3 phase 230v highleg service....

[JacksonburgFarmer]

Kinda goin out on a limb here.....this is probobaly more utility issue, and I dont know much about utility stuff so....


One of our customers (local grain elevator) has a 3 phase 230v highleg service. The highleg (or wildleg as customer calls it) runs around 190 to 200 volts phase to ground. the other legs are 230v phase to phase, or 115 phase to ground.

The utility line on the road has 3 lines (I belive 2 hot and 1 nuetral?) I do know that it is not true 3 phase, and they are doing something with transformers to produce the three phase off of two phases.....clear as mud?


Now the main question....supposedly, one of the POCO lineman told the customer that "a electrician could balance this system for you" So....customer asks me to look into this.....Any ideas??? I do know another previous EC told the customer it couldnt be done without having true 3 phase from the POCO, and that is my inclination also.....


Any ideas??? could someone explain to me how this transformer configuration works??? Thanks in advance....

 

[infinity]

The utility line on the road has 3 lines (I belive 2 hot and 1 nuetral?) I do know that it is not true 3 phase, and they are doing something with transformers to produce the three phase off of two phases.....clear as mud?

Do you meant the utility drop to the service is only 3 wires?

 

[JacksonburgFarmer]

No, the drop is actually underground. The main service is 3phase, 4 wire. The arangement I was speaking of was the utility high voltage on the power poles

 

[Fulthrotl]

One of our customers (local grain elevator) has a 3 phase 230v highleg service. The highleg (or wildleg as customer calls it) runs around 190 to 200 volts phase to ground. the other legs are 230v phase to phase, or 115 phase to ground.

Now the main question....supposedly, one of the POCO lineman told the customer that "a electrician could balance this system for you" So....customer asks me to look into this.....Any ideas??? I do know another previous EC told the customer it couldnt be done without having true 3 phase from the POCO, and that is my inclination also.....

Any ideas??? could someone explain to me how this transformer configuration works??? Thanks in advance....

somebody else will probably come along with a photo or drawing, but it works
like this....

high leg delta 230v / 3 phase / 4 wire service looks like a triangle, with
three windings connected end to end on the secondary of the utility
transformer.

that'll get you 230v / 3 phase / 3 wire.

the neutral wire is attached to the center point of one of those
three windings i mentioned above, and grounded.

that gives you 115v between either of the adjacent winding ends,
and 208 volts to the opposite end.

the reason is there is a triangle formed between two of the phases,
with an angle of 120 degrees between them.

drawing a triangle with one side at 230, and another side at 115,
and the angle between them is 120 degrees, (1/3 of a circle)
and the remaining line comes out at 208.

a squared + b squared = c squared.

so, it's difficult to balance all three legs of this system, cause
you can only put single phase 115 v. loads on two of the phases.

now, the reason this was a popular service is cause the poco
can actually do a 3 phase service with only two transformers.

it's called open delta, and it's transparent to the end user.
you still have a high leg delta system. you can't tell the difference.

the only difference is that you don't have the full ampacity
of three transformers on the pole. you have a percentage of that.

so, the poco can provide service cheaply, saving 1/3 of their
transfomer cost, and if the customer needs a service upgrade,
they throw another can on the pole, wire it up, and voila! the
full ampacity of the service is available.

so, it saves them money on the front end, and is an inconvenience
to you. just like a public utility should operate....

"balancing" the load is limited to equalizing the 115 volt loads on the
two usable single phase legs, or throwing a 230 v 3 phase primary /
120/208 3 phase wye secondary on the service, and setting a new
panel, and then you have a 120/208 panel you can balance nicely...

or, having the poco change the service to a wye service. then you
can balance the existing service equipment nicely.

that isn't gonna be cheap, but if it's a large customer, the poco will
sometimes do it as it is in their best interest to balance their system.

hope this helps.


randy

 

[hardworkingstiff]

Now the main question....supposedly, one of the POCO lineman told the customer that "a electrician could balance this system for you"

Randy gave you the service information.

I have a question about the PoCo lineman. Why is he telling the customer this? Is there a problem?

 

[ceb58]

What Randy said.
As far as balancing the system since this is a grain elevator I know there are a lot of motor loads. If the system were changed to 208/120 3ph so as to get 120 L-N on each you may have problems with the motors.

 

[JacksonburgFarmer]

Yes, to flat out change to a 120/208 service would definatly reek havoc on motors...the cost to change all the motors would be big....real big....Some of their motors are 208-230-460....but not all of them....

As far as the POCO....these guys are Dunkards (german baptist) and always talk to anyone working at their place(s). The head maintenance man knows enough about electric to be dangerous, and dosent like the unbalanced voltage and amperages on his equipment.....

Another question.....when you size overloads on a highleg system, how do you guys figure the amperage for the overloads? average all three??? I was taught this way, but dont really like the idea of it....

 

[roger]

Another question.....when you size overloads on a highleg system, how do you guys figure the amperage for the overloads? average all three??? I was taught this way, but dont really like the idea of it....

No different than you would for any system, you would use the name plate info.

The voltage between any Phase to Phase will be 240 (230).

Roger

 

[JacksonburgFarmer]

Yes, line to line voltage is 240 nominal, however, the amerage on the highleg is higher than that on the other two legs...how do you account for this???

 

[jim dungar]

One of our customers (local grain elevator) has a 3 phase 230v highleg service. The highleg (or wildleg as customer calls it) runs around 190 to 200 volts phase to ground. the other legs are 230v phase to phase, or 115 phase to ground.

The utility line on the road has 3 lines (I belive 2 hot and 1 nuetral?) I do know that it is not true 3 phase, and they are doing something with transformers to produce the three phase off of two phases.....clear as mud?

The nominal voltage is 240V not 230V (I know old habits dies hard).

The system you are describing is called a 240/120V 3 phase 4 wire system. The high leg is a nominal 208V to ground.

It can only come from a true three phase utility service. The utility may be providing an Open-delta or an open-wye on their side, but it is definitely 3-phase. Do not try to estimate the number of phases by simply counting ungrounded conductors.

Why do you think you need to balance the customer's loads?

Three phase loads should inherently be balanced.
Single phase 240V loads can be balanced just like any other 3-phase panel A-B, B-C, and C-A. Just be aware that any breaker connected to the high leg must be rated for a full 240V to ground (120/240V breakers are not allowed)
Single phase 120V loads can only be connected between L-N so they will naturally cause some amount of imbalance on the three phase, however they would be balanced like you would in any single phase panel.

 

[roger]

Yes, line to line voltage is 240 nominal, however, the amerage on the highleg is higher than that on the other two legs...how do you account for this???

How much higher and is it constant or intermittant? Can you tell how many individual loads are supplied using the high leg compared to loads supplied from the other phases?

Roger

 

[LarryFine]

Yes, line to line voltage is 240 nominal, however, the amerage on the highleg is higher than that on the other two legs...how do you account for this???

Jack, start by picturing a typical 120/240v 1ph 3-wire service. That is exactly what your two 120v-to-neutral lines are. The larger (the one with 3 secondary conductors) of the two transformers in the open Delta is exactly the same as the one feeding your house.

The second transformer, wired for 240v only, has one end connected to one of those two line conductors, and the remaining wire forms the high leg. You get real 3-phase power because each transformer's primary is connected to one line conductor and the utility neutral.

As long as the total impedances are low enough, the open-Delta supply will behave as if it were a closed Delta. You can even power line-to-line-only loads across the open side. But, any load with a neutral connection must only receive power from the two 120v lines.

The only reason the high leg has a higher voltage to ground than the other two legs is because of the conductor we choose to bond.

 

[roger]

Very good Larry, but that doesn't answer the question you quoted. :wink:

Here is an illustration and basic explanation of the "high leg" associated with a 4 wire open delta

Roger

 

[LarryFine]

Very good Larry, but that doesn't answer the question you quoted. :wink:

You're right; I missed "amperage."

In my experiences, the high-leg current is usually lower, and the conductor is sized lower than the others.

 

[JacksonburgFarmer]

In my experiance, the amperage on the high leg is higher....dont really understand why, since the voltage is higher. That is where the problem is....higher amperage on one leg of the service to motors....

 

[LarryFine]

In my experiance, the amperage on the high leg is higher....dont really understand why, since the voltage is higher. That is where the problem is....higher amperage on one leg of the service to motors....

It's almost impossible. The only way the high leg current could be highest would be to load it up with 1ph loads across all phases except the center-tap-grounded one.

Added: Of course, the incorrect high-leg-to-neutral connection could do it, but should never be used.

3-ph loads should have no effect on relative phase balance. Any 120/240v or 120v loads should tip the balance away from the high leg. Curiouser and curiouser.

 

[LarryFine]

Here is an illustration and basic explanation of the "high leg" associated with a 4 wire open delta

Roger

Interestingly, I remember from my puppy days that the high leg used to be called 190v, not 208, but the 208v fits the theory.

 

[Fulthrotl]

Yes, line to line voltage is 240 nominal, however, the amerage on the highleg is higher than that on the other two legs...how do you account for this???

we're talking about a individual 3 phase 240 volt squirrel cage motor, right?

the high leg on such a motor will pull exactly the same current as the other
two legs, because in a three phase application, it is not the high leg anymore,
it's just a leg of a three phase system.

i'm not talking about the feeders to the panel, which will have various single
phase loads on them to make them unequal.

 

[JacksonburgFarmer]

we're talking about a individual 3 phase 240 volt squirrel cage motor, right?

the high leg on such a motor will pull exactly the same current as the other
two legs, because in a three phase application, it is not the high leg anymore,
it's just a leg of a three phase system.

i'm not talking about the feeders to the panel, which will have various single
phase loads on them to make them unequal.

I am taking these measurements, both voltage and current at the load of the motor starter. I will get pictures the next time I am on the site...might be a day or two....I assure you that the highleg feed to all their motors draws more amperage than the others.....

 

[winnie]

Have you measured all three 'phase to phase' voltages? My guess is that you will find that the phase to phase voltages will be different, and that the unbalanced supply to the motors is causing unbalanced current consumption.

-Jon