Zero Sequence current during XFMR inrush? 1

[JBinCA]

Greetings All,

I'd like to solicit opinions on the following question - Should there be any significant zero sequence (ground) current during the energization of a XFMR?

The event which gives rise to my question is a ground fault trip that happened when the XFMR was energized with no appreciable load. The XFMR is D-Y, 480/120-208, 300 kVA (364 FLA). It is fed by an ICCB (Insul. case BKR) with a 500 amp Long Time Pick Up (LTPU) and a 360 amp Ground Fault Pick Up (GFPU). The breaker has a microprocessor based trip unit which recorded a GF current of 448 amps! There is no other load on the ICCB. There is not neutral current sensor on this feeder as it feeds only the Delta connected XFMR.

I know that XFMR inrushes vary widely in magnitude based upong residual flux, the point in the sine wave when energized, etc. I also know that the inrush currents have a strong 2nd harmonic and tend to have a significant DC component that decays in a second or so. I have seen multiple recordings that show the DC offset balanced between a couple of phases (one offsets positive while the other ofsets negative), so it seems to me that the resultant vector sum off all currents would still be 0. I'm also aware the the DC current can lead toward CT saturation, and that this can be a problem with sensitive protections. It doesn't seem to me that a 360 Amp GFPU on a 364 amp FLA XFMR is particularly sensitive, though.

Any thoughts would be appreciated.

Best Regards,

JB

 

[JBinCA]

Reactors can be used in the pre-insertion scheme that bronzeado mentions as well.

Regards,

JBinCA

 

[crossconduction]

In the last few years I've been transitioning more to power electronics (merging 450V power system world to the SCR / IGBT electronics world). I once thought that transformers were the most beneign device in any electrical system and pales to the complexity of electronics. I had no idea how wrong I was, putting those two worlds together is the most challenging thing I've ever been involved with !

Thanks to all for replies.

 

[vima]

Since it is operating only during the energisation, you can adopt a simple scheme by using the voltage poarised earth fault relay. This can distinguish exactly between the earth fault and inrush.

 

[REM76]

Generally the inrush lasts about 0.1 second, but it could last longer depends on many factors already mentioned in other responses. I would increase the time delay (GFD) to 0.2 or even 0.3 sec in order ride through the inrush period.

 

[Bronzeado]

In (weak) transmission system I have experienced inrush current lasting more than 10s.

Herivelto