harmonics
harmonics
(OP)
we have two transformers supplying our facility these being 11kv to 415v rated at 2.5mva.
Both are oil cooled and one is running roughly 10 deg hotter than the other.
doing a check on the harmonics we got the following -
cool transformer hot transformer
THD 22.70% 74.60%
THD EVEN 2.50% 5.90%
THD ODD 22.80% 75.40%
K FACTOR 5.3 11.3
C FACTOR 1.9 1.9
further checks reveal that the I4 values are way different
20.79% against 73.42%.
Looking for advice and recommendations as to how to go on from here. I take it that this isnt a good position to be in.
Rugged
Both are oil cooled and one is running roughly 10 deg hotter than the other.
doing a check on the harmonics we got the following -
cool transformer hot transformer
THD 22.70% 74.60%
THD EVEN 2.50% 5.90%
THD ODD 22.80% 75.40%
K FACTOR 5.3 11.3
C FACTOR 1.9 1.9
further checks reveal that the I4 values are way different
20.79% against 73.42%.
Looking for advice and recommendations as to how to go on from here. I take it that this isnt a good position to be in.
Rugged






RE: harmonics
I don't recognize I4, what is that?
RE: harmonics
They are seperated - each supplying a portion of the site load. Each transformer supplies two UPS units and a dirty board. The two UPS supporting a critical board. The UPS units are running in parallel.
Ive had a look through google to try and identify what the I4 was - Im sure that its a neutral current - but thinking about it it may be the fourth harmonic... Google can be good but from time to time it gives you a red herring !
Were using an ION meter on the main boards and they give out readings on I1 I2 I3 I4 I5 and V1 V2 V3 V4.
so working back through the manuals to try and work this one through. I dont think that 10 degree is a major difference in temp if the transformers are equally loaded but this difference in the I4 concerns me....
Rugged
RE: harmonics
You didn't say what temperature the transformers are running at, but unless it is quite low, the hotter transformer will certainly have a shorter life than the cooler transformer. For the protection of the transformer, it would be advantageous to find the source of the harmonics and try to correct it.
RE: harmonics
So I dont think that the temperature on its own is a concern but more so the ten degree difference is a concern for me. especially when the hotter transformer is running with the increased I4. Im going to have to find that out and then see where this takes me. Thanks for the advice and if you have any more then feel free to contribute.
The UPS are indeed n+1 so I can take one out to see if need be - Im looking through the harmonics issue to see if this leads anywhere - the dirty board supplies a number of drives and this may be the problem - only thing is the other side does pretty much the same thing so I maybe comparing and missing the wood for the trees.
Rugged
RE: harmonics
its phase to phase and phase to neutral V1....V4
and
currents phase neutral and earth I1.....I5
Google did come up with the results.
Still going to ahve to look into the issue and see what happens with this I4 figure
Rugged
RE: harmonics
RE: harmonics
RE: harmonics
What is the transformer coupling? If it has one winding in delta, then the triplen harmonics are captured in it and do not propagate ahead of transformer. That might be the reason for overheating. High I4 suggests that there is also high content of triplen harmonics in the neutral wire.
Be aware that transformer manufacturers use K factor to calculate transformers in correspondence to expected power of non-linear loads. K-factor is a weighting of the harmonic load currents according to their effects on transformer heating, as derived from ANSI/IEEE C57.110. A K-factor of 1.0 indicates a linear load (no harmonics). The higher the K-factor, the greater the harmonic heating effects. When a non-linear load is supplied from a transformer, it is sometimes necessary to derate the transformer capacity to avoid overheating and subsequent insulation failure. The reason for this is that the increased eddy currents caused by the harmonics increase transformer losses and thus generate additional heat. Also, the RMS load current could be much higher than the kVA rating of the load would indicate. Hence, a transformer rated for the expected load will have insufficient capacity.
Generally, only substation transformer manufacturers specify K-factor load de-rating for their products. So, for K-factors higher than 1, the maximum transformer load is de-rated. This might mean that your transformer in not adequate according to new circumstances.
If you identify that the load is the problem, then you should either install filters or try to redistribute loads to burden the transformers equally. You can leave it as it is (if there is no harmonic propagation into the network and you are not paying penalties) and be prepared to change transformer sooner than it is planned, but this time with proper ratings. Also, all the infrastructure down from the transformer will also be overloaded due to harmonics.
RE: harmonics
The 10 degree thing isnt such a huge differentce but the harmonic issue is a concern and we will be looking to see if we can pin this one down.
Thanks for your explination on this one as it has confirmed some of my thoughts.
Will need to look into seeing if we can trace the source of those harmonics, and see if they can be identified and any potential fault sorted out.
Rugged