By Steve Stephens, P.Eng.
In recent times, the need for increased economy, capacity, quality and productivity to maintain a competitive edge has motivated many industries to modernize aging plant facilities and processes. One result has been a gradual increase in electrical system capacity, placing higher demands on existing electrical control equipment, sometimes beyond the original design specifications. Generally, electrical equipment does not in itself have a direct influence on the competitive position of an organization. Because of this, there is often a lack of funding for replacement or upgrading. As a result, many users have 20 to 50 year old equipment, which may not only be at or beyond its expected design life, but also undersized due to increased system capacity.
"Equipment life" may be defined as the point when intended function is no longer performed reliably and safely. It may be considered a "grey area" as equipment approaches the end of expected design life. It has limits influenced by such things as electrical operations which erode or degrade materials in the interrupter mechanical operations, affecting the interrupter contacts and mechanism insulation systems. All pieces of equipment have finite lives due to voltage stresses, usually aggravated by moisture and temperature.
Poor Maintenance
Manufacturers design equipment for maximum economically feasible performance lives under known or controlled conditions and parameters. Unfortunately, this ideal approach does not always apply to "real world" applications and installation practices. Consequently, there is no hard basis for accurately predicting the life of electrical equipment. Of course, regular maintenance will help enhance the performance and extend the life of any piece of equipment.
Switchgear and industrial control problems begin as a result of both age and service, manifesting themselves as failure and the non-availability and/or high cost of replacement parts. Aging equipment has a very unique mode of failure, different from other power equipment such as mechanical systems. The latter provides advanced indicators of failure such as noise, vibration, heat, or some change in process parameters. Electrical control equipment, on the other hand, often does not provide any of these indicators -- it is a "silent sentinel" and fails on a "go-no-go" basis, sometimes with catastrophic results.
Generally, the extent of damage is a function of the available short circuit capacity and the time taken to isolate the cause of the failure. With this in mind, attention should be given to preventing or at least minimizing the chances of failure and so extend the life of the installed equipment.
Options Available to Help Extend Equipment Life
Where system and load demands have increased beyond the capabilities of existing equipment and, given the fact that many components in electrical power switchgear equipment have a finite life, some parts will fail. One of the following "3-R" options can be considered to help remedy the situation:
Complete replacement of the original installation, using modern "state-of-the-art" currently manufactured equipment.
Rebuild certain portions of the original equipment utilizing refurbished "used" assemblies to original design specifications.
Retrofit, or selective replacement of the interrupter portion only, using pre-engineered current technology interrupter assemblies.
While each option may yield improvements in system operation, there is a trend towards retrofitting. In many instances, the superior value and performance benefits achievable through retrofit v. total replacement or rebuild of existing systems, where loads or processes must be maintained with minimum disturbance and downtime, can be demonstrated.
Given the magnitude of "avoidance" costs associated with downtime resulting from equipment failure, any of these options may be seen as an "affordable" solution, if equipment cost is the exclusive consideration. It must be recognized, however, that any realistic assessment of relative cost benefit must address a range of additional factors such as:
- safety
- opportunities for increased system or load capacity
- site modification, installation, and downtime costs
- future maintenance and training requirements
- availability and cost of replacement parts
The relative merit of each option is considered in table 4t-1.
Replace
In many instances, conventional analysis would suggest total replacement of the switchgear or control equipment. Replacement is in fact a natural option in situations where corrosive environment is a major factor, or where system expansion/reconfiguration to support increased power or load demand is required.
Thus all new equipment, using higher rated current technology interrupters, is installed and the problems are solved. In effect, the life-cycle timer is turned back to zero, with new everything - steelwork, relaying equipment, mechanisms, etc., are all designed to perform well into the next century.
A further benefit is that new equipment inherently is more reliable. With higher safety considerations, equipment costs less to maintain, and can be repaired using "off the shelf" replacement parts in reasonable time.
Certainly, complete replacement can satisfy virtually any conceivable set of performance needs based on given parameters. The catch is that, overall, it is the most expensive option. Not only must the equipment costs be considered, but also hidden costs associated with the following:
- long delivery time, which may not be acceptable
- downtime to remove the old and install the new equipment
- site modifications to accept the new equipment. This may involve new foundations, relocation of power and control cables, etc.
- start-up and verification tests associated with the new equipment
- staff-training
- all new spare parts
The indirect costs can exceed the cost of replacement equipment.
Rebuild
Initially, this may be the more economical option. It has the added benefit of interchangeability using identical devices, making installation easy. Downtime is kept to a minimum, and no structural modifications or additions should be necessary. Also, maintenance practices will remain unchanged, thus eliminating the need for retraining.
Disadvantages include:
- total investment may still be substantial, depending on the availability of "used" equipment
- many of the benefits available with the other options may be sacrificed:
- no increase in safety levels
- no new technology
- no increased system or load capabilities
- questionable reliability
- questionable mechanical or insulation life
- postponement of the inevitable - possibility of failure relatively high
- past problems may reappear
- marginal extension of service life
- lack of replacement part availability with little cost improvement
- retrofit
-modernizing and upgrading for increased safety and reliability
- taking vintage equipment into the 21st century
Analysis has revealed that power switchgear and industrial control equipment failures and maintenance problems occur mostly in the interrupter assembly. This is not surprising, as the interrupter performs the strenuous switching and interrupting functions that contribute to electrical and mechanical wear shortens the life of equipment.
Based on this analysis, it can be said that power switchgear and industrial control equipment is divided into two areas:
1) Parts that basically have an infinite life
- steelwork
- conduit
- properly braced and insulated bus assemblies.
- insulation and insulating supports, generally are expected to have a long life. In cases where deterioration has taken place, the old insulation can be replaced quite easily and economically using the latest insulating system.
- control wiring
2) Parts that have a finite life:
- the circuit interrupter which is subject to electrical degradation
- associated mechanism which is subject to wear
- Auxiliary devices such as protective and general purpose relays, metering equipment, instrument transformers, generally have a long life or are easily replaceable.
Retrofit
It seems reasonable then that equipment life, reliability, capabilities, and safety can be increased economically by limiting the modifications to simply replacing the interrupter assembly using the latest technology. This is the third option available to the end-user.
At the present time there are two technologies available for interrupter retrofit - Vacuum and SF-6. Both have a proven record, and are well accepted in the market place, each enjoying between 40 per cent and 50 per cent of the total worldwide market for new equipment. There are suggestions of a trend towards vacuum taking over an increasing share of the market, however, as every major medium voltage equipment manufacturer in the world -- with the possible exception of one country -- has now developed its own line of vacuum interrupters. This includes China, South Korea, and India. At least one U.S. manufacturer who initially licensed SF-6 technology, is developing vacuum interrupters.
Some of the benefits of this third option are:
- latest state of the art proven technology interrupter, the component or assembly subjected to the most strenuous duty. This is on par with complete equipment replacement, but at a much lower overall cost. It provides for increased reliability and safety, and helps extend life of the 20 to 40 year old equipment well into the next century. It can be said that the "life cycle" timer has been turned back close to zero, the same as for new equipment.
- relatively short delivery time
- minimum down time, with minimal site modifications
- changeout may be accomplished one or two units at a time
- roll out the old, roll in the retrofit
- maintenance budget rather than capital investment
- increased system and load capabilities. This may require engineering investigation to ensure bus bracing is adequate.
- verification tests, staff retraining minimal
- availability of parts enhanced
- environmentally friendly material used throughout
- interrupters tested to perform for at least 20 years
- it is estimated the vacuum envelope used in the vacuum interrupter has expected life in excess of 100 years. Field data gathered by all manufacturers has confirmed the probability of vacuum loss to be negligible.
There are three major approaches to retrofitting the interrupter equipment:
1) Design interrupters to fit into the old interrupter frame, keeping the old mechanism, linkages, etc.
While this has been successfully carried out by at least one major manufacturer, it can be a very complex undertaking. It involves changing select mechanism parts to reduce the closing stroke, and force balancing other components for opening and closing speeds. Plus there is a possibility that some of the more crucial components subjected to mechanical wear will be reused. Extensive tests have to be carried out to verify the performance of the hybrid design.
2) Replace the original interrupter, mechanism, linkages, etc. ...with VACUUM or SF-6, fully proven and tested modules. In this case, the user's original, spare, or "refurbished" drawout truck is used. The modules are self-contained, fully tested to ANSI or other applicable standards. All safety interlocks to meet the requirements of ANSI standards are included. When replacing a solenoid operated device with a modern spring operated device, an additional safety interlock is required to ensure the complete assembly is not withdrawn from its compartment with the closing spring charged. This is specified in the applicable standards. Means must be provided to operate externally mounted switches (MOC), but care must be exercised to ensure the new interrupter assembly has sufficient closing and tripping forces to handle the additional load imposed by MOC linkage.
While both VACUUM and SF-6 are good candidates for retro-fitting, using the original drawout truck, the end user may wish to remember the following:
- vacuum interrupters tend to be smalle>