5000 KVA, Sub lose at 50 %

[Johnny Johnson]

I have been asked to look for ways to cut cost on energy where I work. We have 24- 2000KVA 13.2/480 subs and 4 -13.2/2400 5000 KVA Subs. The 4 2400 volt subs are only loaded at about 40 % of their rated KVA. I am looking at moving the secondary feeder breaker and cable to the xformer setting beside it and shutting down 2 of the 4. This is based on if it will it be cost effective. I am trying to determine the value on cost saved by having 2 subs fully loaded vs. 4 subs loaded at less than 50 %. I have read that running a sub at 50 % load cost alot extra on VARS wasted. Thanks for any help on determining the lose loaded at 50%.

 

[Johnny Johnson]

Substation Lose Question

Substation Lose Question

I am trying to determine if the cost of operating a 5000 KVA 13.2/2400 substation at about 40% load is costing extra money from xformer lose. In my case it would be simple to move the feeder breaker and cables to the sub beside it and shut one down. This would result in one sub loaded at about 90 % rather than two at 40% each. I have read that subs operating at low loads cost extra money in the form of VARS.

thanks for any help..

 

[Johnny Johnson]

Sorry...I didnt think it took my first post

 

[kingpb]

If both transformers are about the same impedance, then the losses are going to be about the same. I believe the only real savings will be in eliminating the energization current, i.e. no load losses.

 

[Johnny Johnson]

I agree about the load lose, but how can I put some sort of value/saving on shutting one down? Our cost is .05 on peak and .03 off peak with no PF or demand penalty. Our overall PF for the plant is .79

 

[jghrist]

I am trying to determine if the cost of operating a 5000 KVA 13.2/2400 substation at about 40% load is costing extra money from xformer lose. In my case it would be simple to move the feeder breaker and cables to the sub beside it and shut one down. This would result in one sub loaded at about 90 % rather than two at 40% each. I have read that subs operating at low loads cost extra money in the form of VARS.

thanks for any help..

You would need to know the no-load losses and load losses to get a definitive answer. You also need to know the load on each transformer if they are both in service.

Load losses will vary with the square of the kVA load, so if they are evenly loaded, putting all load on one would increase load losses on one tranformer by a factor of 4 (2?), so the total load losses would reduce by a factor of 2 (4?2) because you only have one transformer loaded. No-load losses do not vary with the load, and would be reduced by a factor of 2 if one transformer is deenergized.

Also consider that if you have a transformer or bus outage, you lose all of you load instead of just half. If you don't have one transformer energized, it is more likely to get moisture in the oil. If both transformers are energized, then you know both are available in case one fails. Keeping both energized eliminates the reduction in no-load losses.

Chances are, with 40% loading on each transformer, you will have higher losses by putting all load on one.

 

[zog]

Also keep in mind if you ever plan on using these (I assume dry type) transformers again you will need to place them in lay up and do a full battery of testing before re-energizing.

 

[rcwilson]

Eaton's Engineering guide gives data for 2MVA dry type transformers:
no load loss = 5.7 kW, load loss = 44.2 kW for 150C rise and 6.9 kW and 20 KW for 80C rise transformers. Load losses vary with the square of the load so multiply by 40% squared to get losses at 40% load.

Using data for the 150C rise units, for each set of two transformers at 40% load, your losses will be = 2 x [ 5.7 + (.4 x .4) x 44.2] = 25.5 kW total.

One unit running at 80% will be = [ 5.7 + (.8 x .8) x 44.2]= 33.98 kW

Losses will be 33% higher running only one unit. Plus, since it is running hotter, you will need more HVAC and since there is more current there are more cable losses.

If you have the more efficient 80C rise transformers the figures are:

together @ 40% = 2 x [ 6.9 + (.4 x .4) x 20] = 20.2 kW.

Only one @ 80% = 19.7 kw. Save 0.5 kW = 2.4%.

Answer depends on the loading, the relative ratio of no load to load losses for your transformers.

Note for oil filled units, typical vlaues are 4 kw no load and 22.6 kW load loss.

You could put togehter a spread sheet showing time of day, loads, losses and rates to come up with which set up is most cost effective. But for rule of thumb, splitting the load is usually more efficient and more reliable.

 

[IslandSparky]

It's better to see if you can eliminate a transformation than consolidate substations, unless they are under 20% load. VFDs and innovative controls usually do an order of magnitude in saving energy. You would be amazed about what you can learn about a process by putting meters on all the components for a week or two. We had a client that had controls effectively keeping motors at locked rotors for an hour a day; adding limit switches and a VFD did wonders.