Table of Contents
Advertisement
5 SETPOINTS
The issue of maloperation due to heavy external faults resulting in CT saturation is handled by a programma-
ble timer. The timer provides the necessary delay required for the external fault to be cleared by the appropri-
ate external protection with the added benefit that if the RGF element remains picked up after the timer expires
the 745 will operate and clear the fault. This approach provides backup protection. Since the RGF element is
targeted at detecting low magnitude internal winding fault currents, the time delay for internal faults is of little
consequence since sensitivity and security are the critical parameters.
Calculate
Maximum Phase
Maximum Phase
Ia
Current
Ib
Calculate
Ic
3I
3I
o
o
Ig
Measure
Ig
Figure 5–13: RESTRICTED GROUND FAULT IMPLEMENTATION
b) RESTRICTED GROUND FAULT SETTINGS EXAMPLE
Consider a transformer with the following specifications:
10 MVA, 33 kV to 11 kV, 10% Impedance, Delta/Wye30
Rg = 6.3 ohms
CT Ratio = 600 / 1 Amp
Rated Load Current = I
Maximum Phase-to-Ground Fault Current = I
For a winding fault point at 5% distance from the neutral:
×
I
I
=
0.05
(
)
fault
gf max
From Figure 5–11: FAULT CURRENTS VS. FAULT POINT FROM NEUTRAL on page 5–60, we see that the I
increase due to the fault is negligible and therefore 3 I
Therefore: maximum phase current = I
I
=
3 I
–
I
=
0
–
gd
0
g
I
50 A
gd
Slope
=
=
---------- -
--------------- -
I
525 A
max
Time Delay: dependent on downstream protection coordination (100 ms typical)
GE Power Management
Imax
Slope = I /I
Slope = I /I
Slope > Setpoint
Slope > Setpoint
I
I
gd
gd
Calculate
I
I
> Setpoint
> Setpoint
gd
gd
|3I - I |
|3I - I |
o
o
g
g
= 10 MVA / (√3 x 11 kV) = 525 Amps
rated
gf(max)
×
=
0.05
1000 A
=
50 A
= I
= 525 A (approx.), and
max
rated
I
0 50 A
fault
=
–
=
0.08
---------------------- -
----------- -
CT Ratio
600
=
9.5% (select Slope Setting = 9%)
745 Transformer Management Relay
745
max
max
gd
gd
AND
= 11 kV / (√ 3 x 6.3) = 1000 Amps
= 0 (approx.)
o
×
CT
=
Pickup Setting
5.6 S4 ELEMENTS
Timer
OUTPUT
0 to 0.5s
0 to 0.5s
5-61
5
p
Advertisement
Table of Contents
