Dilemma with a IEEE-1100 recommended practice

[PQexplorer]

Hi all,

What issues or downsides will I have, if i install an step-up autotransformer(208Y/120-->480/277) of 160kVA right after the main facility transformer (13.2KV-208Y/120) of 500kvA. This is needed to supply a 160kVA UPS. Also, knowing that this 500kvA also supplies the rest of loads of the building, except the HVAC and the elevator.

If i can do so, What considerations (harmonics, transformer characteristics, etc) do i have to evaluate?

According (textually) to the IEEE 1100-1999 (emerald book)this is not a recommended practice:

"It is not recommended practice to step-up the voltage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment. Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used"

thanks in advance.

 

[petersonra]

Re: Dilemma with a IEEE-1100 recommended practice

no idea why they would state this. it's a pretty common practice to do exactly what you are proposing.

 

[charlie b]

Re: Dilemma with a IEEE-1100 recommended practice

Why a Wye-Wye?

 

[rcwilson]

Re: Dilemma with a IEEE-1100 recommended practice

The front end of a UPS is a large battery charger that usually draws harmonic currents from the power system during normal oepration. As these currents flow through the system?s impedance harmonic voltage drops occur that show up as harmonics on the 480V bus. When you add a transformer, you add impedance, so the harmonic levels will be increased at the input to the UPS. The UPS can probably take it, but any other 480 V loads may not.

IMO the step up transformer also adds losses and voltage drop.

The transformer does have a minor beneficial effect on the 208/120 V system because it acts like a filter attenuating the amount of harmonics from the UPS back to the 208V system.

I would look at a larger transformer for a 160 KVA UPS. After an outage, the UPS charger will have to recharge the batteries while it is still supplying the full load. The transformer also has to supply the UPS losses. A 250 kVA transformer may be the right size for a 160 kVA UPS.

 

[PQexplorer]

Re: Dilemma with a IEEE-1100 recommended practice

Originally posted by charlie b:
Why a Wye-Wye?

becuase, In the UPS especifications says : "The phases must be symmetrical about ground (i.e., from a Wye source) for proper equipment operation."
Also, as a requirement of the EMC directive, every new Double conversion UPS is seen as a linear load for the upstream electrical system. 5% Current THD at full load

 

[jim dungar]

Re: Dilemma with a IEEE-1100 recommended practice

Get a 208 delta to 480Y/277 transformer instead.

 

[charlie b]

Re: Dilemma with a IEEE-1100 recommended practice

I agree with Jim.

 

[steve66]

Re: Dilemma with a IEEE-1100 recommended practice

I would get a UPS with a 208 volt input and a 480V output. The UPS takes the 208 volt input, converts it to DC, and then converts the DC to a 480V output.

I don't see any reason to add another conversion (208V to 480V) to the above sequence. That would only decrease efficency.


Steve

 

[PQexplorer]

Re: Dilemma with a IEEE-1100 recommended practice

Originally posted by steve66:
I would get a UPS with a 208 volt input and a 480V output. The UPS takes the 208 volt input, converts it to DC, and then converts the DC to a 480V output.

I don't see any reason to add another conversion (208V to 480V) to the above sequence. That would only decrease efficency.


Steve

Thank you steve, but maybe I wasn'tclear.
Our needs for using this voltage (480V) is just because for distributing 160kVA @ 208V we must use 800kcmil conductors, that's too impractical. And By the way the UPS will supply a data center, so the utilization voltage is again 208Y/120.

 

[mhulbert]

Re: Dilemma with a IEEE-1100 recommended practice

What about parallel conductors? If your loads are 120/208, and your service is 120/208, it seems quite backwards to go to all the trouble of stepping up your voltage just so you can run smaller conductors. With the installtaion, and cost, of a transformer as well as secondary protection, bonding, using up rentable space, etc, you may still come out ahead running paralel conductors at 120/208.

It may seem like a PITA now, but then you never have to worry about transformer losses (and their energy costs) down the road. Also, if we are talking about a data center with high reliability (very few aren't), the less parts in the chain, the higher chance of things staying online.

Are we missing a detail that would require 480/277?

 

[jim dungar]

Re: Dilemma with a IEEE-1100 recommended practice

Are you going to stepping down the voltage before the data center?

If you are simply trying to run higher voltage feeders to reduce conductor size, then look into using 600V delta instead of 480Y/277. Larger voltage smaller conductor and why pull a neutral if you don't need it.

 

[PQexplorer]

Re: Dilemma with a IEEE-1100 recommended practice

Originally posted by jim dungar:
Are you going to stepping down the voltage before the data center?

If you are simply trying to run higher voltage feeders to reduce conductor size, then look into using 600V delta instead of 480Y/277. Larger voltage smaller conductor and why pull a neutral if you don't need it.

You're suggestion may sounds good.
But in fact the scenario is as follows:

- The UPS only accept an 480V input and as the manufacturer states: The rectifier feed into this
equipment uses three wires. The phases must be symmetrical about ground (i.e., from a Wye source) for proper equipment operation." that's why my choice is so far 280-480Y/277 (delta-wye)
- The UPS output will be 480V (it doesn incorporate a transf). In this way, i'll have to install an 480V-208Y/120.

Any suggestions??

 

[mhulbert]

Re: Dilemma with a IEEE-1100 recommended practice

PQExplorer,
is the UPS already purchased/on-site? I would still hold with my original suggestion, using 120/208 all the way, if you have a UPS that has 120/208 in and out. Why install 2 transformers and related OCPD/bonding/heat gain/inefficienies, when you can just run a pair of conductors?

The only advantage with a 480delta to 208/120 would be harmonics mitigation. This may or may not matter depending on the UPS you are using.

If you are trying to make an existing 480 input UPS work, then that is another situation.

Mike

 

[PQexplorer]

Re: Dilemma with a IEEE-1100 recommended practice

Originally posted by mhulbert:
What about parallel conductors? If your loads are 120/208, and your service is 120/208, it seems quite backwards to go to all the trouble of stepping up your voltage just so you can run smaller conductors. With the installtaion, and cost, of a transformer as well as secondary protection, bonding, using up rentable space, etc, you may still come out ahead running paralel conductors at 120/208.

Assuming both approaches have the same cost , because anyhow 120/208 requires more conductors and bigger raceways/cable trays ,and also the switchgear is more expensive. What approach would you choose?. in terms of power quality, isn't it better the thansformers approach?

 

[mhulbert]

Re: Dilemma with a IEEE-1100 recommended practice

PQ,
assuming both systems have an equal up-front cost, you are still going to pay more in the long run for 2 transformers... you are paying for the losses of both of the transformers in your electric bill. You are running the HVAC system more to cool both of these units. You have to provide preventive maintenance to both units(ie clean out the dust). You also reduce the amount of space available to rent out (if you are a colo).

Adding two links to your chain of power distribution will lower the reliability of your system. If either one of these units break, you are offline until you can obtain and install a new one. Properly sized & installed wire in pipe is not going to have any problems for at least 50 years.

The only argument against everything above would be harmonics mitigation. I'm not sure what kind of harmonics your UPS puts out on its line side. Some are worse than others. If it was a problem, you could use a 208delta to 208Y next to the unit, and jsut run 3 phases and ground back to your service.

mike

 

[steve66]

Re: Dilemma with a IEEE-1100 recommended practice

What kind of distances are we talking about here? Are the service and UPS and loads very close together? Or is the load at the top of a skyscraper?

I have to agree with Mhulbert, the transformer losses (and cost and maintenance) probably makes sticking with 208V a better solution. The only way I would consider stepping up the voltage is if the run is very long.

You mentioned 160KVA. If we multiply that by 1.25 to include battery charging, and another 1.25 for short overloads on the UPS, that is still only 250KVA. Thats only 700 amps. That current should be easy to wire for short distances.

Steve

 

[steve66]

Re: Dilemma with a IEEE-1100 recommended practice

What kind of distances are we talking about here? Are the service and UPS and loads very close together? Or is the load at the top of a skyscraper?

I have to agree with Mhulbert, the transformer losses (and cost and maintenance) probably makes sticking with 208V a better solution. The only way I would consider stepping up the voltage is if the run is very long.

You mentioned 160KVA. If we multiply that by 1.25 to include battery charging, and another 1.25 for short overloads on the UPS, that is still only 250KVA. Thats only 700 amps. That current should be easy to wire for short distances.

Steve

 

[iwire]

Re: Dilemma with a IEEE-1100 recommended practice

Originally posted by steve66:
Thats only 700 amps. That current should be easy to wire for short distances.

Steve

I agree, two sets of 500 Kcmil CU or three sets of 250 Kcmil CU in separate raceways will take care of 700 amps.

If left up to me it would be two sets of 3" EMT with the 500 Kcmil CUs

 

[PQexplorer]

Re: Dilemma with a IEEE-1100 recommended practice

Originally posted by steve66:
What kind of distances are we talking about here? Are the service and UPS and loads very close together? Or is the load at the top of a skyscraper?

I have to agree with Mhulbert, the transformer losses (and cost and maintenance) probably makes sticking with 208V a better solution. The only way I would consider stepping up the voltage is if the run is very long.

You mentioned 160KVA. If we multiply that by 1.25 to include battery charging, and another 1.25 for short overloads on the UPS, that is still only 250KVA. Thats only 700 amps. That current should be easy to wire for short distances.

Steve

The cabling will run about 70mts (curves, bends, etc), from the service entrance(basement) to the 4th floor of the building.

The max input current according to the UPS specs is 480A (208/120 model).
I did the calculus, considering raceways/cable trays,transformers ,OCPD's, wiring and the installation at 480V will cost 15 USD dollars less than the 208/120 inst. But the electrical cost (24/7, 365 days) of the two transformers (with 98% efic) per year will be around 8 thousand USD... so after 2 year of operation the 208/120 inst. will pay by itself.