harmonics 1

[ruggedscot]

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

 

[davidbeach]

Are these paralleled or do they each supply separate loads? Hopefully it is the latter. If separate, what are the loads on each? If parallel, you probably have a transformer on its way out, but I don't know what might be going on in the transformer. If they are parallel, can you separate them and see if the problem goes away?

I don't recognize I4, what is that?

 

[ruggedscot]

Hi David

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

 

[davidbeach]

My guess is that the front end of one of the UPS units is going bad. Either that or the loads on one of the "dirty boards" is very different than the loads on the other dirty board. Are the UPS units redundant; can you turn them off sequentially? If you can, see if the harmonic problem goes away when one of the UPS units is disconnected from the power system.

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.

 

[ruggedscot]

The temperatures are running about 40 and 50 and the ambient is going about 11 to 13.

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

 

[ruggedscot]

Think Ive cracked the volts and currents -

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

 

[jghrist]

Are there 5 current inputs on the meter and 4 voltage inputs? If so, whatever is connected to the inputs is what is measured on I1-I5 and V1-V4.

 

[LionelHutz]

I would think that if the meter was reading a current it would display it in amps and if it was reading a harmonic it would display it as a percentage. That's usually what I see with the measurements anyways.

 

[sitl]

Your device is measuring to much harmonics in the second transformer. Since it has more than three times bigger THD (odd harmonics are the critical ones) than the first one, one variant is that the second transformer is supplying non-linear load with greater influx of harmonics than the first transformer. If this is the case, I would be also worried about UPS, since he is also liable to harmonics influence. This might be the case since UPS obviously does not have the characteristics of some modern UPS devices that have clear sinusoidal wave at the input no matter the load properties (kVA=kW, less than 3% distortion at the output). However, if load is not the case, than it is possible that UPS is malfunctioning and therefore does not draw sinusoidal voltage from the network.

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.

 

[ruggedscot]

Thanks sitl were looking into this just now and trying to follow through and see where we are picking up the 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