Utility transformer sizing and thoughts on across-the-line motor starts

[fastline]

This is a rural area so there is otherwise no easements with private water well and septic. I agree on the 'utility' easement regarding who can enter but I guess I don't like people having authority to access. However, again, this is a rural area I doubt they will have some interest is making camp in my yard and the underground utility means they won't be in to repair poles, ever....


Regarding the loads, I will have my own transformer but yes, from what I gather, they are primarily looking at how my draw affects others power quality. They are pretty blunt and fixed on this '10hp rule' so I gave up trying to reason with them and will do it anyway. Makes no sense that they will give me as many amps as I want, but limit the motor sizes.

Per the POCO engineer, they had issues with a customer starting a 20hp RPC before. Don't know the specifics on that but I suspect that was on a shared transformer..... The biggest question is what that primary line will be doing when I drop something across-the-line.

 

[kwired]

Someone may have brought it up already but you said "I have more than enough loads to justify the 400A and will certainly use it".

Question becomes what is the demand cycle(s) going to look like. If you only have relatively short periods of peak over 50 kVA POCO will likley only want to supply you with a 50 kVA transformer. If you run at 78 kVA for 8 - 10 hours then have fairly minimal load for at least another 10-16 hours they may be fine with a 75 kVA unit.

Most usually want to work that transformer hard instead of having one that seldom even comes close to it's capacity.

Also consider other characteristics of your load(s). You mentioned a 5 hp saw. Is this saw loaded to 5 hp when it is used? If so is that load constant while in use or does it vary? Saw running but not cutting any material is only loaded to what it takes to maintain the blade at speed and may be well below full load rating. If this is hand fed saw - there is probably a lot of time with little or no load, if it is a constant automated process it may run closer to full load rating for hours at a time.

 

[meternerd]

Don't want to beat this subject to death, but...as a POCO guy, we have what is known as "line losses". That's power we buy and provide that does not translate into revenue. Can be caused by many factors from long, lightly loaded lines to power theft. But one prime contribution is a transformer that is energized but very lightly or not loaded at all. Sometimes customers will move away or own a second home in the area but turn off all of the breakers while they're away. In those cases, we get a "zero or low usage" report which requires a tech to visit the site to see if there is really no load or if there's a meter problem. Same goes for a transformer that feeds a business , school, or whatever that has almost no usage but is being fed from a large service. We, as a utility, sell KWh, but we have to size equipment as KVA. The difference is the reactive power or resistive losses. The more we can keep our transformers at near rated load, where they are the most efficient, the more efficient the system becomes. Unity PF is the goal, and we pay a penalty to our suppliers when we vary from that. As far as utility easement, we seldom enter private property unless there is a problem. In the old days, when we did manual meter reads, we had to visit every meter once every month. Now we just download data remotely. I know many larger utilities still have mechanical meters, but there are fewer every year. But...our responsibility usually ends at the meter location, so if a customer wants a remote service point, the metered conductors and equipment become his responsibility. Sounds good, but can become hugely expensive if a fault happens on an underground service downstream of the meter. Then, the customer calls us and whines. And of course we politely reply...so what!

 

[meternerd]

OOPS! Hit Post button again...

 

[fastline]

Agree with the load/time considerations and certainly something i need to take a look at. My machines are automated and most will start up unloaded BUT you have a bump for spindle accelerations and, as with a lathe, when she spools up, there is some big mass there between the spindle head and a 12ft stick of bar stock. It will also accel/decel that spindle all day so that load will constantly vary between full load and around 20% load.

I am going to look at my hardest load to see what that spindle drive is capable of.

Meterhead, since you are in this business, would these constant fluctuations be something that someone on a separate Xformer, 500ft, or even a mile down the road would realize? Again, I know the primary is 7.2K that they run a little hot for sag, and I think it is 6ga copper OH.

What I really need to drive at is the Vdrop at the Xformer at my highest possible loads. Consider that I have a 10hp compressor kick in with a 500A inrush, while already pulling 200A on the system. I can tell you from experience that I can break 200A of load without breaking a sweat.

I really don't mind the 50KVA if it will hold my loads at reasonable voltage but I am starting to think I might tear on it a bit.

In terms of time, at times, I might guess a 60% duty cycle for 8hrs at 400-500A, but there are days where that might be 50A. I realize about the issue with oversized Xformers eating power and trying to play along here. I always calculated for about 5% loss at idle? I looked at one that appeared to be less than 1% loss?

 

[meternerd]

Agree with the load/time considerations and certainly something i need to take a look at. My machines are automated and most will start up unloaded BUT you have a bump for spindle accelerations and, as with a lathe, when she spools up, there is some big mass there between the spindle head and a 12ft stick of bar stock. It will also accel/decel that spindle all day so that load will constantly vary between full load and around 20% load.

I am going to look at my hardest load to see what that spindle drive is capable of.

Meterhead, since you are in this business, would these constant fluctuations be something that someone on a separate Xformer, 500ft, or even a mile down the road would realize? Again, I know the primary is 7.2K that they run a little hot for sag, and I think it is 6ga copper OH.

What I really need to drive at is the Vdrop at the Xformer at my highest possible loads. Consider that I have a 10hp compressor kick in with a 500A inrush, while already pulling 200A on the system. I can tell you from experience that I can break 200A of load without breaking a sweat.

I really don't mind the 50KVA if it will hold my loads at reasonable voltage but I am starting to think I might tear on it a bit.

In terms of time, at times, I might guess a 60% duty cycle for 8hrs at 400-500A, but there are days where that might be 50A. I realize about the issue with oversized Xformers eating power and trying to play along here. I always calculated for about 5% loss at idle? I looked at one that appeared to be less than 1% loss?

It all depends on what load is already on the primary, but unless your tap is WAY down at the end of a long primary run, your load would not impact any other customers unless they were sharing your transformer. Remember, though, that typical load on a circuit is 200 to 400A at 7200 Volts. Power wise, that's 12,000 to 24,000 Kw. A 50A load at 240V isn't going to make much of a difference. Regardless of transformer sizing, current draw is the same on the primary. The difference would be on the impedance of your transformer. Too small would show sag on your secondary, but would not likely change the primary sag at all. On the other hand, if you are way at the end of a long primary tap, transformer size would not really help any sag you may experience. Primary voltage stability is mainly dependent on wire size and length to the load. We control voltage with regulators, cap banks, etc to try to keep a stable supply, but none are instantaneous. The only folks who see very little fluctuation are those who live next door to the substation....but who wants to listen to transformer hum all night long? Voltages out of the sub are kept somewhat high to allow for line losses, but our regulators are set at 122 +/- 5%. That works out to 114 to 126. Of course, that's X60 because the secondary is 7200. If it's outside that range, the regulators or Load Tap Changers won't step up or down for 30 sec....way too slow for short term sags and surges. Kinda off the subject, but I figured I'd try to sound geeky anyway.

 

[iwire]

You know the name of this thread should be Help me break the rules and screw my neighbors

 

[electrofelon]

You know the name of this thread should be Help me break the rules and screw my neighbors

Aren't you a moderator?

 

[iwire]

Aren't you a moderator?

Yes I am, but I try not to close a thread just because I don't personally agree with its premise.

I can still voice my own opinions on it.

 

[mivey]

As far as utility easement, we seldom enter private property unless there is a problem. In the old days, when we did manual meter reads, we had to visit every meter once every month.

Then your utility should re-think its inspection program. The ENTIRE system should be inspected on a periodic schedule. Also, meter readers rarely fill that role even though many utilities assume they do.

 

[mivey]

I would like to ask if there are any references for actual voltage drop estimations for over current loads? It was mentioned that an undersized transformer will have drop when starting a load and that might drive my sizing.

Within a reasonable limit, his can be approximated by the ratio of overload to full load multiplied by the transformer %Z.

Also, no one has replied in regards to their motor sizing. Does anyone see any other concerns with my thought process regarding larger VFD driven loads? My thought process is this, a 10HP load FLA is about 50A and figure a worst case of 10x for inrush, that is 500A. I have one drive that is 90A 3PH 30min rating and accel is 120% of that so that is 186A inrush on 1PH power. The way I am looking at it is to control my inrush on loads to stay under the 500A threshold. I know the POCO can technically look up data off my meter from their office and probably see my inrush BUT they already know I have several loads and how could they really know if I start 15HP motor across the line or happen to have 3, 5HP motors start up simultaneously? I have brought that up and they said, "just don't have anything bigger than a 10HP connected".

The stated limit is usually for across the line starting so soft starts may allow bigger motors. In your case they probably just gave you the single-phase limit. You can get up to 100 HP single-phase motors but there is also a running limit they will allow because starting alone is not the only issue.

You really have to listen to what they tell you and if you need specifics you will have to reach one of the utility engineering folks.

 

[mivey]

You know the name of this thread should be Help me break the rules and screw my neighbors

I agree it looks like where it went.

Rules? We don't need no stinkin' rules!

 

[kwired]

You know the name of this thread should be Help me break the rules and screw my neighbors

Though I think the OP should probably subscribe to three phase service if easily available, we often run into fairly heavily loaded single phase services in remote locations where single phase is all that is readily available. Lot of similar challenges come up that he has brought up. One has to determine whether to spend money on POCO upgrades so three phase is available or spend it on phase conversion equipment, as well as deal with any limitations the POCO may place on you, like maximum across the line starting values. You may be able to increase that value, but it may cost you something else to get it.

 

[mbrooke]

You know the name of this thread should be Help me break the rules and screw my neighbors

I dont see any rules being broken in this case. POCOs often size transformers as they see fit. If it burns up tomorrow, they are technically off the hook. As for sizing the actual unit correctly, it will involve dozens of 'what if' scenarios everywhere from premature failure to increased fault current. The key is to have the smallest size unit meet the minimum goals and nothing more. Such takes place at POCO countless times per day.

 

[fastline]

You know the name of this thread should be Help me break the rules and screw my neighbors

Trying not to turn this into a flaming thread but I am honestly trying to do quite the opposite by running the numbers now and determining how my neighbors will be affected. If I had no care in the world, I would hammer the line with 50HP motors.


I think some here might agree that the POCO rules seem to be missing a LOT of language regarding loads. They will give me however many amps I want but limit to 10hp motors. We all know a 10hp motor across-the-line is a lot different than a 10hp motor on a VFD with a 5-10sec accel rate. I am trying to look at the big picture to ensure my activities do not affect my neighbors or piss off the POCO. I am trying to determine what the voltage drop might look like on the primary and secondary.

 

[Iron_Ben]

I am trying to look at the big picture to ensure my activities do not affect my neighbors or piss off the POCO. I am trying to determine what the voltage drop might look like on the primary and secondary.

With a 7.2 kv primary, there's pretty much no chance that starting a 10 HP motor across the line will be evident to any other customers on the circuit. We had many industrial customers starting 50's, 75's, and 100's across the line on our 13.2 kv primaries and and I can't recall a single complaint from a neighbor. Now if we're talking something like a 500 HP motor (which we would have put on our 34.5 kv system) yes, then we'd do a motor start study and see if it would be okay. Starting something that large across the line could well be an issue, depending on many variables. As for your 10 HP motor, _you_ could see some objectionable voltage drop/flicker, especially if your service is overly long or sized on the small side.

 

[meternerd]

Then your utility should re-think its inspection program. The ENTIRE system should be inspected on a periodic schedule. Also, meter readers rarely fill that role even though many utilities assume they do.

Ah, yes...the perfect world scenario. Of course it would be a grand idea to visit every service and every pole often, but budgets being what they are, it boils down to cost-effective work. Regular inspections are done, but certainly not the monthly visit to each meter like in the old days of manual reads. In addition, those areas such as ours where deep snow can impact accessibility, it just isn't practical. Residential and small commercial services are pretty much at the bottom of the priority list. Smaller utilities such as ours have few meter readers (we now only have one), and the techs such as myself were only sent out when problems arose or to inspect or wire new services. Remote reading has caused the subject of inspection to become a real hot button, but solutions are few. I can foresee a need for contract inspection services, but again, it costs money. Way off topic, though. Sorry!

 

[mivey]

My thought process is this, a 10HP load FLA is about 50A and figure a worst case of 10x for inrush, that is 500A... The way I am looking at it is to control my inrush on loads to stay under the 500A threshold.

The worst case is usually not allowed. The NEC table shows about 300 amps. Even that is well beyond what many utilities will allow. Many will limit single phase starting amps to around 200 or less. Have seen some as low as 100 amps.

The way YOU are looking at it?: If you connect to the utility's system you should be concerned with the way THEY look at it.

I know the POCO can technically look up data off my meter from their office and probably see my inrush BUT they already know I have several loads and how could they really know if I start 15HP motor across the line or happen to have 3, 5HP motors start up simultaneously? I have brought that up and they said, "just don't have anything bigger than a 10HP connected".

IIRC, they have 6ga, 7.2KV for primary OH.

If you start them simultaneously then that equates to larger than 10 HP.

As to the inrush, I tend to think the this POCO put a very vague limit of 10HP on the books with no provisions for VFDs and such.

Somebody did, either the POCO or their consultant. Does not mean you have reached the right person. That said, IMO a soft start that limits the current to no more than the normally allowed across the line limit is close to okay as a rough estimate.

I have made mention of having multiple smaller motors but some trying to start at the same time. I mentioned a 5hp table saw with a 10hp vac system that is switched to come in when the saw turns on. That is 15hp across-the-line. He did not say much.

Be thankful you don't have a limit less than 10 HP, which is the case with many utilities.

They are pretty blunt and fixed on this '10hp rule' so I gave up trying to reason with them and will do it anyway. Makes no sense that they will give me as many amps as I want, but limit the motor sizes.

That could be read that you meant you will do what you have been discussing previously anyway and not stick with the 10 HP limit. At least that was my first impression.

Per the POCO engineer, they had issues with a customer starting a 20hp RPC before. Don't know the specifics on that but I suspect that was on a shared transformer..... The biggest question is what that primary line will be doing when I drop something across-the-line.

Could cause several percent of voltage drop or more, depending on your source.

Now for some wild guesses on a moderately remote source: With normal 10 HP (not the high LRA you supposed) you would impact the primary less than 2%, likely less than 1%. Your 500 amp scenario could cause a drop of several percent on the primary. The frequency of starts is also important. Some utilities do allow a customer on a transformer by themselves to impact their own service more (that might be on the order of twice the percentage impact you would see on the primary). This could be worse on some system locations.

I don't see any rules being broken in this case.

Depends what the OP intends to do. Started sounding like he wanted to make his own recipe.

I am honestly trying to do quite the opposite by running the numbers now and determining how my neighbors will be affected.

Why do you think that is your call?

If I had no care in the world, I would hammer the line with 50HP motors.

But it sounds like you will only be happy if the POCO agrees with your assessment/analysis. That may not be what you mean but that is how it kinda sounds.

 

[mivey]

Regular inspections are done, but certainly not the monthly visit to each meter like in the old days of manual reads.

Years not months would be the schedule.

Remote reading has caused the subject of inspection to become a real hot button, but solutions are few. I can foresee a need for contract inspection services, but again, it costs money.

You should work towards implementing an inspection program. Some utilities are required to do it. It is a recoverable cost for a rate case.

 

[mivey]

With a 7.2 kv primary, there's pretty much no chance that starting a 10 HP motor across the line will be evident to any other customers on the circuit.

Actually there is a high chance that it will be visible on a small line as described. The odds that it will reach the irritability & complaint level are low.

We had many industrial customers starting 50's, 75's, and 100's across the line on our 13.2 kv primaries and and I can't recall a single complaint from a neighbor. Now if we're talking something like a 500 HP motor (which we would have put on our 34.5 kv system) yes, then we'd do a motor start study and see if it would be okay. Starting something that large across the line could well be an issue, depending on many variables. As for your 10 HP motor, _you_ could see some objectionable voltage drop/flicker, especially if your service is overly long or sized on the small side.

I have seen some industrial loads with high precision equipment that is very sensitive and they can tell when motor loads start on other parts of the system. Why anyone would build equipment that sensitive to source supply issues is beyond me.