CT ratio calculation for the neutral of a power transformer

Status
Not open for further replies.
D

Dan Ground

Member
Location
Spain
How should I calculate the ratio of a current transformer to be installed at the solidly grounded neutral of a power transformer?
I am calculating the CT ratio for protecting a power transformer against over-currents. The CT of each phase (in the secondary side) was calculated based on the nominal current of the transformer with a nominal power of 30 MVA @ 6.6 kV, so the CTs ratio is 3000:1

I hope I'm thinking it right (if not, you will tell), but since in a balanced three-phase system the current through the neutral should be zero, the continuous current rating of the neutral CT could be defined as a percentage of the continuous current rating in the lines. So if it applies, which should be the maximum percentage of the secondary side nominal current that I should use, maybe a percentage of maximum phase imbalance expected/allowed)?

If the former analysis does not apply, how do you define the radio of the neutral CT?

Thank you in advance.
 
Dan...

The neutral CT should be equal in ratio and type to phase CTs if the neutral is 'solidly'-grounded. This is the typical arrangement for LV-winding transformers. MV transformers, however, don't usually have the Y-connected winding's neutral solidly-grounded, although it can be done!

The reason MV systems limit ground-fault current magnitude is to minimize damage to the iron-structure of electrical apparatus rated at, for example, 6.6kV! In this case an impedance is inserted in the neutral-to-ground connection to limit fault-current, say 10-50A... usually called called a High Impedance-Grounded system. When fault-currents are limited to 100-500A, the system is referred to as Low-Impedance-Grounded! In this case the CT can be much smaller.

In most MV cases the impedance is a High-voltage resistor! Less often a reactor is used! Also, a third method is the combination of a LV-resistor connected to the secondary of small distribution-Xfmr. Its primary winding is connected between the Xfmr's neutral and ground. Of course, there are Pro-Con arguments for each application!

The subject of Neutral-Gounding is quite large! Do you want additional information?

Regards, Phil Corso
 
Last edited:
A solidly grounded transformer can have equal to, or even greater ground fault current than a 3-phase fault. Thus, as stated, the CT ratio should be the same.
 
Dan, your assumption is correct. The neutral CT size should be 3000/5 for the solid-grounded xfmr. The reason for this is to avoid over burdening the CT secondary for severe load imbalance. Also, most MV residential distribution substations solidly ground the neutral. I think most industrial substations (where the customer takes MV voltage and has their own xfmr) are impedance or resistance grounded.

Alternatively, if you are using microprocessor relays for xfmr protection then, there is a chance that the neutral CT may be shorted and not used. This is because the microprocessor relays can calculate the 3I0 that is coming from the neutral. The older electro-mechanical relays couldn't do this so, a neutral CT had to be used.

Have fun!
 
Besoeker said:
From post #1:
Click to expand...

which should be the maximum percentage of the secondary side nominal current that I should use, maybe a percentage of maximum phase imbalance expected/allowed)?
Click to expand...

Perhaps I'm wrong but, it seemed to me his statement about a balanced load was just in theory and not his specific application. I doubt his load on the lowside is completely balanced. If it is, then we would need to know what the available ground fault current is on the lowside to size the CT.
 
This was an interesting discussion I was facing the same issue ,
if it is no trouble could tell me also how to chose CT ration if there was an earthing (grounding ) transformer with no external resistor connected on the delta side of a power transformer?
 
Status
Not open for further replies.
Top