Saturation factor definitions compared – Basler Electric BE1-87T User Manual

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A-8

BE1-87T Setting Notes

9171300990 Rev V

Figure A-8. CT Terminal and Excitation Voltages

This first definition of saturation relates the CT terminal voltage to the accuracy class of the CT (effective

class in the case of multi-ratio CTs). It is practical and easy to calculate since it requires only readily

available data. An application is considered reasonably secure when SF is less than 0.5

Saturation Factor Defined from the CT Excitation Curve

The definition of the saturation factor given above appears to be conservative because it assumes the

worst case ratio error. However, a closer look is required since it neglects the CT internal resistance. It

corresponds to an excitation voltage on a curve passing through point Q in Figure A-8 at which the

excitation current is 10 amperes (the maximum error allowed by the accuracy class definition). The

Rsx100 term represents the voltage drop across the CT internal resistance. A new SF which takes the

internal CT resistance into account can be defined on the excitation curve, as:

Where Ve is the internal excitation voltage (VT+Rs.IF) at the maximum fault current IF and V10` is the

voltage of the curve passing through point Q where the exciting is 10A. This voltage is practically close to

the knee-point voltage VK` which would yield nearly the same (a slightly more conservative) result.
Since in all likelihood, the excitation voltage capability of the CT will be higher ( passing through point R in

Figure A-8 for instance), the saturation factor defined on the excitation curve appears to be lower, i.e. -

more favorable. A detailed analysis can be performed to compare the two saturation factor definitions.

Saturation Factor Definitions Compared

Using the equivalent circuit in Figure A-7 and the ANSI Accuracy Class definition that the CT must be able

to source 20 times nominal current into a standard burden Zc, we now develop a comparative analysis

between the two definitions:

200

400

100

C200

C400

IFs

Rs.IF

Rs.100

10A

VK`

EXC

EXCITATION CURRENT

RELAY CURRENT

T T

V10

V10`

SF =

100

)

100

)