Legacy power plant migration

Fully 60% of the 30-800 amp central office and switch site power systems in place today are ferroresonant rectifier technology, many systems having been in service for as long as 30 years. An additional 20% employ silicon controlled rectifier (SCR) or magnetic amplifier (Mag-Amp) technology. While some ferroresonant and SCR systems are still being purchased today, their use is diminishing and switch-mode power conversion technology is gaining ground on these legacy power systems.

Although many of the legacy systems still have valuable service life, it is becoming increasingly difficult to locate spares, service parts and qualified service technicians to maintain and repair them. This is necessitating the migration to the newer switch-mode technology. When replacing legacy systems or increasing capacity at existing sites, the service provider is faced with a variety of migration alternatives, each with its costs, advantages and disadvantages.

Power Technology Outlook

Investing in power equipment with an expected service life of 20+ years, only to find that it is difficult or impossible to obtain parts and service in two or three years, is not a good decision, particularly in today's tough capital market. Examining the outlook for current power conversion technologies is useful in guiding your choice of power systems for replacement or expansion.

Ferroresonant. Although ferro is still the dominant technology in place, parts are getting hard to find, along with qualified technicians to diagnose and install them. While there is a limited amount of circuit design involved, extensive transformer winding and labor-intensive hand wiring contribute to a rising cost per amp to produce ferroresonant equipment.

A reduced number of models and sizes are available today, compared to the early '90s; and there is not very much R&D taking place at the outside equipment manufacturers (OEMs). It is extremely difficult to find direct replacement parts for units built 25 years ago.  Alternatives are available, but matching is sometimes very difficult. Because the installed ferro population is so large, the present prognosis remains fair, but fluctuates with telecom business conditions and will likely diminish in the future.

SCR. Like ferro, the cost of producing SCR technology is growing, and components are becoming more difficult to find. In fact, SCR replacement components are harder to find than ferro components in some cases. Because ferro systems were designed to a common standard that all manufacturers were required to observe, ferro components are more widely interchangeable and available.

Also, ferroresonant performance characteristics make them preferable to SCRs in many telecom applications because of the Telcordia GR specifications these were designed around to meet. Most manufacturing activity is limited to production of replacements for previously installed SCR systems and there is almost no R&D effort underway. On-site repair is costly and the prognosis for this technology in telecom applications is fair to poor.

Mag-Amp. Mag-Amp is the oldest of the legacy technologies discussed here. There is no current production of new rectifiers for telco and obtaining replacement components is next to impossible. The technology is so out of date that no one can afford to manufacture service parts. Any that can be found are so costly that system replacement may become the less expensive alternative. 

Schematics, drawings and service instructions are difficult to find since the information accompanying the original equipment has long-since been lost or destroyed. Manufacturers' decades-old documentation archives are sketchy as well and difficult to access. On-site repair is nearly non-existent and the prognosis for Mag-Amp is poor.

Although it will certainly be replaced someday by some other scheme, [switch-mode]  is the technology of choice for the present and foreseeable future.

Switch-mode. The outlook for switch-mode power conversion is radically different. While it has been around for more than a dozen years, it is new technology compared to the others mentioned above. Although it will certainly be replaced someday by some other scheme, it is the technology of choice for the present and foreseeable future.

Switch-mode rectifier technology is used in many other industries and products in addition to telecom. There is a wide selection of off-the-shelf devices available, from a wide range of suppliers. Essentially all of the OEM R&D budgets, development efforts and continuous improvement programs are focused on expanding switch-mode technology.

The cost-per-amp-to-produce is stable, if not decreasing. Also, with the application of digital control technology, there has been a rapid and constant development of features not possible with the older technologies. These firmware-driven feature sets have reduced the amount of site engineering required, save considerable time in installation and repair, and help to generate extensive, real-time information for operation and planning purposes.

The prognosis for switch-mode technology is excellent because considerable work is being done to upgrade early-generation systems to accommodate the latest control features. Many of these improvements are based on digital controller sets at the supervisory level; when the controller becomes obsolete, it can be replaced without disturbing the already-installed power conversion units (PCUs).

TAKE A LOOK View a powerpoint presentation that shows four different migration paths.

Logical conclusion

As legacy systems become inadequate or obsolete, there is little question that the telecom industry will migrate to switch-mode technology. The most convincing reason is the cost of the rectifier itself. While it may not have been true just a few years ago, a 200-amp ferro or SCR-based rectifier will cost more than a 200-amp switch-mode rectifier. Add to this the communication capability and “plug-and-play” configuration flexibility of digital control, the broad selection of components and widely available technical support, and the conclusion is inescapable.

But what migration strategy will the telecommunications industry employ to handle this unavoidable migration with such a large embedded base of legacy rectifiers, particularly in today's difficult business climate?

Migration strategies

Selection of an affordable migration strategy may be influenced by various factors:

• Equipment cost

• Time to complete

• Degree of complexity

• Engineering content

• Disposal of legacy equipment

• Installation cost

• Integration of alarms

• Integration of controls

• Flexibility of growth

• Space savings

• Features

Let us examine four possible migration strategies, evaluating each of the factors listed above.  These strategies are:

• Complete DC power plant replacement

• Complete rectifier-only replacement

• Hybrid rectifier plant, with independent controller for each rectifier type

• Hybrid rectifier plant, with integrated controller to run all rectifier types

1. Complete DC power plant replacement
When the telecom boom of yesteryear was going on, this was the right strategy, considering the amount of capital that was available. With complete replacement, providers knew they would have the latest technology for years to come and be ready for any increased load.

The best features of this method were easy integration of alarms and controls, flexibility of growth, new control features, and the space savings the newer, smaller power plants afforded. With the cost of floor space ranging from $125 to $300 per square foot, per month, these new power plants would accommodate more traffic or support newer communication technology, often without additional “brick and mortar.”

The biggest downsides of this method are the equipment cost, time to complete, disposal cost of replaced equipment and the installation cost of the new plant. Complete replacement is a long process that can take from six months to one year to complete. Sizing is not difficult, but a complete re-engineering of the site is involved, including a complete survey of termination requirements and the need to re-layout battery strings. Assuming maintenance of traffic during replacement, a complex load transition plan is required.

AC and HVAC requirements may change because, while you can now fit a 6KA switch-mode plant in the same space previously occupied by a 2KA ferro plant, the input power and cooling requirements have increased dramatically. Also this strategy may require discarding some equipment that may not have served out its intended life if the plant was only 10 years old.

2.  Complete rectifier-only replacement
The biggest advantage of leaving the plant alone, other than replacing the old rectifiers with new PCUs and integrating all alarming and control back to the original control bays, is the elimination of any load transitioning. It offers considerable saving in terms of risk, time and expense. Also, some additional space may become available, allowing a larger recharge capacity for the existing plant—assuming other parts of the plant (term bar, shunts, for example) are large enough to handle the increase in power or rectification.

Downsides of the strategy include uncertainties about power density and efficiency, the complex engineering required to bridge the gap between digital and analog circuits and the integration of alarms and controls into either the site controller or the site alarm database. Re-cabling of batteries may be required, as well as installing additional electronics to get the same functionality the plant provided in the past.

3.  Hybrid rectifier plant, with independent controller for each rectifier type
The provider may choose to add a quantity of newer switch-mode systems to an existing base of legacy chargers to save equipment cost, allowing each technology to be independently controlled. This strategy typically avoids all load transitioning, and existing chargers can, to some degree, remain intact.  The provider can then replace legacy units as time goes on. The strategy may also allow for larger charge/re-charge capacity if the rest of the plant is sized appropriately.

On the other hand, getting independent controllers running different types of rectifiers to talk to each other, to give overall plant readings, and to share the load properly can be a monumental challenge. Questions arise about what happens when generator recycling takes place, and what happens when AC is restored. Will the generator become overloaded because rectifier-sequencing functions can be accomplished with one device but not the other? Generators are not cheap, so you can't ignore the fact that an overload on the generator can be a serious condition. Integration of controls and alarms can be done, but the engineer needs to understand the engineering of the plant and how the signals are being processed.

4. Hybrid rectifier plant, with integrated controller to run all rectifier types
In this strategy, you add a quantity of newer switch-mode technology models to an existing base of legacy chargers and control them all with a single, integrated controller. It is an attempt to get the best of both worlds—the utilization of existing serviceable equipment and the implementation of the newest technology and features that will allow your power to grow along with your service needs.

The easiest way to integrate the control features of both systems is with the use of analog-to-digital (A/D) converters for the analog legacy devices, allowing them to mesh with the digitally controlled newer technology. You will still face the same problems of alarm integration encountered in the previous strategies, but with newer controller technologies, it should be manageable.

With this strategy, installation costs and costs of load transitioning are reduced even more, and you now have a common controller that gives the operators a single set of rules to learn and a single, consistent picture of what is happening, rather than guessing which of two controllers is correct.

Much of today's control strategy is based on response to changing environmental conditions. As the operating environment changes, you may want the plant to react a certain way. With a common control set, that action/reaction response is distributed across all the chargers in the system unilaterally. You don't have to worry that some units will respond and some won't.

On the negative side, some additional wiring and hardware will be needed, including the A/D converter, and some additional rack space will be consumed. Also, wiring and integration can be difficult with 30-year-old chargers, but it is achievable if the functioning of the old equipment is clearly understood. There will be some additional rack space consumption. And while there is only one set of rules for operators to learn, learning a new set of rules after 20 years of doing it some other way may be a challenge.

A Heterogeneous DC Power Plant

A recent design exercise was conducted to evaluate the feasibility of digitizing each of the rectifiers in an older ferroresonant power plant. It was determined that the following steps would be required:

• Take the plant off line and disconnect AC service.

• Remove all analog control cards and mark wires.

• Label and remove all wires to the external control panels.

• Install newer “digital” control cards inside the older rectifier.

• Re-terminate required leads out to a new digital controller.

• Adjust the setting through the digital controller.

It quickly became apparent that this would be hard to do, particularly where multiple units are involved at a larger CO or switch site. Gathering technical information to determine which version of circuit controller is in the various types of older chargers is overwhelming. Digitizing a ferroresonant-based plant is time consuming and a high-risk activity when live AC is involved and when there is traffic on the switch or traffic on the loads with which to contend.

Estimates for the work required in this exercise were based on $85/hour for hot work and for transition work for each rectifier. Total estimated cost of materials was $335 per rectifier, engineering was $225 per rectifier, and installation was $516 per rectifier. The total cost: $1016 per rectifier. It also became quickly apparent that, assuming the rectifier will accept a digital interface, spending over $1000 on an older ferro charger, when a new switch-mode charger costs about the same, might not be a wise idea. At the end of the day, you are still left with the same old rectifier.

Field Trial

An actual field trial of migration strategy #4  was conducted at an on-line site to evaluate a new migration device incorporating an A/D converter compatible with a switch-mode-technology rectifier set and digital controller. To conduct this trial, three older 200-amp ferro rectifiers were migrated from the power plant while the plant was still carrying traffic, and they were interfaced with equivalent 200-amp switch-mode rectifiers.

The new migration device was designed to allow the legacy base of analog-controlled ferroresonant chargers to run simultaneously with the newer switch-mode chargers. All chargers would be controlled by a single digital supervisory controller, all alarms would be integrated, all system parameters would be available on a single monitor, and all the latest digital control and maintenance features would be available.

While replacing three of the older ferroresonant rectifiers was straightforward, preserving the remaining ferros was not so simple since the original manuals were not available to show where the control connections should be made.

However, after extensive testing and some educated guesses based on ferroresonant manufacturing experience and the few support documents that were available, we were able to take the 20-year-old control leads from the remaining legacy rectifiers and re-map them to the A/D converter.

The new digital switch-mode controller and the A/D converter were added to the existing control bay and the battery supply was used from the existing positions on the main control bay. 

All alarm leads and controls from the digital switch-mode controller were re-terminated back to the existing site monitor. The monitor was fairly well loaded, and a certain amount of “wire mining” was required to make sure that no functions were lost.

Once the newer switch-mode rectifiers were placed into service, adjustments were made to the digital controller to set alarm and operational set points equal to those of the existing plant.  Alarm integration was verified through rectifier testing, and load sharing characteristics between the different rectifier banks was within 4%. Additional adjustments were made to bring the existing ferro chargers closer to load sharing with one another. All new alarm settings were verified once again as clean up began.

Overall, the plan worked as originally planned, but several key items were noted for future reference. Additional time needs to be allotted for alarm integration into existing monitoring systems. Only two hours were budgeted for alarm integration, but with the extensive runs and varying degree of cable identification needed, this can take the better part of the day. It was also noted that having good measurements where new equipment was being placed with respect to existing equipment would allow for better planning and material allocations. 

Which Path?

Clearly, there are alternatives to scrapping viable legacy power plants in favor of total replacement with the latest technology. However, each has its pros and cons. No single solution stands out as the best one for every application, although some form of heterogeneous DC plant architecture would seem to be the most economical.

Choosing the best path to follow requires an accurate assessment of the current and future status of your CO or switch site, an understanding of the technical issues involved and the establishment of selection criteria to direct your decision. The criteria in the four migration matrices included in this article are a good place to begin.

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David Michlovic is Director—PLM Marconi Outside Plant, Power & Services Group, which is based in Lorain, OH. He can be reached at david.michlovic@marconi.com.

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