Finding transformer magnetizing kVARs?
Finding transformer magnetizing kVARs?
(OP)
Can anyone tell me how to determine the magnetizing kVARs for a transformer? Is there a general value that can be used for estimating purposes based on kVA capacity? The known values of the transformers in question are primary and secondary voltage, rated kVA, and percent impedance.
In order to help understand the nature of my question, my facility has about 9.5MVA of distribution in the form of eight transformers with 13.2kV primaries and various low voltage secondaries. I am in the process of determining our actual power usage, but I am estimating that it is in the range of 350-500kVA continuous with a peak of 1000kVA and a power factor of 80-90%. The difference in usage versus capacity is due to the fact that the building's original purpose required much more power than we do.
The problem is that our power factor as seen by the utility is very low and it is costing us a lot of money in penalties. Since the utility metering is on the primary side, I suspect that the reason for this is that we have so much idle transformer capacity. However, I need to put a number on it. If I can determine the magnetizing kVARS for a given transformer or for a given amount of kVA capacity, I can estimate how much all of that idle transformer capacity is costing and decide if it is worth it to do something about it. Right now, I could eliminate 3 transformers (4250kVA) and de-energize 2 more (2250kVA) if I transfered some loads to the remaining 3 transformers.
In order to help understand the nature of my question, my facility has about 9.5MVA of distribution in the form of eight transformers with 13.2kV primaries and various low voltage secondaries. I am in the process of determining our actual power usage, but I am estimating that it is in the range of 350-500kVA continuous with a peak of 1000kVA and a power factor of 80-90%. The difference in usage versus capacity is due to the fact that the building's original purpose required much more power than we do.
The problem is that our power factor as seen by the utility is very low and it is costing us a lot of money in penalties. Since the utility metering is on the primary side, I suspect that the reason for this is that we have so much idle transformer capacity. However, I need to put a number on it. If I can determine the magnetizing kVARS for a given transformer or for a given amount of kVA capacity, I can estimate how much all of that idle transformer capacity is costing and decide if it is worth it to do something about it. Right now, I could eliminate 3 transformers (4250kVA) and de-energize 2 more (2250kVA) if I transfered some loads to the remaining 3 transformers.






RE: Finding transformer magnetizing kVARs?
RE: Finding transformer magnetizing kVARs?
RE: Finding transformer magnetizing kVARs?
RE: Finding transformer magnetizing kVARs?
For jghrist, you are correct. When asking my question, I was disregarding the real power no-load losses. My beginning focus was on the kVARs since my power factor is lower than expected. However, since my original post I have learned that my "penalties" are based on kVA demand instead of kVAR demand, so the real power no-load losses are worth considering as well. Can you or anyone else provide info on what the no-load kW losses for a transformer are based on kVA rating?
For Edison123, you are correct in stating that a transformer's power factor is determined by the load to which it is connected. However, as stated in my previous post, I am trying to determine the costs of having idle transformers in my facility so the no-load kVAR and kW (thanks again jghrist)losses are my concern.
As I stated in my previous post, I have 9MVA of transformer capacity and my utility metering is on the primary side. I now have detailed info on my power consumption. A typical reading is 249.6kW, 279.1kVAR, 374.4kVA, and 0.6667pf. Using electricuwe's figures (and ignoring kW losses for now), if I eliminated 6500kVA of idle transformer capacity I would also eliminate about 98 kVARs of consumption. This would change the above data to 249.6kW, 181.9kVAR, 308.8kVA, and 0.808pf. If I could waive a magic wand and replace all of the idle transformers with a single 500kVA unit, the results would be 249.6kW, 151.6kVAR, 292.0kVA, and 0.8547 pf.
So...although the actual power consumption is lower than the estimate given in my previous post, the "corrected" power factor of 0.85 is more in line with what I expected. If anyone can provide info on the no-load kW losses, it would allow me to come up with a more accurate estimate of the costs of these idle transformers.
Thanks to all and Merry Christmas
RE: Finding transformer magnetizing kVARs?
If you redistribute loads to eliminate some transformers, you will increase load losses, which are proportional to the square of the load. The load loss is given at rated load.
RE: Finding transformer magnetizing kVARs?
On seeing your load service factors, I can understand why you would want to eliminate idling giants. I agree at the service fcators you have mentioned, the total system pf will be poor. But, if you trafos had different secondary voltages, how do you propose to integrate your loads to a single trafo ? Probably further step down trafos downstream so that all are in series instead the present parallel set up ?
jghrist,
I think the total load losses will remain the same before and after load integration.
RE: Finding transformer magnetizing kVARs?
For edison123, I agree that I can eliminate no-load losses but that my load losses will remain constant. About the question of consolidating the transformers of different secondary voltages..that is the question. Other than the 8 transformers I have mentioned, there are only a few other step down transformers in my facility with a total capacity of less than 150kVA and they are obviously add-ons since the original tenants. This is because the design specification for the original tenants apparently did not allow stepdowns other than the primary substations. For primary substations, there are 4-440V delta, 2-240V delta, and 2-208Y/120V units in service for a total of 9MVA. There is also 1-2400V 1500kVA delta unit that has I haven't mentioned before now since it has already been disconnected at the primary switch.
I am thinking that I can completely remove 2-440V and one 240V units that were each dedicated to primary loads that no longer exist as long as I switch the few remaining circuits to other sources. As an example, one of these is a 750kVA 440V sub dedicated to an large oven that used to be electrically heated. The oven has since been converted to gas, so when you follow the circuit from the 750kVA transformer it leads through the unused oven electric power panel, portions of which are still energized (??), and ends up at the 150VA control power transformer for the new gas controller (LOL).
Anyway, of the remaining five, 2-440V, 1-240V, and 2-208Y/120V, one each of the 440V and the 208V transformers feed sections of the building that are mostly not used, so with the exception of a few circuits that need to be transferred, they can be deenergized at the primary switch until whenever they are truly needed again (if ever). That would leave me with 1-440V at 1000kVA, 1-240V at 750 kVA, and 1-208Y/120V at 1000kVA. This is still way more than I need, but I think at this point the cost to consolidate further would far outway any savings.
RE: Finding transformer magnetizing kVARs?
Fixed kvar and this is related to the magnitizing current
Step kvar and this is dependant on the loading
Such figures are almost known and available at manufactureres.
Try www.trafounion.com
Elecme
RE: Finding transformer magnetizing kVARs?
Load loss will increase if the load is consolidated into fewer transformers because it is proportional to the square of the load. For example:
With 2-750 kVA transformer each 1/2 loaded
%R = 1.94% %Z = 5.75% No-load/Load Loss = 40%
No-load loss = 40% · 1.94% = 0.582%
No-load loss for each xfmr = 750 · 0.582% = 4.365 kW
Total no-load loss = 2 · 4.365 = 8.73 kW
Load loss for each xfmr = 750 · 1.94% · (1/2)² = 3.6375 kW
Total load loss = 2 · 3.6375 = 7.275 kW
With 1-750 kVA transformer fully loaded
No-load loss = 750 · 0.582% = 4.365 kW
Load loss = 750 · 1.94% = 14.55 kW
In addition to the demand savings, you will save energy (kWh). The no-load losses reduction will provide 24 hrs/day energy savings, so multiply the kW savings by the number of hours in a month to find the kWh reduction. Load loss energy increases are more complicated because the transformers are not continuously loaded and the loss is proportional to the square of the load. This means that you can't use the average load but have to look at the load profile.
RE: Finding transformer magnetizing kVARs?
e.g. SKM, Inc. (DAPPER, A-FAULT, etc.), OTI, Inc. (ETAP), EDSA, etc. may help you to find the right solution.
These businesses have their web sites.
http://www.skm.com
http://www.etap.com
http://www.edsa.com
Also, check the facility computers. They may already have some software for the electrical power system modeling installed.
RE: Finding transformer magnetizing kVARs?
RE: Finding transformer magnetizing kVARs?
rhatcher, is there any chance you have access to CTs and PTs in the switchgear that serves the transformers? Excitation characteristics could be measured to compare data for actual costs. It looks like your calculations would allow scaling of a portable power analyzer or simply, voltmeter, ammeter and phase-angle meter connected to existing instrument-transformer secondaries.
RE: Finding transformer magnetizing kVARs?
Another long shot towards billing reduction with respect to no-load losses may be to offset var demand with a single small capacitor bank on the bus between transformer primaries and the utility meter.
Conceivably, small capacitor sets on the >600V secondaries could also offset transformer primary-side var losses.
RE: Finding transformer magnetizing kVARs?
RE: Finding transformer magnetizing kVARs?
To compensate the kVAr by capacitors may be a better option. It would keep the transformers normally functioning, on standby if the power consumption increases. The facility of that size should not have any problems to maintain such power distribution equipment.
It is not clear from the original posting, in what kind of business is the facility in.