Harmonic issues with lightly loaded transformers 1

[arrehman1]

Can harmonic problems arise (particularly 3rd harmonic) for lightly loaded distribution transformers connected Delta Wye-grounded? I've gone through several sources but couldn't find anything substantial on this particular topic... Thank you in advance for your comments!

 

[Skogsgurra]

Lightly loaded means that the voltage is higher than when loaded. And higher voltage always means that you move more into core saturation, which in turn means more odd harmonics - especially third.

So, the answer is YES.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Power0020]

well, lightly loaded means the mangnitinzg current will be appearant as a major component of primary current (delta side, by default), this current usually contains a high thrid harmonic content, but this third harmonic current is a zero sequence one and can never run beyond a delta winding.

On wye side, load should dictate the current and accordingly the waveshape.Overvoltage on secondary may cause overfluxing (this is very rare to occure if the tap changer is not operating and the grid voltage is too high!) which have high content of 5th harmonic, not 3rd, and 5th harmonic will pass the delta and run into the grid upsteam causing harmionic issues within nearby loads (note that 5th harmonic will have a negative sequency effect and may well overheat induction motors nearby)

 

[krisys]

From the OP's description, it does not appear to be a large network transformer. The smaller transformers of lower voltage class are generally not exposed to the frequent over voltages and thus over fluxing. The resulting current has predominantly fifth harmonic components.

While energizing a transformer, second harmonic component is more predominant.

However for the steady state operation, the transformers on their own are not expected to generate harmonics. I guess,, it would be coming from the secondary side. Check what is the load connected in the transformer secondary side.

Unless the transformer design is too poor, like the working flux density is too close to the saturation level or poor core lamination material.

 

[waross]

At no load the primary current is magnetizing. Due to the slight non linearity of the magnetic circuit there will be harmonics generated. Taken as a percentage of the magnetizing current the measured harmonics may appear high. But remember that you are looking at a percentage of a relatively low current.
Once a reasonable load is placed on the transformer the percentage harmonic content may become insignificant even though the actual harmonic content may be the same.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[arrehman1]

Thank you all for your input! It is much appreciated

Here is another question to further my understanding - for a lightly loaded transformer where the magnetizing current as a percentage is pretty comparable to the load current, would 3rd harmonics (generated by magnetizing current) contribute to power quality issues on the load side of the transformer? Or are these harmonics isolated to the transformer core itself?

For example - suppose I had a 50kVA single phase pole-top distribution transformer. And suppose my load was only 500VA (1% of full load rating). Would I get power quality issues (3rd harmonics) on the load side of the transformer as a result of the exciting current?

I've been reading some papers and it appears that transformer exciting current is typically 0.5 to 1% of full load current... and the exciting current generates approx. 20% of 3rd order harmonics. Do these numbers feel right?

Thank you for your help!

 

[Skogsgurra]

Your first question: No. For two reasons.
1. Current harmonics, even if a large percentage of total current, will not generate any voltage distosion problems in a well designed system. Well designed means with Isc at least 15 to 20 times nominal load current.
2. Your primary is Delta, I Think. Then the triplenes are all in phase and you will never see them in the L-L voltage. And, since the secondary voltages are replicas of the primary voltage, you will not have ant third harmonics on the secondary either. Fifth and other harmonics,yes. But no third.

Your second question: Yes, plausible numbers.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

The exciting current values for power transformers is quite low 0.15 % (10 MVA)-0.03 % (1500 MVA)of rated load current.For distribution transformers 3-1 % depending on the rating.The third harmonic current content in three phase delta primary transformers will not be reflected in secondary side as third harmonic will be circulating in delta.
But in case of single phase units, it can be reflected on secondary, but the values will be very small, of the order of 0.6 % of rated load current. As long as there is third harmonic current in excitation current,there will be no third harmonic component in voltages.The purpose of third harmonics in exciting current is to make the secondary voltage sinusoidal.

 

[prc]

Let me make a correction.With the wound core technology and modern core steels,maximum excitation current in distribution transformers is max 0.5 % of full load current. Even with 30 % (usually less than 20 %) third harmonics, it will be less than 0.15% of full load current.

 

[arrehman1]

Thank you all for your contribution! I have a much better understanding now