GE 745 Instruction Manual page 171

CHAPTER 5: SETPOINTS
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
10 MVA, 33 kV to 11 kV, 10% impedance, delta/wye 30,
R = 6.3 ohms,
g
Phase CT ratio = 600 / 1 A,
Rated load current I
rated
Maximum phase-to-ground fault current I
For a winding fault point at 5% distance from the neutral:
I 0.05 I
=
fault
From FIGURE 5–24: Fault currents vs. points from neutral on page 5–73, we see that the I
increase due to the fault is negligible and therefore 3I
maximum phase current = I
I 3I I 0 --------------------------------------------
= – = –
gd 0 g
phase CT primary
I
50 A
gd
Slope --------- - ------------- - 9.5% (select slope setting = 9%)
= = =
I 525 A
max
Time delay: dependent on downstream protection coordination (100 ms typical)
The winding 1 restricted ground fault setpoints are described below:
• W1(3) RESTD GND FAULT PICKUP: Enter the minimum level of ground differential
current (in units of phase CT primary associated with the winding, where the restricted
ground fault is set) for the winding 1(3) restricted ground fault element.
• W1(3) RESTD GND FAULT SLOPE: Enter a slope percentage (of ground differential
current to maximum line current).
• W1(3) RESTD GND FAULT DELAY: Enter the time that the winding 1(3) restricted
ground fault element must remain picked up before the element operates.
FIGURE 5–27: Restricted ground fault scheme logic
= 10 MVA / ( 3 × 11 kV) = 525 A,
= 11 kV / (
gf(max)
× 0.05 × 1000 A
=
( )
gf max
o
= I = 525 A (approximately), and
max rated
I
50 A
fault
0 ---------- -
= – =
600
S4 ELEMENTS
3 × 6.3) = 1000 A.
50 A
=
= 0 (approximately). Therefore, the
0.08 CT × pickup setting
=
(EQ 5.10)
p
(EQ 5.11)
5–75