Basler Electric BE1-11g - Manual (Page 119)

9424200994 Rev N 107
























−










=












Relay 2,
Relay 1,
Relay 0,
Source 2,
Source 1,
Source 0,
Source 2,
Source 1,
Source 0,
Relay 2,
Relay 1,
Relay 0,
I
I
I
Z 0 0
0 Z 0
0 0 Z
V
V
V
V
V
V
Equation 9. Local Substation Voltage
The impedance seen by the BE1-11g is calculated in Equation 10:
Source 2,
Relay 2,
Source 2,
Relay 2,
Relay 2,
Relay 2,
Source 1,
Relay 1,
Source 1,
Relay 1,
Relay 1,
Relay 1,
Source 0,
Relay 0,
Source 0,
Relay 0,
Relay 0,
Relay 0,
Z
I
V
I
V
Z
Z
I
V
I
V
Z
Z
I
V
I
V
Z
− = =
− = =
− = =
Equation 10. Impedance Seen by the BE1-11g
If V
0,Source
and V
2,Source
are very small (Equation 11):
Source 2, Relay 2,
Source , 0 Relay 0,
Z Z
Z
− ≈
− ≈ Z
Equation 11. Impedance with Small V
0
and V
2
Source Voltages
The calculations in the BE1-11g are account for the negative factor in the above equation and hence a
180
° phase shift is implemented in the BE1-11g firmware so that a correct forward/reverse decision is
made.
The positive-sequence impedance as seen by the BE1-11g is quite a bit more complicated since V
1,Source
is
not negligible. One simple application to study is the three-phase fault and the B to C phase fault
(Equation 12):
Line 2, Source 2, Line 1, Source 1,
Line 2, Source 2, Line 1, Source 1,
Source 1,
Source 1,
BC Relay, 1,
Line 1, Source 1,
Line 1, Source 1,
Source 1,
Source 1,
se Relay,3pha 1,
Z Z Z Z
Z Z Z Z
V
V
Z
Z Z
Z Z
V
V
Z
+ + = −
+ + +
=
= −
+
=
Equation 12. Three-Phase/B to C-Phase Faults
Directional relaying would not be of much value in a radial system where all current flow will be forward. In
two source systems, as shown in Figure 67, a profile of sequence voltages in the system will show V
0
and
V
2
at either source. V
0
and V
2
will still be negligible in normal operation, and show up only during fault
conditions with their maximum value being at the fault location, with current flowing from X and Y in a
current division rule applied to the symmetrical component network that represents the fault impedances
and the fault type. Independent of the fault type and how the sequence currents divide in the system, the
sequence voltages and hence sequence impedances measured by the BE1-11g will still be dependent on
V
Relay
=V
Source
- Z
Source
I
Source
. The BE1-11g will sense the zero and negative-sequence impedances in the
opposite direction as the direction to the fault, looking back toward the source. But for the three-phase
fault the BE1-11g will sense the positive-sequence impedance in the line between the BE1-11g and the
fault location. The sensed zero-sequence current can be shifted notably when zero-sequence coupling
between adjacent lines is involved. However, since a large phase angle window of
±90° from the MTA is
being utilized for directional decisions, the direction decision is not highly sensitive to zero-sequence
coupling effects.
BE1-11g Directional Overcurrent (67) Protection