CT accuracy class for Transformer differential protection per IEEE 1

[lakshmanan1974]

I have few queries regarding CT accuracy class selection for Transformer differential protection as per IEEE C37.110-1996.

IEEE C37.110's worked example indicates that only one way CT lead(cable)resistance shall be considered for total burden calculation. Where as IEC/European standards require the return path resistance also to be included for 'knee point voltage' calculation (Vk=2*If*(Rct+2Rc). Could any one please clarify how oneway resistance will be enough to calculate the 'C' voltage rating to minimize the CT saturation.

Further, the indicated burden value in the worked example calculation for C200,4.16KV CT is just 1/4th (0.5 ohm) of the standard CT burden (2 ohm,B-2 burden) as given in Table 9, IEEE C57.13. Will the actual value be normally 1/4 th of standard value for all CTs..?

 

[davidbeach]

The actual value will be what ever it is. With numeric relays it will generally be far less than the standard burden.

I don't have the example at hand, but in a wye connection there is no current in the neutral for balanced operation. With no current it doesn't much matter what the burden of that conductor might be.

 

[stevenal]

The example given checks for saturation during a balanced three phase through fault. For this situation, no current flows through the CT secondary wiring neutral. In general, you will want to look at the SLG through fault also, where the impedance of the return path comes into play. This example has a neutral grounding impedance, so SLG currents are presumably too low to consider saturation.

Most differential relay instruction books will have worked examples also.

 

[lakshmanan1974]

Thanks Stevenal...Table-1 given in the same IEEE C37.110 matches with your reply. Since most of the CTs either connected in delta or wye at the CT itself, the return path resistance need not considered for symmetrical faults.

Also found the answer for the CT burden given in the worked example. The given values are actual CT secondary resistance values which are naturally less than standard CT burden.

But the typial excitation curve(shown in IEEE Std) doesn't have the CT secondary resistance value for 2000/5A CTs.Can any one provide me the approximate secondary resistance value for 2000/5A CTs.?

 

[riteshbharat]

Its hard to find standard values of CT secondary resistances as they differ from manufacturer to manufacturer. Generally they vary between 2.5 to 4 milli ohms per turn. hence 2000/5 CT could have a resistance between 1 to 1.6 ohms. This is just a guideline and depending on the design many CTs could have much different values.