Neutral current in Transformers 2

[prc]

In a substation feeding to arc furnace transformers continuous neutral current is noticed. Transformer 80 MVA 220/33 kV Ynyn connection with a stabilising tertiary winding.. HV transformer neutral is continuously showing 6-7 A irrespective of load.Because of this to avoid tripping on charging REF relay had to be set at 20 %. What can be the reason for this neutral current on HV? How to prevent this?
The current values on HV- 24,22,25,7 A (R,Y, B,Yn)(9.5 % of rated load)
LV- 167,163,168,0.21 A

HV- 3.8,3.8,4.0,6 A
LV- 27,26,28,0.2 A

 

[jghrist]

Arc furnaces have highly distorted currents and even though the average phase currents may be balanced, there will still be an unbalance on an instantaneous basis, so there will be a neutral current. You could eliminate the neutral current by using a delta-delta transformer. Interestingly, this will not eliminate third harmonic current distortion because the third harmonic is not balanced. I have measured significant third harmonic current on the primary of a delta-delta transformer serving an arc furnace.

 

[waross]

You have either a slight voltage unbalance on the primary or a slight phase angle error on the primary or both. You may check your voltage balance by measuring the phase to neutral voltages. You may check for phase angle errors by checking phase to phase voltages. A possible cause may be lack of transpositions on the transmission line feeding the sub. Three possible cures:
1. Correct the unbalance or phase angle errors. (This may not be reasonably possible.)
2. Open the delta.
3. Float the primary neutral. (Not recommended because of possible over-voltage switching surges.)

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

 

[prc]

jghrist, there is HV neutral current even at no-load.There is no neutral current in secondary.So the load cannot be the reason for the neutral current.

Thank you waross for your points esp the transmission line transposition issue.Let me check up.

 

[FPelec]

I definitely agree with waross; the most probable cause should be presence of a zero-sequence component in the primary voltage source, especially if the transformer under question in one of the few with a closed delta winding and with grounded primary neutral.
This issue sometimes happens in our utility, in such cases we float the primary neutral (which for our distribution transformers is generally fully insulated at primary voltage),
If the stabilizing delta winding is buried, this should be the only solution.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[sajuthomas]

Check your earth value and make sure earth resistance value is perfect as per your requirement. This is one chance of your problem

 

[spc300]

The use of filtered relay to detect the fundamental frequency current may resolve your sensitiviy concerns.

 

[PHovnanian]

6-7 A irrespective of load.

If this is with the rectifiers/converters disconnected, then I'm with waross.

Unless there's a mismatched tap on the transformer someplace, its due to in imbabance on the incoming supply.