Three Phase Utility Transformers Configurations

[rhpope]

This is related to the previous thread from yesterday on Load Calculations.

Current Situation:

One 25 KVA transformer is mounted to the utility pole which serves several homes and the machine shop. As noted in the other thread the three phase choices from the power company are 120/240 CT Delta 4 wire or 120/208 Wye 4 wire. The preference that I am leaning toward is the 120/208 Wye since it has the most flexibility and no "wild leg" with the Delta service. My main concern is with the WYE service is whether of not this will affect the single phase service to the homes. Will the power company reconfigure the existing transformer with two more 25 KVA transformers for the WYE service and then the homes will only be able to pull off 120/208 single phase instead of the original 120/240 single phase. This may have adverse effects on water heaters and especially A/C compressors by using 208V instead of 240V? If so, then the better choice for the homes would be for me to go with the Delta to keep the voltage at 240V single phase instead of 208V single phase????

 

[itsmoked]

Hi rhpope,

I'd be pretty chapped if the POCO tried to drop me to 208. I would have several serious issues. I'd lose a VFD, a large but marginal baseboard heater, a 5 HP single phase compressor, and a high-end tig welder to a 208 migration.

A whole lot of places do fine with a stinger leg. It gets everybody what they need.

If this is a whole new installation/facility they might just provide three transformers for it and leave the houses on the single transformer. I also see them do this into a single building. Then you have a 1ph and a 3ph meter. Depends on your POCO.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[dpc]

If the transformer bank is changed to 208/120 V wye, you will no longer have 240 V available. Water heaters won't care much, it will just take longer for the water to heat up. Motors are the main concern. Some may work OK, others may overload and overheat.

It's tough to answer your question without knowing why you want three-phase power and what the various single-phase and three-phase loads are.

In general, the 120/240V wild-leg delta system is intended for services where the large majority of the load is three-phase with only a small amount of single-phase loads. Pump stations were often done this way.

If you have a large amount of single-phase loads, as I suspect you do, I do not recommend the three-phase delta wild-leg.

 

[waross]

I concur with itsmoked and with dpc.
dpc, I understand that you are referring to closed delta installations. I agree but I have found the reverse to be true of open delta installations. Typically an open delta is used for a large single phase load and a small three phase load. It is also often used for small isolated pumping stations or irrigation pumps. Typically up to about 15 hp. or 25 hp. I mention it because it may be the system the poco is proposing here.
The best thing is to ask the poco if the 120/208 will affect everyone or if it will be a separate installation with three new transformers.
Has the poco quoted a price for the installations?
How much three phase load are you planning to add?
If the AHJ and the poco allow it, the most economical may be to keep your existing single-phase service (Assuming that you have an existing single phase service) and add a small three phase service. The 4 wires would run to your panel to provide a ground point.
If this is a totally new service, do a panel breaker layout.
Count off your three phase breakers and then lay out your single-phase breakers. If you have lots of spaces left over go ahead with the Center Tapped Delta.
You have not yet told us what your projected loads are, or if you have an existing service.
The existing 25 KVA transformer will basically support a loaded 100-amp service. If it is already serving several homes it may be maxed out already and the poco may be planning to replace it with a larger unit if you go delta or leave it alone and install three new transformers for your wye service. The loading calculations on an open delta bank are a little different again. In a closed delta bank, the phase current is less than the line current because it is supported by two transformers. In an open delta bank, the line current equals the phase current on the open ends of the delta.
Calculate the single-phase load on the main transformer in amps.
Calculate the three-phase load in line amps.
Add these figures together. The sum should be less than or equal to the rated current of the main transformer.
The three phase line current should be less than or equal to the rated current of the teaser transformer.
respectfully

 

[jghrist]

I don't think you have to concern yourself with the service to the homes. If you take 3ø service at 120/208, then the utility will have to hang a separate bank of transformers. The homes will have to continue being served at 120/240 - this is the utility's problem, not yours.

 

[rhpope]

The building current has a 200amp single phase service taken off of the 25 KVA transformer that I spoke of at the beginning that feeds the other 3 homes also with 200amp services too. There will not be any cost to me to do this upgrade and I have my choice of 208Y/120 or 240/120 Closed Delta service. I have calculated my load to be less than 45 KVA total (single & three phase loads combined). So three 15 KVA transformers in a WYE configuration will work or (1) 25 KVA & (2) 10 KVA transformers in a Closed Delta configuration will work also. I did each calculation for the loads for each type of configuration to come up with this. The three phase load by itself is about 19 KVA and spread across 3 phases or transformer equates to 6.3 KVA per phase or transformer. So as you can see the three phase load is not very big.

waross,

Can you further explain this as I am not sure what you mean by adding in the three phase service & the 4 wires would run in to provide a ground point:

"If the AHJ and the poco allow it, the most economical may be to keep your existing single-phase service (Assuming that you have an existing single phase service) and add a small three phase service. The 4 wires would run to your panel to provide a ground point."

 

[jbelectrical]

To reiterate and add to what was already said...most modern appliances that I've encountered (Water heaters, electric cooktops/ranges, dryers, etc.) are dual-rated for both 208V and 240V. Heating elements are simply de-rated when operating at a lower voltage. For example, an electric dryer rated at 5600W at 240V will operate at 4400W at 208V. However, if existing homes are equipped with HVAC equipment configured to operate at 240V (Which they probably are), the lower voltage will be a problem.

Anyways, I wouldn't worry about it. Like jghrist said, the utility will need to provide a new bank of transformers if you specify 3 phase service. There's absolutely no way they'd suddenly start providing residences with 120/208 service.

 

[waross]

A 120/240 Volt center tapped delta system typically has the center tap grounded to provide a system groung. If you run a three phase only service, the fourth wire from the center tap will almost certainly be brought to the service to provide a system ground point. You may then run three phase only panels with only the three phase wires and a ground, but not a neutral. This assumes another single phase service to supply the single phase loads. I don't think it will apply to you.
Better let the utility size the transformers.
I estimate that your single phase load is about 25 KVA
The 25 KVA will supply about 18 KVA of this.
The 10 KVA transformers will act together as one 10 KVA transformer to supply the other 7 KVA. That leaves you with 3 KVA capacity on each of your 10 kva transformers to supply your 6.3 per phase three phase loads.
Wye do I prefer why.
A 37.5 and a 25 in open delta may work but it's late and I haven't checked the numbers on it.
Good night all
yours

 

[stevenal]

120/208 open wye is commonly used for residential services where three phase loads are served from the same transformer. I would have a problem converting existing services though, as the voltage complaints would be numerous as water heaters and ovens take longer to heat than folks are used to. I agree it's the utility's concern.

 

[dpc]

I agree that an open delta configuration is another option if the motor loads are small. I just hate these setups so much I didn't want to mention them

 

[waross]

Hello dpc
I guess it's different experiences. I like open delta for small three phase loads, but closed delta is the connection I don't like. In this area the 4 wire wye primary is used and problems abound. Last year, I saw three 50 KVA transformer failures within about 6 weeks. Two seperate but related incidents at one installation and one at another location. All resulting from wye-closed delta connections. Primary voltage unbalance and primary phase loss kill these transformers.
respectfully.

 

[dpc]

I don't like either one. But at least you get balanced voltages when there is actually a winding for each phase.

I've had a lot of problems with high negative sequence currents and motor overheating due to voltage imbalance caused by open delta banks. If you keep the motors very small, it works.

The main problem with the 4 wire delta systems is ignorance and attempting to take more single-phase load than the one winding can support.

 

[peebee]

For what it's worth, I'd tend to lean toward the 208/120v service. But I have very little residential experience, so take that with a grain of salt -- the commercial & industrial equipment I'm used to would generally handle a 208v service with no problem.

I'm a bit surprised at some of the reactions to the 208v suggestion like the following: "I would have several serious issues. I'd lose a VFD, a large but marginal baseboard heater, a 5 HP single phase compressor, and a high-end tig welder to a 208 migration." (my apologies to itsmoked, I don't mean to pick on you, yours was just the first post to come out against 208v).

I wouldn't think that you'd "lose" any of that stuff.

Your heating equipment would be derated to 75% of it's original BTU output according to the ratio of the voltages squared, (208/240)^2 = 75%. Heaters would run more often, stoves would cook slower, dryers would dry more slowly. But I'd think they'd all work.

Your motorized equipment would see input current would go up as voltage went down -- if your motors are 1.15 service factor (I don't know if that's common on residential stuff), most motorized equipment should work just fine, you shouldn't even notice the difference, although torque will drop by about 25% (with v-squared).

Your VFD would probably be derated to 87% of its original rating (208/240 = 87%), input current is fixed but voltage is dropping by 13%, which might or might not be a problem.

Regarding the tig welder -- not sure, I suspect you'd only be able to weld about 75% as fast (v-squared).

Moreover, note that utilization equipment is generally not rated at 240v, it's rated at 230v. 208v is only a 10% undervoltage condition from 230, which makes the above analysis look even better when you use 230 as a base voltage.

Lastly, note that a 240-volt nominal system would still be "in spec" with brief (or even extended) excursions down to 212-volts or so -- you'd have no room to complain (or not much room) if your utility delivered you 212-volts continuously on your 240-volt service. The equipment "should" be built to withstand that. And assuming you have a "stiff" 208-volt source, or if you can bump the taps up on your 208-volt transformers, well, what's the difference between 208 and 212 then?

There's lots of other threads on this site about the effects of running 240v equipment at 208v. . . .