risk Significant
- High Risk Significant
- Ensuring operability:
Once established, the criticality of the power supply should dictate the level of preventative maintenance and inventory levels to ensure operating spares are available and operability of the installed unit is monitored.
An example for High Risk Significant Power Supplies might include:
- Monitoring power supply performance at specified intervals in an attempt to predict age related failures. NOTE: Ref. 1 suggests that these monitoring activities may not produce the desired affect and, in some cases, have been determined to be ineffective.
- Establish a replacement frequency for the power supplies based on the risk associated with failure of the power supply. This would involve:
- Maintain sufficient spares in stock and ensure operability through an aggressive In Storage Maintenance program.
- Start with a conservative replacement interval
- Monitoring the “as found” condition of the power supply removed from the plant.
- Refurbish the power supply removed from the plant for re-installation at the end of the next interval.
- Once “as found” data is established, it may be determined that the replacement interval can be increased or decreased.
6.0Replacement Alternatives:
The primary goal of this guidance is to provide potential solutions that address the obsolescence of the power supply. Since obsolescence is only an issue when a replacement is desired, this section of the guide discusses the following alternatives for replacing a power supply:
1)Repair – The first line of defense against replacing a power supply.
2)Refurbishment – Replace the age sensitive subcomponents that are close to the end of their useful life, potentially degrading the power supply performance.
3)Simple Equivalency Evaluation – Locate a similar power supply that meets the original design criteria and perform an evaluation to allow use of the replacement. This may include dedication, as well.
4)Reverse Engineer – Re-engineer the power supply based on the information available for the existing power supply.
5)Design Modification – Redesign the system to use a power supply with different performance characteristics or remove the need for the power supply.
Determining the method to be used is contingent upon several factors including: budget, time constraints, and the criticality of the need. These options should be considered in conjunction with other factors discussed in this guide to develop a long-term strategy for power supply obsolescence.
6.1Repair:
Repair is generally considered a maintenance activity but should be considered as an option in programs designed to manage power supply obsolescence. Power supplies will fail regardless of our best preventative maintenance efforts. In many cases, depending on the resources available at a specific plant, the most economical option would normally be to replace the failed unit. The following are several ways the repair of a power supply may be factored into a proactive obsolescence program:
- A program may determine a power supply used in the plant is not critical to plant operations and established no a “run to failure” criteria for maintenance of the item. Upon failure, repair of the unit should be the first option.
- Another power supply, may fail prior to its established preventative maintenance interval. In cases where a replacement is unavailable, repair should be the first consideration.
- Prior to dispositioning a failed power supply that has been replaced, it should be evaluated to determine whether the unit is obsolete. If obsolete, the unit should be returned to inventory for ProcurementEngineering (PE) disposition. The PE disposition may be to simply repair the unit and return it to inventory.
The following are other factors that should be considered when planning whether to repair a power supply:
- Ensure your facility has the available skills and test equipment to properly troubleshoot and repair the problem.
- Qualified replacement parts may be unavailable. The cost and effort required to locate and dedicate these parts may outweigh the costs of other options presented within this guide.
- Consider third hand supplier capabilities as a potential resource for the repair.
- Ensure the quality level of the repair program meets the quality program of the original power supply manufacturer.
6.2 Refurbishment:
Refurbishment involves replacing the age sensitive subcomponents of the power supply that have been determined to be near their end of life. This, in many cases, can be the most economical path for replacing an obsolete power supply, provided an existing unit is available for refurbishment. The consideration to refurbish is based on the availability of the power supplies subcomponents (or equivalents) and the condition of the unit.