CT Sizing of Transformer Ground Cable

For REF protection all the four CTs to be of same specification and turns ratio.

Earthing of transformer neutral is to be after CT.

As can be seen in the drawing, there is one CT, on the right side, for grounding cable (green cable).How do we calculate the size of the CT? What class should be used?

Local_Eng:

What I see, is the only one smaller green wire connected to the neutral bar. It is Ground Return Sensing Method, isn't it? You just need to size the CT to operate at the current that the grounding wire can carry.

I assume that you are talking about non-restricted earthfault protection. If so, the CT size depends upon the required protection level that you want. At the same tim if you keep it so small, then during a high fault, the CT might saturate. It has to be a balancing act. The class can be 5P10.

That's earth fault for the system, not restricted earth fault.

To be honest the earth (ground) looks far to small, what size are the conductors?

Yes.. It's an earth fault for the system.

The cable size is 185mm2. How do we calculate the size of the CT then?

Your going to have to look at the entire transformer set up if you want restricted earth fault. The neutral earth connection will have to be moved for a start.

Your existing 5000/5A CT's are too small for the maximum transformer rated current of 7600A. What are they used for at the moment, or is it just the E/F C/T you refer to?

What country is this in (this is an international forum)?

Are you using a neutral earthing (or grounding) resistor to limit the earth fault current (may explain the small size of the earth wire)?

Do you know what your (approximate) fault current is and what transformer relay protection settings (especially the time delays relating to earth fault disconnection) are?

Thought I better ask for clarity.

Dear all,

A REF protection requires all CTs with the same CT ratios. Therefore, total five CTs (for 3 phase lines, 1 neutral and 1 neutral earth conductor) are all 5000/5A.

If the protection relay is micro-processor type, the CT ratio for the neutral earth conductor can be selected with a different CT ratio from other 4 CTs. Please refer to your relay manual.

Regards

What does REF mean?

Dear all,

REF: Restricted earth fault is differential protection for transformer Wye winding.

Each of all conductors brought out from the winding must have a CT of the same ratios. The total of the CT secondary currents is zero, if no earth fault on the winding. If non-zero, an earth fault is inside the winding.

Regards

@ TonyS: I'm sorry for the limited information. Actually the 5MVA transformer installation is in progress. The photo is the existing transformer with EF issue, and we have to follow it.

For the new transformer of 5MVA, the ip LN fault current is 211kA with 8000/5 CT.

Is it ok if I use 800/5 CT to detect the EF?

REF requires 4 matched C/T's but you've obviously got that sorted.

As to the general E/F C/T I think you need to talk to others on the board, my main concern is saturation of the C/T under fault conditions. I've never fitted a neutral/earth C/T that was underrated.

it depends.. know the acceptable unbalance current as well as the maximum fault current that could pass through the current transformer.. you may change the ratio of your CT if it will lessen the burden that the CT will handle..

Dear friend,

There are two things, one is EARTH FAULT i e it is a line protection,second REF it is for transformer winding earth fault protection.

It is not necessary to provide C.T. in neutral conductor for line earth fault detection but earth fault current can be sense by providing an earth fault relay in between common point of R_Y_B O/C element and C.T.star point.

For REF protection we needs all C.T. of same ratio and same protection class generally it is PS class having ratio near to but more than full load of the protected object.

RAKHOLIA B.K.

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