Dual Cb Application With Different Phase Ct Ratios; Figure 84: Low-Z Ref For Dual Cb Application With Different Phase Ct Ratios - GE P642 Technical Manual

Chapter 8 - Restricted Earth Fault Protection
Is1 is set to 10% of the winding nominal current:
6
= (0.1 x 90 x 10
= 39 Amps primary
= 39/400 = 0.0975 Amps secondary (approx 0.1 A)
Is2 is set to the rated current of the transformer:
Ö 3 x 132 x 10
6
= 90 x 10 / (
= 390 Amps primary
= 390/400 = 0.975 Amps secondary (approx 1 A)
Set K1 to 0% and K2 to 150%
5.2.4

DUAL CB APPLICATION WITH DIFFERENT PHASE CT RATIOS

The following diagram shows the situation where low impedance REF is being used in a dual breaker (breaker-and-
a-half) application where the phase CT ratios (CT
conventional transformer is shown, but the explanation is also applicable to autotransformers.
V00754

Figure 84: Low-Z REF for dual CB application with different phase CT ratios

The low impedance REF function be used in dual breaker (breaker-and-a-half) applications. The line CT ratios can
be different. In this case the low impedance REF differential and bias current formulae are calculated as follows:
(
I  IA
=
CTx
diff REF ( )

1
{
I max
=
bias REF ( )
2
where:
● CT and CT (T1, T2, T3, T4 or T5) are the current inputs associated with a particular winding (given in the
x y
settings HV CT Terminals, LV CT Terminals and TV CT Terminals respectively)
K1 = CT Ratio/CT
●
y
K2 = Neutral CT Ratio/CT
●
Reference: CT .
●
x
Note:
The above formulae are valid for autotransformers and conventional transformers when the Phase CT Ratios are different
(CTx ≠ CTy) and the reference is CTx
180
Ö 3 x 132 x 10 3
) / ( )
3
)
IB IC K IA
+ + +
CTx
CTx
 (
IA K IA
+
CTx CTy
1

Ratio (Scaling Factor K1)
x
Ratio (Scaling Factor K2)
x
and CT ) are different. In this example, one phase of a
x y
CT
y
CT
x
Neutral CT
N
K IB K IC
+ +
CTy CTy
1 1 1
) ( )
IB K IB
+
CTx CTy
1
)
K I N N 
+
CTy
2

( ) 
IC K IC
+
CTx CT y y
1

P64x
}
K IN
+
2
P64x-TM-EN-1.3