Transformer Load Balancing

OK, I have another question related to the one I had yesterday. Let me explain where I'm coming from. I'm writing an enhancement to a Utility Billing program (for PUDs, cities, etc...) that will track transformers. Part of that is a 'load balancing' report. The example of a 'load balancing' report that I have gives actual usage (in KWH) of all the meters connected to it, converted to KVA and the % it is of the maximum KVA rating of the transformer. My question is - should I be concerned with the actual usage or the potential usage? My guess is that if every meter reads the max usage, it will overload 99% of their transformers. Is this typical in the energy industry? Are the transformers usually balanced with the actual usage or the potential usage? Thanks, Wes

Reply to
Wes Faul
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you cannot convert kWH to kVA except as an average power used over the period in question.

the maximum kVA rating of a transformer is of interest to the engineers and electricians who install the service but i can not see how it would be of any interest to a billing program.

My question is - should I be concerned with the actual

potential usage is 100 % of the power grid. at that point fuses open hopefully before cable melts.

watt-hour meters do not read "max usage' they read watt-hours. the meter will have a max amperage rating suitable for the service load. (i.e. 200A,

400A 800A and so on) some W-H meters used for industrial purposes are called "Demand meters". these show the peak usage over the meter reading period. it is my understanding that these are used by power companies to adjust the rate (higher) for those users who may place a heavy load on the power grid during peak demand hours. in others words a user with 800A service who never exceeds 200A during the billing period gets a better rate.

Is this typical in the

unless we are taking about the sub-station transformers there is nothing the power company can do other then provide power. the balancing part is done (or not done) by the user. a competent electrician will attempt to wire circuits in such a way as to evenly distribute load among the branches or phases but there is no accounting for what the user will switch on or off or what equipment will fail.

my response here may be simplistic and incomplete, or i may not fully understand the question and its relation to the billing problem.

i do have the feeling, based on long experience with accounting and management types that they will have no interest in this unless it somehow impacts the "bottom line".

Reply to
Tim Perry

snipped

Short, sweet and to the point. For my view RIGHT ON.

Your application could vary

Reply to
SQLit

OK, let me clarify a few things. We are just providing our customers a way to determine if they need to rewire, install new, or change out to higher KVA transformers. We are not billing this in any way.

The kWH is the monthly usage, and we are converting it to average kVA (dividing the kWH by 720 - 30 days/month * 24 hrs/day). I guess the purpose of this load balancing is to answer the question "On average, is the transformer overloaded?".

By potential usage, I mean all the meters attached to that transformer are using the maximum amount it can read.

Our software will allow them to bill these demand meters, but I'm not sure if that will effect the load balancing aspect of our software.

Thanks for the info. I think I will just include both figures on the load balancing report - what the actual average load was on the transformer, and what the load would be in every meter was measuring the maximum amount it can handle. Thanks, Wes

Reply to
Wes Faul

What ever you monitor will tell you what % of capacity you have. I think your really looking for the rate/size of change. I would guess if your monitoring period was say every five minutes per meter, after a couple of years you would be able to start predictions.

Do not fall into the trap of comparing June to July. You can only compare to the same month in different years. I set up a program for a University that did this. After the first summer I realized that I needed an constant that was not being recorded. For my application I also recorded temperature. Since 2/3 of the load was a/c. The temperature constant helped us understand spikes that were not consistent over time. Your application could be different. Just a suggestion

Reply to
SQLit

It sounds like you want to do a study of loading, demand, and diversity factors. If the customer is already being metered and billed for demand, that is a valuable source of data for your study.

There is a well known technique described in The Electric City: Energy and the Growth of the Chicago Area, 1880-1930 by Harold L. Platt that describes how Samuel Insull did this in the early years of the last century for a one block area of Chicago's North Side.

Reply to
Beachcomber

Basically, this report is to notify them which transformers are being overloaded (on average - by month). Unfortunately, they only have demand meters at some of the locations. I'm trying to come up with a way to convert between the kWH that is read at the meter to some sort of kVA to compare it against the rating on the transformer. They came up with a conversion calculation of .0035 * kWH = KVA. They're not sure where they got it from, but they think it was from a federal study done back in the 60's. They said this seems to work fairly well for single phase residential meters, but don't have a good way to convert for single phase commercial/industrial meters or for three phase meters without a demand reading.

Does the above book have info on how to do this? Thanks, Wes

Reply to
Wes Faul

The problem you have is that your customer can't tell you the peak to average loading.

After all, if we're talking just two shifts, five days a week in a manufacturing facility, your 720 hour month is more like just 320 hours, with a very load load during the night and week-end. Some industries have different load profiles than others (conveyor, compressor, furnaces, all have different demands). Commercial building load can depend on the business(s) inside. Hours of operation, elevators to take people to work in the morning, HVAC demands, computer rooms, etc.... All sorts of variables to be considered that you're not supplying (and even if you do, I doubt anyone here is willing to undertake the calculations pro bono).

Such load analyses are what EE and PE get paid the 'big bucks' for. Trying to use a 'rule of thumb' from residential usage for commercial, isn't going to cut it.

daestrom

Reply to
daestrom

this is done in the field by a lineman with a clamp on ammeter. (or an ammeter with a chart recorder)

you are beating your head against a rock here. the average power used over

30 days can not be made into peak power used over a period or hours (or minutes of one day). the "constant" will be different for each and every type of service. it might be "safe" to lump residential loads together but light and heavy industry will have huge differences in peak loads.

a real world example would be an AM broadcast transmitter which is daytime operation only. sunrise to sunset it draws a steady load... lets say 25,000 watts. sunset to sunrise it is off... zero watts (relatively speaking). the total "on" time changes every month. you long term average will show approximately half of the true peak load.

by the time your monthly average can show an overload condition the transformer has already caught on fire and will have been replaced.

if it a long term constant overload the voltage drop will be such that the user will be complaining about brown outs and burnt up motors.

They're not sure where they got it from, but they think it was from a

there were a lot of hallucinogens floating around back in the 60's :)

They said this seems to work fairly

any power usage calculation that would work for single phase would work as well for 3 phase as long as the proper formula is used.

Reply to
Tim Perry

Hello, Wes. I apologize for being negative, but I think you're flogging a dead horse. I interface regularly with utility companies and generally hear these two comments:

  1. Our summertime overload rating on transformers is 133% of rating.
  2. We don't perform preventive maintenance anymore; we just replace something when it fails. The more load the utility can supply from a specific transformer the more revenue goes to their bottom line. The only incentive for the utility is, if the customer pays a transformer rental charge, to increase the transformer size and rental charge.

Disclosure: I am a consulting engineer with 34 years of experience, mainly in the SE. I have limited experince in the SW and NE, none on the West Coast.

Cheers, Chris Johnston

Reply to
Chris Johnston

Well, I think the OP is asking a semi-valid question, but addressing the wrong group.

For many years, my day job was to represent a large electrical equipment manufacturer, and it was not uncommon for me to be involved in the sale of distribution equipment.

In that era (1960's -1970's) the standard method of sizing residential distribution was based on pure and utter statistics. One customer of mine used a table based on service entrance sizes. Each 60 ampere service needed

1.5 kVA of transformer rating, a 100 ampere service needed 3.0 kVA, and so forth up to 8.0 kVA at 200 amperes. By this method, a 10 kVA transformer would supply 6 - 60 amp services. This system worked fine. Why? Because residential loads have very well-defined load curves. Indeed, transformers were frequently overloaded, but were usually underloaded, and the thermal properties prevented the overloads from causing trouble. I am sure that the tables have since been updated to reflect changes in the load profile, but I am positive that the average residential distribution design still relies on the purely statistical approach, which "equates" service entrance size with transformer rating.

I imagine that some utilities could have derived similar tables based on monthly kWh usage, again assuming the standard residential load profile and diversity, and I think this is where the "formula" mentioned by the OP originated. Such a formula could indeed evaluate installed transformer capacity against known consumption. I see no way, however, to transfer this concept to a commercial or industrial situation, which have unique and variable load profiles, which is what the OP is apparently trying to do.

Reply to
BFoelsch

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