Charging Current Calculation Through Transformers

[Bigjabe]

Hi all, just a quick question about charging current for NGR sizing and such. Do the effects of a charging current pass through transformers?

Reason I ask, we’re about to add a very large network of MV cable to our 13.8kV main distribution network (at a single tap point) which would render our NGRs at the main sub undersized. We can’t upsize the NGRs for various reasons, so I was thinking of adding a 13.8-13.8kV transformer with NGR sized for the new network at the tap off point. Any thoughts?

 

[Julius Right]

At first see-for instance: IEEE 142/2007 1.4.3.1 High-resistance grounding and further something like: Cable Manufacturer’s Catalogue or Electrical Transmission and Distribution Reference Book in order to calculate the charging current. If you'll neglect the grounding returning current [through the Earth] the neutral resistance current it is equal to 3Ico.

 

[Bigjabe]

At first see-for instance: IEEE 142/2007 1.4.3.1 High-resistance grounding and further something like: Cable Manufacturer’s Catalogue or Electrical Transmission and Distribution Reference Book in order to calculate the charging current. If you'll neglect the grounding returning current [through the Earth] the neutral resistance current it is equal to 3Ico.

Right but the question is, if you have a transformer with a bunch of cables connected to its secondary side, do you count all them when calculating your charging current on the primary side?

 

[Ingenieur]

what is the xfmr mva?
what is the ngr value?
what current is flowing thru the ngr now? how do you know?

if you tap off with a new 2 winding xfmr and derive a seperate ngr neut/gnd on the sec you do not add that charging current to the ngr on the primary

 

[Bigjabe]

what is the xfmr mva?
what is the ngr value?
what current is flowing thru the ngr now? how do you know?

if you tap off with a new 2 winding xfmr and derive a seperate ngr neut/gnd on the sec you do not add that charging current to the ngr on the primary

Awesome thanks!

 

[Ingenieur]

Awesome thanks!

what is the xfmr mva
ngr value
current thru ngr

 

[Bigjabe]

Main sub is 42MVA, 50A NGR. Not sure what it actually is, but original charging current calculation was 11A. Charging current calc for new additional network is 10A.

 

[Ingenieur]

Main sub is 42MVA, 50A NGR. Not sure what it actually is, but original charging current calculation was 11A. Charging current calc for new additional network is 10A.

thanks
the charging current associated with a given xfmr/ngr will return to that xfmr's X0 (thru the ngr)