Standby Power Consumption of a Residential Utility Transformer

[DanS26]

Any member here know the standby power use of a residential utility transformer.....10kVa, 15kVa or 25kVa?

My limited on line search indicated a 1% to 1.5% consumption based on nameplate capacity. That amount does not seem correct to me but average line losses for utilities run around 5 to 7%, so a 1% loss just from transformer operation may not be so bad after all.

Local REMC has 800 residential transformers in the 10 to 25kVa range sitting idle. They are service disconnects waiting for re-connection. Just wondering how much power is being wasted because these transformers are NOT disconnected from the primary......just sitting there humming along waiting for the meter re-connection.

 

[Ingenieur]

likely less than 1% with sec open

you can get an estimate with the v, kva, x/r and pu Z
calc r
that is winding i^2r losses for prim/sec
don't how to apportion z between sec/prim
75/25 seems reasonable since prim is a smaller gauge and more windings

 

[Ingenieur]

simple example
100 kva
2400:240
pu z 2%
x/r = 10 or x = 10r

ref to prim
z base = 2400^2/100000 = 57.6 Ohm
z = 0.02 x 57.6 = 1.152

z = sqrt (x^2 + r^2) = sqrt(100r^2 + r^2) = sqrt(101r^2) = 1.152
r = sqrt((1.152^2)/101) = 0.1146
assume 75% prim 0.086

the only i that flows is magnetizing (ignore C very small)
even if 100 A losses only 0.86 kw or <1%
highly doubt it's anywhere close to 100 A

i rated = 100000/2400 = 41
if mag i is 10% or 4 A
losses 14 W or so, negligible

 

[gar]

180604-2012 EDT

5 W to a 175 VA at 123 V in vs nominal 120 for transformer. Thus, about 3% on a small transformer.

.

 

[DanS26]

simple example
100 kva
2400:240
pu z 2%
x/r = 10 or x = 10r

ref to prim
z base = 2400^2/100000 = 57.6 Ohm
z = 0.02 x 57.6 = 1.152

z = sqrt (x^2 + r^2) = sqrt(100r^2 + r^2) = sqrt(101r^2) = 1.152
r = sqrt((1.152^2)/101) = 0.1146
assume 75% prim 0.086

the only i that flows is magnetizing (ignore C very small)
even if 100 A losses only 0.86 kw or <1%
highly doubt it's anywhere close to 100 A

i rated = 100000/2400 = 41
if mag i is 10% or 4 A
losses 14 W or so, negligible

So lets say 14w constant draw per transformer 24/7 for which the REMC pays 7.5 cents per kWh, then.......

14w * 24h * 365d * 800 / 1000 * $0.075 = $7,358.40 cost per year.

Thanks for your input.

 

[retirede]

So lets say 14w constant draw per transformer 24/7 for which the REMC pays 7.5 cents per kWh, then.......

14w * 24h * 365d * 800 / 1000 * $0.075 = $7,358.40 cost per year.

Thanks for your input.

No - the 0.075 is per kWh not wh.

 

[Ingenieur]

So lets say 14w constant draw per transformer 24/7 for which the REMC pays 7.5 cents per kWh, then.......

14w * 24h * 365d * 800 / 1000 * $0.075 = $7,358.40 cost per year.

Thanks for your input.

the lines likely are an order of magnitude higher

you might find this interesting table 1 shows est no load losses
http://www.powersystem.org/docs/publications/triplett_ieee-2010-repc-final.pdf

50 kva 140 w or 0.28%

 

[kwired]

Any member here know the standby power use of a residential utility transformer.....10kVa, 15kVa or 25kVa?

My limited on line search indicated a 1% to 1.5% consumption based on nameplate capacity. That amount does not seem correct to me but average line losses for utilities run around 5 to 7%, so a 1% loss just from transformer operation may not be so bad after all.

Local REMC has 800 residential transformers in the 10 to 25kVa range sitting idle. They are service disconnects waiting for re-connection. Just wondering how much power is being wasted because these transformers are NOT disconnected from the primary......just sitting there humming along waiting for the meter re-connection.

Are you talking about situation like customer didn't pay bill and is now disconnected - losses are an energized but unloaded transformer? If so, how many of said transformers are serving just one customer? In rural areas maybe nearly 100%, in cities/villages, not so likely to be that many individual transformers per service in most residential areas.

Our rural POCO does have slightly different rates for customers within villages they serve vs the rural customers - such losses and other costs are probably the reason.

 

[Ingenieur]

No - the 0.075 is per kWh not wh.

isn't the 1000 to convert w to kw?

 

[retirede]

Standby Power Consumption of a Residential Utility Transformer

isn't the 1000 to convert w to kw?

Apparently so. I didn’t know what the 800/1000 was so I ignored it. Went back to the OP and see the he is talking about 800 transformers.

My bad.

 

[gadfly56]

isn't the 1000 to convert w to kw?

Then what is the 800 for?

ETA: OK, just saw retirede's post. Now I get it.

 

[Ingenieur]

if we use 60 w avg/xfmr (10-25 kva)

800 xfmr x 60 w/xfmr x 24 hr/day x 365 day/yr x 0.001 kw/w = 420000 kwh/yr

420000 kwh/yr x 0.075 $/kwh = $31,500 per year

 

[kwired]

if we use 60 w avg/xfmr (10-25 kva)

800 xfmr x 60 w/xfmr x 24 hr/day x 365 day/yr x 0.001 kw/w = 420000 kwh/yr

420000 kwh/yr x 0.075 $/kwh = $31,500 per year

Seems it would be wise to disconnect primary on a transformer supplying a single customer if you are expecting them to be off for any length of time. Many that don't pay their bill often will pay within just a few days after disconnection. Majority of them likely within hours of being disconnected - seems to motivate them to find some way to pay or at least make arrangements with the billing office.

 

[Ingenieur]

Seems it would be wise to disconnect primary on a transformer supplying a single customer if you are expecting them to be off for any length of time. Many that don't pay their bill often will pay within just a few days after disconnection. Majority of them likely within hours of being disconnected - seems to motivate them to find some way to pay or at least make arrangements with the billing office.

like you said it may be 800 avg for a year but each is off for a few days/weeks then back and another off, and so on

easier to pull the meter than a truck and hotstick ?

 

[DanS26]

No - the 0.075 is per kWh not wh.

Take a look at the formula again.....it is adjusted for kWh.

 

[kwired]

like you said it may be 800 avg for a year but each is off for a few days/weeks then back and another off, and so on

easier to pull the meter than a truck and hotstick ?

The guy that goes around doing disconnects/reconnects for non payment can use a pickup truck instead of a bucket truck, and can probably knock out more in a day then he can if he has to set up bucket truck (deploy outriggers, climb into bucket, tie off fall arrest harness.... then reverse procedure when finished) at each location. Might even have some that are not so easy to get truck into desired position. Pulling meters and putting isolation tabs on them is faster and simpler. Electrical risk hazards of either method probably somewhat of a wash. Pulling meter is bigger arc flash hazard, using hot stick on primary - pretty safe procedure for the most part - but is a very unforgiving voltage if you do mess up.

They can use the longer extendable hot sticks and switch primary while standing on the ground - not so easy even in light winds sometimes though, and maybe a little more shock risk then when standing in the bucket of the truck.

 

[DanS26]

Seems it would be wise to disconnect primary on a transformer supplying a single customer if you are expecting them to be off for any length of time. Many that don't pay their bill often will pay within just a few days after disconnection. Majority of them likely within hours of being disconnected - seems to motivate them to find some way to pay or at least make arrangements with the billing office.

We are trying to convince our REMC to not only disconnect the transformer from the primary but to also take down the transformer after 3 years of hanging there without use. The transformer can then be reconditioned and re-purposed to new service somewhere else in the system.

Also these new procedures will discourage and/or eliminate power theft.

 

[Ingenieur]

3 years!
pull it
might be some benefit keeping it warm? moisture/condensation/corrosion?

might slightly improve pf if pulled?

what is the disconnect/delinquent rate? 5%? less?
must be a big system

 

[DanS26]

3 years!
pull it
might be some benefit keeping it warm? moisture/condensation/corrosion?

might slightly improve pf if pulled?

what is the disconnect/delinquent rate? 5%? less?
must be a big system

About 7,800 residential services and around 50 disconnects and subsequent reconnects per month.....say 7.7% annual rate. Line loss running over 6%. Just trying to get a handle on this.

 

[texie]

Any member here know the standby power use of a residential utility transformer.....10kVa, 15kVa or 25kVa?

My limited on line search indicated a 1% to 1.5% consumption based on nameplate capacity. That amount does not seem correct to me but average line losses for utilities run around 5 to 7%, so a 1% loss just from transformer operation may not be so bad after all.

Local REMC has 800 residential transformers in the 10 to 25kVa range sitting idle. They are service disconnects waiting for re-connection. Just wondering how much power is being wasted because these transformers are NOT disconnected from the primary......just sitting there humming along waiting for the meter re-connection.

Most of the coops I work with pull the primary fuses on transformers that feed only one customer if the meter is pulled due to the losses. This is cost effective for them, as unlike larger investor owned POCOs with separate lineman and meter folks, as it is almost always a lineman doing the meter pull and he can just pull the fuse while there to pull the meter. I think the economics of this are a lot less attractive if a POCO has to roll 2 separate truck/crew to do this as some would have to do. Of course it also depends how long they expect the customer service to be removed but they regularly pull the fuses on irrigation services in the fall for the winter months.