Harmonics on feeder circuit

[mull982]

I was checking some of the power meters on some of our 4.16kV feeder breakers the other day when I noticed a particular feeder circuit having a somewhat high Total Harmonic Distortion (THD) on one of the phase currents. The %THD on the phase currents were:

Ia = 5.5%
Ib = 13.5%
Ic = 4.3%

I know the IEEE standard (I believe standard 519) states that the maximum %THD for current should be 5%. Seeing that I have slightly over 5% on Ia and almost 3x that on Ib, do you think this warrants a cause for concern and additional investigation? I also set the meter up to record max values and at some points it shows %THD on current as being as high as 181%.

The 4.16-480V delta-wye transformer that this feeder feeds seems somewhat louder than the remaining similar transformers that we have. Could this be a result of this high harmonic content on phase Ib?

There are no VFD's on the secondary of this transformer, and the only non-linear loads are really just lighting. I briefly looked at the harmonic spectrum and most of the distortion seemed to be from the 5th harmonic which would indicate a 6-pulse drive on the system. Any ideas what else could be causing harmonics?

On the same lineup as this feeder breaker there is a main breaker who's meter only shows 2-3% THD on the phase currents. Is this value less here, b/c the % distorted current on the feeder breaker is a much smaller portion of the toal current on the main breaker? In other words is THD the acutal % of the total current so for a smaller current on a feeder breaker it may be large but when mixed with the toal current on a main breaker it may become smaller?

 

[kingpb]

It would be interesting to know what type of facility this is, and what are the load types.

Since your utilizing MV switchgear, I would presume you have a large amount of load.

 

[jim dungar]

I usually don't care much %THD current, until it reaches the point of of causing more than 5% voltage distortion.

 

[Besoeker]

I usually don't care much %THD current, until it reaches the point of of causing more than 5% voltage distortion.

I agree. Voltage distortion should be the measure.

 

[mull982]

It would be interesting to know what type of facility this is, and what are the load types.

Since your utilizing MV switchgear, I would presume you have a large amount of load.

This is a cement plant. All 4.16kV feeders feed lineups downstream which either feed 4.16kV motors directly or serve 1000-1500kVA transformers. Total load capacity right now is at about 1000A at 4.16kV. Plant is under construction so when its all said and done we expect to see about 2500A at 4.16kV.

The particular circuit I am referencing is only feeding a 1000kVA transformer with 480V motor loads on the secondary. There are 277V lighting loads fed from an isolation transformer on the 480V system.

I usually don't care much %THD current, until it reaches the point of of causing more than 5% voltage distortion.

So we shouldn't be concerned with current distortion at all no matter how high it is as long as voltage is below 5%? Why is this? Is there not any negative effects from strictly having harmonic distortion on the current?

I believe the meters picked up max voltage THD's of about 7-9% but its hard to say how long these lasted for. It may have been intermitent. Could a transient cause such a reading?

 

[rcwilson]

You are on the right track and have the correct answers.

Voltage distortion affects other equipment. Current distortion only affects the equipment that the current flows through.

IEEE 519 current limits are for %TDD, Total Demand Distortion. ( I may have the terms incorrect, don't have my reference with me.) The limits are based on the amout of harmonics compared to the largest demand current, not the instantaneous current. Your meter reads THD, which is instantaneous. I recall the TDD limit is around 8-12% and it depends on the size of your system.

Your guesses on the transformer hum sound good. (No pun intended.)

 

[jim dungar]

So we shouldn't be concerned with current distortion at all no matter how high it is as long as voltage is below 5%? Why is this? Is there not any negative effects from strictly having harmonic distortion on the current?

I once looked at a system that had power monitoring equipment installed. They had an alarm set for %THD_I that was being triggered every weekend.

When I looked at their readings the %THD_I was >20%. But the loading was <100A on a 1000KVA 480Y/277 transformer. It is amazing how much 'trash' emergency light battery chargers put out.

And remember, I did not say never consider current.

 

[PowerQualityDoctor]

The limit for voltage harmonics is given at IEEE 519 in THD and for specific harmonics. 5% for THD, 3% for each specific harmony.

The limit for current harmonics is more complicated - there are limits for THD and specific harmonics which depends on the ratio between the load and the short circuit current. Other standards ignore short circuit value. In addition, there is a limit for 5% in TDD. TDD is the ratio between the harmonics current and the maximum current (either measured or nominal).

Logging the maximum THD is not relevant at all. If you have harmonic source which operates 24h/day, when other loads are turned off, you will receive extremely high THD but this is not harmonic issue.

About the values that you measure - I usually recommend 10% of current THD as the point to start monitoring the harmonics. Do you have over the time measurements of harmonics? If you do, you are welcome to send me the complete file and I will analyze it for you.