Portable and Distributed Power for Telecommunications

8/6/2019 Portable and Distributed Power for Telecommunications

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Portable and Distributed Power for TelecommunicationsJimmy Godby

Telecom Power Consultants

PO Box 271004

Littleton, CO 80127

[email protected]

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mailto:[email protected]





Portable and Distributed power for Telecommunications

Contents 1. INTRODUCTION ................................................................................................................... 3 2. INTEGRATED POWER ......................................................................................................... 3

A. RECTIFIERS ................................................................................................................... 4 B. BATTERIES .................................................................................................................... 4

3. VOLTAGE CONSIDERATIONS........................................................................................ 5 4. BATTERY RESERVE .......................................................................................................... 5 5. DISTRIBUTION.................................................................................................................... 6 SECTION 2: DISTRIBUTED ARCHITECTURE POWER .......................................................... 6

A. BATTERIES ................................................................................................................... 7 B. LOAD ALGORITHMS ................................................................................................. 7 C. DISTRIBUTED BAY LOAD REQUIREMENTS ...................................................... 8 D. BAYS TO BE FED ......................................................................................................... 8 E. DISTRIBUTION ............................................................................................................ 9 F. RECTIFIERS ............................................................................................................... 10 G. AC CABLE ROUTING ............................................................................................... 11 H. GROUNDING FOR DISTRIBUTED POWER ........................................................ 12 I. CONTROLLER ............................................................................................................... 12

6. DISTRIBUTED POWER CONSIDERATIONS .............................................................. 13

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1. INTRODUCTION This document discusses distributed power for telecommunications.

In today’s rapidly changing environment, there is an ever increasing need for a

distributed power solution. With today’s available technology in both DC power

plants and batteries, there is a real economical ability to provide these unique

solutions. The implementation of a “Distributed Power Solution” is a solution that

“grows” distributed architecture power as opposed to BDFBs in a traditional

central office. This session will provide insight into the engineering rational behind

this solution. These will include space, capacity, and size considerations for the AC

required, DC plant with distribution, and associated batteries.

This presentation will discuss two aspects of each of the above solutions. The first

aspect is the application. When would you use a distributed architecture bay, where

should you use it, and how would you implement the solution? The second aspect

will tackle design and sizing of that solution.

2. INTEGRATED POWER

Several suppliers have small (one or two shelf switch mode rectifier plants. For the

purpose of this document, the various power systems will be labeled as Micro

Power Systems (MPS) for the duration of this document. The MPS is the heart of

an integrated power solution for distributed architecture. The MPS provides control

and rectification – along with intelligent communication via the customer’s

intranet for the distributed architecture plant.

In keeping with the tradition of all true power engineers and technicians, the load is

just that. It is “something” hanging off the end of the power plant. In these

discussions, the nature of the load is only important as to the answer to these

questions: What is the acceptable voltage range of all of the components of the

load to be fed from this power plant? What are the acceptable parameters of the

DC output (noise, ripple, etc)? What are the acceptable parameters of feedback onthe AC CIRCUITS?

Of course, to be a complete DC power plant, there must be some form of energy

reserve. That reserve will also be discussed.

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A. RECTIFIERS The heart of a DC plat is the rectifier. As a user, there are several primary

considerations when looking for a rectifier. They include conformation to standard

requirements (NEBS – GR63 CORE from Telcordia), NEC requirements, UL

listing or tested, etc. Exactly which requirements are to be considered and how

they are applied are beyond the scope of this presentation and will not be discussed

further.

The Voltage Requirements are actually on both sides of the rectifier: First, what

AC voltage levels are going to be required? (208, or 240 VAC, and phase – single

or 3 phase AC service.) 480 VAC is not a viable solution for this small

application.

On the output side, do you need 52.08 or 52.80 VDC to match the existingcentralized plant, or can you use a 54 VDC solution as the equipment fed from the

distributed architecture bay is isolated from the central office bulk power supply.

Of course rectifier density is the real key to viability of these applications. When

determining the maximum rectifier size, you must consider whether you have one

or several rectifiers per shelf to be powered from the same AC feed. The two

parameters here are the input circuit breaker size and the ac cable size. Done

incorrectly, the input could call for a non standard AC breaker which will make

your AC feed much more expensive. The second consideration is the AC cablesize. When large AC feeds are required in very limited space, just running the AC

cable can be a real issue. The MPS shelf is designed to eliminate the problems

associated with routing and connecting the AC input cables.

B. BATTERIES

The first technology to be looked at is Valve Regulated Lead Acid (VRLA)

batteries. This is a “known item”. These cells require periodic testing and physical

inspection, they have a much higher energy density that traditional “flooded”

batteries, and have a predictable failure mechanism. (They are prone to thermalrunaway when subjected to high temperatures. This can result in a release of

hydrogen sulfide and be hazardous to people and equipment.)

The real problem with VRLA batteries is that they are not maintenance free, but

maintenance proof. Placing this technology in a distributed architecture bay would

result in an embedded technology that would require quarterly maintenance visits

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and frequent battery replacement. Furthermore, due to the required maximum bay

width and depth in equipment line-ups, VRLA technology would require the use of

smaller capacity batteries and two bays of these batteries to provide the required

capacity where one bay of lithium batteries would suffice.

NICAD technology is mentioned here just to observe that the technology is not

being ignored. At the same time, the technology is not being considered. Cost and

disposal considerations are the primary reasons that the NICAD technology is not

being discussed.

3. VOLTAGE CONSIDERATIONS

The real issue here is that flooded lead acid batteries that have been traditionally

used in telecommunications float service operate between 52.08 and 52.8 volts per

string. Where VRLA batteries are used in telecommunications, the power plant isdesigned to float at around 54.4 volts.

The concern is not at the minimum volts per string, but the normal float voltage of

the plant. This concern is due solely to the fact that some older equipment will not

work at the 54 volt level... Specifically some of the older switch technologies are

upper voltage level limited at 53 volts.

If all of your equipment is less than 10 years old, the higher voltage is probably not

an issue. However, it is necessary to ensure that all older equipment will function

at these higher voltages before any transition can be considered.

4. BATTERY RESERVE

Battery reserve is mentioned here because it is a critical engineering element. Short

battery reserves can be acceptable with an auto start/auto transfer stand by

engine/alternator for the essential loads at the site.

With an engine/alternator and 3 or 4 hours of battery reserve, we will see that it is

possible to provide all of the power (rectification) needed, the distribution, and thebattery reserve required in a relatively small space. It the case of distributed power,

that would be all of the equipment in a single bay as opposed to a bay for the plant

and an additional bay for batteries.

Reality here is that distributed power is generally not going to be a viable option in

a Community Dial Office (CDO) environment. Due to the small size of CDOs,

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generally if there is an existing power plant, there are no economics in the

distributed architecture technology because the cost effectiveness of this solution

only comes in to play with long DC cable runs to BDFBs. The solution in a small

site would be to replace the traditional power plant with a one or two bay

distributed architecture plant and “recover” the space currently occupied by the

traditional plant and it’s associated flooded batteries.

5. DISTRIBUTION

The decision to use fuses or breakers is a customer preference. Breakers can be

reset, but over time may weaken and need to be replaced. When a fuse fails it

always needs to be replaced so the “deterioration” clock starts over every time a

fuse is replaced.

Fuse/breaker positions for distributed architecture will be in the 0 to 70 amp range.This will be discussed more thoroughly later.

SECTION 2: DISTRIBUTED ARCHITECTURE POWER Distributed Architecture is essentially a power plant that exists in the equipment

space that a traditional BDFB would. It will have all of the components of a

complete power plant – batteries, distribution, rectifier, and controller.

The following will discuss each of these and develop a “typical size” for the

distributed architecture power bay.

This type of bay was originally proposed by Ed Silverman in the early 1980s.

However, the VRLA batteries available at the time were not reliable enough nor

did they have the power density required for this solution.

The key developments that make this approach practical are the highly dense MPS

rectifier plant, lithium batteries.

The critical physical requirement for this bay is that it be no deeper than 15 inchesso that the bay will fit in a standard relay rack aisle. In addition (for the American

RBOC market), the bay should be 23” wide.

As we develop the distributed architecture model, we need to thank Curtis Ashton

from Qwest Communications who contributed the fact that the “average” transport

equipment bay draws approximately 7.5 amps so that each A or B load to any bay

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will be approximately 4 amps. (NOTE: This low drain is largely due to the fact that

there are a lot of vacant slots in each bay. If a site has an appreciably higher drain

per bay, a proportionally smaller number of bays will be fed than described below.)

A. BATTERIES

Since the batteries are a critical element, and Lithium batteries are required in

order for this solution to be maintainable, because of rapid technology advances, it

will be necessary for the reader to research lithium battery options.

Because of the circuit density of the rectifier plant used, the distributed architecture

bay will have approximately 2/3s of the frame space available for batteries. The

capacity required will need to fit in this 2/3 of a bay in order for distributed

architecture to be a viable solution.

B. LOAD ALGORITHMS

Earlier I stated that Curtis Ashton of Qwest provided informaiton that the average

load on a typical toll bay in a RBOC central office was 7 ½ amps. This average

was derived across an entire central office where a large number of bays with

various loads are averaged. This accounts for areas where a bay or several bays

may pull 1 amp or less added with bays that draw 40 amps. With a centralized

plant, if the load increases in one are of the building while decreasing in another,

the net result is a wash to the power plant.

With distributed power plant architecture, it is essential to look at a smaller

“universe”. It is also critical to look at growth patterns in the equipment to be

served. If the load in an office moves from one area to another, then one distributed

plant can be essentially deloaded while another is overloaded. By the same token,

if the distributed architecture plant is designed with a 200 amp maximum capacity

and say 20 bays, if 5 bays draw 40 amps each, it will be necessary to place an

additional distributed architecture power plant(s). (More about this later.)

The engineering questions that come out of this situation are: 1) Does bay spaceneed to be reserved for additional distributed architecture power bays? – 2) If so,

how much space should be reserved for additional power?

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C. DISTRIBUTED BAY LOAD REQUIREMENTS

A key element to the decision of Distributed Power or not is the load that each bay

will draw from the plant. This load can be measured as the Average and Worst

Case drains as shown in the slide. The question that you have to ask yourself is “Is

the drain that I see on this bay now all that I can expect on the bay? Some

“specialized bays are required for certain equipment types. The office might need

only one or two shelves of this equipment, so although the bay has a capacity of 20

or 30 amps the actual load may never exceed 5 amps. Other bays may start with

only one or two amps of load and then eventually blossom to 40 amps. (If a

distribution bay is gong to draw 60 or more amps, it would probably be more

efficient to feed that bay directly from the centralized power plant provided

capacity and distribution fuse/breaker space exists on the centralized plant.

As the decision is made to use distributed power, careful consideration must bemade to determine not only the initial load on the power plant from the bay, but

also the projected load over the life of the plant.

To that end, for this section we will assume that each bay draws a total of 10 amps

– 5 amps each for the A and B feeds. This should be a safe average in the smaller

universe to be served by the distributed power plant.

D. BAYS TO BE FED

The goal of this technology is to provide the most economical power. To that end,

it was first necessary to determine the amount of battery capacity available in 2/3

of a distributed architecture power bay. As the battery technologies being

discussed are still in development, the battery capacity of approximately 200 amps

is available with the lithium batteries that have been researched.

The second step was to determine the amount of current drawn by each bay. Based

on estimates above, for the purpose of this discussion we will use 5 amps each for

the A and B loads on the equipment bays for a total of 10 amps per equipment bay.

W we will use a battery capacity of 200 amps for 4 hours. From the above, we can

calculate that the 200 amps will provide power to 20 bays or to the A or B side of

40 bays.

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E. DISTRIBUTION

Since this is a single bay solution, there is limited distribution space. In a 7 foot

relay rack, when you remove the top and bottom space for iron work, there are 6 ½

feet of space available for equipment. Reserving 2/3 of that space for batteries and

3 inches for the DC plant leaves approximately 1 ½ feet of space for distribution

The decision to use fuse or circuit breaker distribution again is a decision specific

to the customer. However, the decision can be made easier now because there are

panels available that will accept either.

Distribution fuses and breakers are sized to protect the cable at maximum load to

the bay. These fuse/breakers are generally 0 to 60 amps.

There are several manufacturers that can provide the required distribution. Anacceptable solution will provide a total of 44 zero to seventy amp fuse positions.

NOTE: Distributed architecture provides a significant savings in DC distribution

cable over a centralized power solution. There are two areas of savings. First, the

need for large cables from the centralized power plant to the BDFB does not exist.

Second, and providing an even greater savings, is the cable from the distributed

power plant to the load. As was discussed earlier, there could be as little as a .5 V

drop from the distributed plant to the load. However, this requires relatively large

cable, and is not necessary. Because of the discharge characteristics of the Lithium

battery and the fact that the “load” equipment will work down to 42 volts, it is

possible to use much smaller cable (and smaller fuses) from the distributed

architecture power plant to the load than would allowable from the BDFB solution.

This results in a significant savings in cable with distributed architecture. (Fuse

prices do not generally vary by size so there is no real dollar savings there.

However, care must be taken to correctly size the fuses.)

All traditional BDFBs are rear access. With this configuration, the fuse or breaker

is in the front while cabling is done to the rear. There are three things that happen

with traditional BDFBs. First, the DC feeder cable and all distribution cables areall wired in the back. This can result in cable congestion and routing problems with

a large number of loads in a BDFB. Second, with the cable termination in the back

and the fuse or breaker position in the front there is a possibility of misidentifying

a position and placing the wrong size fuse or breaker resulting in a power failure to

equipment when the only problem is the fuse is too small. Third, this

configuration requires rear access to the bay.

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Distributed architecture frames can be produced that are 100% front access. This

allows the user to place the bay against the wall if so desired. Also, with front

access, the cable is terminated directly above and on the same side as the

distribution fuse eliminating the possibility of placing the wrong size fuse on a

power feed. On the same lines, since there is no DC input to the distributed

architecture bay, and only a maximum of 40 outputs, DC cable congestion in the

bay is eliminated.

F. RECTIFIERS

Our original goal was to develop a distributed architecture bay. Since the

discussion revolves around a single bay, the rectifiers must be chosen to provide

the power and redundancy required in the space available. There are generally

three rectifier sizes that can be placed in a 15 inch deep relay rack. (All of thesolutions will be 208/240 VAC – 120 VAC and 480 VAC solutions are not

practical for this architecture.) Rectifiers for plants that will meet these

requirements are in the 20 to 50 amp range. We will look at 20, 30, and 50 AMP

rectifiers.

One of the critical considerations is whether to go with N+1 reliability in the

rectifier plant or – since the two bays cross feed – to go with 2N redundancy. If the

configuration is going to power both the A and B side of up to 20 bays, the typical

solution – and one that provides the same reliability as most existing bulk DC

plants, N+1 philosophy can be used.

If the configuration is a two bay pairing where each distributed architecture power

bay provides power to either the A or B load of 40 frames, the 2N rectifier

redundancy design looks practical. This would provide added reliability of the

system by feeding each of the mated bays from a different commercial AC service

entrance. Using separate AC service entrances (usually available only in the largest

offices) decreases the probability of a simplex AC failure causing the power

system to go on batteries.

The 2N solution provides an AC electrical advantage. By using a 2N configuration

in a MPS power plant with two rectifier shelves, it is possible to feed each pair of

rectifiers with a single AC breaker. This solution results in 4 rectifiers per shelf

each shelf balance fed with two AC breakers and a bulk output to the distribution

panels.

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G. AC CABLE ROUTING

The first question to ask – and this is another first and last question is “Is AC

available?” This is not a capacity question but it is a distribution question. It is the

first and last thing to consider when contemplating this technology. It is an

economic question. When AC fuse/breaker panels are assigned, there is an

assumption that there will be no point in time when all of the circuits draw full

load. Therefore typically (you can see this in your home) a 150 amp AC panel will

have 300 to 500 amps of fuse or breaker positions. As there is never a time when

everything in the home is on and attempting to pull full power, this is not a

problem. However, when rectifiers are installed, THERE WILL BE TIMES

WHEN ALL RECTIFIERS PULL 100% OF THEIR MAXIMUM DRAIN

SIMULTANEOUSLY. Having said that, it is essential that the breaker panel that

provides power to the rectifiers be dedicated to those rectifiers and have enough

capacity to handle all of the rectifiers pulling 100% - this is not 100% of thebreaker, but 100% of the AC rectifier current required at full load. So a 50 amp

rectifier running at 208 VAC and 95% efficiency will pull approximately

(54*50/.05)/208 = 14.4 amps. NOTE: This is with the rectifier running at 100% not

110% (which some rectifiers can be set to).

A breaker panel must be available (this could be installed for the application). If

two pairs of distributed architecture power bays are being installed, the panel will

need 16 positions. Each position will have approximately 30 amps of drain at full

load so the panel must be sized for and capable of 480 amps. Adding 20% for a

“safety margin” means that the source feeding this panel needs to be capable of

576 Amps. (NOTE: This discussion is not based on available fuse sizes, but on

projected loads.)

At this point it is necessary to go back to the house service entrance. There are two

questions to ask here. First, is the site being fed with enough AC to handle the

additional load? The second question is “Is there a breaker position on the panel

with enough available capacity to handle the load? (If as an alternative to powering

4 bays from one panel the decision is made to power 1 bay from each distribution

panel, then it will be necessary to have 4 feeds with a minimum of 144 amps each.If it is necessary to replace the house service entrance to install distributed power,

then it may not be the best solution.

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H. GROUNDING FOR DISTRIBUTED POWER

Bonding and grounding this system is critical to safe and proper operation.

Integrated Versus isolated grounding is generally beyond the scope of this paper.

However, it is critical to bond and route the AC conduit correctly. The first general

assumption is that distributed power will be used for toll room transport equipment

as opposed to switch. To that end, distributed architecture will always power

integrated equipment. If the distributed power bay is within 6 feet of any isolated

equipment bays, it will be necessary to run the AC conduit past and bond it to the

ground window.

The same situation exists with the DC ground and return. Equipment positioning

will be required to determine whether the return on the distributed architecture

should be bonded to the ground window, or merely tied to the vertical riser and

then back to the OPGP.

I. CONTROLLER

It is the nature of these small switch mode rectifier plants that the controller and

rectifiers are in integrated unit. The MPS controller must be capable of providing

everything from dry contact alarms to a full featured web based interface for

managing the power plant and alarms from a centralized location.

The advanced features deal with the operation and maintainability of the plant.

There are also several questions that must be asked. The first question “Does this

equipment require remote monitoring?” is always yes in today’s environment. For

functionality it is necessary that monitoring of the distributed power component fit

in with the architecture of the user’s monitoring system. As there are several of

these available, this document will hit the highlights.

The first monitoring would be a customer that required dry contact major and

minor alarms. These would be routed to the site alarm panel or the remote alarm

center to provide major and minor alarms for rectifier fail and fuse fail.

The next configuration would be an “intelligent” controller that provided full

power plant information to a down-stream intelligent alarm center. Because of the

distributed nature of the power, it is critical that any alarm or information (load,

battery, discharge state, etc.) include enough information to uniquely identify the

power bay. For example, the RBOCS use a CLLI code to define a site –

ALBQNMMACGO – where ALBQ means Albuquerque NM is the state – New

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Mexico, MA is the site location (Main) and CGO is traditionally the switch

associated with the power plant. To use this scheme for distributed architecture, the

CGO could be replaced with the bay location (ALBQNMM214). Of course, this

would only work in a site with less than 10 floors and fewer than 100 bay locations

on each floor.

The next function is to get the informaiton out. Because ports in an office may be

limited, one possibility is to daisy chain all of the power plants together so that

there is only one physical appearance for all of the distributed plants at the site. A

second solution is to provide each power plant with a unique address and have

intranet access to each bay from a private network.

Since the concept is for a distributed architecture plant to be truly maintenance

free, the down stream alarm should uniquely identify the plug and play part that

has failed or the status of the plant. This would include distribution, rectifiers, andespecially batteries.

6. DISTRIBUTED POWER CONSIDERATIONS

So far this presentation has pointed out the technical issues and sizing

considerations to be addressed when making a choice between centralized power

and distributed power. One of the first considerations is to determine if converting

to distributed power would leave a lot of “stranded capacity” in the existing DC

power plant. This would mean that the rectifiers, batteries, controller, and adequate

AC is available for the proposed additional load. The decision in this situation can

be simple or thorough. The simple decision is merely one of comparing the cost of

feeding from the power board or placing a BDFB VS the costs associated with a

distributed architecture bay. The thorough solution would additionally look at the

condition of the existing DC plant. For example, if the existing plant had one or

two 400 Amp Manufacturer Discontinued rectifiers in service, the customer could

remove those rectifiers and replace them with the distributed power option. This

would 1) get rid of obsolete equipment; and 2) provide AC breaker capacity for the

distributed architecture bays.

When the new bays can be fed from the main power board (for example in a small

CDO where the outside room dimensions are approximately 40 by 80 feet), there is

no need for a BDFB. In this situation, there is only the ½ volt drop to the served

equipment, there are no 750 MCM DC distribution cables, and the savings

associated with distributed power are questionable. (NOTE: the exception to this

would be where one or two bays of distributed power could be used to replace the

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entire existing DC plant with one or two distributed architecture power bays. This

would allow removal of the entire existing DC plant (including batteries) and

provide prized growth space in a small CDO.

An entire dimension is added when growth of the main DC plant is required. The

economic comparison here is much more favorable to distributed architecture.

When this is coupled with the need for a BDFB to be placed a considerable

distance from the existing power plant, then the economic consideration will favor

distributed architecture even more.

In the end, the decision to grow distributed or centralized power could be a

corporate decision based on reduced maintenance requirements or the increased

overall site reliability because the catastrophic failure of one DC power plant

would not take the entire site down.

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Portable and Distributed Power for Telecommunications