thrid harmonics vs transformer grounding

[cmelguet]

We are in the process of selecting a grounding criteria for a Dy1 power transformer. We have two alternatives:

1. Use a low resistance grounding of neutral point of secondary. (1500 A)
2. Use a high resistance grounding of neutral point of secondary. (1500 A). (300 or 400 A)

My question is regarding the third harmonics in the system and secondary.

According to my theory if you use a direct neutral grounding (no resistance) you allow third harmonic currents to flow through the neutral and the lines in the secondary, and probably dont have third harmonic voltages distortions. On the other hand if i dont earth the neutral (floating neutral) you dont allow third harmonic currents but have big distortion on the voltages on the secondary.

How the resistance grounding affect this theory?
Whats better? have third harmonic voltages or third harmonic currents?

Thanks and regards.

 

[odlanor]

cmelguet ,
what kind of load at low voltage, MVA and kVs of transnformer?

 

[electricpete]

According to my theory if you use a direct neutral grounding (no resistance) you allow third harmonic currents to flow through the neutral and the lines in the secondary, and probably dont have third harmonic voltages distortions. On the other hand if i dont earth the neutral (floating neutral) you dont allow third harmonic currents but have big distortion on the voltages on the secondary.

fwiw, your conclusion is very logical to me, assuming the secondary supplies three-phase balanced but non-sinusoidal loads like 3-phase diode/scr bridge.

=====================================
(2B)+(2B)' ?

 

[Masbibe]

Third harmonic component of current will only flow if your load has star point connected to ground and if you have as electricpete said 3-phase diode/scr bridge third harmonic component of current will not flow no mather if star point of transformer is connected or not connected to ground.



Milovan Milosevic

 

[cmelguet]

Thanks electricpete and milovan. I have some comments on this.

1. Assume a 50 MVA transformer 23 kv / 4.16 kv.

2. the load are MV motors, VFD, other transformers, etc.

3. I though transfomers itself introduce a third harmonic component due to magnetizing current. In delta transformers the third harmonic is kept in the delta of the primary. On the secondary side, if i dont allow the third harmonic to flow in the system you will get voltage distortion, is this not correct?. How the groundging (hight or low resistance) affect the third harmonic currents and voltages?

Thanks again.