Power Sags

[Bill Arden]

The power company always uses the smallest transformers possible.

Worse yet if they don't have an AMR (Automatic meter reading) system they decide the transformer size on guesses of you peak load.

In order for the voltage sag to be noticeable you would need a voltage drop of 120Volts / 255(intensity levels) = .5 volts (1 volt at 240 Volts)

It would be very hard to get that kind of voltage drop across the house wiring without getting some smoke, so it's probably the transformer. ("Across" as in the voltage difference between the breakers and the voltage at the transformer)

*evil grin* I would turn on every device I have at the same time during a warm day... Burn Baby Burn...

On a more sane thought. Did they say what the KVA of the transformer was?
Most homes have a 15KVA transformer. (66 amps max)

 

[Jdoll42]

Worse yet if they don't have an AMR (Automatic meter reading) system they decide the transformer size on guesses of you peak load.

I know they have AMR. They talked about all the problems they had when we had a huge ice storm a little over a year ago. Evidently they use the AMR system to also determine whether or not you have power, so theoretically they could know your power is out before you do.

In order for the voltage sag to be noticeable you would need a voltage drop of 120Volts / 255(intensity levels) = .5 volts (1 volt at 240 Volts)

If you are able to notice a power sag at just 1V drop from 240V, is that considered a problem with the service? At what point is a voltage drop considered excessive enough to warrant the power company to do something?

*evil grin* I would turn on every device I have at the same time during a warm day... Burn Baby Burn...

I like the way you think...he he he. In reality, though, I put quite a load on the transformer for about a week. My geothermal system wasn't completely hooked up and I was running on all electric strip heat in the unit. Even the 3/0 wires coming in to the main panel were warm. I wonder if that possibly "broke" something in the transformer?

On a more sane thought. Did they say what the KVA of the transformer was? Most homes have a 15KVA transformer. (66 amps max)

They've told me before, but I can't remember. The transformer is in one of those green boxes on the ground (buried service). I'll look out there in the daylight sometime and see if it marked somewhere on the box. If not, I guess I'll have to give them a call and see if they'll tell me. If it is a 15KVA, would a "bigger" transformer help at all? I wonder how willing they would be to swap it out....

 

[Sponsor]

 

[Bill Arden]

I know they have AMR. They talked about all the problems they had when we had a huge ice storm a little over a year ago.

What brand?

Some systems have "15 minute peak" measuring ability.
1. Itron RF does not the last time I checked.
2. Twax DCSI, Hunt turtle, and some others have the ability to read the 15 minute peak.
3. I am not sure about the newer cannon emetcon systems.

If you are able to notice a power sag at just 1V drop from 240V, is that considered a problem with the service? At what point is a voltage drop considered excessive enough to warrant the power company to do something?

When multiple customers complain or when something goes poof.

I like the way you think...he he he. In reality, though, I put quite a load on the transformer for about a week. My geothermal system wasn't completely hooked up and I was running on all electric strip heat in the unit. Even the 3/0 wires coming in to the main panel were warm. I wonder if that possibly "broke" something in the transformer?

*ponders* The only thing I can think of that would cause the voltage to sag while not causing other HUGE problems would be if the transformer boiled off the oil and is running hot.

*eliminates other thoughts*
1. Losing the neutral on the house side would cause high voltages on half of the house.
2. Loosing the neutral on the high voltage side would start a nasty failure mode. It would cause sags, but the transformer would also smoke and go poof after a weeks.

If it is a 15KVA, would a "bigger" transformer help at all? I wonder how willing they would be to swap it out....

Larger transformers tend to have lower resistance and lower leakage inductance, but we found some variations to that rule when we were doing research for a power line carrier system.

 

[Mikey]

In order for the voltage sag to be noticeable you would need a voltage drop of 120Volts / 255(intensity levels) = .5 volts (1 volt at 240 Volts)

Bill, could you explain this a little more or provide a reference?

mike

 

[Lakee911]

*ponders* The only thing I can think of that would cause the voltage to sag while not causing other HUGE problems would be if the transformer boiled off the oil and is running hot.

I'm not so sure that this is possible. Generally they are sealed. If it did "boil off" as you say, where would it go? It would explode. If you see it leaking, then that could be a possibility. I would guess that it would catch fire long before...

15KVA seems kind of small for even a single 200A service. That's 62A at 240V Single phase. Rule of thumb is to load dry type transformers to 80%, but liquid filled can go to 100%, possibily even more with appropriate cooling. I don't know about where this install is, but sometimes there are as many as four services on one pad mount transformer.

I would suspect that it's undersized, but the OP needs to find out how many services are on it, their demands and the size of the transformer.

Your incoming service cables should not be getting excessivly warm. You mentioned that they're 3/0 cables. I'm assuming that they're direct bury triplex UF and that would be fine if they're copper. If they are Aluminum (typical) they would be slightly undersized. Generally 2/0 Cu or 4/0 Al is used for 200A service.

Jason

 

[Lakee911]

1. Losing the neutral on the house side would cause high voltages on half of the house.
2. Loosing the neutral on the high voltage side would start a nasty failure mode. It would cause sags, but the transformer would also smoke and go poof after a weeks.

1. Not necessarily. It would cause Leg A to be wired in series with Leg B. 240V devices not requiring a neutral would still be fine. Depending on the loading, you may never know (unlikely). Assuming you have mettallic water pipes, it's also possible that a slight unbalanced loading would travel on the ground to a neighbor's house and back to the transformer through their neutral! Would seem just fine.

2. How do you figure? Single phase services are connected to a phase conductor and a multi-grounded neutral. There is no center tap on the high side. If you lost the phase, it would not function. If you lost the nuetral, then it would still function just fine. The center tap would still be connected to the same nuetral and that's usually internally connected inside of the transformer. Chances are it would go unnoticed.

Jason

 

[Bill Arden]

Bill, could you explain this a little more or provide a reference?

256 levels of light intensity (per color) is generally regarded as the standard for images since it roughly matches the limits of what people can perceive in terms of intensity changes.

In reality it is non-linear in that you can perceive smaller changes at low light.
http://en.wikipedia.org/wiki/Visual_perception

The exact number varies from person to person, but a 1% to 2% change is generally detectable by the human eye if the change occurs repeatably.

-
The lights are also non-linear and that makes it easier to detect voltage changes.

1) Incandescent lights turn more red and that change is easier to see.
2) CF lights dim linearly, but then blink and start to flicker as the voltage continues to drop.
3) Magnetic ballast lights will dim. I don't know how they respond at lower voltages. I do know that they can make audible sounds if you have audio frequency voltage changes on the power line. (early Cannon Emetcon Load control systems used ~9Khz as the power line carrier frequency)
4) LED lights need regulators or they change a lot with just a small voltage change.
5) Mercury vapor and sodium vapor can blink out and then restart.

-
Other devices can be even more sensitive. For example the small UPS I have in one place cycles every time my compressor kicks in. luckily it had dip switches that let me turn off the "beep"

 

[Bill Arden]

1. Not necessarily. It would cause Leg A to be wired in series with Leg B. 240V devices not requiring a neutral would still be fine. Depending on the loading, you may never know (unlikely). Assuming you have mettallic water pipes, it's also possible that a slight unbalanced loading would travel on the ground to a neighbor's house and back to the transformer through their neutral! Would seem just fine.

Loss of neutral has been discussed a lot and you are correct in that it can cause a lot of weird effects. Things can get even stranger if you loose the neutral between two ground rods.

2. How do you figure? Single phase services are connected to a phase conductor and a multi-grounded neutral. There is no center tap on the high side. If you lost the phase, it would not function. If you lost the neutral, then it would still function just fine. The center tap would still be connected to the same neutral and that's usually internally connected inside of the transformer. Chances are it would go unnoticed.

If the neutral wire of the high side winding opened, there is a good chance that the 7.2Kv would start a small plasma arc and then a carbon path.
This would keep the transformer running, but not for very long...

 

[Bill Arden]

I'm not so sure that this is possible. Generally they are sealed. If it did "boil off" as you say, where would it go? It would explode. If you see it leaking, then that could be a possibility. I would guess that it would catch fire long before...

Oil filled transformers are required to have pressure relief valves. They are similar to T&P valves and look like small tubes with a metal pull ring.

They are known to vent and smoke for quite some time as the boiling process cools the unit.

15KVA seems kind of small for even a single 200A service. That's 62A at 240V Single phase.

Larger transformers have higher standby power usage. The power utility changes out transformers as needed.

 

[Bob NH]

Your incoming service cables should not be getting excessivly warm. You mentioned that they're 3/0 cables. I'm assuming that they're direct bury triplex UF and that would be fine if they're copper. If they are Aluminum (typical) they would be slightly undersized. Generally 2/0 Cu or 4/0 Al is used for 200A service.

Jason

Size of service cables in my area bears no relationship to the code-specified size for residential services.

The POCO cable from the pole is #2 aluminum. When I upgraded my 1964-vintage 100 Amp service to 200 Amps they didn't change the conductors from the pole.

The nearest transformer serves 5 houses.

 

[Jdoll42]

I don't know about where this install is, but sometimes there are as many as four services on one pad mount transformer.

I would suspect that it's undersized, but the OP needs to find out how many services are on it, their demands and the size of the transformer.

There is only one service on this transformer. They had to add it to get back to my house. There is nothing else even close.

Your incoming service cables should not be getting excessivly warm.

They are not warm now. The only time they were warm was when I was relying on ~100A of electric strip heat. That was only for about 5 days or so until they got the geothermal loops hooked up. I didn't know my meter could spin that fast....

You mentioned that they're 3/0 cables.

Well, it's 3/0 copper from the meter base to the main panel (about 10'). Not sure what the power company ran from the transformer to the meter.

 

[Jdoll42]

Things can get even stranger if you loose the neutral between two ground rods.

You think maybe I have a grounding problem? Would adding an additional grounding rod near the existing one help? I do have the one from when I had my temporary construction service up. I could sink that and tie it into the existing one.

 

[JWelectric]

You think maybe I have a grounding problem? Would adding an additional grounding rod near the existing one help? I do have the one from when I had my temporary construction service up. I could sink that and tie it into the existing one.

No. Earth connections plays no role in the systen function at all and I had a little laugh when I read that other post.

Current does not flow through earth to make our circuits work nor does it flow through earth for the power company to work.

 

[Lakee911]

Size of service cables in my area bears no relationship to the code-specified size for residential services.

The POCO cable from the pole is #2 aluminum. When I upgraded my 1964-vintage 100 Amp service to 200 Amps they didn't change the conductors from the pole.

The nearest transformer serves 5 houses.

That's true, Bob. The NEC does not govern the Utility Company. Generally speaking though, for new installs it does loosely follow that of the NEC. At least from what I've seen, that is true. That does seem kind of small though.

I'll have to pull out our AEP Distribution Standards book and see if it has anything about sizes of service connections. Most likely it is not in that book.

Jason

 

[Lakee911]

I'd be curious to see what happens when you fire up the strip heating and then start the geothermal unit! (Don't run it long though if you do).

 

[Bill Arden]

You think maybe I have a grounding problem?

No. I was just exploring very unlikely possibilities.

Would adding an additional grounding rod near the existing one help? I do have the one from when I had my temporary construction service up. I could sink that and tie it into the existing one.

Yes. The more the merrier when it comes to ground rods during lighting strikes.

Back on topic.

You most likely are on either a small or overly cheep transformer.

Are there any numbers on the outside of the unit. They generally have the KVA number on the outside. Note: the number is not likely to be labeled and they sometimes run the numbers together so customers think it's a serial number.

Example: 24015 could be 240V, 15Kva

 

[Bill Arden]

There is only one service on this transformer. They had to add it to get back to my house. There is nothing else even close.

*spots something*

How far from the city are you?

Many miles of power lines can add significant resistance.

The source impedance (resistance to voltage sags) increases at a 1/33 rate. (Assuming 7.2Kv)

So many miles of low current lines can add up.

Edit:
Or maybe it's just a number of factors that add up to one large issue.
1. Power line resistance if it's long.
2. Transformer.
3. Wire to house. "75' of unknown wire from utility"

Q: Do the lights dim if you turn on the electric?
Taking a voltage reading with it on and off will allow us to calculate the "source impedance"

 

[JWelectric]

There are a couple of things that is for sure and certain with this installation.

• If the problem is with the transmission lines that supply the transformer there is nothing that can be done
• If the problem is with the transformer then the homeowner could buy a larger transformer to solve this problem
• If the problem is with the service drop or lateral then the homeowner can pay to have larger conductors installed
• There has not been one person as yet to give this poor man one thing that will help him with his problem but instead there has been a lot of talk about the power company and their problems

If this is in fact a 5 horse power (240 volt 27 RLA) motor and it is starting under load by having to push a liquid like glycol through a inch and a quarter or larger pipe for several hundred feet then there will be a large voltage drop across the entire system until the motor reaches close to synchronous speed.

Now the question remains;
What if anything can the homeowner do to stop this voltage drop across his entire house?

 

[Bill Arden]

There has not been one person as yet to give this poor man one thing that will help him with his problem but instead there has been a lot of talk about the power company and their problems

If this is in fact a 5 horse power (240 volt 27 RLA) motor and it is starting under load by having to push a liquid like glycol through a inch and a quarter or larger pipe for several hundred feet then there will be a large voltage drop across the entire system until the motor reaches close to synchronous speed.

Now the question remains;
What if anything can the homeowner do to stop this voltage drop across his entire house?

It's the freon compressor and the blower fan motor that are drawing the load. The recirculating pumps are generally very small.

A VFD was suggested earlier.

A "soft start" device would be less expensive, but that could cause the motor to stall since it will be starting against higher back pressure. (similar to starting just after it shuts off.)

 

[Jdoll42]

What brand?

Some systems have "15 minute peak" measuring ability.
1. Itron RF does not the last time I checked.
2. Twax DCSI, Hunt turtle, and some others have the ability to read the 15 minute peak.
3. I am not sure about the newer cannon emetcon systems.

Maybe this photo will help: (I see it says DCSI on it. Maybe you'll recognize the model and capabilities.)